PSG 372 Note PDF

ORGANIZATION OF THE NERVOUS SYSTEM Nervous system is one of the smallest and yet the most complex of the 11 body systems. This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: the central nervous system and the periphera...

ORGANIZATION OF THE NERVOUS SYSTEM Nervous system is one of the smallest and yet the most complex of the 11 body systems. This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: the central nervous system and the peripheral nervous system. Central Nervous System The central nervous system (CNS) consists of the brain and spinal cord (Figure 1). The brain is the part of the CNS that is located in the skull and contains about 85 billion neurons. The spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. The spinal cord contains about 100 million neurons. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS. Peripheral Nervous System The peripheral nervous system (PNS) (pe-RIF-e-ral) consists of all nervous tissue outside the CNS (Figure 1). Components of the PNS include nerves, ganglia, enteric plexuses, and sensory receptors. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge from the spinal cord. Each nerve follows a defined path and serves a specific region of the body. Ganglia (swelling or knot; singular is ganglion) are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. Ganglia are closely associated with cranial and spinal nerves. Enteric plexuses (PLEK- sus-ez) are extensive networks of neurons located in the walls of organs of the gastrointestinal tract. The neurons of these plexuses help regulate the digestive system. The term sensory receptor refers to a structure of the nervous system that monitors changes in the external or internal environment. Examples of sensory receptors include touch receptors in the skin, photoreceptors in the eye, and olfactory receptors in the nose. The PNS is divided into a somatic nervous system (SNS), an autonomic nervous system (ANS), and an enteric nervous system (ENS). The SNS consists of (1) sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell, and (2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary. The ANS consists of (1) sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, and (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. With a few exceptions, effectors receive nerves from both divisions, and usually the two divisions have opposing actions. For example, sympathetic neurons increase heart rate, and parasympathetic neurons slow it down. In general, the sympathetic division helps support exercise or emergency actions, the “fight or- flight” responses, and the parasympathetic division takes care of “rest-and-digest” activities. 1 Figure1: Organization of the nervous system: subdivision of the nervous system The operation of the ENS, the “brain of the gut,” is involuntary. Once considered part of the ANS, the ENS consists of over 100 million neurons in enteric plexus that extend most of the length of the gastrointestinal (GI) tract. Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons. Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. Enteric motor neurons govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs (such as acid from the stomach), and activities of GI tract endocrine cells, which secrete hormones Functions of the Nervous System The nervous system carries out a complex array of tasks. It allows us to sense various smells, produce speech, and remember past events; in addition, it provides signals that control body movements and regulates the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory (input), integrative (process), and motor (output). Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. Integrative function. The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration. Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete. 2 The three basic functions of the nervous system occur, for example, when you answer your cell phone after hearing it ring. The sound of the ringing cell phone stimulates sensory receptors in your ears (sensory function). This auditory information is subsequently relayed into your brain where it is processed and the decision to answer the phone is made (integrative function). The brain then stimulates the contraction of specific muscles that will allow you to grab the phone and press the appropriate button to answer it (motor function). 3  SYNAPTIC TRANSMISSION Synapse (SIN-aps) is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell). The term presynaptic neuron (pre- before) refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal. A postsynaptic cell is the cell that receives a signal. It may be a nerve cell called a postsynaptic neuron (post- after) that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse. Most synapses between neurons are axodendritic (from axon to dendrite), while others are axosomatic (from axon to cell body) or axoaxonic (from axon to axon). In addition, synapses may be electrical or chemical and they differ both structurally and functionally. Synapses are essential for homeostasis because they allow information to be filtered and integrated. During learning, the structure and function of particular synapses change. The changes may allow some signals to be transmitted while others are blocked. For example, the changes in your synapses from studying will determine how well you do on your physiology tests! Synapses are also important because some diseases and neurological disorders result from disruptions of synaptic communication, and many therapeutic and addictive chemicals affect the body at these junctions. Electrical Synapses At an electrical synapse, action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions. Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell. Gap junctions are common in visceral smooth muscle, cardiac muscle, and the developing embryo. They also occur in the brain. Electrical synapses have two main advantages: 1. Faster communication. Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. At an electrical synapse, the actionpotential passes directly from the presynaptic cell to the postsynaptic cell. The events that occur at a chemical synapse take some time and delay communication slightly. 2. Synchronization. Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract. Chemical Synapses Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch. They are separated by the synaptic cleft, a space of 20– 50 nm that is filled with interstitial fluid. Nerve impulses cannot conduct across the synaptic cleft, so an alternative, indirect form of communication occurs. In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). The time required for these processes at a chemical synapse, a synaptic delay of 4 about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses. A typical chemical synapse transmits a signal as follows (Figure 2): 1 A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon. 2 The depolarizing phase of the nerve impulse opens voltagegated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels. An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic vesicle contains several thousand molecules of neurotransmitter The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane. The receptor shown in Figure 2 is part of a ligand-gated channel; you will soon learn that this type of neurotransmitter receptor is called an ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to metabotropic receptors. Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane. As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition). For example, opening of Na+ channels allows inflow of Na+, which causes depolarization. However, opening of Cl- or K+channels causes hyperpolarization. Opening Cl- channels permits Cl- to move into the cell, while opening the K+ channels allows K+ to move out—in either event, the inside of the cell becomes more negative. When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron. At most chemical synapses, only one-way information transfer can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell. For example, synaptic transmission at a neuromuscular junction (NMJ) proceeds from a somatic motor neuron to a skeletal muscle fiber (but not in the opposite direction). Only synaptic end bulbs of presynaptic neurons can release neurotransmitter, and only the postsynaptic neuron’s membrane has the receptor proteins that can recognize and bind that neurotransmitter. As a result, action potentials move in one direction. 5 Figure 2: Excitatory and Inhibitory Postsynaptic Potentials A neurotransmitter causes either an excitatory or an inhibitory graded potential. A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs. A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state. A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP). SPINAL CORD PHYSIOLOGY The spinal cord has two principal functions in maintaining homeostasis: nerve impulse propagation and integration of information. The white matter tracts in the spinal cord are highways for nerve impulse propagation. Sensory input travels along these tracts toward the brain, and motor output travels from the brain along these tracts toward skeletal muscles and other effector tissues. The gray matter of the spinal cord receives and integrates incoming and outgoing information. Sensory and Motor Tracts As noted previously, one of the ways the spinal cord promotes homeostasis is by conducting nerve impulses along tracts. Often, the name of a tract indicates its position in the white matter and where it begins and ends. For example, the anterior corticospinal tract is located in the anterior white column; it begins in the cerebral cortex (superficial gray matter of the cerebrum of the brain) and ends in the spinal cord. Notice that the location of the axon terminals comes last in the name. This regularity in naming allows you to determine the 6 direction of information flow along any tract named according to this convention. Because the anterior corticospinal tract conveys nerve impulses from the brain toward the spinal cord, it is a motor (descending) tract. Figure 3 highlights the major sensory and motor tracts in the spinal cord. Sensory tracts are summarized in Table 1. Figure 3: Locations of major sensory and motor tracts, showing in a transverse section of the spinal cord. Nerve impulses from sensory receptors propagate up the spinal cord to the brain along two main routes on each side: the spinothalamic tract and the posterior column. The spinothalamic tract (spıˉ_-noˉ-tha-LAM-ik) conveys nerve impulses for sensing pain, warmth, coolness, itching, tickling, deep pressure, and crude touch. The posterior column consists of two tracts: the gracile fasciculus (GRAS-ıˉl fa-SIK-uˉ-lus) and the cuneate fasciculus (KUˉ -neˉ-aˉt). The posterior column tracts convey nerve impulses for discriminative touch, light pressure, vibration, and conscious proprioception (the awareness of the positions and movements of muscles, tendons, and joints). The sensory systems keep the CNS informed of changes in the external and internal environments. The sensory information is integrated (processed) by interneurons in the spinal cord and brain. Responses to the integrative decisions are brought about by motor activities (muscular contractions and glandular secretions). The cerebral cortex, the outer part of the brain, plays a major role in controlling precise voluntary muscular movements. Other brain regions provide important integration for regulation of automatic movements. Motor output to skeletal muscles travels down the spinal cord in two types of descending pathways: direct and indirect. The direct motor pathways include the lateral corticospinal (kor_-ti-koˉ-SPIˉ-nal), anterior corticospinal, and corticobulbar tracts (kor_-ti-koˉ-BUL-bar). They convey nerve impulses that originate in the cerebral cortex and are destined to cause voluntary movements of skeletal muscles. Indirect motor pathways include the rubrospinal (ROO-broˉ-spıˉ-nal), tectospinal Table 1 7 8 (TEKto ˉ-spıˉ-nal), vestibulospinal (ves-TIB-uˉ loˉ-spıˉ-nal), lateral reticulospinal (re-TIK- uˉ-loˉ-spıˉ-nal), and medial reticulospinal tracts. These tracts convey nerve impulses from the brain stem to cause automatic movements and help coordinate body movements with visual stimuli. Indirect pathways also maintain skeletal muscle tone, sustain contraction of postural muscles, and play a major role in equilibrium by regulating muscle tone in response to movements of the head. Refl exes and Refl ex Arcs The second way the spinal cord promotes homeostasis is by serving as an integrating center for some reflexes. A reflex is a fast, involuntary, unplanned sequence of actions that occurs in response to a particular stimulus. Some reflexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot. Other reflexes are learned or acquired. For instance you learn many reflexes while acquiring driving expertise. Slamming on the brakes in an emergency is one example. When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex. An example is the familiar patellar reflex (knee jerk). If integration occurs in the brain stem rather than the spinal cord, the reflex is called a cranial reflex. An example is the tracking movements of your eyes as you read this sentence. You are probably most aware of somatic reflexes, which involve contraction of skeletal muscles. Equally important, however, are the autonomic (visceral) reflexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands. Body functions such as heart rate, digestion, urination, and defecation are controlled by the autonomic nervous system through autonomic reflexes. Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination. The pathway followed by nerve impulses that produce a reflex is a reflex arc (reflex circuit). A reflex arc includes the following five functional components (Figure 4): 11 Sensory receptor. The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a specific stimulus—a change in the internal or external environment—by producing a graded potential called a generator (or receptor) potential. