Chapter 13- PNS
reflex arc
1 sensory(afferent) division- PNS detects and delivers stimulus ot CNS 2. CNS- integrates stimulus 3. Motor(efferent) division- PNS delivers motor response from CNS to effectors
Golgi Tendon Reflex
A polysynaptic reflex that also protects muscles and tendons from damage is the Golgi tendon reflex. However, the effect of this reflex is opposite that of the simple stretch reflex—it causes muscle relaxation. As its name implies, this reflex involves Golgi tendon organs. When tension in the muscle and tendon increases dramatically, the Golgi tendon organs signal the spinal cord and cerebellum. This leads to inhibition of the motor neurons supplying the contracting agonist muscles and simultaneous activation of antagonist muscles. As a result, the agonist muscle relaxes and the antagonist muscles contract, protecting the muscle and tendon from potentially damaging forces. The Golgi tendon reflex is part of the reason why you should perform certain weight-lifting exercises with a spotter—if the weight is too heavy, the Golgi tendon reflex might cause you to drop the weight. Although the reflex would prevent muscle and tendon damage, you could drop the weight on yourself and be seriously injured.
crossed-extension reflex
A second reflex that occurs simultaneously with the flexion reflex is the crossed-extension reflex, which triggers extension of the opposite limb to help preserve balance. Note in Figure 13.20b that the spinal interneurons can synapse on two different populations of motor neurons: some that supply the flexor muscles of the affected limb, and some that supply the extensor muscles of the opposite limb. For this reason, when the flexion reflex is stimulated, step 2a, flexion of the affected limb, and step 2b, extension of the opposite limb, occur simultaneously. In our example, muscles that flex the right lower limb contract at the same time as muscles that extend the left lower limb. Together, these two motions allow for withdrawal from the painful stimulus (the broken glass) while providing balance and postural support. Without the crossed-extension reflex and flexion reflex happening simultaneously, we would likely lose our balance and fall when withdrawing our limb from the painful stimulus.
structure of sensory neurons
A typical somatic sensory neuron is shown in Figure 13.13. Recall that we refer to these as first-order neurons because they are the first neurons to detect and transmit sensory stimuli along the way to the primary somatosensory cortex in the CNS (see Chapter 12). As you can see, first-order somatic sensory neurons are pseudounipolar neurons with three main components—a cell body and two axons:
Sensory Division
As its name implies, the sensory division of the PNS consists of sensory (or afferent) neurons that detect various sensory stimuli and bring them to the CNS (see the left side of Figure 13.1). The sensory division has two anatomical subdivisions: The somatic Sensory division contains neurons that detect sensory stimuli from the skin and structures of the musculoskeletal system. Its neurons respond to stimuli of the general senses that arise external to the body, such as touch, temperature, and pain, as well as those originating from within the body, such as muscle stretch and the concentration of different chemicals in the body's fluids. The somatic sensory division also contains special sensory neurons that are responsible for detecting stimuli of the special senses (vision, hearing, equilibrium, taste, and smell). The visceral sensory division contains neurons that relay sensory stimuli from the organs of the abdominopelvic and thoracic cavities. Its neurons detect internal changes such as blood pressure and the degree to which organs such as the urinary bladder are stretched.
brachial plexus nerves
Axillary nerve. The axillary nerve (AKS-ih-lehr-ee) is a branch of the posterior cord of the brachial plexus. True to its name, the axillary nerve serves structures near the axilla, including the deltoid and teres minor muscles and the skin over the deltoid region. Radial nerve. As the posterior cord descends in the posterior arm, it becomes the radial nerve. In the posterior arm, this nerve supplies the triceps brachii muscle, after which it supplies most of the muscles in the forearm that extend the hand. A small branch continues down to the hand, where it supplies the skin over the posterior thumb, the second and third digits, and the lateral half of the fourth digit. Musculocutaneous nerve. The continuation of the lateral cord of the brachial plexus is the musculocutaneous nerve (muss′-kyoo-loh-kyoo-TAYN-ee-us). This nerve supplies most of the muscles that flex the forearm, including the biceps brachii muscle, and the skin over the lateral forearm. Median nerve. If you look back at Figure 13.7a, you can see that the lateral and medial cords unite to form the median nerve, so named because it runs approximately down the middle of the arm and forearm. In the forearm this nerve innervates the muscles in the forearm that flex the hand and the digits, and in the hand it supplies some of the intrinsic hand muscles. It also provides sensory innervation to the skin over the anterior thumb, the second and third digits, and the lateral half of the fourth digit. As the median nerve enters the hand, it passes under a band of connective tissue that you encountered in the muscular system chapter, called the flexor retinaculum (see Figure 9.19). This anatomical arrangement unfortunately sometimes causes the median nerve to become inflamed and "trapped" under the retinaculum, resulting in carpal tunnel syndrome. This painful condition is generally treated by making a small incision in the retinaculum and releasing the pressure on the nerve. Ulnar nerve. The ulnar nerve is the continuation of the medial cord of the brachial plexus. It begins in the posterior arm, then passes over the medial epicondyle of the humerus to enter the forearm. At this point, the ulnar nerve is very superficial, a fact of which you may become acutely aware if you bang your elbow on a hard surface. The painful, tingling, electrical sensation that results from hitting your "funny bone" (although it is decidedly unfunny) is the effect of the slight contusion this nerve receives with such an injury. Once the ulnar nerve enters the forearm, it travels along the ulna and supplies the muscles in the forearm that flex the hand but that are not supplied by the median nerve, most of the intrinsic hand muscles, and the skin of the fifth digit and the medial side of the fourth digit.
types of sensory receptors
Different types of sensory receptors respond to stimuli with different speed, intensity, and duration. Some receptors respond rapidly and with high intensity but stop sending the stimuli after a certain period, a phenomenon known as adaptation. These rapidly adapting receptors are important for detecting the initiation of stimuli, but they ignore ongoing stimuli. Rapidly adapting receptors are the reason why you can walk around your home in search of your sunglasses only to find out that they were on top of your head the whole time. Other receptors, called slowly adapting receptors, respond to stimuli with constant action potentials that do not diminish with time. The dull, throbbing pain you feel for a week after spraining your ankle is the work of slowly adapting receptors.
