Chapter 9 Nervous System

Transcription

1 Chapter 9 Nervous System Introduction: A. The nervous system is composed of neurons and neuroglia. 1. Neurons transmit nerve impulses to other neurons. 2. Nerves are made up of bundles of nerve fibers or axons. 3. Neuroglia aid and protect components of the nervous system divisions: central nervous system (CNS), the brain and spinal cord Nervous System Functions: sensory, integrative, and motor. (think pair share: 3 types of sensory input and 3 types of motor responses) peripheral nervous system (PNS), made up of peripheral nerves that connect the CNS to the rest of the body. (Think pair share: come up with 3 main roles neurons can play) 3 1. Motor functions somatic nervous system: conscious control autonomic nervous system: Unconscious control Sympathetic: Fight or flight Parasympathetic: at rest 4 Supporting cells A. Neuroglial Cells 1. fill spaces, support neurons, provide structural frameworks, produce myelin, and carry on phagocytosis. 4 in the CNS and 1 in the PNS

2 Microglial cells -phagocytosis Oligodendrocytes form myelin Schwann Cell Astrocytes provide nutrients, aid metabolism, structural support respond to brain injury by filling in spaces. Ependymal cells line ventricles, form choroid plexuses Schwann cells form myelin in PNS 7 8 Classification of Neurons grouped two ways: Structural 1- structural differences (bipolar, unipolar, and multipolar neurons), 2- functional differences (sensory neurons, interneurons, and motor neurons) Structural classification 1. Bipolar neurons Sensory neurons: eyes, nose, ears 2. Unipolar neurons - in ganglia outside the CNS - sensory neurons 3. Multipolar neurons - brain and spinal cord - interneurons - motor neurons 4. Sensory (afferent ) neurons usually unipolar, some are bipolar. conduct impulse from body to CNS 5. Interneurons - multipolar conduct impulse between neurons 6. Motor (efferent) neurons - multipolar Conduct impulse from CNS to body

3 Neuron Structure Nerve fibers = one axon, many dendrites. dendrites carry impulses toward the cell body. axons transmit the impulse away from the cell body Larger axons are enclosed by sheaths of myelin provided by Schwann cells = myelinated. Nodes of Ranvier = gaps in the myelin sheath between Schwann cells White matter in the CNS is due to myelin sheaths in this area. gray matter is unmyelinated nerve tissue in the CNS The smallest axons lack a myelin sheath and are unmyelinated fibers. 17 Nerve Impulse physiology 1- cells keep positive charge outside, negative charge inside = membrane potential. controlling ion concentrations. 2- Sodium/Potassium pumps create concentration gradient for Na+ and K+ ions. cause membrane potential. depolarization (flow of + ions into cell) caused by stimulation to neuron. 18 3

4 19 20 Summation: = depolarizations pile up on each other. (Like 2 small waves becoming a larger one) Threshold: the amount of stimulation needed to cause an action potential to fire. Action Potential: Rapid depolarization of cell membrane caused by the opening of Na+ channels. (Nerve Impulse) Nerve Impulse Conduction speed 1. Unmyelinated fibers = slow Conduct impulses over entire membrane. 2. Myelinated fibers = fast conduct impulses from node to node Nerve Impulse: rippling of action potentials down an 21 axon. 22 The Synapse The junction (space) between two neurons. way of controlling impulse transmission Where many drugs take action Where neurotransmitters released Synaptic Transmission process where impulse in presynaptic neuron transmitted across synapse to postsynaptic neuron

