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Chapter 9 The Nervous System. Organs and Divisions of the Nervous System. Central Nervous System (CNS) brain and spinal cord centrally located Peripheral Nervous System (PNS) nerves extending to outlying parts of the body Autonomic Nervous System (ANS)

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Chapter 9 The Nervous System

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Chapter 9The Nervous System


Organs and Divisions of the Nervous System

  • Central Nervous System (CNS)

    brain and spinal cord centrally located

  • Peripheral Nervous System (PNS)

    nerves extending to outlying parts of the body

  • Autonomic Nervous System (ANS)

    regulates automatic or involuntary functions

    • Sympathetic - “fight-or-flight”

    • Parasympethetic “rest, feed and breed”


Cells of the Nervous System

Two Types of Nervous System Cells

  • Neurons

    nerve cells conduct impulses

  • Glia

    connective tissue cells supporting neurons


Cells of the Nervous System

Neurons

Dendrites

conduct impulses to cell body of neuron

Cell body

main, central part with nucleus

Axons

conducts impulses away from cell body of neuron


Cells of the Nervous System

Classification of Neurons according to Function

  • Sensory or Afferent

    conduct impulses to the spinal cord and brain;

  • Motor or Efferent

    conduct impulses away from brain and spinal cord to muscles and glands

  • Interneurons

    conduct impulses from sensory neurons to motor neurons


Cells of the Nervous System

Glia (neuroglia)

Support cells, bringing the cells of nervous tissue together structurally and functionally (like glue)

3 main types of connective tissue cells

Astrocytes

star-shaped cells that anchor small blood vessels to neurons

Microglia

smallest of the glial cells that act as phagocytes in the brain

Oligodendrocytes

form myelin sheaths on axons in the CNS (Schwann cells form myelin sheaths in PNS only)


Cells of the Nervous System

Disorders of Nervous Tissue

Multiple Sclerosis

  • characterized by myelin loss in central nerve fibers

    resulting in conduction impairment

  • most common in younger women

    Tumors

    • General name for nervous system tumors is neuroma

    • Most neuromas are gliomas, glial tumors

    • Multiple neurofibromatosis - characterized by numerous benign tumors


Nerves

Nerves are bundles of peripheral axons

Tract

bundle of central axons

White matter

brain or cord tissue composed primarily of myelinated

axons (tracts)

Gray matter

brain or cord tissue composed primarily of cell bodies

and unmyelinated fibers


Nerves

Nerve coverings are made of fibrous connective tissue

Endoneurium

surrounds individual fibers within a nerve

Perineurium

surrounds a group (fascicle) of nerve fibers

Epineurium

surrounds the entire nerve


Reflex Arcs

  • Reflex arcs by-pass the brain - occur in the spinal chord in situations requiring immediate response

  • Nerve impulses are conducted from receptors to effectors

  • conduction by a reflex arc results in a reflex, i.e.;

    contraction by a muscle or secretion by a gland

  • Two-neuron arcs

    the simplest arc, consisting of sensory neurons synapsing in the spinal cord with motor neurons

  • Three-neuron arcs

    consist of sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons


Nerve Impulses

  • Definition -self-propagating wave of electrical activity that travels along the surface of a neuron membrane

  • Mechanism

    • A stimulus triggers the opening of Na+channels in the plasma membrane of the neuron

    • Inward movement of positive sodium ions allows sodium into the cell, and marks the beginning of a nerve impulse


The Synapse

  • Synapse

    The space between the axon of one neuron (presynaptic) - across the synaptic cleft - and the dendrite of the next neuron (postsynaptic)

  • Neurotransmitters

    chemical compounds released from axon terminals of a presynaptic neuron into the synaptic cleft

    bind to specific receptor molecules of a postsynaptic neuron, opening ion channels, and thereby stimulating impulse conduction by the membrane


The Synapse

Names of Neurotransmitters

  • acetylcholine

  • catecholamines - “fight-or-flight” hormones

    norepinephrine, dopamine, and serotonin

  • endorphins, enkephalins, nitric oxide (NO)

    Parkinson Disease

  • abnormally low levels of dopamine in motor control areas of the brain

  • involuntary trembling and muscle rigidity


Central Nervous System

Major Divisions of the Brain

1.Brainstem- vital functions

medulla oblongata, pons, midbrain

2. Cerebellum - movement, posture

and balance. Lies under the cerebrum.

