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48 BRS Physiology Rotation of head ~ Right horizontal semicircular canal Initial direction of endolymph movement Stereocilia \ Hair cell Depolarized (excited) Hyperpolarized {inhibited) Afferent vestibular fiber FIGURE Z.7. The semicircular canals and vestibular transduction during counterclockwise...
48 BRS Physiology Rotation of head ~ Right horizontal semicircular canal Initial direction of endolymph movement Stereocilia \ Hair cell Depolarized (excited) Hyperpolarized {inhibited) Afferent vestibular fiber FIGURE Z.7. The semicircular canals and vestibular transduction during counterclockwise rotation. 3. Vestibular-ocular reflexes a. Nystagmus An initial rotation of the head causes the eyes to move slowly in the opposite direction to maintain visual fixation. When the limit of eye movement is reached, the eyes rapidly snap back (nystagmus) and then move slowly again. The direction of the nystagmus is defined as the direction of the fast (rapid eye) movement. Therefore, the nystagmus occurs in the same direction as the head rotation. b. Postrotatory nystagmus occurs in the opposite direction of the head rotation. F. Olfaction 1. Olfactory pathway a. Recaptor calls are located in the olfactory epithelium. are true neurons that conduct action potentials into the CNS. Basal cells of the olfactory epithelium are undifferentiated stem cells that continuously turn over and replace the olfactory receptor cells (neurons). These are the only neurons in the adult human that replace themselves. b. CN I (olfactoryt carries information from the olfactory receptor cells to the olfactory bulb. The axons of the olfactory nerves are unmyelinated Cfibers and are among the smallest and slowest in the nervous system. Olfactory epithelium is also innervated by CNV (trigeminal), which detects noxious or painful stimuli, such as ammonia l!1lD'!ltlfj Neurophysiology 49 The olfactory nerves pass through the cribriform plate on their way to the olfactory bulb. Fractures of the cribriform plate sever input to the olfactory bulb and reduce (hyposmia) or eliminate (anosmia) the sense of smell. The response to ammonia, however, will be intact after fracture of the cribriform plate because this response is carried on CN V. c. Mitral cells in the olfactory bulb are second-order neurons. Output of the mitral cells forms the olfactory tract, which projects to the prepiriform cortex. 2. Steps in transduction in the olfactory receptor neurons a. Odorant molecules bind to specific olfactory receptor proteins located on cilia of the olfactory receptor cells. b. When the receptors are activated, they activate G proteins (Golf), which in turn activate adenylate cyclase. c. There is an increase in intracellular cAMP that opens Na+ channels in the olfactory receptor membrane and produces a depolarizing receptor potential. d. The receptor potential depolarizes the initial segment of the axon to threshold, and action potentials are generated and propagated. G. Taste 1. Taste pathways a. Taste receptor cells line the taste buds that are located on specialized papillae. The receptor cells are covered with microvilli, which increase the surface area for binding taste chemicals. In contrast to olfactory receptor cells, taste receptors are not neurons. b. The anterior two-thirds of the tongue has fungiform papillae. detects salty, sweet, and umami sensations. is innervated by CNVII (chorda tympani). c. The posterior one-third of the tongue has circumvallate and foliate papillae. detects sour and bitter sensations. is innervated by CN IX (glossopharyngeal). The back of the throat and the epiglottis are innervated by CN X. d. CNVII, CN IX, and CN X enter the medulla, ascend in the solitary tract, and terminate on second-order taste neurons in the solitary nucleus. They project, primarily ipsilaterally, to the ventral posteromedial nucleus of the thalamus and, finally, to the taste cortex. 2. Steps in taste transduction Taste chemicals (sour, sweet, salty, bitter, and umami) bind to taste receptors on the microvilli and produce a depolarizing receptor potential in the receptor cell. IV. MOTOR SYSTEMS A. Motor unit consists of a single motoneuron and the muscle fibers that it innervates. For fine control (e.g., muscles of the eye), a single motoneuron innervates only a few muscle fibers. For larger movements (e.g., postural muscles), a single motoneuron may innervate thousands of muscle fibers. The motoneuron pool is the group of motoneurons that innervates fibers within the same muscle. 50 BRS Physiology The force of muscle contraction is graded by recruitment of additional motor units (size principle). The size principle states that as additional motor units are recruited, more motoneurons are involved and more tension is generated. 1. Small motoneurons innervate a few muscle fibers. have the lowest thresholds and, therefore, fire first. generate the smallest force. 2. Large motoneurons innervate many muscle fibers. have the highest thresholds and, therefore, fire last. generate the largest force. B. Muscle sensor 1. Types of muscle sensors (see Table 2.5) a. Muscle spindles (groups Ia and II afferents) are arranged in parallel with extrafusal fibers. They detect both static and dynamic changes in muscle length. b. Golgi tendon organs (group lb afferents) are arranged in series with extrafusal muscle fibers. They detect muscle tension. c. Pacinian corpuscles (group II afferents) are distributed throughout muscle. They detect vibration. d. Free nerve endings (groups Ill and N afferents) detect noxious stimuli. 