Thoracic spinal nerves
Except for T1, the nerves coming from the thoracic spinal cord do not form plexuses. Instead, each posterior ramus serves the deep back muscles, and each anterior ramus travels between two ribs as an intercostal nerve. The pattern of innervation for the intercostal nerves is as follows: The intercostal branch of the anterior ramus of T1 travels in the first intercostal space, where it innervates the intercostal muscles and the skin of the axilla. The anterior rami of T2−T6 travel in their respective spaces and serve the intercostal muscles and the skin of the chest wall. The anterior rami of T7−T12 also travel in their respective spaces and serve the intercostal muscles and overlying skin. However, they continue from the intercostal spaces into the anterior abdominal wall, where they supply the abdominal muscles and the overlying skin.
sacral plexus
Inferior to the lumbar plexuses against the posterior pelvic wall we find the right and left sacral plexuses, which form from the anterior rami of spinal nerves L4−S4 (Figure 13.9a). The nerves from the sacral plexuses innervate structures of the pelvis and gluteal region and much of the lower extremity. Like the nerves of the brachial and lumbar plexuses, those of the sacral plexus separate into anterior and posterior divisions.
Functional Overview of the PNS
Let's look at how the PNS functions and how those functions are integrated with the CNS. We start with the sensory arm of the PNS. Sensory neurons detect stimuli at structures known as sensory receptors. The detected stimuli are transmitted along the sensory neurons via spinal or cranial nerves to sensory neurons of the CNS, which transmit the impulses to the cerebral cortex for interpretation and integration. Depending on the nature of the sensory stimuli, an appropriate motor response is initiated. For example, if you hear the microwave beep, your CNS interprets this input to mean that your dinner is ready, and initiates a motor response so you can walk to the microwave and remove, and presumably eat, your dinner. The motor response is initiated by commands from the motor areas of the brain to the upper motor neurons. These impulses travel to the spinal cord, where they synapse on local interneurons and then lower motor neurons of the PNS. The lower motor neurons then carry the impulses to the appropriate muscle fibers via specific cranial or spinal nerves, where they trigger their contraction. Later modules will delve into the specifics of these processes.
chapter 13 summary
Lower motor neuron disorders have very different symptoms from those of upper motor neuron disorders (note that upper motor neuron disorders are not peripheral neuropathies, as they impact neurons of the CNS). These conditions can result from damage or disease anywhere along the pathways from the motor cortices to the spinal cord. The body's initial response to upper motor neuron damage is spinal shock, characterized by paralysis. Spinal shock is believed to result from the "shock" experienced by spinal cord circuits when input from the upper motor neurons is removed. For reasons that are not well understood, after a few days spinal shock wears off and in its place spasticity often develops. Spasticity is characterized by an increase in stretch reflexes, an increase in muscle tone, and a phenomenon called clonus, or the alternating contraction and relaxation of a stretched muscle. Spasticity is likely due to a loss of normal inhibition mediated by upper motor neurons. In addition to spasticity, the Babinski sign (bah-BIN-skee) develops. This sign is elicited by stroking the bottom of the foot, which in a healthy adult will cause flexion of the toes, a response known as the plantar reflex (Figure 13.21a). However, a patient with an upper motor neuron disorder will extend the hallux (first toe) and splay out the other toes (Figure 13.21b). A positive Babinski sign is often present in infants up to 18 months old and does not signify pathology, but the same response in an adult is always considered abnormal.
upper motor neuron disordes
Lower motor neuron disorders have very different symptoms from those of upper motor neuron disorders (note that upper motor neuron disorders are not peripheral neuropathies, as they impact neurons of the CNS). These conditions can result from damage or disease anywhere along the pathways from the motor cortices to the spinal cord. The body's initial response to upper motor neuron damage is spinal shock, characterized by paralysis. Spinal shock is believed to result from the "shock" experienced by spinal cord circuits when input from the upper motor neurons is removed. For reasons that are not well understood, after a few days spinal shock wears off and in its place spasticity often develops. Spasticity is characterized by an increase in stretch reflexes, an increase in muscle tone, and a phenomenon called clonus, or the alternating contraction and relaxation of a stretched muscle. Spasticity is likely due to a loss of normal inhibition mediated by upper motor neurons. In addition to spasticity, the Babinski sign (bah-BIN-skee) develops. This sign is elicited by stroking the bottom of the foot, which in a healthy adult will cause flexion of the toes, a response known as the plantar reflex (Figure 13.21a). However, a patient with an upper motor neuron disorder will extend the hallux (first toe) and splay out the other toes (Figure 13.21b). A positive Babinski sign is often present in infants up to 18 months old and does not signify pathology, but the same response in an adult is always considered abnormal.
