For this assignment, make sure you post your initial response to the Discussion Area by the due date assigned.
Start reviewing and responding to the postings of your classmates as early in the week as possible. Respond to at least two of your classmates. Participate in the discussions (which are your peer responses) by providing a statement of clarification, providing a point of view with a rationale, challenging an aspect of the discussion, or by indicating a relationship between two or more lines of reasoning in the discussion. Your peer responses should be about 150 words each, follow APA format, and include academic citations. Complete your participation for this assignment by the end of the week.
To successfully complete this assignment, first read the following exercises from the Laboratory Manual: Exercise 15: Histology of Nervous Tissue, Exercise 16: Neurophysiology of Nerve Impulses, and Exercise, and 19 The Spinal Cord and Spinal Nerves.
Student Discussion Assignment
- Briefly identify and describe the structure and function of a typical motor neuron.
- Briefly discuss the function(s) of axons, dendrites, and the role of Schwann cells in the formation of the myelin sheath.
- Briefly identify and discuss one major excitatory and one inhibitory neurotransmitter not mentioned in the Excitatory and Inhibitory Neurotransmitters lecture.
- Briefly identify and describe the superficial and internal anatomy of the spinal cord
Write your responses in a minimum of 250 words in APA format.
As in all assignments, cite your sources in your work and provide references for the citations in APA format. Support your work, using your course lectures and textbook readings. Helpful APA guides and resources are available in the South University Online Library. Below are guides that are located in the library and can be accessed and downloaded via the South University Online Citation Resources: APA Style page. The American Psychological Association website also provides detailed guidance on formatting, citations, and references at APA Style.
For this assignment, make sure you post your initial response to the Discussion Area by the due date assigned. Start reviewing and responding to the postings of your classmates as early in the week as
Neuroglia The neuroglia (“nerve glue”), or glial cells, of the CNS include astrocytes, oligodendrocytes, microglial cells, and ependymal cells ( Figure 15.1 ). The neuroglia found in the PNS include Schwann cells, also called neurolemmocytes, and satellite cells. Neuroglia serve the needs of the delicate neurons by supporting and protecting them. In addition, they act as phagocytes (microglial cells), myelinate the cytoplasmic extensions of the neurons (oligodendrocytes and Schwann cells), play a role in capillary-neuron exchanges, and control the chemical environment around neurons (astrocytes). Although neuroglia resemble neurons in some ways (many have branching cellular extensions), they are not capable of generating and transmitting nerve impulses. In this exercise, we focus on the highly excitable neurons. Neurons Neurons, or nerve cells, are the basic functional units of nervous tissue. They are highly specialized to transmit messages from one part of the body to another in the form of nerve impulses. Although neurons differ structurally, they have many identifiable features in common ( Figure 15.2a and b). All have a cell body from which slender processes extend. The cell body is both the biosynthetic center of the neuron and part of its receptive region. Neuron cell bodies make up the gray matter of the CNS, and form clusters there that are called nuclei. In the PNS, clusters of neuron cell bodies are called ganglia. The neuron cell body contains a large round nucleus surrounded by cytoplasm. Two prominent structures are found in the cytoplasm: One is cytoskeletal elements called neurofibrils, which provide support for the cell and a means to transport substances throughout the neuron. The second is darkly staining structures called chromatophilic substance, clusters of rough endoplasmic reticulum and ribosomes involved in the metabolic activities of the cell. Figure 15.2 Structure of a typical motor neuron. (a) Diagram. (b) Photomicrograph (4503). (c) Enlarged diagram of a synapse. Neurons have two types of processes. Dendrites are receptive regions that bear receptors for neurotransmitters released by the axon terminals of other neurons. Axons, also called nerve fibers when they are long, form the impulse generating and conducting region of the neuron. The white matter of the nervous system is made up of axons. Neurons may have many dendrites, but they have only a single axon. The axon may branch, forming one or more processes called axon collaterals. In general, a neuron is excited by other neurons when their axons release neurotransmitters close to its dendrites or cell body. The electrical signal produced travels across the cell body and if it is great enough, it elicits a regenerative electrical signal, a nerve impulse or action potential, that travels down the axon. The axon in motor neurons begins just distal to a slightly enlarged cell body structure called the axon hillock (Figure 15.2a). The axon ends in many small structures called axon terminals, or terminal boutons, which form synapses with neurons or effector cells. These terminals store the neurotransmitter chemical in tiny vesicles. Each axon terminal of the neuron is separated from the cell body or dendrites of the next neuron by a tiny gap called the synaptic cleft (Figure 15.2c). Most long nerve fibers are covered with a fatty material called myelin, and such fibers are referred to as myelinated fibers. Nerve fibers in the peripheral nervous system are typically heavily myelinated by special supporting cells called Schwann cells, which wrap themselves tightly around the axon in jelly roll fashion ( Figure 15.3 , p. 260). The wrapping is the myelin sheath. Since the myelin sheath is formed by many individual Schwann cells, it is a discontinuous sheath. The gaps, or indentations, in the sheath are called myelin sheath gaps or nodes of Ranvier (see Figure 15.2a). Figure 15.3 Myelination of a nerve fiber (axon) by Schwann cells. (a) Nerve fiber myelination. (b) Electron micrograph of cross section through a myelinated axon (75003). Within the CNS, myelination is accomplished by neuroglia