5 Figure 9.13 Synaptic Transmission 1- impulse reaches synaptic knob (end of axon) 2- synaptic vesicles release neurotransmitter to synaptic cleft. (synapse) 3- Neurotransmitters bind to specific receptors on postsynaptic neuron (dendrite) 4- if Threshold stimulus is reached, action potential triggered 5- Nerve impulse travels to cell body and down axon Neurotransmitters At least 50 kinds Most made in synaptic knobs stored in synaptic vesicles. Ex: Acetylcholine = neurotransmitter that controls skeletal muscle actions. Nerve Pathways routes nerve impulses travel simplest is a reflex arc. Reflex Arcs sensory receptor - sensory neuron interneuron (in spinal cord ) - motor neuron - effector. Serotonin deficits associated with depression (SSRI-medications Zoloft, Paxil, Prozac) Serotonin reuptake inhibitors Reflexes automatic, subconscious response to stimuli Maintain homeostasis (heart rate, blood pressure, etc.) responses = vomiting, sneezing, swallowing, etc. knee-jerk reflex (patellar tendon reflex) sprain protection monosynaptic reflex (no interneuron). withdrawal reflex Injury protection sensory neurons, interneurons, motor neurons. At the same time, antagonistic extensor muscles inhibited

6 Meninges Membranes that surround the brain and spinal cord between bone and soft tissues. 1- Dura mater: outermost, is made of tough, white, dense connective tissue, contains many blood vessels. forms the inner periosteum of skull bones. In some areas, forms partitions between lobes of the brain in others, forms dural sinuses arachnoid mater, Middle membrane, thin & lacks blood vessels. does not follow convolutions of brain. subarachnoid space: between arachnoid and pia maters contains cerebrospinal fluid Pia Matter: Innermost is thin and contains many blood vessels and nerves. Spinal Cord extends to the level of the intervertebral disk between the first and second lumbar vertebrae. is attached to the surface of the brain and spinal cord and follows their contours

7 37 38 Structure of the Spinal Cord 31 segments, each gives rise to a pair of spinal nerves. cervical enlargement gives rise to nerves leading to the upper limbs, lumbar enlargement gives rise to those innervating the lower limbs Two deep longitudinal grooves (anterior median fissure and posterior median sulcus) divide the cord into right and left halves. 4. White matter, made up of bundles of myelinated nerve fibers (nerve tracts), surrounds a butterflyshaped core of gray matter housing interneurons. Functions of the Spinal Cord two major functions: transmit impulses to and from the brain house spinal reflexes. 5. A central canal contains cerebrospinal fluid

8 Brain A. The brain is the largest, most complex portion of the nervous system, containing 100 billion multipolar neurons. B. can be divided into the cerebrum (largest portion and associated with higher mental functions), the diencephalon (processes sensory input), the cerebellum (coordinates muscular activity), and the brain stem (coordinates and regulates visceral activities) Cerebrum 1. largest portion of the mature brain, consisting of two cerebral hemispheres. 2. A deep ridge of nerve fibers called the corpus callosum connects the hemispheres. 3. The surface of the brain is marked by convolutions, sulci,andfissures. Copyright The McGaw-Hill Companies, Inc. Permission required for reproduction or display. 4. The lobes are named according to the bones they underlie and include the frontal lobe, parietal lobe, temporal lobe, occipital lobe, and the insula. 5. A thin layer of gray matter, the cerebral cortex, lies on the outside of the cerebrum and contains 75% of the cell bodies in the nervous system Beneath the cortex lies a mass of white matter made up of myelinated nerve fibers connecting the cell bodies of the cortex with the rest of the nervous system

9 Functions of the Cerebrum 1. provides higher brain functions, such as interpretation of sensory input, initiating voluntary muscular movements, memory, and integrating information for reasoning. Hemisphere Dominance a. Both cerebral hemispheres function in receiving and analyzing sensory input and sending motor impulses to the opposite side of the body. b. Most people exhibit hemisphere dominance for the language-related activities of speech, writing, and reading c. The left hemisphere is dominant in 90% of the population, although some individuals have the right hemisphere as dominant, and others show equal dominance in both hemispheres. d. The non-dominant hemisphere specializes in nonverbal functions and controls emotions and intuitive thinking. Diencephalon 1. The diencephalon lies above the brain stem and contains the thalamus and hypothalamus. 2. The thalamus functions in sorting and directing sensory information arriving from other parts of the nervous system, performing the services of both messenger and editor The hypothalamus maintains homeostasis by: regulating heart rate and arterial blood pressure, body temperature, water and electrolyte balance, hunger and body weight, movements and secretions of the digestive tract, growth and reproduction, and sleep and wakefulness. Brain Stem 1. consists of the midbrain, pons, and medulla oblongata. lies at the base of the cerebrum, and connects the brain to the spinal cord