The Role of Lower Motor Neurons
Lower motor neurons are multipolar neurons whose cell bodies are located within the CNS (in either the anterior horn of the spinal cord or the brainstem) and whose large, myelinated axons are located in the PNS. As we covered in the muscle tissue chapter, each lower motor neuron innervates skeletal muscle fibers within a single skeletal muscle (see Chapter 10). Groups of lower motor neurons that innervate the same muscle, called motor neuron pools, are located together in the anterior horn in rod-shaped clusters. Most lower motor neurons within a motor neuron pool are the large α-motor neurons. α-Motor neurons stimulate skeletal muscle fibers to contract by the excitation-contraction mechanism (see Chapter 10). Also present within a motor neuron pool are smaller γ-motor neurons. These lower motor neurons innervate muscle fibers called intrafusal fibers that are part of specialized stretch receptors, discussed in Module 13.6.
referred pain
Many spinal nerves carry both first-order somatic and visceral sensory neurons. This anatomical arrangement has a curious consequence known as referred pain, whereby pain that originates in an organ is perceived as cutaneous pain. The pain is generally located along the dermatome for that nerve. For example, pain caused by a heart attack is often perceived as pain in the anterior chest wall and left arm. This is because the first-order visceral sensory neurons from the heart travel with the first-order somatic sensory neurons of the T1-T5 dermatomes. As you can see in Figure 13.15a, these dermatomes map to the anterior chest wall and arm. Other examples of referred pain are shown in Figure 13.15b. Dermatomes are also clinically relevant in shingles, also known as herpes zoster. Shingles is due to the chickenpox virus (this is not the same virus that causes either form of herpes, but the viruses do belong to the same family). After the initial infection, the virus usually persists in the cell bodies of first-order somatic sensory neurons in posterior root ganglia, where it remains inactive, often for decades, until it is reactivated for unknown reasons. In patients with compromised immune systems, the virus proliferates and migrates down the axon of the peripheral process to the skin, causing a painful rash with blisters. The virus may also migrate to neighboring ganglia and infect one or more entire dermatomes. This leads to the characteristic presentation of a painful unilateral rash (on one side of the body) along dermatomes. No known treatments cure shingles, but it may be prevented effectively with a vaccine.
five types of receptors
Mechanoreceptor. Mechanoreceptors (mek′-ah-noh-ree-SEP-terz) are encapsulated exteroceptors or interoceptors found in the skin, the musculoskeletal system, and many different organs. They depolarize in response to anything that mechanically deforms the tissue, including external stimuli such as light touch and vibration and internal stimuli such as stretch and pressure. The mechanism behind their sensory transduction is found within their specialized ion channels, called mechanically gated ion channels, which are shown in Figure 13.11. Thermoreceptor. As you can probably guess by their name, thermoreceptors are exteroceptors that respond to thermal stimuli, depolarizing in response to temperature changes. Most thermoreceptors are slowly adapting receptors. Separate thermoreceptors detect hot and cold stimuli. Chemoreceptor. A chemoreceptor (kee′-moh-ree-SEP-ter) is an exteroceptor or interoceptor that depolarizes in response to certain chemicals in body fluids or in the air. Chemicals that are specific for the receptor bind and trigger ion channels to open, which generates a receptor potential and perhaps an action potential. Internal chemoreceptors detect the hydrogen ion concentration, the level of carbon dioxide, and the level of oxygen in the body's fluids. External chemoreceptors are responsible for the special senses of smell and taste. Photoreceptor. Photoreceptors, found only in the eye, are special sensory exteroceptors whose membrane potentials change in response to light. Nociceptor. As you learned in the CNS chapter, nociception is how you detect noxious stimuli, and pain is how you perceive and interpret these stimuli (see Chapter 12). The receptors that depolarize in response to these stimuli are accordingly called nociceptors (noh-sih-SEP-terz) and are generally exteroceptors. Like thermoreceptors, nociceptors are slowly adapting receptors.
types of mechanoreceptors
Merkel cell fibers. Recall from the integumentary chapter that Merkel cell fibers, also called tactile cell fibers, consist of a nerve ending surrounded by a capsule of Merkel, or tactile, cells (see Chapter 5). We find these slowly adapting receptors in the floors of the epidermal ridges, where they are most numerous in the skin of the hands, especially the fingertips. Action potentials appear to stem from mechanically gated ion channels in the nerve ending. Merkel cell fibers have the finest spatial resolution of any of the skin mechanoreceptors. For this reason, they primarily detect discriminative touch stimuli such as form and texture. Tactile corpuscles. Tactile corpuscles, also known as Meissner corpuscles (MYS-ner KOHR-pus-ulz), are found in the dermal papillae, projections of the dermis into the epidermis (see Chapter 5). These rapidly adapting receptors are more numerous than Merkel cell fibers. Like Merkel cell fibers, tactile corpuscles transmit discriminative touch stimuli, although their resolution is not as fine. Ruffini endings. The spindle-shaped Ruffini endings, also called as bulbous corpuscles, are located in the dermis and the hypodermis, as well as in ligaments. They are slowly adapting receptors that respond to stretch and movement; they do not transmit discriminative touch stimuli. Lamellated corpuscles. Lamellated corpuscles, formerly called Pacinian corpuscles (pah-SIN-ee-an), are named for their layered, onion-like appearance and are located deep within the dermis. The layered capsule enables these receptors to perform their function—the layers act somewhat like a filter that allows only high-frequency vibratory stimuli and deep pressure to activate them. This is an example of the Structure-Function Core Principle (Module 1.5.5). Lamellated corpuscles are rapidly adapting and, like Ruffini endings, do not transmit discriminative touch stimuli. Hair follicle receptors. Recall that Hair follicle receptors are free nerve endings wrapped around the base of a hair follicle in the dermis or hypodermis (see Chapter 5). These receptors are not found in thick skin, the type of skin shown in Figure 13.12. Hair follicle receptors respond to stimuli that cause the hair to bend, such as an insect landing on your arm. Proprioceptors. We find proprioceptors in the musculoskeletal system, where they detect the movement and position of a joint or body part (they are not found in the skin, so they are not included in Figure 13.12). These receptors are integral to the body's ability to sense its position in space and to monitor ongoing movement. In other words, they are critical to the integration of sensory and motor functions. Figure 13.12
lower motor neurons disorders
Motor peripheral neuropathies are also known as lower motor neuron disorders. They most often result from injury to a spinal or cranial nerve or to lower motor neuron cell bodies in the spinal cord. Such injuries prevent an α-motor neuron from stimulating a skeletal muscle fiber to contract. For this reason, lower motor neuron disorders may result in paralysis (inability to move a given muscle) or paresis (weakness of a given muscle). The damaged α-motor neuron is also unable to respond to feedback from muscle spindles. This causes a reduction or absence of stretch reflexes and loss of muscle tone.