called oligodendrocytes (see Figure 15.1d). Because of its chemical composition, myelin electrically insulates the fibers and greatly increases the transmission speed of nerve impulses. Activity 1 Identifying Parts of a Neuron Study the illustration of a typical motor neuron (Figure 15.2), noting the structural details described and then identify these structures on a neuron model. Obtain a prepared slide of the ox spinal cord smear, which has large, easily identifiable neurons. Study one representative neuron under oil immersion and identify the cell body; the nucleus; the large, prominent “owl’s eye” nucleolus; and the granular chromatophilic substance. If possible, distinguish the axon from the many dendrites. Sketch the neuron in the space provided below, and label the important anatomical details you have observed. Compare your sketch to Figure 15.2b. Obtain a prepared slide of teased myelinated nerve fibers. Identify the following (use Figure 15.4 as a guide): myelin sheath gaps, axon, Schwann cell nuclei, and myelin sheath. Figure 15.4 Photomicrograph of a small portion of a peripheral nerve in longitudinal section (400×). Sketch a portion of a myelinated nerve fiber in the space provided below, illustrating a myelin sheath gap. Label the axon, myelin sheath, myelin sheath gap, and Schwann cell nucleus. Do the gaps seem to occur at consistent intervals, or are they irregularly distributed? Explain the functional significance of this finding: Neuron Classification Neurons may be classified on the basis of structure or of function. Classification by Structure Figure 15.5 Classification of neurons according to structure. (a) Classification of neurons based on structure (number of processes extending from the cell body). (b) Structural variations within the classes. Structurally, neurons may be differentiated by the number of processes attached to the cell body ( Figure 15.5a ). In unipolar neurons, one very short process, which divides into peripheral and central processes, extends from the cell body. Functionally, only the most distal parts of the peripheral process act as receptive endings; the rest acts as an axon along with the central process. Unipolar neurons are more accurately called pseudounipolar neurons because they are derived from bipolar neurons. Nearly all neurons that conduct impulses toward the CNS are unipolar. Bipolar neurons have two processes attached to the cell body. This neuron type is quite rare, typically found only as part of the receptor apparatus of the eye, ear, and olfactory mucosa. Many processes issue from the cell body of multipolar neurons, all classified as dendrites except for a single axon. Most neurons in the brain and spinal cord and those whose axons carry impulses away from the CNS fall into this last category. Activity 2 Studying the Microscopic Structure of Selected Neurons Obtain prepared slides of pyramidal cells of the cerebral cortex, Purkinje cells of the cerebellar cortex, and a dorsal root ganglion. As you observe them under the microscope, try to pick out the anatomical details and compare the cells to Figure 15.5b and Figure 15.6 . Notice that the neurons of the cerebral and cerebellar tissues (both brain tissues) are extensively branched; in contrast, the neurons of the dorsal root ganglion are more rounded. The many small nuclei visible surrounding the neurons are those of bordering neuroglia. Which of these neuron types would be classified as multipolar neurons? As unipolar? Figure 15.6 Photomicrographs of neurons. (a) Pyramidal neuron from the cerebral cortex (6003). (b) Purkinje cell from the cerebellar cortex (6003). (c) Dorsal root ganglion cells (2453). Figure 15.7 Classification of neurons on the basis of function. Sensory (afferent) neurons conduct impulses from the body’s sensory receptors to the central nervous system; most are unipolar neurons with their cell bodies in ganglia in the peripheral nervous system (PNS). Motor (efferent) neurons transmit impulses from the CNS to effectors (muscles). Interneurons complete the communication line between sensory and motor neurons. They are typically multipolar, and their cell bodies reside in the CNS. Classification by Function In general, neurons carrying impulses from sensory receptors in the internal organs (viscera), the skin, skeletal muscles, joints, or special sensory organs are termed sensory, or afferent, neurons ( Figure 15.7 ). The receptive endings of sensory neurons are often equipped with specialized receptors that are stimulated by specific changes in their immediate environment. (The structure and function of these receptors are considered separately in Exercise 22, General Sensation.) The cell bodies of sensory neurons are always found in a ganglion outside the CNS, and these neurons are typically unipolar. Neurons carrying impulses from the CNS to the viscera and/or body muscles and glands are termed motor, or efferent, neurons. Motor neurons are most often multipolar, and their cell bodies are almost always located in the CNS. The third functional category of neurons is interneurons, which are situated between and contribute to pathways that connect sensory and motor neurons. Their cell bodies are always located within the CNS, and they are multipolar neurons structurally. Structure of a Nerve In the CNS, bundles of axons are called tracts. In the PNS, bundles of axons are called nerves. Wrapped in connective tissue coverings, nerves extend to and/or from the CNS and visceral organs or structures of the body periphery such as skeletal muscles, glands, and skin. Like neurons, nerves are classified according to the direction in which they transmit impulses. Sensory (afferent) nerves conduct impulses only toward the CNS. A few of the cranial nerves are pure sensory nerves. Motor (efferent) nerves carry impulses only away from the CNS. The ventral roots of the spinal cord are motor nerves. Nerves carrying both sensory (afferent) and motor (efferent) fibers are called mixed nerves; most nerves of the body, including all spinal nerves, are mixed nerves. Within a nerve, each axon is surrounded by a delicate connective tissue sheath called an endoneurium, which insulates it from the other neuron processes adjacent to it. The endoneurium is often mistaken for the myelin sheath; it is instead an additional sheath that surrounds the myelin sheath. Groups of