3. Diencephalon-sensations, emotions

hypothalamus, thalamus (endocrine glands)

4. Cerebrum - higher brain functions

consciousness, thought, speech,

memory, decision-making, judgment


Central Nervous System

Brainstem

  • in ascending order: medulla oblongata, pons, and midbrain (vital functions)

  • Structure

    white matter with bits of gray matter scattered through it

  • Function

    gray matter in the brainstem functions as reflex centers (for example, for heartbeat, respirations, and blood vessel diameter)

    sensory tracts in the brainstem conduct impulses to the higher parts of the brain

    motor tracts conduct from the higher parts of the brain to the spinal cord


Central Nervous System

Diencephalon

  • Structure of the hypothalamus

    Consists mainly of the posterior pituitary gland, pituitary stalk, and gray matter

  • Functions of the hypothalamus

    • acts as the major center for controlling the ANS

    • helps control the functioning of most internal organs

    • controls hormone secretion by anterior and posterior pituitary glands; therefore, it indirectly helps control hormone secretion by most other endocrine glands

    • controls appetite, wakefulness, pleasure


Central Nervous System

  • Structure of the Thalamus

    dumbbell-shaped mass of gray matter in each cerebral hemisphere

  • Function of the Thalamus

    • Relays sensory impulses to cerebral

      cortex sensory areas

    • produces the emotions of pleasantness

      or unpleasantness associated with

      sensations


Central Nervous System

Cerebellum

Structure

  • Second largest part of the human brain

  • Lies under the cerebrum (largest part of the brain)

    Function

  • Helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal postures


Central Nervous System

Cerebrum

Structure

  • Largest part of the human brain

  • Outer layer of gray matter is the cerebral cortex; made up of lobes; composed mainly of dendrites and cell bodies of neurons

  • Interior of the cerebrum composed mainly of white matter (that is nerve fibers arranged in bundles called tracts)

    Function

  • mental processes of all types, including sensations, consciousness, memory, and voluntary control of movements


Central Nervous System

Brain Disorders

Cerebrovascular Accident (CVA)

“stroke” - hemorrhage from or cessation of blood flow through cerebral blood vessels

Cerebral Palsy

condition in which damage to motor control areas of the brain before, during, or shortly after birth causes paralysis (usually spastic) of one or more limbs


Central Nervous System

Dementia

Syndrome that includes progressive loss of memory, shortened attention span, personality changes, reduced intellectual capacity, and motor control deficit

  • Alzheimer Disease (AD) - brain disorder of the middle and late adult years characterized by dementia

  • Huntington Disease (HD) - inherited disorder characterized by chorea (purposeless movement) progressing to severe dementia

  • HIV - can infect neurons and thus cause dementia


Central Nervous System

Seizure Disorders

Seizure

sudden burst of abnormal neuron activity that results in temporary changes in brain function

Epilepsy

many forms, all characterized by recurring seizures

Electroencephalogram (EEG)

graphic representation of voltage changes in the brain used to evaluate brain activity