Classification of Sensory Neurons
Sensory neurons are generally classified according to the speed with which their peripheral axons conduct action potentials. Recall from the nervous tissue chapter that two factors determine the speed at which action potentials are conducted along an axon—the diameter of the axon and the thickness of its myelin sheath (see Chapter 11). The axons with the largest diameter and the thickest myelin sheaths conduct impulses the fastest. Such axons include those that convey proprioceptive stimuli to the CNS. It's critical for these stimuli to travel quickly because the CNS needs them to make adjustments during movement to maintain posture and balance. The axons that convey both discriminative and nondiscriminative touch stimuli are also fairly large and heavily myelinated, and so conduct action potentials rapidly. The smallest-diameter axons with the least amount of myelin transmit action potentials the slowest. Both pain and temperature stimuli are conveyed to the CNS by such axons. Indeed, conduction through these axons is so slow that a noticeable delay occurs between the initiation of the stimulus and its interpretation by the CNS. This explains why, after you touch an object, it may take a moment to determine whether it is hot or cold. It also accounts for the "throbbing" nature of pain—each "wave" of pain you feel is actually one continuous stimulus. The delay between waves is just the time it takes for the painful stimulus to travel up the sensory neurons and be delivered to the CNS.
Classification of Sensory Receptors
Sensory receptors can be broadly classified according to the location of the stimuli they detect. Exteroceptors (ek′-ster-oh-SEP-terz) are typically close to the surface of the body. They detect stimuli originating outside the body, including an object's texture, temperature, and color; chemical odors in the air; and the level of light. Interoceptors (in′-ter-oh-SEP-terz) lie generally within the body's interior, where they detect stimuli originating inside the body, including blood pressure, the stretch of an organ such as a skeletal muscle or the urinary bladder, the concentration of certain chemicals in body fluids, and body temperature.
Cranial Nerve Reflexes
Several polysynaptic reflex arcs involve the cranial nerves. Recall that some structures of the head and the neck are supplied with motor and sensory neurons from different nerves. For this reason, these cranial nerve reflexes involve two separate nerves—one afferent and one efferent. Among the more important of these cranial nerve reflexes, and ones you have probably experienced, are the gag and corneal blink reflexes. The gag reflex is triggered when the visceral sensory nerve endings of the glossopharyngeal nerve in the posterior throat are stimulated unilaterally. This stimulus is brought back to a medullary nucleus shared by the glossopharyngeal and vagus nerves. When integration is complete, somatic motor neurons of the vagus nerve trigger contractions of the muscles of the pharynx, producing the familiar "gagging" sound and action. When something comes in contact with your eye, generally you blink, thanks to the corneal blink reflex. This reflex is triggered when a stimulus reaches the somatic sensory receptors of the trigeminal nerve in the thin outer covering of the eye called the cornea. These stimuli are returned to the pons for integration, and the orbicularis oculi muscle is triggered to contract, producing a blink, via the facial nerve's somatic motor neurons.
From CNS to PNS: Motor Output
Skeletal muscle fibers are voluntary and contract only when stimulated to do so by a somatic motor neuron. In the CNS chapter, you explored how the CNS initiates movement— upper motor neurons in the primary motor cortex of the cerebrum make the "decision" to move and initiate movement (with the help of the basal nuclei; see Chapter 12). However, the upper motor neurons do not contact skeletal muscle fibers, and so by themselves cannot stimulate a muscle contraction. Instead, the messages from upper motor neurons are relayed to lower motor neurons, which release acetylcholine onto the muscle fiber and initiate a muscle contraction.
The Role of Stretch Receptors in Skeletal Muscles
Some of the most common reflexes occur without our realizing it. These reflexes are part of normal movement and allow the CNS to correct motor error and prevent muscle damage. The sensory component of such reflexes is detected by mechanoreceptors within muscles and tendons called muscle spindles and Golgi tendon organs. These mechanoreceptors monitor muscle length and the force of contraction and communicate this information to the spinal cord, cerebellum, and cerebral cortex.
The basic pathway of information flow is as follows:
Stimulus is detected by sensory receptors of the PNS → transmitted by PNS sensory neurons to the CNS → integrated and interpreted by CNS neurons.
Central process
The central process exits the cell body and travels through the posterior root of the spinal cord to enter the posterior horn (or into the brainstem for cranial nerves).
cranial nerves
The cranial nerves attach to the brain and mainly innervate structures of the head and neck. Unlike spinal nerves, cranial nerves are not formed from the fusion of motor and sensory nerve roots, and so some cranial nerves are purely sensory, others are mixed, and others are predominantly motor. Module 13.2 covers the 12 pairs of cranial nerves.