axons are bound by a coarser connective tissue, called the perineurium, to form bundles of fibers called fascicles. Finally, all the fascicles are bound together by a white, fibrous connective tissue sheath called the epineurium, forming the cordlike nerve ( Figure 15.8 , p. 264). In addition to the connective tissue wrappings, blood vessels and lymphatic vessels serving the fibers also travel within a nerve. Figure 15.8 Structure of a nerve showing connective tissue wrappings. (a) Three-dimensional view of a portion of a nerve. (b) Photomicrograph of a cross-sectional view of part of a peripheral nerve (5103). Activity 3 Examining the Microscopic Structure of a Nerve Use the compound microscope to examine a prepared cross section of a peripheral nerve. Using the photomicrograph (Figure 15.8b) as an aid, identify axons, myelin sheaths, fascicles, and endoneurium, perineurium, and epineurium sheaths. If desired, sketch the nerve in the space below. Neurons are excitable; they respond to stimuli by producing an electrical signal. Excited neurons communicate—they transmit electrical signals to neurons, muscles, glands, and other tissues of the body, a property called conductivity. In a resting neuron, the interior of the cell membrane is slightly more negatively charged than the exterior surface ( Figure 16.1 ). The difference in electrical charge produces a resting membrane potential across the membrane that is measured in millivolts. As in most cells, the predominant intracellular cation is K+; Na+ is the predominant cation in the extracellular fluid. In a resting neuron, Na+ leaks into the cell and K+ leaks out. The resting membrane potential is maintained by the sodium-potassium pump, which transports Na+ back out of the cell and K+ back into the cell. Figure 16.1 The action potential. (a) Resting membrane potential (RMP). There is an excess of positive ions at the external cell surface, with Na+ the predominant extracellular fluid ion and K+ the predominant intracellular ion. The plasma membrane has a low permeability to Na+. (b) Depolarization—reversal of the RMP. Application of a stimulus changes the membrane permeability, and Na+ ions are allowed to diffuse rapidly into the cell. (c) Generation of the action potential. If the stimulus is of adequate intensity, the depolarization wave spreads rapidly along the entire length of the membrane. (d) Repolarization—reestablishment of the RMP. The negative charge on the internal plasma membrane surface and the positive charge on its external surface are reestablished by diffusion of K+ ions out of the cell, proceeding in the same direction as in depolarization. (e) In the resting state, Na+ ions leak into the cell and K+ ions leak out. The RMP is maintained by the active sodium-potassium pump. (f) The action potential is caused by permeability changes in the plasma membrane. When a neuron receives an excitatory stimulus, the membrane becomes more permeable to sodium ions, and Na+ diffuses down its electrochemical gradient into the cell. As a result, the interior of the membrane becomes less negative (Figure 16.1b), an event called depolarization. If the stimulus is great enough to depolarize the initial segment of the axon to threshold, an action potential is generated. The initial segment of the axon in multipolar neurons is at the axon hillock of the cell body. In peripheral sensory neurons, the initial segment is just proximal to the sensory receptor, far from the cell body located in the dorsal root ganglion. When the threshold voltage is reached, the membrane permeability to Na+ increases rapidly (Figure 16.1f). As the neuron depolarizes, the polarity of the membrane reverses: the interior surface now becomes more positive than the exterior (Figure 16.1c). As the membrane permeability to Na+ falls, the permeability to K+ increases, and K+ diffuses down its electrochemical gradient and out of the cell (Figure 16.1d). Once again the interior of the membrane becomes more negative than the exterior. This event is called repolarization. As you can see, the action potential is a brief reversal of the neuron’s membrane potential. The period of time when Na+ permeability is rapidly changing and maximal, and the period immediately following when Na+ permeability becomes restricted, together correspond to a time when the neuron is insensitive to further stimulation and cannot generate another action potential. This period is called the absolute refractory period. As Na+ permeability is gradually restored to resting levels during repolarization, an especially strong stimulus to the neuron may provoke another action potential. This period of time is the relative refractory period. Restoration of the resting membrane potential restores the neuron’s normal excitability. Once generated, the action potential propagates along the entire length of the axon. It is never partially transmitted. Furthermore, it retains a constant amplitude and duration; the action potential is not small when a stimulus is small and large when a stimulus is large. Since the action potential of a given neuron is always the same, it is said to be an all-or-none response. When an action potential reaches the axon terminals, it causes neurotransmitter to be released. The neurotransmitter may be excitatory or inhibitory to the next cell in the transmission chain, depending on the receptor types on that cell. (The experiments in this exercise consider only excitatory neurotransmitters.) Physiology of Nerves The sciatic nerve is a bundle of axons that vary in diameter. An electrical signal recorded from a nerve represents the summed electrical activity of all the axons in the nerve. This summed activity is called a compound action potential. Unlike an action potential in a single axon, the compound action potential varies in shape according to which axons are producing action potentials. When a nerve is stimulated by external electrodes, as in our experiments, the largest axons reach threshold first and generate action potentials. Higher-intensity stimuli are required to produce action potentials in smaller axons. In this laboratory session, you will investigate the functioning of a nerve by subjecting the sciatic nerve of a frog to various types of stimuli and blocking agents. Work in groups of two to four to lighten the workload. Dissection Isolating the Gastrocnemius Muscle