Human Nervous System:

Transcription

1 OLLI Brain: Making Sense of Our World: Lecture 3 Human Nervous System: The Motor & Sensory Divisions Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings Organization of the Nervous System 1. Central nervous system (CNS) a. Brain (in the cranial volt) b. Spinal cord (within the vertebral canal) c. Function: integration and command center 2. Peripheral nervous system (PNS) a. Spinal (31 pairs) & Cranial Nerves (12 pairs) b. Function: carries messages to & from the CNS c. Two functional divisions: sensory & motor Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings 1

2 The Human Brain Corpus callosum Cerebrum 2

3 Cranial Nerves Twelve pairs of cranial nerves arise from the brain They have sensory, motor, or both sensory and motor functions Each nerve is identified by a number (I through XII) and a name Summary of Function of Cranial Nerves Cranial nerve Type General function Olfactory (I) Sensory Olfaction Optic (II) Sensory Vision Oculomotor (III) Motor Most eye movement Adjust lenses and light input Trochlear (IV) Motor Directs the eyeball Trigeminal (V) Motor & sensory Sensation of face and mouth Mastication command Abducens (VI) Motor Eye muscle movement Facial (VII) Motor & sensory Facial expression Sense of taste Vestibulocochlear (VIII) Sensory Sense of hearing & balance Glossopharyngeal (IX) Motor & sensory Taste & swallowing muscles Vagus (X) Motor & sensory Heart and breathing rate Swallowing and speech Spinal Accessory (XI) Motor Movement of back and neck Hypoglossal (XII) Motor Allows tongue movement 3

4 Spinal Cord Slender column of nerve fibers continuous with the brain and extending downward through the vertebral canal Consists of 31 segments, each of which gives rise to a pair of spinal nerves (total of 31 pairs) that branch to various body parts and connect them with the CNS The spinal cord has two major functions: 1. Pathways for nerve impulses to and from the brain 2. It is a center for spinal reflexes Anatomy of the Spinal Cord 4

5 Histology of the Spinal Cord The spinal cord has a butterfly-shaped inner core of gray matter and an outer layer of white matter (unlike in the brain!) Spinal Nerves The 31 pairs of spinal nerves originate in the spinal cord and exit through the vertebrae all along the vertebral column They are designated according to the level of the vertebral column at which they emerge: Cervical nerves (8) Thoracic nerves (12) Lumbar nerves (5) Sacral nerves (5) 5

6 Function of Spinal Nerves Cervical nerves: supply the back of the head, neck, shoulders, arms, hands and diaphragm Thoracic nerves: supply the chest and parts of the abdomen Lumbar nerves: supply lower back as well as parts of thighs and legs Sacral nerves: supply the buttocks, most parts of the legs and feet, as well as anal and genital areas Spinal Nerves The spinal nerves are all mixed nerves: both sensory and motor axons run together through them Most spinal nerves innervate specific areas of the skin called dermatomes. By testing each dermatome, the health care professional can determine which spinal nerve have been damaged Damage to a spinal nerve can lead to muscular paralysis and loss of sensation in the area supplied by the nerve 6

7 Functional Nervous System Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.1 Functional Subdivisions of the PNS 1. Sensory afferent division a. Somatic sensory division: afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain b. Visceral sensory division: afferents fibers transmit impulses from visceral organs to the brain 2. Motor efferent division a. Somatic motor division: allow conscious control of skeletal muscles b. Visceral motor division (ie, autonomic nervous system; ANS): regulate smooth muscle, cardiac muscle and glands Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings 7

8 Classification of Neurons Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings Somatic Motor Subdivision Motor areas control movements of voluntary skeletal muscles Motor homunculus caricature of relative amounts of cortical tissue devoted to each motor function: 8

9 Autonomic Nervous System The autonomic nervous system (ANS) is the motor division of the peripheral nervous system (PNS) that functions independently and continuously, without conscious effort. This system controls visceral activities by regulating the action of smooth muscles, cardiac muscles, and various glands The ANS oversees heart rate, blood pressure, breathing rate, body temperature, and other visceral activities that help in maintaining homeostasis. Portions of the ANS also respond during times of emotional stress and prepare the body to meet the demands of strenuous physical activity. Divisions of the ANS The two divisions of the ANS are the sympathetic and parasympathetic The sympathetic system mobilizes the body during extreme situations The parasympathetic system performs maintenance activities and conserves body energy The sympathetic system originate from the lateral horns of the first thoracic through h first lumbar segments of the spinal cord The parasympathetic system consists of cranial nerves (ie. vagus) and spinal nerves originating in the sacral region of the spinal cord. 9

10 Anatomy of the ANS Role of the Sympathetic Division The sympathetic division is the fight-or-flight system It involves the E activities: exercise, excitement, emergency, and embarrassment Promotes adjustment during exercise: blood flow to organs is reduced, flow to muscles is increased Its activity is illustrated in a person who is threatened Heart rate increases, and breathing is rapid and deep Gastrointestinal tract activity is low The skin is cold and sweaty, and the pupils dilate 10

11 Role of the Parasympathetic Division This division is concerned with keeping body energy use low It involves the D activities: digestion, defecation, and diuresis Its activity is illustrated in a person who relaxes after a meal: Blood pressure, heart rate, and respiratory rates are low Gastrointestinal tract activity is high The skin is warm and the pupils are constricted Interactions of the Autonomic Divisions Most visceral organs are innervated by both sympathetic and parasympathetic fibers. This results in dynamic counterbalancing effects that precisely control visceral activity: Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes The sympathetic division can override the effects of the parasympathetic division during times of stress Cooperation between ANS divisions is best seen in control of the external genitalia: Parasympathetic fibers cause vasodilatation and are responsible for erection of the penis and clitoris Sympathetic fibers cause ejaculation in males and reflex peristalsis in females. 11

12 Short Term vs. Prolonged Stress 12