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

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 Posterior semicircular canal Lateral semicircular canal Utricle (otoliths are within the utricle) Saccule Cochlea Auditory nerve Vestibular nerve

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