and Sciatic Nerve Don gloves to protect yourself from any parasites the frogs might have. Obtain a pithed frog from your instructor, and bring it to your laboratory bench. Also obtain dissecting instruments, a tray, thread, two glass rods or probes, and frog Ringer’s solution at room temperature from the supply area. Figure 16.2 Removal of the sciatic nerve and gastrocnemius muscle. Cut through the frog’s skin around the circumference of the trunk. Pull the skin down over the trunk and legs. Make a longitudinal cut through the abdominal musculature, and expose the roots of the sciatic nerve (arising from spinal nerves 7–9). Ligate the nerve and cut the roots proximal to the ligature. Use a glass probe to expose the sciatic nerve beneath the posterior thigh muscles. Ligate the calcaneal tendon, and cut it free distal to the ligature. Release the gastrocnemius muscle from the connective tissue of the knee region. Prepare the sciatic nerve as illustrated ( Figure 16.2 ). Place the pithed frog on the dissecting tray, dorsal side down. Make a cut through the skin around the circumference of the frog approximately halfway down the trunk, and then pull the skin down over the muscles of the legs. Open the abdominal cavity, and push the abdominal organs to one side to expose the origin of the glistening white sciatic nerve, which arises from the last three spinal nerves. Once the sciatic nerve has been exposed, keep it continually moist with room-temperature frog Ringer’s solution. Using a glass probe, slip a piece of thread moistened with frog Ringer’s solution under the sciatic nerve close to its origin at the vertebral column. Make a single ligature (tie it firmly with the thread), and then cut through the nerve roots to free the proximal end of the sciatic nerve from its attachments. Using a glass rod or probe, carefully separate the posterior thigh muscles to locate and then free the sciatic nerve, which runs down the posterior aspect of the thigh. Tie a piece of thread around the calcaneal tendon of the gastrocnemius muscle, and then cut through the tendon distal to the ligature to free the gastrocnemius muscle from the heel. Using a scalpel, carefully release the gastrocnemius muscle from the connective tissue in the knee region. At this point you should have completely freed both the gastrocnemius muscle and the sciatic nerve, which innervates it. Activity 1 Stimulating the Nerve In this first set of experiments, stimulation of the nerve and generation of the compound action potential will be indicated by contraction of the gastrocnemius muscle. Because you will make no mechanical recording (unless your instructor asks you to), you must keep complete and accurate records of all experimental procedures and results. Obtain a glass slide or plate, ring stand and clamp, stimulator, electrodes, salt (NaCl), forceps, filter paper, 0.01% hydrochloric acid (HCl) solution, Bunsen burner, and heat-resistant mitts. With glass rods, transfer the isolated muscle-nerve preparation to a glass plate or slide, and then attach the slide to a ring stand with a clamp. Allow the end of the sciatic nerve to hang over the free edge of the glass slide, so that it is easily accessible for stimulation. Remember to keep the nerve moist at all times. You are now ready to investigate the response of the sciatic nerve to various stimuli, beginning with electrical stimulation. Using the stimulator and platinum electrodes, stimulate the sciatic nerve with single shocks, gradually increasing the intensity of the stimulus until the threshold stimulus is determined. The muscle as a whole will just barely contract at the threshold stimulus. Record the voltage of this stimulus: Threshold stimulus: V Continue to increase the voltage until you find the point beyond which no further increase occurs in the strength of muscle contraction—that is, the point at which the maximal contraction of the muscle is obtained. Record this voltage below. Maximal stimulus: V Delivering multiple or repeated shocks to the sciatic nerve causes volleys of impulses in the nerve. Shock the nerve with multiple stimuli. Observe the response of the muscle. How does this response compare with the response to the single electrical shocks? To investigate mechanical stimulation, pinch the free end of the nerve by firmly pressing it between two glass rods or by pinching it with forceps. What is the result? Test chemical stimulation by applying a small piece of filter paper saturated with HCl solution to the free end of the nerve. What is the result? Drop a few crystals of salt (NaCl) on the free end of the nerve. What is the result? Now test thermal stimulation. Wearing the heat-resistant mitts, heat a glass rod for a few moments over a Bunsen burner. Then touch the rod to the free end of the nerve. What is the result? What do these muscle reactions say about the excitability and conductivity of neurons? Most neurons within the body are stimulated to the greatest degree by a particular stimulus (in many cases, a chemical neurotransmitter), but a variety of other stimuli may trigger nerve impulses, as seen in the experimental series just conducted. Generally, no matter what type of stimulus is present, if the affected part responds by becoming activated, it will always react in the same way. Familiar examples are the well-known phenomenon of “seeing stars” when you receive a blow to the head or press on your eyeball, both of which trigger impulses in your optic nerves. Activity 2 Inhibiting the Nerve Numerous physical factors and chemical agents can impair the ability of nerve fibers to function. For example, deep pressure and cold temperature both block nerve impulse transmission by preventing the local blood supply from reaching the nerve fibers. Local anesthetics, alcohol, and numerous other chemicals are also very effective at blocking nerve transmission. Ether, one such chemical blocking agent, will be investigated first. Since ether is extremely volatile and explosive, perform this experiment in a vented hood. Don safety goggles before beginning this procedure. Obtain another glass slide or plate, absorbent cotton, ether, and a pipette. Clamp the new glass slide to the ring stand slightly below the first slide of the apparatus setup for the previous experiment. With glass rods, gently position the sciatic nerve on this second slide, allowing a