10 Midbrain a. located between the diencephalon and pons, contains bundles of myelinated nerve fibers that convey impulses to and from higher parts of the brain, and masses of gray matter that serve as reflex centers. b. contains centers for auditory and visual reflexes Pons a. lies between the midbrain and medulla oblongata, transmits impulses between the brain and spinal cord, and contains centers that regulate the rate and depth of breathing. Medulla Oblongata a. transmits all ascending and descending impulses between the brain and spinal cord. b. Functions include: heart rate control, blood pressure control, works along with the pons to control the rate and depth of breathing and is associated with coughing, sneezing, swallowing, and vomiting Cerebellum 1. The cerebellum is made up of two hemispheres connected by a vermis. 2. The cerebellum functions to integrate sensory information about the position of body parts and coordinates skeletal muscle activity and maintains posture. Peripheral Nervous System A. The peripheral nervous system (PNS) consists of the cranial and spinal nerves that arise from the central nervous system and travel to the remainder of the body. B. The PNS is made up of the somatic nervous system that oversees voluntary activities, and the autonomic nervous system that controls involuntary activities

11 C. Cranial Nerves 1. Twelve pairs of cranial nerves arise from the underside of the brain, most of which are mixed nerves. 2. The 12 pairs are designated by number and name and include the olfactory, optic, oculomotor, trochlear, trigenimal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves Refer to Figure 9.31 and Table 9.6 for cranial nerve number, name, type, and function. D. Spinal Nerves 1. Thirty-one pairs of mixed nerves make up the spinal nerves. 2. Spinal nerves are grouped according to the level from which they arise and are numbered in sequence, beginning with those in the cervical region. 3. Each spinal nerve arises from two roots: a dorsal, or sensory, root, and a ventral, or motor, root The main branches of some spinal nerves form plexuses. 5. Cervical Plexuses a. The cervical plexuses lie on either side of the neck and supply muscles and skin of the neck. 6. Brachial Plexuses a. The brachial plexuses arise from lower cervical and upper thoracic nerves and lead to the upper limbs

12 7. Lumbrosacral Plexuses a. The lumbrosacral plexuses arise from the lower spinal cordandleadtothelower abdomen, external genitalia, buttocks, and legs. Autonomic Nervous System A. maintains homeostasis of visceral activities without conscious effort. B. General Characteristics 1. two divisions: sympathetic and parasympathetic divisions, exert opposing effects on target organs. parasympathetic operates under normal conditions. 67 sympathetic operates under conditions of stress or emergency. 68 Autonomic Nerve Fibers motor pathways include two fibers: preganglionic fiber that leaves the CNS postganglionic fiber that innervates the effector. Sympathetic Division Fibers arise from the thoracic and lumbar regions of the spinal cord, and synapse in paravertebral ganglia close to the vertebral column. Postganglionic axons lead to an effector organ Parasympathetic Division Fibers arise from the brainstem and sacral region of the spinal cord, and synapse in ganglia close to the effector organ

13 Autonomic Neurotransmitters Preganglionic fibers of both sympathetic and parasympathetic divisions release acetylcholine. Parasympathetic postganglionic fibers are cholinergic fibers and release acetylcholine Sympathetic postganglionic fibers are adrenergic and release norepinephrine. The effects of these two divisions, based on the effects of releasing different neurotransmitters to the effector, are generally antagonistic. Determine what action some of your organs would have if they were sympathetically stimulated verses parasympathetically stimulated. (Ex. What happens in the pancreas? Blood vessels? Intestines?) Control of Autonomic Activity largely controlled by reflex centers in the brain and spinal cord. The limbic system and cerebral cortex alter the reactions of the autonomic nervous system through emotional influence