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron. 2 Sensory neuron. The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem. From here, relay neurons send nerve impulses to the area of the brain that allows conscious awareness that the reflex has occurred. 3 Integrating center. One or more regions of gray matter within the CNS acts as an integrating center. In the simplest type of reflex, the integrating center is a single synapse between a sensory neuron and a motor neuron. A reflex pathway having only one synapse in the CNS is termed a monosynaptic reflex arc (mon_-oˉ-si-NAP-tik; mono- _ one). More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A polysynaptic reflex arc (poly- _ many) involves more than two types of neurons and more than one CNS synapse. 4 Motor neuron. Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond. 5 Effector. The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector. Its action is called a reflex. If the effector is skeletal muscle, the reflex is a somatic reflex. If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an autonomic (visceral) reflex. Because reflexes are normally so predictable, they provide 9 useful information about the health of the nervous system and can greatly aid diagnosis of disease. Damage or disease anywhere along its reflex arc can cause a reflex to be absent or abnormal. For example, tapping the patellar ligament normally causes reflex extension of the knee joint. Absence of the patellar reflex could indicate damage of the sensory or motor neurons, or a spinal cord injury in the lumbar region. Somatic reflexes generally can be tested simply by tapping or stroking the body surface. Figure 4:General components of a reflex arc The Stretch Refl ex A stretch reflex causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reflex is the patellar reflex (knee jerk). A stretch reflex operates as follows (Figure 5): 1 Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle. In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord. In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with, and thereby activates, a motor neuron in the anterior gray horn. If the excitation is strong enough, one or more nerve impulses arises in the motor neuron and propagates, along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle. 10 Figure5: 5 Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching. In the reflex arc just described, sensory nerve impulses enter the spinal cord on the same side from which motor nerve impulses leave it. This arrangement is called an ipsilateral reflex (ip-si-LAT-er-al _ same side). All monosynaptic reflexes are ipsilateral. In addition to the large-diameter motor neurons that innervate typical skeletal muscle fibers, smaller-diameter motor neurons innervate smaller, specialized muscle fibers within the muscle spindles themselves. The brain regulates muscle spindle sensitivity through pathways to these smaller motor neurons. This regulation ensures proper muscle spindle signaling over a wide range of muscle lengths during voluntary and reflex contractions. By adjusting how vigorously a muscle spindle responds to stretching, the brain sets an overall level of muscle tone, which is the small degree of contraction present while the muscle is at rest. Because the stimulus for the stretch reflex is stretching of muscle, this reflex helps avert injury by preventing overstretching of muscles. Although the stretch reflex pathway itself is monosynaptic (just two neurons and one synapse), a polysynaptic reflex arc to the antagonistic muscles operates at the same time. This arc involves 11 three neurons and two synapses. An axon collateral (branch) from the muscle spindle sensory neuron also synapses with an inhibitory interneuron in the integrating center. In turn, the interneuron synapses with and inhibits a motor neuron that normally excites the antagonistic muscles (Figure 5). Thus, when the stretched muscle contracts during a stretch reflex, antagonistic muscles that oppose the contraction relax. This type of arrangement, in which the components of a neural circuit simultaneously cause contraction of one muscle and relaxation of its antagonists, is termed reciprocal innervation (reˉ-SIP-ro_-kal in_- er-VAˉ - shun). Reciprocal innervation prevents conflict between opposing muscles and is vital in coordinating body movements. Axon collaterals of the muscle spindle sensory neuron also relay nerve impulses to the brain over specific ascending pathways. In this way, the brain receives input about the state of stretch or contraction of skeletal muscles, enabling it to coordinate muscular movements. The nerve impulses that pass to the brain also allow conscious awareness that the reflex has occurred. The stretch reflex can also help maintain posture. For example, if a standing person begins to lean forward, the gastrocnemius and other calf muscles are stretched. Consequently, stretch reflexes are initiated in these muscles, which cause them to contract and reestablish the body’s upright posture. Similar types of stretch reflexes occur in the muscles of the shin when a standing person begins to lean backward. The Tendon Refl ex The stretch reflex operates as a feedback mechanism to control muscle length by causing muscle contraction. In contrast, the tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Although the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when tension is great, making you drop a very heavy weight, for example. Like the stretch reflex, the tendon reflex is ipsilateral. The sensory receptors for this reflex are called tendo (Golgi tendon) organs, which lie within a tendon near its junction with a muscle. In contrast to muscle spindles, which are sensitive to changes in muscle length, tendon organs detect and respond to changes in muscle tension that are caused by passive stretch or muscular contraction. A tendon reflex operates as follows (Figure 6): 1 As the tension applied to a tendon increases, the tendon organ (sensory receptor) is stimulated (depolarized to threshold). 2 Nerve impulses arise and propagate into the spinal cord along a sensory neuron. Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that synapses with a motor neuron. 