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38 BRS Physiology II. ORGANIZATION OF THE NERVOUS SYSTEM A. Divisions of the nervous system The nervous system is composed of the CNS and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The major divisions of the CNS are the spinal cord, brain stem (medulla, pons, an...

38 BRS Physiology II. ORGANIZATION OF THE NERVOUS SYSTEM A. Divisions of the nervous system The nervous system is composed of the CNS and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The major divisions of the CNS are the spinal cord, brain stem (medulla, pons, and midbrain), cerebellum, diencephalon (thalamus and hypothalamus), and cerebral hemispheres (cerebral cortex, basal ganglia, hippocampus, and amygdala). Sensory or afferent nerves bring information into the nervous system. Motor or efferent nerves carry information out of the nervous system. B. Cells of the nervous system 1. Structure of the neuron a. Cell body surrounds the nucleus and is responsible for protein synthesis. b. Dendrites arise from the cell body and receive information from adjacent neurons. c. Axon projects from the axon hillock, where action potentials originate and send information to other neurons or muscle. 2. Glial cells function as support cells for neurons a. Astrocytes supply metabolic fuels to neurons, secrete trophic factors, and synthesize neurotransmitters. b. Oligodendrocytes synthesize myelin in the CNS (whereas Schwann cells synthesize myelin in the PNS). c. Microglial cells proliferate following neuronal injury and serve as scavengers for cellular debris. Ill. SENSORY SYSTEMS A. Sensory receptors-general are specialized epithelial cells or neurons that transduce environmental signals into neural signals. The environmental signals that can be detected include mechanical force, light, sound, chemicals, and temperature. 1. Types of sensory transducers a. Mechanoreceptors Pacinian corpuscles Joint receptors Stretch receptors in muscle Hair cells in auditory and vestibular systems Baroreceptors in carotid sinus b. Photoreceptors Rods and cones of the retina c. Chemoreceptors Olfactory receptors Taste receptors Osmoreceptors Carotid body 0 2 receptors d. Extremes of temperature and pain Nociceptors I!1[Jt!jfj t a b I e 2.5 39 Neurophysiology Characteristics of Nerve Fiber Types General Fiber Type and Example Sensory Fiber Type and Example Diameter Conduction Velocity A-alpha Ia largest Fastest Large a-motoneurons Muscle spindle afferents largest Fastest Medium Medium Medium Medium Small Medium Small Medium Smallest Slowest lb Golgi tendon organs A-beta II Touch, pressure Secondary afferents of muscle spindles; touch and pressure A-gamma y-Motoneurons to muscle spindles (intrafusal fibers) A-delta Ill Touch, pressure, temperatura, and pain Touch, pressure, fast pain, and temperatura B Preganglionic autonomic fibers c IV Slow pain; postganglionic autonomic fibers Pain and temperature (unmyelinated) 2.. Fiber types and conduction velocity (Table 2.5) 3. Receptive field is an area of the body that, when stimulated, changes the firing rate of a sensory neuron. If the firing rate of the sensory neuron is increased, the receptive field is excitatory. If the firing rate of the sensory neuron is decreased, the receptive field is inhibitory. 4. Staps in sensory transduction a. Stimulus arrives at the sensory recaptor. The stimulus may be a photon of light on the retina, a molecule of NaQ on the tongue, a depression of the skin, and so forth. b. lon channels are opened in the sensory receptor, allowing current to flow. Usually, the current is inward, which produces depolarization of the receptor. The exception is in the photoreceptor, where light causes decreased inward current and hyperpolarization. c. The change in membrane potential produced by the stimulus is the receptor potential, or generator potential (Figure 2.2). RGURE 2.2. Receptor {generator! potential and how it may lead to an action potential. 40 BRS Physiology If the receptor potential is depolarizing, it brings the membrane potential closer to threshold. If the receptor potential is large enough, the membrane potential will exceed threshold, and an action potential will fire in the sensory neuron. Receptor potentials are graded in size depending on the size of the stimulus. 5. Adaptation of sensory receptors a. Slowly adapting, or tonic, receptors (muscle spindle; pressure; slow pain) respond repetitively to a prolonged stimulus. detect a steady stimulus. b. Rapidly adapting, or phasic, receptors (pacinian corpuscle; light touch) show a decline in action potential frequency with time in response to a constant stimulus. primarily detect onset and offset of a stimulus. 