Central Nervous System

Spinal cord

Structure

  • outer part is composed of white matter made

    up of many bundles of axons called tracts

  • interior composed of gray matter made up

    mainly of neuron dendrites and cell bodies

    Function

  • center for all spinal cord reflexes

  • sensory tracts conduct impulses to the brain

  • motor tracts conduct impulses from the brain


Central Nervous System

Coverings and Fluid Spaces of the CNS

Coverings

  • Cranial bones and vertebrae

  • Cerebral and spinal meninges

    outermost > innermost: the dura mater,

    arachnoid mater, and the pia mater

    Fluid spaces

  • subarachnoid spaces of meninges

  • central canal inside cord

  • ventricles in brain


  • Peripheral Nervous System

    Cranial Nerves

    Twelve pairs attached to undersurface of the brain

    Connect brain with the neck and structures in the

    thorax and abdomen

    Spinal Nerves

    contain dendrites of sensory neurons and axons of

    motor neurons

    conduct impulses necessary for sensations and

    voluntary movements


    Peripheral Nervous System

    Peripheral Nerve Disorders

    Neuritis - nerve inflammation

    • Sciatica is inflammation of the sciatic nerve that innervates the legs

    • Neuralgia, or muscle pain, often accompanies neuritis

      Trigeminal neuralgia

      recurring episodes of stabbing pain along one or more

      branches of the trigeminal (5th cranial) nerve in the head

      Bell’s palsy

      paralysis of facial features resulting from damage to the

      facial (7th cranial) nerve

      Herpes zoster or ‘Shingles’

      viral infection caused by chickenpox virus that has invaded the dorsal root ganglion and remained dormant until an episode of shingles

      produces characteristic painful plaques or vesicles


    Autonomic Nervous System

    Composed of Two Divisions

    1. Sympathetic Nervous System

    2. Parasympathetic Nervous System

    General Functions

    • Regulates the body’s automatic or involuntary

      functions (cardiac, respiratory, digestive, sensory)

    • Motor neurons (efferent neurons) conduct impulses

      away from the CNS > cardiac muscle, smooth

      muscle, and glandular tissue

    • the 2 systems tend to work in opposites, and many

      organs are innervated by both.


    Autonomic Nervous System

    Sympathetic Nervous System

    Structure

    • Originates in the spinal chord

    • Dendrites and cell bodies of the first neuron (the preganglionic neuron) are located in the thoracic and upper lumbar segments of the spinal cord

    • Axons from these neurons project to a chain of ganglia located near the spinal chord.

    • These axons synapse with another neuron (post-ganglionic neuron)in the ganglion.

    • A chain of sympathetic ganglia is in front of and at each side of the spinal column


    Autonomic Nervous System

    Sympathetic Nervous System

    Function

    • serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and strong emotions (anger, fear, hate, or anxiety)

    • group of changes induced by sympathetic control is called the “fight-or-flight” response


    Autonomic Nervous System

    Parasympathetic Nervous System

    Structure

    • Parasympathetic preganglionic neurons have dendrites and cell bodies in the medulla and the sacral segments of the spinal cord.

    • Parasympathetic preganglionic neurons project to ganglia located in the head and the thoracic and abdominal cavities close to their target organs

    • Each parasympathetic preganglionic neuron synapses with postganglionic neurons to only one target organ


    Autonomic Nervous System

    Parasympathetic Nervous System

    Function

    • dominates control of many visceral

      effectors under normal, everyday conditions

      i.e: heart rate, blood pressure, respirations,

      digestion, urination


    Autonomic Nervous System

    Autonomic Neurotransmitters

    Cholinergic Fibers

    • cholinergic substances produce acetylcholine.

    • All preganglionic axons of parasympathetic and

      sympathetic systems and parasympathetic

      postganglionic axons release acetylcholine

      Adrenergic Fibers

    • adrenergic substances produce norepinephrine.

    • All postganglionic axons of the sympathetic nervous system release norepinephrine (noradrenaline)


    Autonomic Nervous System

    Autonomic Nervous System as a Whole

    • Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis

    • Many visceral effectors are doubly innervated (that is, they receive fibers from parasympathetic and sympathetic divisions) and are influenced in opposite ways by the two divisions.


    Autonomic Nervous System

    Disorders of the Autonomic Nervous System

    Stress-induced disease -

    prolonged or excessive response to stress can disrupt

    normal functioning throughout the body

    • Heart disease

    • Digestive problems

    • Reduced resistance to disease

      Neuroblastoma

  • highly malignant tumor of the sympathetic

    nervous system

  • primarily affects young children


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