4 The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses. The muscle relaxes and relieves excess tension. Thus, as tension on the tendon organ increases, the frequency of inhibitory impulses increases; inhibition of the motor neurons to the muscle developing excess tension (effector) causes relaxation of the muscle. In this way, the tendon reflex protects the tendon and muscle from damage due to excessive tension. Note in Figure 6 that the sensory neuron from the tendon organ also synapses with an excitatory interneuron in the spinal cord. The excitatory interneuron in turn synapses with motor neurons controlling antagonistic muscles. Thus, while the tendon reflex brings about relaxation of the muscle attached to the tendon organ, it also triggers contraction of antagonists. This is another example of reciprocal innervation. The sensory neuron also relays nerve impulses to the brain by way of sensory tracts, thus informing the brain about the state of muscle tension throughout the body. 12 Figure 6: THALAMUS The thalamus (THAL-a-mus _ inner chamber), which measures about 3 cm (1.2 in.) in length and makes up 80% of the diencephalon, consists of paired oval masses of gray matter organized into nuclei with interspersed tracts of white matter (Figure 7). The thalamus is the principal relay station for sensory impulses that reach the cerebral cortex from other parts of the brain and the spinal cord. The thalamus is the major relay station for most sensory impulses that reach the primary sensory areas of the cerebral cortex from the spinal cord and brain stem. In addition, the thalamus contributes to motor functions by transmitting information from the cerebellum and basal nuclei to the primary motor area of the cerebral cortex. The thalamus also relays nerve impulses between different areas of the cerebrum and plays a role in the maintenance of consciousness. Based on their positions and functions, there are seven major groups of nuclei on each side of the thalamus (Figure 7c, d): 1. The anterior nucleus receives input from the hypothalamus and sends output to the limbic system. It functions in emotions and memory. 13 2. The medial nuclei receive input from the limbic system and basal nuclei and send output to the cerebral cortex. They function in emotions, learning, memory, and cognition (thinking and knowing). 3. Nuclei in the lateral group receive input from the limbic system, superior colliculi, and cerebral cortex and send output to the cerebral cortex. The lateral dorsal nucleus functions in the expression of emotions. The lateral posterior nucleus and pulvinar nucleus help integrate sensory information. 4. Five nuclei are part of the ventral group. The ventral anterior nucleus receives input from the basal nuclei and sends output to motor areas of the cerebral cortex; it plays a role in movement control. The ventral posterior nucleus relays impulses for somatic sensations such as touch, pressure, vibration, itch, tickle, temperature, pain, and proprioception from the face and body to the cerebral cortex. The lateral geniculate nucleus (je-NIK-uˉ-lat _ bent like a knee) relays visual impulses for sight from the retina to the primary visual area of the cerebral cortex. The medial geniculate nucleus relays auditory impulses for hearing from the ear to the primary auditory area of the cerebral cortex. 5. Intralaminar nuclei (in_-tra-LA-mi_-nar) lie within the internal medullary lamina and make connections with the reticular formation, cerebellum, basal nuclei, and wide areas of the cerebral cortex. They function in arousal (activation of the cerebral cortex from the brain stem reticular formation) and integration of sensory and motor information. 6. The midline nucleus forms a thin band adjacent to the third ventricle and has a presumed function in memory and olfaction. 7. The reticular nucleus surrounds the lateral aspect of the thalamus, next to the internal capsule. This nucleus monitors, filters, and integrates activities of other thalamic nuclei. 14 Figure 7 CEREBRAL CORTEX The cerebrum is the “seat of intelligence.” It provides us with the ability to read, write, and speak; to make calculations and compose music; and to remember the past, plan for the future, and imagine things that have never existed before. The cerebrum consists of an outer cerebral cortex, an internal region of cerebral white matter, and gray matter nuclei deep within the white matter. The cerebrum is the “seat of intelligence”; it provides us with the ability to read, write, and speak; to make calculations and compose music; to remember the past and plan for the future; and to create. Specific types of sensory, motor, and integrative signals are processed in certain regions of the cerebral cortex (Figure 8). Generally, sensory areas receive sensory information and are involved in perception, the conscious awareness of a sensation; motor areas control the execution of voluntary movements; and association areas deal with more complex integrative functions such as memory, emotions, reasoning, will, judgment, personality traits, and intelligence. Figure 8: Sensory Areas Sensory impulses arrive mainly in the posterior half of both cerebral hemispheres, in regions behind the central sulci. In the cerebral cortex, primary sensory areas receive sensory information that has been relayed from peripheral sensory receptors through lower regions of 15 the brain. Sensory association areas often are adjacent to the primary areas. They usually receive input both from the primary areas and from other brain regions. Sensory association areas integrate sensory experiences to generate meaningful patterns of recognition and awareness. For example, a person with damage in the primary visual area would be blind in at least part of his visual field, but a person with damage to a visual association area might see normally yet be unable to recognize ordinary objects such as a lamp or a toothbrush just by looking at them. The following are some important sensory areas (Figure 8; the significance of the numbers in parentheses is explained in the figure caption): The primary somatosensory area (areas 1, 2, and 3) is located directly posterior to the central sulcus of each cerebral hemisphere in the postcentral gyrus of each parietal lobe. It extends from the lateral cerebral sulcus, along the lateral surface of the parietal lobe to the longitudinal fissure, and then along the medial surface of the parietal lobe within the longitudinal fissure. The primary somatosensory area receives nerve impulses for touch, pressure, vibration, itch, tickle, temperature (coldness and warmth), pain, and proprioception (joint and muscle position) and is involved in the perception of these somatic sensations. A “map” of the entire body is present in the primary somatosensory area: Each point within the area receives impulses from a specific part of the body (see Figure 9a). The size of the cortical area receiving impulses from a particular part of the body depends on the number of receptors present there rather than on the size of the body part. For example, a larger region of the somatosensory area receives impulses from the lips and fingertips than from the thorax or hip. This distorted somatic sensory map of the body is known as the sensory homunculus (homunculus _ little man). The primary somatosensory area allows you to pinpoint where somatic sensations originate, so that you know exactly where on your body to swat that mosquito. The primary visual area (area 17), located at the posterior tip of the occipital lobe mainly on the medial surface (next to the longitudinal fissure), receives visual information and is involved in visual perception. The primary auditory area (areas 41 and 42), located in the superior part of the temporal lobe near the lateral cerebral sulcus, receives information for sound and is involved in auditory perception. The primary gustatory area (area 43), located at the base of the postcentral gyrus superior to the lateral cerebral sulcus in the parietal cortex, receives impulses for taste and is involved in gustatory perception and taste discrimination. The primary olfactory area (area 28), located in the temporal lobe on the medial aspect (and thus not visible in, receives impulses for smell and is involved in olfactory perception. 