small portion of the nerve’s distal end to extend over the edge. Place a piece of absorbent cotton soaked with ether under the midsection of the nerve on the slide, prodding it into position with a glass rod. Using a voltage slightly above the threshold stimulus, stimulate the distal end of the nerve at 2-minute intervals until the muscle fails to respond. (If the cotton dries before this, re-wet it with ether using a pipette.) How long did it take for anesthesia to occur? sec Once anesthesia has occurred, stimulate the nerve beyond the anesthetized area, between the ether-soaked pad and the muscle. What is the result? Remove the ether-soaked pad and flush the nerve fibers with Ringer’s solution. Again stimulate the nerve at its distal end at 2-minute intervals. How long does it take for recovery? Does ether exert its blocking effect on the nerve or on the muscle fibers? Explain your reasoning. If sufficient frogs are available and time allows, you may do the following experiment. Curare was used by some South American Indian tribes to tip their arrows. Victims struck with these arrows were paralyzed, but the paralysis was not accompanied by loss of sensation. Prepare another frog as described in steps 1 through 3 of the dissection instructions. However, in this case position the frog ventral side down on a frog board. In exposing the sciatic nerve, take care not to damage the blood vessels in the thigh region, as the success of the experiment depends on maintaining the blood supply to the muscles of the leg. Expose and gently tie the left sciatic nerve so that it can be lifted away from the muscles of the leg for stimulation. Slip another length of thread under the nerve, and then tie the thread tightly around the thigh muscles to cut off circulation to the leg. The sciatic nerve should be above the thread and not in the ligated tissue. Expose and ligate the sciatic nerve of the right leg in the same manner, but this time do not ligate the thigh muscles. Take great care in handling tubocurarine, because it is extremely poisonous. Do not get any on your skin. Get a syringe and needle, and a vial of 0.5% tubocurarine, the main toxin in curare. Obtain 1 cc of the tubocurarine by injecting 1 cc of air into the vial through the rubber membrane, and then drawing up 1 cc of the chemical into the syringe. Slowly and carefully inject 1 cc of the tubocurarine into the dorsal lymph sac of the frog. The dorsal lymph sacs are located dorsally at the level of the scapulae, so introduce the needle of the syringe just beneath the skin between the scapulae and toward one side of the spinal column. Wait 15 minutes after injection of the tubocurarine to allow it to be distributed throughout the body in the blood and lymphatic stream. Then electrically stimulate the left sciatic nerve. Be careful not to touch any of the other tissues with the electrode. Gradually increasing the voltage, deliver single shocks until the threshold stimulus is determined for this specimen. Threshold stimulus: V Anatomy of the Spinal Cord Enclosed within the vertebral canal of the spinal column, the spinal cord extends from the foramen magnum of the skull to the first or second lumbar vertebra, where it terminates in the cone-shaped conus medullaris. Like the brain, the cord is cushioned and protected by meninges (Figure 19.1). The dura mater and arachnoid mater extend beyond the conus medullaris, approximately to the level of S2, and the filum terminale, a fibrous extension of the conus medullaris covered by pia mater, extends even farther to attach to the posterior coccyx (Figure 19.2). Denticulate ligaments, saw-toothed shelves of pia mater, secure the spinal cord to the dura mater (Figure 19.2c). The cerebrospinal fluid–filled meninges extend beyond the end of the spinal cord, providing an excellent site for removing cerebrospinal fluid without endangering the spinal cord. Analysis of the fluid can provide information about suspected bacterial or viral infections of the meninges. This procedure, called a lumbar puncture, is usually performed below L3. In humans, 31 pairs of spinal nerves arise from the spinal cord and pass through intervertebral foramina to serve the body area near their level of emergence. The cord is about the circumference of a thumb for most of its length, but there are enlargements in the cervical and lumbar areas where the nerves serving the upper and lower limbs arise. Because the spinal cord does not extend to the end of the vertebral column, the lumbar and sacral spinal nerve roots must travel through the vertebral canal before reaching their intervertebral foramina. This collection of spinal nerve roots is called the cauda equina (Figure 19.2a and d) because of its similarity to a horse’s tail. Figure 19.1 (p. 314) illustrates the spinal cord in cross section. Notice that the gray matter looks like a butterfly. Figure 19.1 Cross section through the spinal cord illustrating its relationship to the surrounding vertebra. Figure 19.2 Gross structure of the spinal cord, dorsal view. Activity 1 Identifying Structures of the Spinal Cord Obtain a three-dimensional model or laboratory chart of a cross section of a spinal cord, and identify its structures as they are described in Table 19.1 on p. 314 and shown in Figure 19.3 . Table 19.1 Anatomy of the Spinal Cord in Cross Section (Figure 19.3) Structure Description Gray matter Located in the center of the spinal cord and shaped like a butterfly or the letter H. Areas of the gray matter are individually named and described below. Dorsal (posterior) horns Posterior projections of the gray matter that contain primarily interneurons and the axons of sensory neurons. Ventral (anterior) horns Anterior projections of the gray matter that contain the cell bodies of somatic motor neurons and some interneurons. Lateral horns Small lateral projections that are present only in the thoracic and lumbar regions of the gray matter. When present, they contain the cell bodies of motor neurons of the autonomic nervous system. Gray commissure The cross bar of the H that surrounds the central canal. Central canal A narrow central cavity that is continuous with the ventricles of the brain. Dorsal root A nerve root that fans out into dorsal rootlets to connect to the posterior spinal cord. It contains the axons of sensory neurons. Dorsal root ganglion A bulge on the dorsal