Largest nerve in sacral plexus
The largest nerve of the sacral plexus—indeed, the largest and longest nerve in the body—is the sciatic nerve (sy-AT-ik; sciatic = "of the hip"). Unlike the nerves in other plexuses, the sciatic nerve actually contains axons from both the anterior and posterior divisions that travel together and share an epineurium. The sciatic nerve travels from the pelvis through the greater sciatic notch and enters the thigh by passing between the greater trochanter and the ischial tuberosity. Within the posterior thigh, it serves the hip joint before it splits into its two main branches, typically near the lower third of the thigh: the tibial nerve and the common fibular nerve.
lumbar plexus
The left and right lumbar plexuses arise from the anterior rami of L1−L4 (Figure 13.8a). These plexuses lie anterior to the vertebrae, embedded within the posterior part of the psoas major muscle. Their many branches primarily serve structures of the pelvis and the lower extremity. As with the brachial plexus, most nerve roots of the lumbar plexus also separate into anterior and posterior divisions; Figure 13.8b shows the nerves of the anterior division. The largest nerve of this division is the obturator nerve (AHB-too-ray-ter). This nerve travels through the pelvis and enters the thigh by passing through the obturator foramen. Its branches serve the adductor muscles in the thigh, the hip joint, and the skin over the medial part of the thigh. The largest nerve from the posterior division, and of the lumbar plexus as a whole, is the femoral nerve. It passes from the psoas major muscle through the pelvis and under the inguinal ligament to enter the thigh. The branches of the femoral nerve supply the muscles in the anterior thigh that extend the knee, including the quadriceps femoris muscle group, the skin over the anterior and medial thigh and leg, and the knee joint.
peripheral nerves
The main organs of the PNS are its peripheral nerves (this term is generally shortened to just nerves), which consist of the axons of many neurons bound together by a common connective tissue sheath (Figure 13.2). The many nerves of the PNS contact, or innervate (IN-er-vayt), the majority of structures in the body. Most nerves are mixed nerves, meaning they contain both sensory and motor neurons. This is why a damaged nerve affects both sensation and movement to some degree. There are, however, sensory nerves that contain only sensory neurons and motor nerves that contain mostly motor neurons. All motor nerves contain a small population of sensory neurons that carry stimuli pertaining to muscle stretch and tension, so no nerve is a pure motor nerve.
The Motor Cranial Nerves
The motor cranial nerves—the oculomotor (III), trochlear (IV), abducens (VI), accessory (XI), and hypoglossal (XII) nerves—also technically contain axons of proprioceptive sensory neurons. In spite of these sensory axons, they are still viewed as motor nerves because the main function of their sensory neurons is to allow the brain to monitor the contraction of the muscles they innervate.
motor division
The motor division of the PNS consists of motor neurons that carry out the motor functions of the nervous system (see the right side of Figure 13.1). Like the sensory division, it may be further classified—based on the organs that the neurons contact—into the somatic motor division and the visceral motor division, or autonomic nervous system. Somatic Motor Division The somatic motor division is responsible for the body's voluntary motor functions. It is made up of lower motor neurons (or somatic motor neurons) that directly contact skeletal muscle fibers and trigger a contraction when stimulated by upper motor neurons in the CNS. We'll see in Module 13.6 that the activities of the somatic sensory and somatic motor divisions are closely linked. Visceral Motor Division:The Autonomic Nervous System As we discussed in the chapters on nervous tissue and the CNS (see Chapters 11 and 12, respectively), the visceral motor division, or autonomic nervous system (ANS), is responsible for maintaining the homeostasis of many physiological variables through its control of the body's involuntary motor functions. Its neurons innervate cardiac muscle cells, smooth muscle cells, and the secretory cells of glands. The ANS has two divisions: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is usually described as the "fight or flight" division of the ANS—this name reflects its role in preparing the body for emergency situations that would involve fighting off an attacker or fleeing from danger. Its role is broader than this, however, as it maintains homeostasis when the body is engaged in any type of physical work and mediates the body's visceral responses to emotion. The parasympathetic nervous system is often described as the "rest and digest" division of the ANS, which reflects its role in digestion and in maintaining the body's homeostasis at
peripheral process
The peripheral process of the neuron is a long axon. At one end, it splits into nerve endings; associated with each nerve ending is a sensory receptor. At the other end, the peripheral process terminates near the neuron's cell body.
two-point discrimination threshold
The relative sizes of receptive fields can be measured by the two-point discrimination threshold. In this test, two stimuli (usually a pair of calipers or applicator sticks) are placed closely together on the skin (Figure 13.14b). The stimuli are then moved apart until the subject can feel two distinct points. The two-point discrimination threshold for the fingertips is quite small (about 3 mm) compared to that of the forearm (about 40 mm).
Cervical Plexuses
The right and left cervical plexuses are located deep in the neck lateral to the first through the fourth cervical vertebrae. These plexuses consist largely of the anterior rami of C1−C4, although they also receive small contributions from C5 and the hypoglossal nerve (cranial nerve XII). As you can see in Figure 13.6, the nerves of each cervical plexus contain cutaneous branches that supply the skin of the neck and portions of the head, chest, and shoulders. They also contain motor branches that supply muscles of the neck, including the infrahyoid and scalene muscles. The major named motor branch is the phrenic nerve (FREN-ik), which contains axons from C3 to C5, although its main contributor is C4. The phrenic nerve supplies the diaphragm muscle and is the main nerve that drives ventilation (a good mnemonic to remember the roots of the phrenic nerve is "3, 4, 5 to stay alive")
Dermatomes and Referred Pain
The skin can be divided into segments called dermatomes (DER-muh-tohmz; derma = "skin," -tome = "section") based on the spinal nerve that supplies the region with somatic sensation. Dermatomes can be combined and assembled into a dermatome map that represents all the spinal nerves (except the first cervical spinal nerve) (Figure 13.15a). This map can be used clinically to test the integrity of the sensory pathways to different parts of the body. For example, a patient with a cervical spinal cord injury may be able to feel his lateral hand, his thumb, and his second and third digits, but be unable to feel his medial hand or fourth or fifth digits. This would tell you the location of the spinal cord injury—about the level of C8.
fibular nerve
The smaller common fibular nerve, also known as the common peroneal nerve (pehr-OH-nee-ul; peron- = "fibula"), contains axons from the posterior division of the sacral plexus. This nerve descends along the lateral leg, where it supplies part of the knee joint and the skin of the anterior and distal leg. It terminates by dividing into superficial and deep branches. The superficial branch supplies the skin on the dorsum of the foot and the muscles in the lateral leg that evert the foot, whereas the deep branch supplies the muscles in the anterior leg that dorsiflex the foot and two muscles on the dorsum of the foot.