6. Sensory pathways from tha sensory recaptor to the cerebral cortex a. Sensory receptors are activated by environmental stimuli. may be specialized epithelial cells (e.g., photoreceptors, taste receptors, auditory hair cells). may be primary afferent neurons (e.g., olfactory chemoreceptors). transduce the stimulus into electrical anergy (i.e., receptor potential). b. First-order neurons are the primary afferent neurons that receive the transduced signal and send the information to the CNS. Cell bodies of the primary afferent neurons are in dorsal root or spinal cord ganglia. c. Second-order neurons are located in the spinal cord or brain stem. receive information from one or more primary afferent neurons in relay nuclei and transmit it to the thalamus. Axons of second-order neurons may cross the midline in a relay nucleus in the spinal cord before they ascend to the thalamus. Therefore, sensory information originating on one side of the body ascends to the contralateral thalamus. d. Third-order neurons are located in the relay nuclei of the thalamus. From there, encoded sensory information ascends to the cerebral cortex. e. Fourth-order neurons are located in the appropriate sensory area of the cerebral cortex. The information received results in a conscious perception of the stimulus. B. Somatosensory system includes the sensations of touch, movement, temperature, and pain. 1. Pathways in the somatosensory system a. Dorsal column system processes sensations of fine touch, pressure, two-point discrimination, vibration, and proprioception. consists primarily of group II fibers. Course: primary afferent neurons have cell bodies in the dorsal root. Their axons ascend ipsilaterally to the nucleus gracilis and nucleus cunaatus of the medulla. From the medulla, the second-order neurons cross the midline and ascend to the contralateral thalamus, where they synapse on third-order neurons. Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order neurons. I!1[Jt!jfj t a b I e 2.6 Neurophysiology Types of Mechanoreceptors Type vf Mechanoreceptor Description Sensation Encoded Adaptation Pacinian corpuscle Onion-like structures in the subcutaneous skin (surrounding unmyelinated nerve endings) Vibration; tapping Rapidly adapting Meissner corpuscle Present in nonha iry skin Velocity Rapidly adapting Ruffini corpuscls Encapsulated Pressurs Slowly adapting Merkel disk Transducer is on spithelial cells location Slowly adapting 2. 1 4. 5. 41 b. Anterolateral system processes sensations oftemperature, pain, and light touch. consists primarily of group Ill and IV fibers, which enter the spinal cord and tenninate in the dorsal hom. Course: second-order newons cross the midline to the anterolateral quadrant of the spinal cord and ascend to the contralateral thalamus, where they synapse on thirdorder newons. Third-order neurons ascend to the somatosensory cortex, where they synapse on fourth-order newons. Mechanoreceptars for touch and pressure (Table 2.6) Thalamus Information from different parts of the body is arranged somatotopically. Destruction of the thalamic nuclei results in loss of sensation on the contralateral side of the body. Somatosensory cortex-the sensory homunculus The major somatosensory areas of the cerebral cortex are Sl and Sll. SI has a somatotopic representation similar to that in the thalamus. This "map" of the body is called the sensory homunculus. The largest areas represent the face, hands, and fingers, where precise localization is most important. Pain is associated with the detection and perception of noxious stimuli (nociception). The receptors for pain are free nerve endings in the skin, muscle, and viscera. Neurotransmitters for nociceptors include substance P. Inhibition of the release of substance P is the basis of pain relief by opioids. a. Fibers for fast pain and slow pain Fast pain is carried by group III fibers. It has a rapid onset and offset, and is localized. Slow pain is carried by C fibers. It is characterized as aching, burning, or throbbing that is poorly localized. b. Referred pain Pain ofvisceral origin is referred to sites on the skin and follows the dermatome rule. These sites are innervated by nerves that arise from the same segment of the spinal cord. For example, ischemic heart pain is referred to the chest and shoulder. C. Vision 1. Optics a. Refractive power of a lens is measured in dioptars. equals the reciprocal of the focal distance in meters. Example: 10 diopters =1110 m = 10 em 42 BRS Physiology Pigment cell layer Photoreceptor layer External limiting membrane -Outer nuclear layer Outer plexiform layer.E g Inner nuclear layer 0 c: 0 ~ Inner plexiform layer i5 Ganglion cell Ganglion cell layer Optic nerve layer Internal limiting membrane ----------------------------------------- FIGURE 2.3. Cellular layers of the retina. (Reprinted with permission from Bullock J, Boyle J Ill, Wang MB. Physiology. 4th ed. Baltimore: Lippincott Williams & Wilkins, 2001:77.) b. Refractive errors (1) Emmetropia-normal. Light focuses on the retina. (2) Hyperopia-farsighted. Light focuses behind the retina and is corrected with a convex lens. (3) Myopia-nearsighted. Light focuses in front of the retina and is corrected with a biconcave lens. (4) Astigmatism. Curvature of the lens is not uniform and is corrected with a cylindric lens. (5) Presbyopia is a result of loss of the accommodation power of the lens that occurs with aging. The near point (closest point on which one can focus by accommodation of the lens) moves farther from the eye and is corrected with a convex lens. 2. Layers of the retina (Figure 2.3) a. Pigment epithelial cells absorb stray light and prevent scatter of light. convert 11-cis retinal to all- trans retinal. b. Receptor cells are rods and cones (Table 2. 7). Rods and cones are not present on the optic disk; the result is a blind spot. t a b I e 2.7 Function Functions of Rods and Cones Rods Cones Sensitivity to light Sensitive to low-intensity light night vision Sensitive to high-intensity light; day vision Acuity Lower visual acuity Not present in fovea Higher visual acuity Present in fovea Dark adaptation Rods adapt later Cones adapt first Color vision No Yes