Chapter 4 The Nervous System Organization of the nervous system Anatomical organization Central nervous system Peripheral nervous system Functional organization Autonomic nervous system Sympathetic Parasympathetic Somatic nervous system Visit The Autonomic Nervous System at www ndrf org ans htm Diagram of the peripheral nerves depicting the extensive nature of the nervous system Functional divisions of the nervous system Cells of the nervous system A neuron a single nerve cell is made up of Dendrites receive nerve impulses and conduct them to the cell body Cell body Axon conducts impulses away from the cell body Cells of the nervous system cont Afferent sensory neurons conduct sensory impulses from the periphery to the central nervous system Efferent motor neurons conduct impulses from the central nervous system to the muscles The neuron and its components The spinal reflex Involves a minimum of two neurons The myotatic reflex A simple two neuron reflex An example is the knee jerk response Diagram of a two neuron reflex from a spindle in a muscle back to the muscle fibers of the same muscle The flexion reflex A simple reflex involving an internuncial neuron Moving your hand away from a hot surface is an example Diagram illustrating how impulses from a cutaneous receptor reach an effector skeletal muscle by a three neuron arc at the level of entrance Proprioception The feedback of sensory information about movement and body position Two types of receptors are involved Vestibular receptors in the semicircular canals of the inner ear Vestibular receptors in the utricle The vestibular system consists of Vestibular receptors found in the nonauditory labyrinths of the inner ear Vestibule found in each labyrinth endolymph fluid within the canals Sacule involved in sensory perception of vibration Utricle contains the otolith organ which is the source of data that informs us of our posture in space Structural relations of innervation of the human labyrinth Anterior semicircular canal Vestibular nerve Posterior semicircular canal Auditory nerve Lateral semicircular canal Utricle otoliths are within the utricle Cochlea Saccule Kinesthesis The sense of movement and position of the body parts in space Main receptors involved are Muscle spindles found in the muscle tissue Golgi tendon organs found at junction between muscle and tendon Muscle spindle Golgi tendon organ Voluntary control of muscular activity Pyramidal system Extrapyramidal system Proprioceptive cerebellar system The pyramidal system System originates in Area 4 in large nerve cells shaped like pyramids Axons from cells form large descending motor pathways pyramidal tracts to synapses with the motor neurons in the ventral horn of the spinal column 85 percent of nerve fibers of pyramidal tract cross from one side to the other Motor cortex area 4 Area of cortex devoted to a body part is Not proportional to amount of tissue served Proportional to complexity of movement potential e g hands have disproportionate share of motor neurons Motor cortex oriented by movement not muscle Stimulation results not in twitch of one muscle but in smooth synergistic movement of a group of muscles Important differences between the pyramidal and extrapyramidal systems Electrical stimulation of the pyramidal system area 4 or motor cortex produces specific movements so it is thought that learning a new skill involves this area Stimulation of the extrapyramidal system area 6 or premotor cortex produces large general movement patterns As skill increases it is thought that the origin of the movement shifts to this area The role of the cerebellum in the proprioceptive cerebellar system Receives sensory information regarding position balance and movement from receptors in muscles joints tendons and skin as well as from visual auditory and vestibular organs Modifies muscular activity during movement The areas of the human cerebral cortex Posture Upright posture mainly brought about through reflexes particularly the myotatic reflex Postural reflexes depend on effects of the extrapyramidal system Balance Coordination of otolith system and muscle and tendon receptors provide the perception of the body s orientation and position of the body parts in space Righting reflex is an example Analysis of neural activity in a simple voluntary movement Example of right arm bringing right index fingertip to the end of the nose 1 Neural activity originates in the arm section of area 4 of the motor cortex 2 Proceeds by way of the pyramidal tracts to synapse with the lower motor neuron in the cord and 3 Out to the appropriate muscles by way of the brachial plexus 4 To the network of nerves in the axilla Analysis of neural activity in a simple voluntary movement cont 5 At the same time kinesthetic impulses traverse the afferent pathways to the cerebellum to bring about proper control and coordination 6 Kinesthetic impulses act reflexively to relax the antagonists through reciprocal inhibition 7 Gamma efferent system is busy all the while innervating the muscle spindles to feed constant measurements of the movement s progress 8 As movement accelerates more motor units are activated 9 Movement has to decelerate in reverse fashion Neurological disorders and injuries Common symptom is neuromuscular weakness Even a small improvement in muscle function may result in a tremendous improvement in functional performance Resistance training may improve strength in such disorders as stroke partial paralysis and polio