spinal nerves
The spinal nerves originate from the spinal cord and mainly innervate structures inferior to the head and neck. As you can see in Figure 13.2a, two groups of axons connect the PNS with the spinal cord's gray matter—an anterior root containing the axons of motor neurons exiting the anterior horn, and a posterior root containing the axons of sensory neurons entering the posterior horn. Note that the posterior root features an enlarged area that houses the cell bodies of sensory neurons just lateral to the spinal cord. A collection of cell bodies in the PNS is called a ganglion, and accordingly, this swollen area in the posterior root is the posterior root ganglion (or dorsal root ganglion). Lateral to the posterior root ganglion, the anterior and posterior roots fuse to form a spinal nerve. Each spinal nerve contains both sensory and motor neurons, so all are mixed nerves. There are 31 pairs of spinal nerves, which are discussed fully in Module 13.3.
The overall pathway for the detection and perception of somatic sensory stimuli
The stimulus is conducted to the first-order sensory neuron's central process, which enters the brainstem or the spinal cord and transmits the action potential to a second-order sensory neuron in the CNS. Recall that the pathway a stimulus takes once it enters the CNS depends on the type of stimulus (see Chapter 12). In the case of a pressure stimulus, it will synapse on a third-order sensory neuron in the thalamus and be relayed to the primary and secondary somatosensory cortices for processing and interpretation. Be aware that many proprioceptive stimuli are routed to the cerebellum instead for initial processing.
posterior root ganglion
The swollen area of the posterior root where the cell bodies of somatic sensory neurons are housed.
Simple Stretch Reflexes
The upper motor neurons in the CNS have a set "idea" of the optimal length for each skeletal muscle. This optimal length applies to both contracting and resting muscles. At rest, a few of the muscle's motor units remain activated to maintain this optimal length, which produces muscle tone (see Chapter 10). When a muscle is stretched (lengthened), it deviates from this optimal length in a way that could be damaging—muscle fibers can stretch only so much before they rupture. To prevent damage to the muscle fibers, the body responds with a monosynaptic reflex that shortens the muscle so that it returns to its optimal length.
The Sensory Cranial Nerves
There are three cranial nerves that contain the axons of only sensory neurons: the olfactory (I), optic (II), and vestibulocochlear (VIII) nerves. Each of these nerves is involved in one of the special senses.
Types of Thermoreceptors
Thermoreceptors are generally small knobs on the ends of free nerve endings in the skin. There are two types of thermoreceptors: so-called cold receptors that respond to temperatures between 10° C and 40° C (50-104° F) and hot receptors that respond to warmer temperatures between 32° C and 48° C (about 90-118° F). Cold receptors are located in the superficial dermis, whereas hot receptors lie in the deep portion of the dermis. Temperatures outside the ranges of hot and cold receptors are detected by nociceptors, which is why very hot or very cold objects feel painful to the touch.
Flexion (Withdrawal) and Crossed-Extension Spinal Reflexes
When you touch a very hot object or step on a tack, you almost immediately pull your hand or foot away from the source of the pain. These are examples of the flexion, or withdrawal, reflex. As you can see in Figure 13.20a, the flexion reflex involves rapidly conducting nociceptive afferents and multiple synapses in the spinal cord, which makes it a polysynaptic reflex. ❶❶ When stimulated, the nociceptive afferents transmit the stimulus to interneurons in the spinal cord. These interneurons then synapse on to α-motor neurons. 2a The α-motor neurons generate an action potential and stimulate contraction of muscles that flex the limb receiving the painful stimulus. This withdraws the affected limb from the stimulus. Figure 13.20
muscle spindles
apered structures that are found embedded among the regular contractile muscle fibers, which are also known as extrafusal muscle fibers (ek-strah-FYOO-zul; extrafusal = "outside the spindle") (Figure 13.18a). Within each spindle are 2-12 specialized muscle fibers called intrafusal muscle fibers ("within the spindle"). As you can see in Figure 13.18a, intrafusal fibers have contractile filaments composed of actin and myosin at their poles and a central area where contractile filaments are absent. These contractile poles are innervated by γ-motor neurons. Two classes of sensory neurons innervate the intrafusal fibers. Both types of neurons contain mechanically gated ion channels that open when the intrafusal fiber is stretched. The first type of neuron, known as a primary afferent, responds to stretch when it is first initiated. The second type, known as a secondary afferent, responds to both the static length of a muscle and the position of a limb. Muscle groups differ in their number of muscle spindles. Those groups that produce fine movements, such as those of the hand and the extrinsic eye muscles, have large numbers of muscle spindles. This allows for precise control of muscle contractions. Conversely, muscle groups that produce coarse movements, such as the postural muscles of the back, have relatively few muscle spindles.
nerve anatomy pns
he nerve as a whole is wrapped by a connective tissue sheath called the epineurium (ep′-ih-NOOR-ee-um). Within the nerve, axons are bundled into smaller groups known as fascicles (FASS-ih-kulz). Fascicles are in turn bound by another connective tissue sheath, the perineurium (pehr′-ih-NOOR-ee-um). Nestled between the fascicles are blood vessels that supply the axons with oxygen and nutrients. Each axon within a fascicle is surrounded by its own connective tissue sheath, the endoneurium (en′-doh-NOOR-ee-um) (note that although the figure shows a spinal nerve, the arrangement within spinal and cranial nerves is the same). Notice how the arrangement of a nerve closely resembles that within a skeletal muscle, which we discussed in the
types of relfexes
he simplest reflex arcs involve only a single synapse within the spinal cord between the sensory and motor neurons; this is called a monosynaptic reflex. A more complicated type of reflex arc, the polysynaptic reflex, involves multiple synapses. The second way to classify reflex arcs is according to the type of organ in which the reflex takes place. Visceral reflexes involve neurons that innervate our internal organs and are largely connected with the autonomic nervous system. We discuss some of these reflexes in the ANS chapter (see Chapter 14), and others are discussed in specific chapters throughout the book. The reflexes covered in the upcoming subsections are primarily somatic reflexes, which involve somatic sensory and motor neurons. We'll look closely at four examples of somatic reflexes: the simple stretch, flexion and crossed-extension, Golgi tendon, and cranial nerve reflexes.
