Transcript Nerve
The Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nervous System o Three specific functions that work to maintain homeostasis: • • • o o Sensory input Integration Motor output Divisions of the Nervous System Nervous Tissue Nervous System o Divisions of the Nervous System • Central nervous system (CNS) • Peripheral nervous system (PNS) – includes all the cranial and spinal nerves o Brain Spinal cord Afferent (sensory) division Efferent (motor) division Nervous Tissue Fig 8.1 Nervous System o Nervous Tissue • • Neurons (nerve cells) – transmit impulses Neuroglia – support and nourish neurons Nervous System • Neuron structure Cell body – contains nucleus and other organelles Dendrite – receive signals from sensory receptors or other neurons Axon Conducts nerve signals away from cell body Nerve – bundle of parallel axons in the PNS Tract – bundle of parallel axons in the CNS May be covered by myelin (lipid coating) Formed by Schwann cells or neurolemmocytes in PNS Formed by oligodendrocytes in CNS Fig 8.2 Nervous System • Types of neurons Motor neurons Take nerve impulses from the CNS to muscles, organs, or glands Multipolar – have many dendrites Sensory neurons Take nerve impulses from sensory receptors to the CNS Almost all are unipolar Interneurons All are in the CNS Typically multipolar Convey nerve impulses between various parts of the CNS Nervous System • Nerve signal conduction Resting potential Neuron possesses potential energy The cell membrane is polarized positively charged outside the cell negatively charged inside Action potential Process of conduction of nerve signals Occurs in the axons Begins with a stimulus Channels in the cell membrane opens and sodium ions rush into the cell (depolarization) Sodium channels close and the cell repolarizes Fig 8.3 Nervous System Conduction of action potentials (APs) In In - unmyelinated axons Slow (~1 meter/second) Each section of the axon must be stimulated myelinated axons Called saltatory conduction An AP at one node of Ranvier can “jump” over myelinated portion of axon Much faster (>100 meters/second) Is an all-or-none event Refractory period Axon cannot conduct an AP Ensures one-way direction of an impulse Fig 8.4 Nervous System • Transmission across a synapse Axon terminal – small swelling at tips of the branched end of an axon Synapse Region of close proximity between two neurons Presynaptic membrane – membrane of the first neuron Postsynaptic membrane – membrane of the next neuron Synaptic cleft – small gap between the presynaptic and the postsynaptic neuron Neurotransmitters – molecules that transmit a nerve impulse across a synapse Fig 8.5 Nervous System Graded potentials and synaptic integration Graded potentials – each small signal from a synapse Excitatory neurotransmitters produce a graded potential that promotes an AP Inhibitory neurotransmitters produce a graded potential that inhibits an AP Integration involves summing up the excitatory and inhibitory signals Nervous System Neurotransmitter Molecules At least 50 have been identified Two well-known neurotransmitters: Acetylcholine (Ach) Norepinephrine (NE) After a neurotransmitter has initiated a response it is removed from the synaptic cleft Enzymes may inactivate the neurotransmitter The neurotransmitter may be reabsorbed by the presynaptic membrane Prevents continuous stimulation (or inhibition) of postsynaptic membranes Central Nervous System o Gray matter – contains cell bodies and unmyelinated fibers White matter – contains myelinated axons Meninges and Cerebrospinal Fluid o o • Meninges – protective membranes of the CNS Dura mater – outer menix composed of tough, fibrous connective tissue Arachnoid mater – middle menix composed of spider-web-like connective tissue Pia mater – deepest menix Fig 8.6 Central Nervous System • Cerebrospinal fluid (CSF) Produced by ependymal cells Fills the following: Subarachnoid space – space between the arachnoid mater and the pia mater Ventricles – hollow, interconnecting cavities of the brain Central canal – hollow, space of the spinal cord Central Nervous System • The spinal cord Extends from the base of the brain to the lumbar vertebra Protected by vertebral column Central Nervous System Structure of the spinal cord Central canal contains CSF Centrally located gray matter Posterior (dorsal) root – contains sensory fibers Anterior (ventral) root – contains motor fibers interneurons White matter Posterior white matter composed of ascending tracts carrying sensory information to the brain Anterior white matter composed of descending tracts carrying motor information from the brain Fig 8.7 Central Nervous System Functions of the spinal cord Provides communication between the brain and the peripheral nerves Reflexes Central Nervous System • The brain Cerebrum Diencephalon Cerebellum Brain stem Ventricles Two lateral ventricles (cerebrum) Third ventricle (diencephalon) Fourth ventricle (cerebellum) Fig 8.8 Central Nervous System • The Cerebrum Largest portion of the brain Voluntary motor responses Coordinates the activities of the other parts of the brain Involved in higher thought processes The cerebral hemispheres Longitudinal fissure divides the left and right Connected by the corpus callosum Gyri (ridges) are separated by sulci (shallow grooves) Central Nervous System Lobes Frontal lobe Parietal lobes Occipital lobe Temporal lobes Insula Fig 8.9 Central Nervous System • The cerebral cortex Outer layer of gray matter Accounts for sensation, voluntary movement, and consciousness Motor and sensory areas Primary motor area In frontal lobe anterior to central sulcus Voluntary commands to skeletal muscle Primary