2. Types of muscle fibers a. Extrafusal fibers make up the bulk of muscle. are innervated by a-motoneurons. provide the force for muscle contraction. b. lntrafusal fibers are smaller than extrafusal muscle fibers. are innervated by y-motoneurons. are encapsulated in sheaths to form muscle spindles. run in parallel with extrafusal fibers, but not for the entire length of the muscle. are too small to generate significant force. 3. Muscle spindles are distributed throughout muscle. consist of small, encapsulated intrafusal fibers connected in parallel with large (forcegenerating) extrafusal fibers. The finer the movement required, the greater the number of muscle spindles in a muscle. a. Types of intrafusal fibers in muscle spindles (Figure 2.8) (1) Nuclear bagfibers detect the rate of change in muscle length (fast, dynamic changes). are innervated by group Ia afferents. have nuclei collected in a central "bag'' region. (2) Nuclear chain fibers detect static changes in muscle length. are innervated by group II afferents. are more numerous than nuclear bag fibers. have nuclei arranged in rows. b. How the muscle spindle works (see Figure 2.8) Muscle spindle reflexes oppose (correct for) increases in muscle length (stretch). (1) Sensory information about muscle length is received by group Ia (velocity) and group II (static) afferent fibers. l!1lD'!ltlfj Neurophysiology 51 Dynamic y-motor fiber v Nuclear bag fiber Nuclear chain fiber Trail ending Primary ending Secondary ending FIGURE 2.8. Organization of the muscle spindle. (Modified with permission from Matthews PBC. Muscle spindles and their motor control. Physiol Rev 1964;44:232.) (2) When a muscle is stretched (lengthened), the muscle spindle is also stretched, stimulating group Ia and group II afferent fibers. (3) Stimulation of group Ia afferents stimulates a-motoneurons in the spinal cord. This stimulation in turn causes contraction and shortening of the muscle. Thus, the original stretch is opposed and muscle length is maintained. c. Function of y-motoneurons innervate intrafusal muscle fibers. adjust the sensitivity of the muscle spindle so that it will respond appropriately during muscle contraction. a-Motoneurons and y-motoneurons are coactivated so that muscle spindles remain sensitive to changes in muscle length during contraction. C. Muscle reflexes (Table 2.8) 1. Stretch (myotatic) reflex-knee jerk (Figure 2.9) a. b. c. d. is monosynaptic. Muscle is stretched, and the stretching stimulates group Ia afferent fibers. Group Ia afferents synapse directly on a-motoneurons in the spinal cord. The pool of a-motoneurons that is activated innervates the homonymous muscle. Stimulation of a-motoneurons causes contraction in the muscle that was stretched. As the muscle contracts, it shortens, decreasing the stretch on the muscle spindle and returning it to its original length. At the same time, synergistic muscles are activated and antagonistic muscles are inhibited. t a b I e 2.8 Summary of Muscle Reflexes Reflex Number of Synapses Stimulus Afferent Fibers Response Stretch ref Iex (knee-jerk) Monosynaptic Muscle is stretched. Ia Contraction of the muscle Golgi tendon reflex (clasp-knife) Disynaptic Muscle contracts lb Relaxation of the muscle Flexor withdrawal reflex (after touching a hot stove) Polysynaptic Pain II, Ill, and IV Ipsilateral flexion; contralateral extension 52 BRS Physiology Ia afferent a-motoneuron Homonymous muscle FIGURE 2.9. The stretch reflex. e. Example of the knee-jerk reflex. Tapping on the patellar tendon causes the quadriceps to stretch. Stretch of the quadriceps stimulates group Ia afferent fibers, which activate a-motoneurons that make the quadriceps contract. Contraction of the quadriceps forces the lower leg to extend. Increases in y-motoneuron activity increase the sensitivity of the muscle spindle and therefore exaggerate the knee-jerk reflex. 2. Golgi tendon reflex (inverse myotatic) is disynaptic. is the opposite, or inverse, of the stretch reflex. a. Active muscle contraction stimulates the Golgi tendon organs and group lb afferent fibers. b. The group lb afferents stimulate inhibitory interneurons in the spinal cord. These interneurons inhibit a-motoneurons and cause relaxation of the muscle that was originally contracted. c. At the same time, antagonistic muscles are excited. d. Clasp-knife reflex, an exaggerated form of the Golgi tendon reflex, can occur with disease of the corticospinal tracts (hypertonicity or spasticity). For example, if the arm is hypertonic, the increased sensitivity of the muscle spindles in the extensor muscles (triceps) causes resistance to flexion of the arm. Eventually, tension in the triceps increases to the point at which it activates the Golgi tendon reflex, causing the triceps to relax and the arm to flex closed like a jackknife. 3. Flexor withdrawal reflex is polysynaptic. results in flexion on the ipsilateral side and extension on the contralateral side. Somatosensory and pain afferent fibers elicit withdrawal of the stimulated body part from the noxious stimulus. a. Pain (e.g., touching a hot stove) stimulates the flexor reflex afferents of groups II, Ill, and IV. b. The afferent fibers synapse polysynaptically (via interneurons) onto motoneurons in the spinal cord. c. On the ipsilateral side of the pain stimulus, flexors are stimulated (they contract) and extensors are inhibited (they relax), and the arm is jerked away from the stove. On the contralateral side, flexors are inhibited and extensors are stimulated (crossed extension reflex) to maintain balance. d. As a result of persistent neural activity in the polysynaptic circuits, an afterdischarge occurs. The afterdischarge prevents the muscle from relaxing for some time.