root that contains the cell bodies of sensory neurons. Ventral root A nerve root that is formed by the ventral rootlets connected to the anterior spinal cord. It contains the axons of motor neurons. Spinal nerve Formed by the fusion of dorsal and ventral roots. They are mixed nerves because they contain both sensory and motor fibers. White matter Forms the outer region of the spinal cord and is composed of myelinated and nonmyelinated axons organized into tracts. Ventral median fissure The anterior, more open of the two grooves that partially divide the spinal cord into left and right halves. Dorsal median sulcus The posterior, shallower of the two grooves that partially divide the spinal cord into left and right halves. White columns Each side of the spinal cord has three funiculi (columns): dorsal (posterior) funiculus, lateral funiculus, and ventral (anterior) funiculus, which are further divided into tracts. Ascending (sensory) tracts Carry sensory information from the sensory neurons to the brain. Descending (motor) tracts Carry motor instructions from the brain to the body’s muscles and glands. Figure 19.3 Anterior view of the spinal cord in cross section and its meninges, thoracic region. Why this Matters Shingles Did you have chickenpox as a child or perhaps were immunized with the vaccine? If you had chickenpox, you have a one in three chance of experiencing shingles in your lifetime. Receiving the vaccine appears to prevent both chickenpox and shingles. The virus that causes both diseases is the varicella-zoster virus. After a person recovers from chickenpox, the virus lies dormant (latent) in the cell body of sensory neurons in the dorsal root ganglion. The virus can lie dormant in multiple levels of the spinal cord and in cranial nerves, as well. Later in life, the virus can be reactivated and travel within a sensory neuron to the surface of the skin, usually in response to stress or a weakened immune response. The later activation of the virus, called shingles, results in a rash and extreme pain. Even though the spinal cord is protected by meninges and cerebrospinal fluid in the vertebral canal, it is highly vulnerable to traumatic injuries, such as might occur in an automobile accident. When the cord is damaged, both motor and sensory functions are lost in body areas normally served by that region and lower regions of the spinal cord. Injury to certain spinal cord areas may even result in a permanent flaccid paralysis of both legs, called paraplegia, or of all four limbs, called quadriplegia. Activity 2 Identifying Spinal Cord Tracts With the help of your textbook, label the spinal cord diagram (Figure 19.4) with the tract names that follow. Each tract is represented on both sides of the cord, but for clarity, label the motor tracts on the right side of the diagram and the sensory tracts on the left side of the diagram. Color ascending (sensory) tracts blue, and descending (motor) tracts red. Then fill in the functional importance of each tract beside its name below. As you work, try to be aware of how the naming of the tracts is related to their anatomical distribution. Figure 19.4 Cross section of the spinal cord showing the relative positioning of its major tracts. Dorsal columns Fasciculus gracilis Fasciculus cuneatus Dorsal spinocerebellar Ventral spinocerebellar Lateral spinothalamic Ventral spinothalamic Lateral corticospinal Ventral corticospinal Rubrospinal Tectospinal Vestibulospinal Medial reticulospinal Lateral reticulospinal Dissection Spinal Cord Obtain a dissecting tray and instruments, disposable gloves, and a segment of preserved spinal cord (from a cow or saved from the brain specimen used in Exercise 17). Identify the dura mater and the weblike arachnoid mater. What name is given to the third meninx, and where is it found? Peel back the dura mater, and observe the fibers making up the dorsal and ventral roots. If possible, identify a dorsal root ganglion. Cut a thin cross section of the cord, and identify the ventral and dorsal horns of the gray matter with the naked eye or with the aid of a dissecting microscope. Notice that the dorsal horns are more tapered than the ventral horns and that they extend closer to the edge of the spinal cord. Also identify the central canal, white matter, ventral median fissure, dorsal median sulcus, and dorsal, ventral, and lateral funiculi. Obtain a prepared slide of the spinal cord (cross section) and a compound microscope. Examine the slide carefully under low power (refer to Figure 19.5 to identify spinal cord features). Observe the shape of the central canal. Is it basically circular or oval? Name the type of neuroglia that lines this canal. Figure 19.5 Light micrograph of cross section of the spinal cord (8×). Spinal Nerves and Nerve Plexuses The 31 pairs of human spinal nerves arise from the fusions of the ventral and dorsal roots of the spinal cord and are therefore mixed nerves containing both sensory and motor fibers (see Figure 19.6). There are 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves (Co1) (Figure 19.6a). The first pair of spinal nerves leaves the vertebral canal between the base of the skull and the atlas, but all the rest exit via the intervertebral foramina. The first through seventh pairs of cervical nerves emerge above the vertebra for which they are named; C8 emerges between C7 and T1. (Notice that there are 7 cervical vertebrae, but 8 pairs of cervical nerves.) The remaining spinal nerve pairs emerge from the spinal cord area below the same-numbered vertebra. Almost immediately after emerging, each nerve divides into dorsal and ventral rami. Thus each spinal nerve is only about 1 or 2 cm long. The rami, like the spinal nerves, contain both motor and sensory fibers. The smaller dorsal rami serve the skin and musculature of the posterior body trunk at their approximate level of emergence. The ventral rami of spinal nerves T2 through T12 pass anteriorly as the intercostal nerves to supply the muscles of intercostal spaces, and the skin and muscles of the anterior and lateral trunk. The ventral rami of all other spinal nerves form complex networks of nerves called nerve plexuses. These plexuses primarily serve the muscles and skin of the limbs. The fibers of the ventral rami unite in the plexuses (with a few rami supplying fibers to more than one plexus). From the plexuses the fibers