somatosensory area Posterior to central sulcus in parietal lobes Receives sensory information from the skin and skeletal muscles Fig 8.10 Central Nervous System Primary taste area Located in insula Taste sensations Primary visual area Located in the occipital lobe Receives information from our eyes Primary auditory area Located in the temporal lobe Receives information from our ears Central Nervous System Association areas Where integration occurs Where memories are stored Processing centers Prefrontal area Uses information from other association areas Reasoning and planning actions Motor speech area (Broca’s area) Wernicke’s area (general interpretive area) Central Nervous System Central white matter Tracts communicate information between the different sensory, motor, and association areas Corpus callosum join the two cerebral hemispheres Basal nuclei Masses of gray matter Integrate motor commands Limbic system Inferior to the cerebral cortex Contains neural pathways that connect portions of the cerebral cortex and the temporal lobes with the thalamus and the hypothalamus Causes emotion Involved in memory and learning Hippocampus involved in processing short-term memory to become long-term memory Central Nervous System • The diencephalon Hypothalamus Thalamus Forms the floor of the third ventricle Integrating center involved in homeostasis Regulates hunger, sleep, thirst, body temperature, and water balance Link between nervous and endocrine systems Located in sides and roof of the third ventricle Functions as a sensory relay center Involved in arousal of the cerebrum Participates in memory and emotions Pineal gland regulates body’s daily rhythms Central Nervous System • The cerebellum Separated from brain stem by the fourth ventricle Two hemispheres composed primarily of white matter Receives sensory input from the eyes, ears, joints, and muscles Receives motor output from the cerebral cortex Maintains posture and balance and ensures smooth, coordinated voluntary movement Assists in learning of new motor skills Central Nervous System • The brain stem Midbrain Relay station between cerebrum and the spinal cord or cerebellum Reflex centers for visual, auditory, and tactile responses Pons Contains bundles of axons traveling between the cerebellum and the rest of the CNS Helps regulate breathing rate Medulla oblongata Vital reflex centers Reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing Reticular formation Involved in maintaining muscle tone Assists in regulating respiration, heart rate, and blood pressure Helps rouse a sleeping person Peripheral Nervous System o o o • Lies outside of CNS Composed of nerves and ganglia Subdivisions Afferent (sensory) Somatic sensory • Serves the skin, skeletal muscles, joints, and tendons Special senses Visceral sensory supplies the internal organs Efferent (motor) Somatic motor system carries commands from CNS to the skeletal muscles Autonomic motor system regulates cardiac and smooth muscle and glands Peripheral Nervous System o Cranial nerves • • • • • • 12 pairs Sensory nerves contain only sensory fibers Motor nerves contain only motor fibers Mixed nerves contain both sensory and motor fibers Mostly involved with the head, neck, and facial regions of the body The vagus nerve (cranial nerve X) has sensory and motor branches to the face and most of the internal organs Fig 8.11a Peripheral Nervous System o Spinal nerves • • 31 pairs Designated according to their location in relation to the vertebrae Mixed nerves • • Sensory fibers enter at the posterior root Motor fibers exit at the anterior root The cell body of a sensory neuron is in a posterior-root ganglion Fig 8.11b Peripheral Nervous System o Somatic Motor Nervous System and Reflexes • Voluntary actions that originate in the motor cortex Reflexes are automatic involuntary responses • Cranial reflexes involve the brain Spinal reflex involves only the spinal cord Peripheral Nervous System o Autonomic Motor Nervous System and Visceral Reflexes • Two divisions of ANS • Function automatically and involuntarily Innervate all internal organs Utilize two motor neurons and one ganglion for each impulse Visceral reflexes are important to the maintenance of homeostasis Peripheral Nervous System • Sympathetic division of ANS (“Fight or Flight”) Preganglionic fibers arise from the thoracic-lumbar portion of the spinal cord Preganglionic fibers are short and postganglionic fibers are long Accelerates heartbeat and dilates the bronchi Inhibits the digestive tract Neurotransmitter released by the postganglionic neuron is primarily norepineprhine (NE) TA 8.3 Peripheral Nervous System • Parasympathetic division of ANS (“Rest and Digest”) Includes a few cranial nerves and preganglionic fibers that arise from the sacral portion of the spinal cord (craniosacral portion of ANS) Preganglionic fiber is long and postganglionic fiber is short Promotes digestion of food, slows heart rate, and decreases the strength of cardiac contraction Acetylcholine (Ach) is the neurotransmitter of the parasympathetic division TA 8.4 Table 8.4 not available in this printed version Fig 8.14 Effects of Aging o o o o Brain mass decreases Learning, memory, and reasoning decline Neurotransmitter production decreases Although structural changes occur, mental impairment is not an automatic consequence of getting older Homeostasis o o o o Detects, interprets, and responds to changes in the internal and external environment With the endocrine system, it coordinates and regulates the functioning of other systems The hypothalamus and medulla oblongata control vital functions Sympathetic division of the ANS works to keep us from danger