16 Figure 9: SENSATION Sensation is the conscious or subconscious awareness of changes in the external or internal environment. The nature of the sensation and the type of reaction generated vary according to the ultimate destination of nerve impulses that convey sensory information to the CNS. Sensory impulses that reach the spinal cord may serve as input for spinal reflexes, such as the stretch reflex. Sensory impulses that reach the lower brain stem elicit more complex reflexes, such as changes in heart rate or breathing rate. When sensory impulses reach the cerebral cortex, we become consciously aware of the sensory stimuli and can precisely locate and identify specific sensations such as touch, pain, hearing, or taste. Perception is the conscious interpretation of sensations and is primarily a function of the cerebral cortex. We have no perception of some sensory information because it never reaches the cerebral cortex. For example, certain sensory receptors constantly monitor the pressure of blood in blood vessels. Because the nerve impulses conveying blood pressure information propagate to the cardiovascular center in the medulla oblongata rather than to the cerebral cortex, blood pressure is not consciously perceived. Sensory Modalities Each unique type of sensation—such as touch, pain, vision, or hearing—is called a sensory modality. A given sensory neuron carries information for only one sensory modality. Neurons relaying impulses for touch to the somatosensory area of the cerebral cortex do not transmit impulses for pain. Likewise, nerve impulses from the eyes are perceived as sight, and those from the ears are perceived as sounds. 17 The different sensory modalities can be grouped into two classes: general senses and special senses. 1. The general senses refer to both somatic senses and visceral senses. Somatic senses (somat- _ of the body) include tactile sensations (touch, pressure, vibration, itch, and tickle), thermal sensations (warm and cold), pain sensations, and proprioceptive sensations. Proprioceptive sensations allow perception of both the static (nonmoving) positions of limbs and body parts (joint and muscle position sense) and movements of the limbs and head. Visceral senses provide information about conditions within internal organs, for example, pressure, stretch, chemicals, nausea, hunger, and temperature. 2. The special senses include the sensory modalities of smell, taste, vision, hearing, and equilibrium or balance. In this chapter we discuss the somatic senses and visceral pain. The Process of Sensation The process of sensation begins in a sensory receptor, which can be either a specialized cell or the dendrites of a sensory neuron. As previously noted, a given sensory receptor responds vigorously to one particular kind of stimulus, a change in the environment that can activate certain sensory receptors. A sensory receptor responds only weakly or not at all to other stimuli. This characteristic of sensory receptors is known as selectivity. For a sensation to arise, the following four events typically occur: 1. Stimulation of the sensory receptor. An appropriate stimulus must occur within the sensory receptor’s receptive field, that is, the body region where stimulation activates the receptor and produces a response. 2. Transduction of the stimulus. A sensory receptor transduces (converts) energy in a stimulus into a graded potential. Recall that graded potentials vary in amplitude (size), depending on the strength of the stimulus that causes them, and are not propagated. Each type of sensory receptor exhibits selectivity: It can transduce only one kind of stimulus. For example, odorant molecules in the air stimulate olfactory (smell) receptors in the nose, which transducer the molecules’ chemical energy into electrical energy in the form of a graded potential. 3. Generation of nerve impulses. When a graded potential in a sensory neuron reaches threshold, it triggers one or more nerve impulses, which then propagate toward the CNS. Sensory neurons that conduct impulses from the PNS into the CNS are called first-order neurons 4. Integration of sensory input. A particular region of the CNS receives and integrates the sensory nerve impulses. Conscious sensations or perceptions are integrated in the cerebral cortex. Assignment Study sensory receptor SOMATIC SENSATIONS Somatic sensations arise from stimulation of sensory receptors embedded in the skin or subcutaneous layer; in mucous membranes of the mouth, vagina, and anus; in muscles, tendons, and joints; and in the inner ear. The sensory receptors for somatic sensations are distributed unevenly—some parts of the body surface are densely populated with receptors, and others contain only a few. The areas with the highest density of somatic sensory receptors are the tip of 18 the tongue, the lips, and the fingertips. Somatic sensations that arise from stimulating the skin surface are cutaneous sensations (kuˉ-TAˉ - neˉ-us; cutane- _ skin). There are four modalities of somatic sensation: tactile, thermal, pain, and proprioceptive. Tactile Sensations The tactile sensations (TAK-tıˉl; tact- _ touch) include touch, pressure, vibration, itch, and tickle. Although we perceive differences among these sensations, they arise by activation of some of the same types of receptors. Several types of encapsulated mechanoreceptors attached to large-diameter myelinated A fibers mediate sensations of touch, pressure, and vibration. Other tactile sensations, such as itch and tickle sensations, are detected by free nerve endings attached to small-diameter, unmyelinated C fibers. Recall that larger-diameter, myelinated axons propagate nerve impulses more rapidly than do smaller-diameter, unmyelinated axons. Tactile receptors in the skin or subcutaneous layer include corpuscles of touch, hair root plexuses, type I cutaneous mechanoreceptors, type II cutaneous mechanoreceptors, lamellated corpuscles, and free nerve endings Touch Sensations of touch generally result from stimulation of tactile receptors in the skin or subcutaneous layer. There are two types of rapidly adapting touch receptors. Corpuscles of touch or Meissner corpuscles (MIˉS-ner) are touch receptors that are located in the dermal papillae of hairless skin. Each corpuscle is an egg-shaped mass of dendrites enclosed by a capsule of connective tissue. Because corpuscles of touch are rapidly adapting receptors, they generate nerve impulses mainly at the onset of a touch. They are abundant in the fingertips, hands, eyelids, tip of the tongue, lips, nipples, soles, clitoris, and tip of the