Structure of Spinal Nerves and Spinal Nerve Plexuses
he spinal nerve itself is actually quite short, because about 1-2 cm after it forms and leaves the vertebral cavity, it splits into two nerves: (1) a posterior ramus (RAY-muss; plural, rami; ramus = "branch"), which travels to the posterior side of the body; and (2) an anterior ramus, which travels to the anterior side of the body and/or the upper and lower limbs. Each ramus typically branches multiple times along its course through the body and ultimately supplies different skeletal muscles and regions of the skin. Both the anterior and posterior rami are mixed nerves, as they contain sensory and somatic motor axons Note that another small branch stems from the anterior ramus. These small branches are called rami communicantes (singular, ramus communicans), and they contain visceral motor or autonomic neurons of the sympathetic nervous system (see Figure 13.4). Unlike the other branches of the anterior and posterior rami, the rami communicantes contain visceral motor axons only and so are not mixed nerves. The 31 pairs of spinal nerves consist of 8 pairs of cervical spinal nerves (C1−C8), 12 pairs of thoracic spinal nerves (T1−T12), 5 pairs each of lumbar and sacral spinal nerves (L1−L5 and S1−S5), and 1 tiny pair of coccygeal nerves (Co1) (Figure 13.5). The anterior rami of the cervical, lumbar, and sacral spinal nerves come together and merge to form complicated networks of nerves called nerve plexuses (PLEK-suss-ez). The axons of each spinal nerve cross over one another to enter different plexus branches. For this reason, the muscles supplied by a single branch of a nerve plexus are often served by two or more different spinal nerves. This works to our advantage, as it means that injury to one spinal nerve does not completely cut off motor or sensory innervation to that body part.
Sensory Neuron Disorders
he symptoms and severity of sensory peripheral neuropathy depend on which spinal or cranial nerve is injured. Damage to visceral sensory neurons can produce a variety of symptoms, such as urinary and/or fecal incontinence. Injury to somatic sensory neurons often decreases or eliminates sensation, including pain, in the affected part of the body. This can be dangerous, as the person with such an injury doesn't recognize and respond to painful stimuli, which can lead to tissue damage. Occasionally, the opposite phenomenon results, and the individual experiences burning and "shooting" pain. This is generally due to inflammation of the neurons and inappropriate firing of nociceptors and other sensory receptors. Another type of sensory peripheral neuropathy involves injury to proprioceptive neurons. This condition is generally due to damaged ligaments and tendons in which proprioceptors are housed. It results in difficulty with monitoring and controlling movement, as the axons do not effectively relay information on joint position to the cerebellum and other parts of the CNS.
Golgi Tendon Organs
mechanoreceptors within tendons near the muscle-tendon junction; they monitor the tension generated by a muscle contraction. Note in Figure 13.18b that a Golgi tendon organ consists of an encapsulated bundle of collagen fibers attached to about 20 extrafusal muscle fibers. Each Golgi tendon organ contains a single somatic sensory axon whose endings are wrapped around its enclosed collagen fibers. The rate at which these neurons fire depends on the amount of muscle tension generated with each contraction—the greater the tension, the more rapidly they fire. This feedback provides the CNS with information about the force being generated by each muscle contraction.
cell body location
posterior root ganglion (or dorsal root ganglion) found lateral to the spinal cord. The cell bodies of cranial nerves are situated in cranial nerve ganglia present in the head and neck.
Reflex Arcs
programmed, automatic responses to stimuli, called reflexes. Most reflexes are protective, preventing tissue damage in some way. Notice that each of these reflexes begins with a sensory stimulus and finishes with a rapid motor response. Between the sensory stimulus and the motor response, neural integration takes place within the CNS, typically within the brainstem or the spinal cord. So the overall sequence of a reflex is this:
brachial plexus
provide motor and sensory innervation to the upper limb. You can see in the following lateral view that they originate from the anterior rami of spinal nerves C5−T1; the first structures formed in the brachial plexus are its large trunks. Typically, C5 and C6 unite to form the large superior trunk, C7 forms the middle trunk, and C8 and T1 combine to form the inferior trunk. This next step is where things get tricky: Each trunk splits into an anterior division and a posterior division that become the cords of the plexus. The anterior division of the inferior trunk forms the medial cord, which descends in the medial arm. The anterior divisions of the superior and middle trunks combine to form the lateral cord, which descends in the lateral arm. The posterior divisions of each trunk unite to form the posterior cord, which lies in the posterior arm; you can see it here:
receptive fields
the area served by that neuron (Figure 13.14a). Each first-order somatic sensory neuron has an extensive network of branches in the skin, and the more branching that exists, the larger the neuron's receptive field. How much a neuron branches is generally determined by the number of neurons that serve a particular region of the body. The skin of the fingertips is richly innervated with sensory neurons, and each of these neurons has a relatively small receptive field. However, the skin of the forearm is innervated by a much smaller number of neurons, so each neuron has a somewhat larger receptive field. Here is another example of the Structure-Function Core Principle (Module 1.5.5): Body regions that have a primary function of sensing the environment (such as fingertips) contain more neurons and smaller receptive fields, which enables them to perform this job.
sensory transduction
the conversion of a stimulus into an electrical signal—takes place. Sensory transduction begins at a region of the nerve ending called a sensory receptor, which can be of several types. Some sensory receptors are surrounded by specialized supportive cells; these receptors are known as encapsulated nerve endings. Others are free nerve endings that lack specialized supportive cells and are "naked" (these were formerly called free dendritic endings, but they are part of an axon, not a dendrite, so this name is misleading). the basic mechanism behind sensory transduction. The axolemma of a nerve ending contains many gated ion channels that respond to various stimuli (such as the mechanically gated ion channels shown in this figure). In response to the stimulus, one or more types of ion channel open or close, changing the flow of ions across the membrane.