diverge again to form peripheral nerves, each of which contains fibers from more than one spinal nerve. (The four major nerve plexuses and their chief peripheral nerves are described in Tables 19.2– 19.5 and illustrated in Figures 19.7– 19.10. Their names and site of origin should be committed to memory). The tiny S5 and Co1 spinal nerves contribute to a small plexus that serves part of the pelvic floor. Figure 19.6 Human spinal nerves. (a) Spinal nerves are shown at right; ventral rami and the major nerve plexuses are shown at left. (b) Relative distribution of the ventral and dorsal rami of a spinal nerve (cross section of thorax). Cervical Plexus and the Neck The cervical plexus ( Figure 19.7 and Table 19.2 , p. 319) arises from the ventral rami of C1 through C5 to supply muscles of the shoulder and neck. The major motor branch of this plexus is the phrenic nerve, which arises from C3 through C4 (plus some fibers from C5) and passes into the thoracic cavity in front of the first rib to innervate the diaphragm. The primary danger of a broken neck is that the phrenic nerve may be severed, leading to paralysis of the diaphragm and cessation of breathing. A rhyme to help you remember the rami (roots) forming the phrenic nerves is “C3, C4, C5 keep the diaphragm alive.” Brachial Plexus and the Upper Limb The brachial plexus is large and complex, arising from the ventral rami of C5 through C8 and T1 (Table 19.3, p. 319). The plexus, after being rearranged consecutively into trunks, divisions, and cords, finally becomes subdivided into five major peripheral nerves (Figure 19.8, p. 318). Figure 19.7 The cervical plexus. The nerves colored gray connect to the plexus but do not belong to it. (See Table 19.2.) Figure 19.8 The brachial plexus. (See Table 19.3.) Table 19.2 Branches of the Cervical Plexus (Figure 19.7) Nerves Ventral rami Structures served Cutaneous Branches (Superficial) Lesser occipital C2 (C3) Skin on posterolateral aspect of neck Greater auricular C2, C3 Skin of ear, skin over parotid gland Transverse cervical C2, C3 Skin on anterior and lateral aspect of neck Supraclavicular (medial, intermediate, and lateral) C3, C4 Skin of shoulder and clavicular region Motor Branches (Deep) Ansa cervicalis (superior and inferior roots) C1–C3 Infrahyoid muscles of neck (omohyoid, sternohyoid, and sternothyroid) Segmental and other muscular branches C1–C5 Deep muscles of neck (geniohyoid and thyrohyoid) and portions of scalenes, levator scapulae, trapezius, and sternocleidomastoid muscles Phrenic C3–C5 Diaphragm (sole motor nerve supply) Table 19.3 Branches of the Brachial Plexus (Figure 19.8) Nerves Cord and ventral rami Structures served Axillary Posterior cord (C5, C6) Muscular branches: deltoid and teres minor muscles Cutaneous branches: some skin of shoulder region Musculocutaneous Lateral cord (C5–C7) Muscular branches: flexor muscles in anterior arm (biceps brachii, brachialis, coracobrachialis) Cutaneous branches: skin on anterolateral forearm (extremely variable) Median By two branches, one from medial cord (C8, T1) and one from the lateral cord (C5–C7) Muscular branches to flexor group of anterior forearm (palmaris longus, flexor carpi radialis, flexor digitorum superficialis, flexor pollicis longus, lateral half of flexor digitorum profundus, and pronator muscles); intrinsic muscles of lateral palm and digital branches to the fingers Cutaneous branches: skin of lateral two-thirds of hand on ventral side and dorsum of fingers 2 and 3 Ulnar Medial cord (C8, T1) Muscular branches: flexor muscles in anterior forearm (flexor carpi ulnaris and medial half of flexor digitorum profundus); most intrinsic muscles of hand Cutaneous branches: skin of medial third of hand, both anterior and posterior aspects Radial Posterior cord (C5–C8, T1) Muscular branches: posterior muscles of arm and forearm (triceps brachii, anconeus, supinator, brachioradialis, extensors carpi radialis longus and brevis, extensor carpi ulnaris, and several muscles that extend the fingers) Cutaneous branches: skin of posterolateral surface of entire limb (except dorsum of fingers 2 and 3) Dorsal scapular Branches of C5 rami Rhomboid muscles and levator scapulae Long thoracic Branches of C5–C7 rami Serratus anterior muscle Subscapular Posterior cord; branches of C5 and C6 rami Teres major and subscapularis muscles Suprascapular Upper trunk (C5, C6) Shoulder joint; supraspinatus and infraspinatus muscles Pectoral (lateral and medial) Branches of lateral and medial cords (C5–T1) Pectoralis major and minor muscles The axillary nerve, which serves the muscles and skin of the shoulder, has the most limited distribution. The large radial nerve passes down the posterolateral surface of the arm and forearm, supplying all the extensor muscles of the arm, forearm, and hand and the skin along its course. The radial nerve is often injured in the axillary region by the pressure of a crutch or by hanging one’s arm over the back of a chair. The median nerve passes down the anteromedial surface of the arm to supply most of the flexor muscles in the forearm and several muscles in the hand (plus the skin of the lateral surface of the palm of the hand). Hyperextend at the wrist to identify the long, obvious tendon of your palmaris longus muscle, which crosses the exact midline of the anterior wrist. Your median nerve lies immediately deep to that tendon, and the radial nerve lies just lateral to it. The musculocutaneous nerve supplies the arm muscles that flex the forearm and the skin of the lateral surface of the forearm. The ulnar nerve travels down the posteromedial surface of the arm. It courses around the medial epicondyle of the humerus to supply the flexor carpi ulnaris, the ulnar head of the flexor digitorum profundus of the forearm, and all intrinsic muscles of the hand not served by the median nerve. It supplies the skin of the medial third of the hand, both the anterior and posterior surfaces. Trauma to the ulnar nerve, which often occurs when the elbow is hit, is commonly referred to as “hitting the funny bone” because of the odd, painful, tingling sensation it causes. Severe injuries to the brachial plexus cause weakness or paralysis of the entire upper limb. Such injuries may occur when the upper limb is pulled hard and the plexus is stretched (as when a football tackler yanks the arm of the halfback), and by blows to the