penis. Hair root plexuses are rapidly adapting touch receptors found in hairy skin; they consist of free nerve endings wrapped around hair follicles. Hair root plexuses detect movements on the skin surface that disturb hairs. For example, an insect landing on a hair causes movement of the hair shaft that stimulates the free nerve endings. There are also two types of slowly adapting touch receptors. Type I cutaneous mechanoreceptors, also called tactile discs, are saucer-shaped, flattened free nerve endings that make contact with tactile epithelial cells (Merkel cells) of the stratum basale These touch receptors are plentiful in the fingertips, hands, lips, and external genitalia. Type II cutaneous mechanoreceptors, or Ruffini corpuscles, are elongated, encapsulated receptors located deep in the dermis, and in ligaments and tendons. Present in the hands and abundant on the soles, they are most sensitive to stretching that occurs as digits or limbs are moved Pressure Pressure, a sustained sensation that is felt over a larger area than touch, occurs with deformation of deeper tissues. Receptors that contribute to sensations of pressure include corpuscles of touch, type I cutaneous mechanoreceptors, and lamellated corpuscles. A lamellated corpuscle or pacinian corpuscle (pa-SIN-eˉ-an) is a large oval structure composed of a multilayered connective tissue capsule that encloses a dendrite. Like corpuscles of touch, lamellated corpuscles adapt rapidly. They are widely distributed in the body: in the dermis and subcutaneous layer; in submucosal tissues that underlie mucous and serous membranes; around joints, tendons, and muscles; in the periosteum; and in the mammary glands, external genitalia, and certain viscera, such as the pancreas and urinary bladder. Vibration Sensations of vibration, result from rapidly repetitive sensory signals from tactile receptors. The receptors for vibration sensations are corpuscles of touch and lamellated corpuscles. 19 Corpuscles of touch can detect lower-frequency vibrations, and lamellated corpuscles detect higher-frequency vibrations. Itch The itch sensation results from stimulation of free nerve endings by certain chemicals, such as bradykinin or antigens in mosquito saliva injected from a bite, often because of a local inflammatory response (bradykinin, a kinin, is a potent vasodilator). Tickle Free nerve endings are thought to mediate the tickle sensation. This intriguing sensation typically arises only when someone else touches you, not when you touch yourself. The solution to this puzzle seems to lie in the impulses that conduct to and from the cerebellum when you are moving your fingers and touching yourself that don’t occur when someone else is tickling you. Thermal Sensations Thermoreceptors are free nerve endings that have receptive fields about 1 mm in diameter on the skin surface. Two distinct thermal sensations—coldness and warmth—are detected by different receptors. Cold receptors are located in the stratum basale of the epidermis and are attached to medium-diameter, myelinated A fibers, although a few connect to small-diameter, unmyelinated C fibers. Warm receptors, which are not as abundant as cold receptors, are located in the dermis and are attached to small-diameter, unmyelinated C fibers. Cold and warm receptors both adapt rapidly at the onset of a stimulus, but they continue to generate impulses at a lower frequency throughout a prolonged stimulus. Pain Sensations Pain is indispensable for survival. It serves a protective function by signaling the presence of noxious, tissue-damaging conditions. From a medical standpoint, the subjective description and indication of the location of pain may help pinpoint the underlying cause of disease. Nociceptors, the receptors for pain, are free nerve endings found in every tissue of the body except the brain. Intense thermal, mechanical, or chemical stimuli can activate nociceptors. Tissue irritation or injury releases chemicals such as prostaglandins, kinins, and potassium ions (K+) that stimulate nociceptors. Pain may persist even after a pain-producing stimulus is removed because pain-mediating chemicals linger, and because nociceptors exhibit very little adaptation. Conditions that elicit pain include excessive distension (stretching) of a structure, prolonged muscular contractions, muscle spasms, or ischemia (inadequate blood flow to an organ). Types of Pain There are two types of pain: fast and slow. The perception of fast pain occurs very rapidly, usually within 0.1 second after a stimulus is applied, because the nerve impulses propagate along medium-diameter, myelinated A fibers. This type of pain is also known as acute, sharp, or pricking pain. The pain felt from a needle puncture or knife cut to the skin is fast pain. Fast pain is not felt in deeper tissues of the body. The perception of slow pain, by contrast, begins a second or more after a stimulus is applied. It then gradually increases in intensity over a period of several seconds or minutes. Impulses for slow pain conduct along small diameter, unmyelinated C fibers. This type of pain, which may be excruciating, is also referred to as chronic, burning, aching, or throbbing pain. Slow pain can occur both in the skin and in deeper tissues or internal organs. An example is the pain associated with a toothache. Pain that arises from stimulation of receptors in the skin is called superficial somatic pain; stimulation of receptors in skeletal muscles, joints, tendons, and fascia causes deep somatic 20 pain. Visceral pain results from stimulation of nociceptors in visceral organs. If stimulation is diffuse (involves large areas), visceral pain can be severe. Diffuse stimulation of visceral nociceptors might result from distension or ischemia of an internal organ. For example, a kidney stone or a gallstone might cause severe pain by obstructing and distending a ureter or bile duct. Localization of Pain Fast pain is very precisely localized to the stimulated area. For example, if someone pricks you with a pin, you know exactly which part of your body was stimulated. Somatic slow pain also is well localized but more diffuse (involves large areas); it usually appears to come from a larger area of the skin. In some instances of visceral slow pain, the affected area is where the pain is felt. If the pleural membranes around the lungs are inflamed, for example, you experience chest pain. However, in many instances of visceral pain, the pain is felt in or just deep to the skin that overlies the stimulated organ, or in a surface area far from the stimulated organ. This phenomenon is called referred pain. Figure 10 shows skin regions to which visceral pain may be referred. In general, the visceral organ involved and the area to which the pain is referred are served by the same segment of the spinal cord. For example, sensory fibers from the heart, the skin superficial to the heart, and the skin along the medial aspect of the left arm enter spinal cord segments T1 to T5. Thus, the pain of a heart attack typically is felt