The Mixed Cranial Nerves
the mixed cranial nerves—the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves—are generally large and have a fairly wide distribution. All but one (the trigeminal nerve) contain somatic sensory, somatic motor, and parasympathetic neurons wrapped up in the same nerve. The mixed cranial nerves have multiple and diverse functions due to their size and the different types of neurons they house
Two main division of pns
the sensory (or afferent) division and the motor (or efferent) division (see Chapter 11). Each division is further categorized anatomically—by the types of structures that are innervated—into somatic (body) and visceral (internal organ) branches
tibial nerve
the tibial nerve is the larger branch—it contains the axons from the anterior division of the sacral plexus. In the thigh its branches supply muscles that extend the thigh and flex the leg, including almost all the muscles of the hamstring group (note that sometimes the sciatic nerve also supplies this muscle group before it splits into the tibial and common fibular nerves). As implied by its name, it then descends through the leg alongside the tibia, where it innervates part of the knee joint, the ankle joint, and the muscles of the leg that plantarflex the foot, such as the gastrocnemius muscle. One of the tibial nerve's early branches, at about the level of the gastrocnemius, is the sural nerve (SOO-rul; sural = "calf"), which supplies the posterior and lateral skin of the distal leg and part of the foot. At the level of the medial malleolus, the tibial nerve divides into its terminal branches, which supply the muscles and skin of the foot.
simple stretch reflex is easily elicited
y tapping on certain tendons. Perhaps the best-known example is the patellar, or knee-jerk, reflex. The patellar reflex is elicited by using a blunt object such as a reflex hammer to tap the patellar tendon. This stretches the quadriceps femoris muscle group of the thigh, producing a quick "knee jerk" with extension of the leg. Although you likely associate this reflex with trips to the doctor's office, it is critical in keeping you upright when you're standing. When you're standing upright, any slight buckling of the knee joint will also stretch the muscles of the quadriceps femoris group. The muscle spindles detect this stretch and initiate the reflex, which results in these muscles firing to straighten the lower limb and prevent the knee joint from buckling further. There are many examples of similar simple stretch reflexes that may be elicited with a tap. For example, the jaw-jerk reflex, which involves the trigeminal nerve, is elicited by tapping the chin. This slight stretch leads to a reflexive contraction of the masseter and temporalis muscles. Reflex contractions may also be generated by tapping the triceps brachii tendon and the calcaneal (Achilles) tendon. But you can see these reflexes in action in an even easier way: Simply stand up and bend over to touch your toes. When doing so, feel your posterior lower limb muscles as they become progressively tighter the more you try to stretch them. This tightening occurs because they are contracting in response to the stretch.
steps of a simple stretch reflex
❶❶ An external force stretches the muscle. In Figure 13.19, the muscle is stretched by the external force of the extra weight that results from adding the liquid to the glass. ❷❷ Muscle spindles detect the stretch, and primary and secondary afferents transmit an action potential to the spinal cord. The stretching force stretches the intrafusal fibers in the muscle spindles, which opens the mechanically gated ion channels in their primary and secondary afferents. If the stretch is strong enough to depolarize the neuronal membranes to threshold, an action potential is initiated and propagated along their axons to the spinal cord (or the brainstem for cranial nerves). ❸❸ In the spinal cord, sensory afferents synapse on α- motor neurons and trigger an action potential. The sensory afferents synapse on α-motor neurons in the spinal cord; note that upper motor neurons are not directly involved in this process. This causes the α-motor neurons to generate action potentials. ❹❹ The α-motor neurons stimulate the muscle to contract, and it returns to its optimal length. The action potentials are propagated along the axons of the α-motor neurons, and when they arrive at the neuromuscular junction, they trigger the muscle to contract. In Figure 13.19, the biceps brachii muscle is stimulated to contract, and the forearm is returned to its original position. Despite being classified as a monosynaptic reflex, this reflex arc has more going on than the stimulation of a single synapse. The sensory afferents also synapse with interneurons in the spinal cord that inhibit the antagonist muscles (in Figure 13.19, the triceps brachii muscle is the antagonist muscle that would be inhibited). This permits the agonist muscle to contract with no interference from the antagonist muscle groups.
Steps in sensory transduction
❶❶ Before any stimulus arrives, the ion channels in the axolemma of the somatic sensory neuron are closed. The process of transduction begins with a stimulus, which in our example is pressure on a finger from a probe. ❷❷ When a stimulus such as pressure is applied, mechanically gated sodium ion channels open. Sodium ions enter the axoplasm, generating a temporary depolarization referred to as a Receptor potential. ❸❸ If enough sodium ions enter that the membrane potential reaches threshold, voltage-gated sodium ion channels open. This triggers an action potential, which will be propagated along the axon to the spinal cord.
first order somatic sensory neuron flow
❶❶, the peripheral process transmits an action potential from the sensory receptor to the neuron's other axon, the central process. In step ❷❷, the central process transmits an action potential from the peripheral process to the posterior horn, eventually synapsing on a second-order neuron in the spinal cord or brainstem. An action potential propagated down the peripheral process does not generally reach the cell body. Instead, the stimulus is usually transmitted to the central process in the area where the peripheral and central processes come into contact near the cell body at the axon "stem."
