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SENSORY, MOTOR, AND INTEGRATIVE SYSTEMS A. SENSATION Identify the three essential functions of the nervous system. 1. 2 3. receive sensory information integrate and interpret the information initiate motor responses, if necessary What is the basis for selecting some things for consciousness? The CNS requires a continuous flow of sensory information to regulate homeostasis and initiate appropriate responses to changes in the internal and external environments. The CNS selects for consciousness only those pieces of information that are of consequence at a given moment. All other information remains subconscious or unconscious. Define the concept of a sensation. In its broadest sense, a sensation is the conscious, subconscious, or unconscious awareness of external and internal stimuli. 1. LEVELS OF SENSATION What are the levels of CNS translation? The nature of the sensation and the type of reaction generated by the CNS vary with the level of the CNS at which the stimulus is translated: a. spinal reflexes b. visceral reflexes (brainstem) c. crude identification (thalamus) d. precise identification (cerebral cortex) 2. MODALITY What is modality? Each specific type of sensation is called a sensory modality. The distinct quality that makes one sensation different from another is its modality. A given sensory neuron carries only one modality. 127 3. COMPONENTS OF A SENSATION In the proper order, list and describe the four processes that must occur in order to produce a sensation. 4. 1. stimulation -- a stimulus (change in the environment) capable of activating a sensory neuron must be present 2. transduction -- a sensory receptor or sensory organ must respond to the stimulus and transduce it to a generator potential 3. conduction -- at threshold, an action potential must be conducted along a sensory neuron 4. translation -- a region of the CNS must translate the nerve impulse into a sensation GENERATOR POTENTIALS AND RECEPTOR POTENTIALS Define the concept of generator potential. A generator potential is the graded depolarization that results in a change in resting membrane potential in a receptor (specialized neuronal ending). Describe some of the characteristics of a generator potential. 1. 2. 3. always produces depolarization will initiate an action potential in a sensory neuron if the stimulus is strong enough to form threshold depolarization all receptors, except those for vision, hearing, equilibrium, and taste, create generator potential. Define the concept of a receptor potential. A receptor potential is depolarization or hyperpolarization of the plasma membrane of a receptor cell that alters release of neurotransmitter from the cell. Describe some of the characteristics of a receptor potential. 1. 2. 3. may produce depolarization or hyperpolarization never directly initiates an action potential in a sensory neuron directly regulates release of neurotransmitter onto a sensory neuron that may initiate threshold depolarization. 128 5. ADAPTATION OF SENSORY RECEPTORS What is adaptation of a receptor? Adaptation is a change in sensitivity, usually a decrease, to a longlasting stimulus. Receptors vary in their ability to adapt. What is the difference between fast-adapting and slow-adapting receptors? Fast-adapting (phasic) receptors -- pressure, touch, and smell receptors adapt quickly and play a major role in signaling changes in a stimulus. Slow-adapting (tonic) receptors -- pain, proprioceptors, and chemoreceptors adapt slowly. They are important in signaling information regarding the steady states of the body. B. GENERAL SENSES 1. CUTANEOUS SENSATIONS Name the cutaneous sensations? Cutaneous sensations include tactile (touch, pressure, vibration), thermal (hot, cold) and pain. Where are these receptors located? The receptors for these sensations are located in the skin, connective tissues under the skin, mucous membranes, mouth, and anus. These receptors are distributed across the body such that some areas are heavily populated (very sensitive) while other areas contain only a few receptors (poorly sensitive). In general, describe the structure of cutaneous receptors. Cutaneous receptors consist of the dendritic ends of sensory neurons that may be enclosed in a specialized encapsulated structure or have no special structure at all. Pain receptors, for instance, are really just bare dendrites. 2. PROPRIOCEPTIVE SENSATIONS What is the proprioceptive sense? An awareness of the activities of the muscles, joints, and tendons, and of balance is provided by the proprioceptive (kinesthetic) sense. 129 Where are proprioceptors located? Skeletal muscles, joint capsules, and tendons What is the role of proprioceptors in homeostasis? They apprise the CNS of muscle and tendon tension, change of position, and the orientation of the head. They adapt very slowly, allowing the CNS to be constantly aware of body position so that fine adjustments can be made. C. PHYSIOLOGY OF SENSORY PATHWAYS Most information from somatic receptor decussates. What does this mean? Most input from somatic receptors on one side of the body crosses over (decussates) to the opposite side of the CNS before ascending to the thalamus. From the thalamus, where does somatic sensory information go? From the thalamus, the sensory information proceeds to the somatosensory (primary sensory cortex) cortex of the postcentral gyrus in the parietal lobe of the cerebrum. It is in this cortical region that the sensory input becomes conscious (perception). Three neurons are required to complete the sensory pathway from somatic receptor to the brain. Name and describe them. First-order neuron -- The first-order neuron is always the sensory neuron bearing the receptor on its dendritic end. Its cell body is in a sensory ganglion. Second-order neuron -- The second-order neuron arises in the gray matter. Its axon travels in an ascending tract and decussates prior to entering the thalamus. Third-order neuron -- The third-order neuron arises in the thalamus. Its axon passes to the somatosensory cortex where it synapses on a cortical neuron. 