The Nervous System

The Nervous System 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Nervous System o

Three specific functions that work to maintain homeostasis: • • •

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Sensory input Integration Motor output

Divisions of the Nervous System Nervous Tissue

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Nervous System o

Divisions of the Nervous System •

Central nervous system (CNS)  

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Peripheral nervous system (PNS) – includes all the cranial and spinal nerves  

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Brain Spinal cord

Afferent (sensory) division Efferent (motor) division

Nervous Tissue 3

Fig 8.1

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Nervous System o

Nervous Tissue • •

Neurons (nerve cells) – transmit impulses Neuroglia – support and nourish neurons

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Nervous System :Neuroglia

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Nervous System :Neuroglia o

Neuroglia (more numerous nervous tissue cells, half of the volume) Support and nourish neurons, cannot conduct impulse Types of neuroglia found in the brain :

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Microglia Astrocytes Oligodendrocytes Ependymal cells

Schwann cells are neuroglia located outside the brain or spinal cord (PNS)

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Nervous System •

Neuron structure Cell body – contains nucleus and other organelles Dendrite – receive signals from sensory receptors or other neurons Axon

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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

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Fig 8.2

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Nervous System •

Types of neurons Motor neurons

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Take nerve impulses from the CNS to muscles, organs, or glands Multipolar – have many dendrites

Sensory neurons

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Take nerve impulses from sensory receptors to the CNS Almost all are unipolar

Interneurons

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All are in the CNS Typically multipolar Convey nerve impulses between various parts of the CNS 10

Nervous System Nerve signal conduction

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Resting potential

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Neuron possesses potential energy The cell membrane is polarized positively charged outside the cell ­ negatively charged inside ­

Action potential

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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 11

Fig 8.3

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Resting Membrane Potential

• inside is negative relative to the outside • polarized membrane • due to distribution of ions • Na+/K+ pump

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Action Potentials • at rest membrane is polarized • threshold stimulus reached • sodium channels open and membrane depolarizes • potassium leaves cytoplasm and membrane repolarizes 14

Action Potentials

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Action Potentials

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Nervous System Conduction of action potentials (APs)

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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 all­or­none event Refractory period ­ Axon cannot conduct an AP ­ Ensures one­way direction of an impulse 17

Fig 8.4

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Nervous System •

Transmission across a synapse Axon terminal – small swelling at tips of the branched end of an axon Synapse

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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 19

Transmission between neurons:

1. When nerve impulses traveling along an axon (action potential) reach an axon terminal, Calcium channels open and Ca2+ enters the terminal. 2. Rise in Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane, and neurotransmitter molecules are released to the synaptic cleft. 20

Fig 8.5

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Nervous System Graded potentials and synaptic integration

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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

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Nervous System Neurotransmitter Molecules

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At least 50 have been identified Two well­known 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 23

Neurotransmitters

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Central Nervous System o

Gray matter – contains cell bodies and unmyelinated fibers White matter – contains myelinated axons Meninges and Cerebrospinal Fluid

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Meninges – protective membranes of the CNS 

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Dura mater – outer meninx composed of tough, fibrous connective tissue Arachnoid mater – middle meninx composed of spider­web­like connective tissue Pia mater – deepest meninx 25

Fig 8.6

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Central Nervous System •

Cerebrospinal fluid (CSF) Produced by ependymal cells Fills the following:

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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

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Central Nervous System •

The spinal cord 

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Extends from the base of the brain to the lumbar vertebra Protected by vertebral column

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Central Nervous System Structure of the spinal cord

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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

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Fig 8.7

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Central Nervous System Functions of the spinal cord

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Provides communication between the brain and the peripheral nerves Reflexes

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Central Nervous System •

The brain Cerebrum Diencephalon Cerebellum Brain stem Ventricles

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Two lateral ventricles (cerebrum) Third ventricle (diencephalon) Fourth ventricle (cerebellum)

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Fig 8.8

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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

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Longitudinal fissure divides the left and right Connected by the corpus callosum Gyri (ridges) are separated by sulci (shallow grooves) 34

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Lobes     

Frontal lobe Parietal lobes Occipital lobe Temporal lobes Insula

Fig 8.9

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Central Nervous System •

The cerebral cortex Outer layer of gray matter Accounts for sensation, voluntary movement, and consciousness Motor and sensory areas

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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 36

Fig 8.10

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Central Nervous System Primary taste area

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Located in insula Taste sensations

Primary visual area

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Located in the occipital lobe Receives information from our eyes

Primary auditory area

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Located in the temporal lobe Receives information from our ears

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Central Nervous System Association areas

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Where integration occurs Where memories are stored Premotor area organizes motor functions for skilled motor activities and then the primary motor area sends signals to the cerebellum and the basal nuclei, which integrate them. Cerebral palsy

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Processing centers Prefrontal area  Uses information from other association areas & perform reasoning and planning actions Motor speech area (Broca’s area) in L. frontal lobe ­speak is partially depend on it ­damage ability to control the muscles of the face and neck that allow speech Wernicke’s area (general interpretive area) ­receives infomration from all other sensory association areas. ­damage hinder a person’s ability to interpret written and spoken messages

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Broca’s area & Wernicke’s area responsible for communication 40

Central Nervous System Central white matter

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Tracts communicate information between the different sensory, motor, and association areas Corpus callosum join the two cerebral hemispheres

Basal nuclei

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Masses of gray matter Integrate motor commands

Limbic system

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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 short­term memory to become long­term memory 41

Basal Nuclei

• masses of gray matter • deep within cerebral hemispheres • caudate nucleus, putamen, globus pallidus • produce dopamine • control certain muscular activities • primarily by inhibiting motor functions 42

Central Nervous System •

The diencephalon Hypothalamus

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Thalamus

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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 43

Central Nervous System •

The cerebellum 

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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 44

Central Nervous System The brain stem

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Midbrain

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Relay station between cerebrum and the spinal cord or cerebellum Reflex centers for visual, auditory, and tactile responses

Pons

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Contains bundles of axons traveling between the cerebellum and the rest of the CNS Helps regulate breathing rate

Medulla oblongata

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Vital reflex centers Reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing

Reticular formation

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Involved in maintaining muscle tone Assists in regulating respiration, heart rate, and blood pressure Helps rouse a sleeping person 45

Peripheral Nervous System o o o

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Lies outside of CNS Composed of nerves and ganglia Subdivisions Afferent (sensory) Somatic sensory

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Serves the skin, skeletal muscles, joints, and tendons Special senses

Visceral sensory supplies the internal organs

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Somatic motor system carries commands from CNS to the skeletal muscles Autonomic motor system regulates cardiac and smooth muscle and glands 46

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 47

Fig 8.11a

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Peripheral Nervous System o

Spinal nerves • •

31 pairs Designated according to their location in relation to the vertebrae Mixed nerves

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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 49

Fig 8.11b

Fig 8.12

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Peripheral Nervous System Somatic Motor Nervous System and Reflexes •

Voluntary actions that originate in the motor cortex Utilize 1 somatic motor neuron to travel directly to effector e.g. skeletal muscle fiber without synapse at a ganglion Reflexes are automatic involuntary responses

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Cranial reflexes involve the brain Spinal reflex involves only the spinal cord

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Fig 8.13

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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 (somatic motor neurone traevls directly to its effector without synapsing at a ganglion) 

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Visceral reflexes are important to the maintenance of homeostasis 54

Peripheral Nervous System Sympathetic division of ANS (“Fight or Flight”)

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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)

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Peripheral Nervous System Parasympathetic division of ANS (“Rest and Digest”) 

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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 56

Fig 8.14

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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 58

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 59