in the skin over the heart and along the left arm. 21 Figure 10: Proprioceptive Sensations Proprioceptive sensations (proprius _ self or one’s own) are also called proprioception (proˉ-preˉ-oˉ-SEP-shun). Proprioceptive sensations allow us to recognize that parts of our body belong to us (self). They also allow us to know where our head and limbs are located and how they are moving even if we are not looking at them, so that we can walk, type, or dress without using our eyes. Kinesthesia (kin_-es-THEˉ -zeˉ-a; kin- _ motion; -esthesia _ perception) is the perception of body movements. Proprioceptive sensations arise in receptors termed proprioceptors. Those proprioceptors embedded in muscles (especially postural muscles) and tendons inform us of the degree to which muscles are contracted, the amount of tension on tendons, and the positions of joints. Hair cells of the inner ear monitor the orientation of the head relative to the ground and head position during movements. Because proprioceptors adapt slowly and only slightly, the brain continually receives nerve impulses related to the position of different body parts and makes adjustments to ensure coordination. Proprioceptors also allow weight discrimination, the ability to assess the weight of an object. This type of information helps to determine the muscular effort necessary to perform a task. Here we discuss three types of proprioceptors: muscle spindleswithin skeletal muscles, tendon organs within tendons, and joint kinesthetic receptors within synovial joint capsules. Muscle Spindles Muscle spindles are the proprioceptors in skeletal muscles that monitor changes in the length of skeletal muscles and participate in stretch reflexes (shown in Figure 5). By adjusting how vigorously a muscle spindle responds to stretching of a skeletal muscle, the brain sets an overall level of muscle tone, the small degree of contraction that is present while the muscle is at rest. Each muscle spindle consists of several slowly adapting sensory nerve endings that wrap around 3 to 10 specialized muscle fibers, called intrafusal fibers ( within a spindle). A connective tissue capsule encloses the sensory nerve endings and intrafusal fibers and anchors the spindle to the endomysium and perimysium (Figure 10). Muscle spindles are interspersed among most skeletal muscle fibers and aligned parallel to them. In muscles that produce finely controlled movements, such as those of the fingers or eyes as you read music 22 and play a musical instrument, muscle spindles are plentiful. Muscles involved in coarser but more forceful movements, like the quadriceps femoris and hamstring muscles of the thigh, have fewer muscle spindles. The only skeletal muscles that lack spindles are the tiny muscles of the middle ear. The main function of muscle spindles is to measure muscle length—how much a muscle is being stretched. Either sudden or prolonged stretching of the central areas of the intrafusal muscle fibers stimulates the sensory nerve endings. The resulting nerve impulses propagate into the CNS. Information from muscle spindles arrives quickly at the somatic sensory areas of the cerebral cortex, which allows conscious perception of limb positions and movements. At the same time, impulses from muscle spindles pass to the cerebellum, where the input is used to coordinate muscle contractions. In addition to their sensory nerve endings near the middle of intrafusal fibers, muscle spindles contain motor neurons called gamma motor neurons. These motor neurons terminate near both ends of the intrafusal fibers and adjust the tension in a muscle spindle to variations in the length of the muscle. For example, when your biceps muscle shortens in response to lifting a weight, gamma motor neurons stimulate the ends of the intrafusal fibers to contract slightly. This keeps the intrafusal fibers taut even though the contractile muscle fibers surrounding the spindle are reducing spindle tension. This maintains the sensitivity of the muscle spindle to stretching of the muscle. As the frequency of impulses in its gamma motor neuron increases, a muscle spindlebecomes more sensitive to stretching of its midregion. Surrounding muscle spindles are ordinary skeletal muscle fibers, called extrafusal muscle fibers (extrafusal - outside a spindle), which are supplied by large-diameter A fibers called alpha motor neurons. The cell bodies of both gamma and alpha motor neurons are located in the anterior gray horn of the spinal cord (or in the brain stem for muscles in the head). During the stretch reflex, impulses in muscle spindle sensory axons propagate into the spinal cord and brain stem and activate alpha motor neurons that connect to extrafusal muscle fibers in the same muscle. In this way, activation of its muscle spindles causes contraction of a skeletal muscle, which relieves the stretching. Tendon Organs Tendon organs are located at the junction of a tendon and a muscle. By initiating tendon reflexes, tendon organs protect tendons and their associated muscles from damage due to excessive tension. (When a muscle contracts, it exerts a force that pulls the points of attachment of the muscle at either end toward each other. This force is the muscle tension.) Each tendon organ consists of a thin capsule of connective tissue that encloses a few tendon fascicles (bundles of collagen fibers). Penetrating the capsule are one or more sensory nerve endings that entwine among and around the collagen fibers of the tendon. When tension is applied to a muscle, the tendon organs generate nerve impulses that propagate into the CNS, providing information about changes in muscle tension. The resulting tendon reflexes decrease muscle tension by causing muscle relaxation. 23 Figure : 10 Somatic Sensory Pathways Somatic sensory pathways relay information from the somatic sensory receptors to the primary somatosensory area in the cerebral cortex and to the cerebellum. The pathways to the cerebral cortex consist of thousands of sets of three neurons: a first order neuron, a second- order neuron, and a third-order neuron. 