1. POSTERIOR COLUMN-MEDIAL LEMNISCUS PATHWAY Describe the posterior column-medial lemniscus pathway. What are the names of the tracts involved and what modalities do they convey? 130 Fasciculus gracilis and fasciculus cuneatus carry sensory information related to discriminative touch, stereognosis, proprioception, vibration, and weight discrimination. The axon of the first-order neuron enters the dorsal horn, passes into either fasciculus gracilis or cuneatus, then ascends ipsilaterally to the white matter of the medulla. In the medulla, the axon enters either nucleus gracilis or nucleus cuneatus, where it synapses with the second-order neuron. The axon of the second-order neuron decussates in the medulla and enters a projection tract known as the medial lemniscus to ascend to the thalamus. The second-order neuron synapses in the thalamus with its thirdorder neuron, the axon of which passes into the internal capsule to reach the somatosensory cortex, where consciousness of the sensation occurs. 2. ANTEROLATERAL (SPINOTHALAMIC) PATHWAYS Describe the anterolateral (spinothalamic) pathways. What are the names of the tracts involved and what modalities do they convey? Anterolateral (spinothalamic) pathways are the lateral spinothalamic tract (pain and temperature) and the anterior (ventral) spinothalamic tract (crude touch, tickle, pressure, itch). For both tracts, the first-order neuron enters the dorsal gray horn and immediately synapses with the second-order neuron. The axon of the second-order neuron decussates through the gray commissure and enters either the lateral or anterior tract, depending upon the modality being carried. Each tract passes through the brainstem to the thalamus where the axon synapses with the third-order neuron. The axon of the third-order neuron passes through the internal capsule to the somatosensory cortex and synapses with the appropriate cortical neuron so that perception of the modality occurs. 3. SOMATOSENSORY CORTEX Describe the somatosensory cortex. What is a homunculus? 131 Areas of the somatosensory cortex in the postcentral gyrus of the parietal lobe have been mapped to form a homunculus, the areas of representation of body parts in the cerebral cortex. Some areas of representations are huge compared to other areas. In particular the hands and face have a tremendous amount of representation in the cortex. The size of cortical areas given to a particular structure is indicative of the number of receptors located in that body part. The size of somatosensory cortex and the number of receptors in a given area are directly related to the functional importance of that part. For each cutaneous receptor, there is a corresponding first-, second-, and third-order neuron in the somatosensory cortex. D. PHYSIOLOGY OF MOTOR PATHWAYS Describe the motor cortex? To direct voluntary motion there is the motor cortex, neurons found in the precentral gyrus of the frontal lobe of the cerebrum. The functional importance of an area is indicated by the amount of motor cortex devoted to movement of that body part. As with the somatosensory cortex, the hands, face, and mouth have the largest area of cortex given to them. What is the difference between an upper motor neuron and a lower motor neuron? Each motor unit of skeletal muscle in the body has a corresponding neuron in the motor cortex. Such a neuron is called an upper motor neuron. The axons of upper motor neurons descend through the CNS to synapse on lower motor neurons located in the brainstem gray matter or the ventral horns of the spinal cord. Lower motor neurons, in turn, leave the CNS to stimulate skeletal muscles. 1. DIRECT (PYRAMIDAL) PATHWAYS Name the tracts of the pyramidal pathways and describe the means by which voluntary motor information reaches skeletal muscles. Voluntary motor impulses are propagated from the motor cortex via upper motor neurons in the direct (pyramidal) tracts, so called because the axons pass through the pyramids of the medulla. There are three primary pyramidal tracts: 132 1. 2. 3. lateral corticospinal tracts anterior corticospinal tracts corticobulbar tracts Upper motor neurons for all tracts originate in motor cortex. Their axons pass through the internal capsule then through the brainstem. Axons of the lateral tracts and corticobulbar tracts decussate in the medulla, while those of the anterior tracts decussate at the level of appropriate lower motor neurons in the spinal cord. The upper motor neurons synapse on the lower motor neurons located in the ventral horns. Axons of all lower motor neurons pass into the periphery stimulate the appropriate motor units. Lower motor neurons associated with the lateral and anterior corticospinal tracts pass into the periphery via spinal nerves while those associated with the corticobulbar tracts pass via the cranial nerves. 2. INDIRECT (EXTRA-PYRAMIDAL) PATHWAYS Give a brief description of the indirect (extrapyramidal) pathways. The indirect pathways include all motor tracts other than the corticospinal and corticobulbar tracts. The upper motor neurons carry information from the motor cortex, basal ganglia, thalamus, cerebellum, reticular formation, and brainstem nuclei to influence the activity of the lower motor neurons. 133