shoulder that force the humerus inferiorly (as when a cyclist is pitched headfirst off his motorcycle and grinds his shoulder into the pavement). Lumbosacral Plexus and the Lower Limb The lumbosacral plexus, which serves the pelvic region of the trunk and the lower limbs, is actually a complex of two plexuses, the lumbar plexus and the sacral plexus (Figures 19.9 and 19.10). These plexuses interweave considerably and many fibers of the lumbar plexus contribute to the sacral plexus. The Lumbar Plexus The lumbar plexus arises from ventral rami of L1 through L4 (and sometimes T12). Its nerves serve the lower abdominopelvic region and the anterior thigh ( Table 19.4 and Figure 19.9 ). The largest nerve of this plexus is the femoral nerve, which passes beneath the inguinal ligament to innervate the anterior thigh muscles. The cutaneous branches of the femoral nerve (median and anterior femoral cutaneous and the saphenous nerves) supply the skin of the anteromedial surface of the entire lower limb. Figure 19.9 The lumbar plexus (anterior view.) (a) Spinal roots (ventral rami) and major branches of the lumbar plexus. (b) Distribution of the major peripheral nerves of the lumbar plexus in the lower limb. (See Table 19.4.) Figure 19.10 The sacral plexus (posterior view). (a) The spinal roots (ventral rami) and major branches of the sacral plexus. (b) Distribution of the major peripheral nerves of the sacral plexus in the lower limb. (See Table 19.5.) Table 19.4 Branches of the Lumbar Plexus (Figure 19.9) Nerves Ventral rami Structures served Femoral L2–L4 Skin of anterior and medial thigh via anterior femoral cutaneous branch; skin of medial leg and foot, hip and knee joints via saphenous branch; motor to anterior muscles (quadriceps and sartorius) of thigh and to pectineus, iliacus Obturator L2–L4 Motor to adductor magnus (part), longus, and brevis muscles, gracilis muscle of medial thigh, obturator externus; sensory for skin of medial thigh and for hip and knee joints Lateral femoral cutaneous L2, L3 Skin of lateral thigh; some sensory branches to peritoneum Iliohypogastric L1 Skin of lower abdomen and hip; muscles of anterolateral abdominal wall (internal obliques and transversus abdominis) Ilioinguinal L1 Skin of external genitalia and proximal medial aspect of the thigh; inferior abdominal muscles Genitofemoral L1, L2 Skin of scrotum in males, of labia majora in females, and of anterior thigh inferior to middle portion of inguinal region; cremaster muscle in males The Sacral Plexus Arising from L4 through S4, the nerves of the sacral plexus supply the buttock, the posterior surface of the thigh, and virtually all sensory and motor fibers of the leg and foot ( Table 19.5 , p. 322, and Figure 19.10 ). The major peripheral nerve of this plexus is the sciatic nerve, the largest nerve in the body. The sciatic nerve leaves the pelvis through the greater sciatic notch and travels down the posterior thigh, serving its flexor muscles and skin. In the popliteal region, the sciatic nerve divides into the common fibular nerve and the tibial nerve, which together supply the balance of the leg muscles and skin, both directly and via several branches. Table 19.5 Branches of the Sacral Plexus (Figure 19.10) Nerves Ventral rami Structures served Sciatic nerve L4–S3 Composed of two nerves (tibial and common fibular) in a common sheath; they diverge just proximal to the knee • Tibial (including sural, medial and lateral plantar, and medial calcaneal branches) L4–S3 Cutaneous branches: to skin of posterior surface of leg and sole of foot Motor branches: to muscles of back of thigh, leg, and foot (hamstrings [except short head of biceps femoris], posterior part of adductor magnus, triceps surae, tibialis posterior, popliteus, flexor digitorum longus, flexor hallucis longus, and intrinsic muscles of foot) • Common fibular (superficial and deep branches) L4–S2 Cutaneous branches: to skin of anterior and lateral surface of leg and dorsum of foot Motor branches: to short head of biceps femoris of thigh, fibularis muscles of lateral leg, tibialis anterior, and extensor muscles of toes (extensor hallucis longus, extensors digitorum longus and brevis) Superior gluteal L4–S1 Motor branches to gluteus medius and minimus and tensor fasciae latae Inferior gluteal L5–S2 Motor branches to gluteus maximus Posterior femoral cutaneous S1–S3 Skin of buttock, posterior thigh, and popliteal region; length variable; may also innervate part of skin of calf and heel Pudendal S2–S4 Supplies most of skin and muscles of perineum (region encompassing external genitalia and anus and including clitoris, labia, and vaginal mucosa in females, and scrotum and penis in males); external anal sphincter Injury to the proximal part of the sciatic nerve, as might follow a fall or disc herniation, results in a number of lower limb impairments. Sciatica, characterized by stabbing pain radiating over the course of the sciatic nerve, is common. When the sciatic nerve is completely severed, the leg is nearly useless. The leg cannot be flexed and the foot drops into plantar flexion (it dangles), a condition called footdrop. Activity 3 Identifying the Major Nerve Plexuses and Peripheral Nerves Identify each of the four major nerve plexuses and their major nerves (Figures 19.7– 19.10) on a large laboratory chart or model. Trace the courses of the nerves and relate those observations to the information provided in Tables 19.2– 19.5. Group Challenge Fix the Sequence Listed below are sets consisting of a plexus, a nerve, and a muscle possibly innervated by the listed nerve. Working in a small group, one student will act as the recorder. For each set, the recorder will write each structure on a separate Post-it Note. If the sequence is correct, simply write “all correct.” If incorrect, identify the structure that doesn’t match, cross it out on the Post-it Note, and write the correct structure. Note that there may be more than one way to correct the sequence. Explain how you corrected the sequence for each numbered set. Cervical plexus, phrenic nerve, diaphragm Brachial plexus, ulnar nerve, palmaris longus Brachial plexus, radial nerve, triceps brachii Cervical plexus, axillary nerve, deltoid Lumbar plexus, femoral nerve, gracilis Lumbar plexus, sciatic nerve, common fibular nerve, tibialis anterior Sacral plexus, superior gluteal nerve, gluteus maximus