1. First-order neurons conduct impulses from somatic receptors into the brain stem or spinal cord. From the face, mouth, teeth, and eyes, somatic sensory impulses propagate along cranial nerves into the brain stem. From the neck, trunk, limbs, and posterior aspect of the head, somatic sensory impulses propagate along spinal nerves into the spinal cord. 2. Second-order neurons conduct impulses from the brain stem and spinal cord to the thalamus. Axons of second-order neurons decussate (cross over to the opposite side) in the brain stem or spinal cord before ascending to the ventral posterior nucleus of the thalamus. Thus, all somatic sensory information from one side of the body reaches the thalamus on the opposite side. 24 3. Third-order neurons conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side. Regions within the CNS where neurons synapse with other neurons that are a part of a particular sensory or motor pathway are known as relay stations because neural signals are being relayed from one region of the CNS to another. For example, the neurons of many sensory pathways synapse with neurons in the thalamus; therefore the thalamus functions as a major relay station. In addition to the thalamus, many other regions of the CNS, including the spinal cord and brain stem, can function as relay stations. Somatic sensory impulses ascend to the cerebral cortex via three general pathways: (1) the posterior column–medial lemniscus pathway, (2) the anterolateral (spinothalamic) pathway, and (3) the trigeminothalamic pathway. Somatic sensory impulses reach the cerebellum via the spinocerebellar tracts. Posterior Column–Medial Lemniscus Pathway to the Cortex Nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head ascend to the cerebral cortex along the posterior column– medial lemniscus pathway (lem-NIS-kus _ ribbon). The name of the pathway comes from the names of two white-matter tracts that convey the impulses: the posterior column of the spinal cord and the medial lemniscus of the brain stem. First-order neurons in the posterior column– medial lemniscus pathway extend from sensory receptors in the limbs, trunk, neck, and posterior head into the spinal cord and ascend to the medulla oblongata on the same side of the body. The cell bodies of these first-order neurons are in the posterior (dorsal) root ganglia of spinal nerves. In the spinal cord, their axons form the posterior (dorsal) columns, which consist of two parts: the gracile fasciculus (GRAS-ıˉl fa-SIK-uˉ-lus) and the cuneate fasciculus (KUˉ-neˉ-aˉt). The axons synapse with the dendrites of second-order neurons whose cell bodies are located in the gracile nucleus or cuneate nucleus of the medulla. Nerve impulses for touch, pressure, vibration, and conscious proprioception from the upper limbs, upper trunk, neck, and posterior head propagate along axons in the cuneate fasciculus and arrive at the cuneate nucleus. Nerve impulses for touch, pressure, and vibration from the lower limbs and lower trunk propagate along axons in the gracile fasciculus and arrive at the gracile nucleus. The axons of the second-order neurons cross to the opposite side of the medulla and enter the medial lemniscus, a thin ribbonlike projection tract that extends from the medulla to the ventral posterior nucleus of the thalamus. In the thalamus, the axon terminals of second-order neurons synapse with third-order neurons, which project their axons to the primary somatosensory area of the cerebral cortex. Anterolateral Pathway to the Cortex Nerve impulses for pain, temperature, itch, and tickle from the limbs, trunk, neck, and posterior head ascend to the cerebral cortex along the anterolateral (spinothalamic) pathway (spıˉ-noˉ-tha-LAM-ik). Like the posterior column–medial lemniscus pathway, the anterolateral pathway is composed of three-neuron sets. The first-order neurons connect a receptor of the limbs, trunk, neck, or posterior head with the spinal cord. The cell bodies of the first order neurons are in the posterior root ganglion. The axon terminals of the first-order neurons synapse with second-order neurons, whose cell bodies are located in the posterior gray horn of the spinal cord. The axons of the second-order neurons cross to the opposite side of the spinal cord. Then, they pass upward to the brain stem as the spinothalamic tract. The axons of the second-order neurons end in the ventral posterior nucleus of the thalamus, where they synapse with the third-order neurons. The axons of the third order neurons project to the primary somatosensory area on the same side of the cerebral cortex as the thalamus. 25 Trigeminothalamic Pathway to the Cortex Nerve impulses for most somatic sensations (tactile, thermal, and pain) from the face, nasal cavity, oral cavity, and teeth ascend to the cerebral cortex along the trigeminothalamic pathway (trıˉ-jem_-i-noˉ-tha-LAM-ik). Like the other somatosensory pathways just described, the trigeminothalamic pathway consists of three neuron sets. First-order neurons extend from somatic sensory receptors in the face, nasal cavity, oral cavity, and teeth into the pons through the trigeminal (V) nerves. The cell bodies of these first-order neurons are in the trigeminal ganglion. The axon terminals of some first-order neurons synapse with second- order neurons in the pons. The axons of other first order neurons descend into the medulla to synapse with second order neurons. The axons of the second-order neurons cross to the opposite side of the pons and medulla and then ascend as the trigeminothalamic tract to the ventral posterior nucleus of the thalamus. In the thalamus, the axon terminals of the second order neurons synapse with third-order neurons, which project their axons to the primary somatosensory area on the same side of the cerebral cortex as the thalamus. Mapping the Primary Somatosensory Area Specific areas of the cerebral cortex receive somatic sensory input from particular parts of the body. Other areas of the cerebral cortex provide output in the form of instructions for movement of particular parts of the body. The somatic sensory map and the somatic motor map relate body parts to these cortical areas. Precise localization of somatic sensations occurs when nerve impulses arrive at the primary somatosensory area (areas 1, 2, and 3 in Figure 8 ), which occupies the post central gyri of the parietal lobes of the cerebral cortex. Each region in this area receives sensory input from a different part of the body. The left cerebral hemisphere has a similar primary somatosensory area that receives sensory input from the right side of the body. Note that some parts of the body - chiefly the lips, face, tongue, and hand - provide input to large regions in the somatosensory area. Other parts of the body, such as the trunk and lower limbs, project to much smaller cortical regions. The relative sizes of these regions in the somatosensory area are proportional to the number of specialized sensory receptors within the corresponding part of the body. For example, there are many sensory receptors in the skin of the lips but few in the skin of the trunk. This distorted somatic sensory map of the body is known as the sensory homunculus. The size of the cortical region that represents a body part may expand or shrink somewhat, depending on the quantity of sensory impulses received from that body part. For example, people who learn to read Braille eventually have a larger cortical region in the somatosensory area to represent the fingertips. Somatic Sensory Pathways to the Cerebellum Two tracts in the spinal cord—the posterior spinocerebellar tract (spıˉ-noˉ-ser-e-BEL-ar) and the anterior spinocerebellar tract— are the major routes proprioceptive impulses take to reach the cerebellum. Although they are not consciously perceived, sensory impulses conveyed to the cerebellum along these two pathways are critical for posture, balance, and coordination of skilled movements. Table 1 summarizes the major somatic sensory tracts and pathways 26