The Nervous System

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The Nervous System
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Objectives

• Describe the functions of the nervous system
• Identify and describe the functions of various
  types of nervous cells
• Describe how a nerve impulse is generated and
  transmitted
• Identify and describe the divisions of the
  nervous system
• Identify the major structures of the central
  nervous system
• Describe the major structure of the peripheral
  nervous system
• List and describe diseases and disorders of the
  nervous system

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Functions of the Nervous System

• Sensory input gathering information
• To monitor changes occurring inside and outside
  the body
• Changes stimuli
• Integration
• To process and interpret sensory input and decide
  if action is needed
• Motor output
• A response to integrated stimuli
• The response activates muscles or glands
  (effector organs)

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Functions of the Nervous System
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Divisions of the Nervous System

• Central nervous system (CNS)
• Brain
• Spinal cord
• Peripheral nervous system (PNS)
• Nerve outside the brain and spinal cord
• Sensory (afferent) and motor neurons (efferent)

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Divisions of the Peripheral Nervous System

• Sensory (afferent) division
• Nerve fibers that carry information to the
  central nervous system. Receptors of stimulus.
• Motor (efferent) division
• Nerve fibers that carry impulses away from the
  central nervous system to effector organs.

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Divisions of the Efferent Peripheral Nervous
System

• Motor (efferent) division
• Two subdivisions
• Somatic nervous system voluntary
• Autonomic nervous system involuntary
• The Autonomic nervous system has two divisions
• Sympathetic Becomes active when body
  is
• stressed,
  fight of flight
• Parasympathetic Becomes active when
  the
• body
  is relaxed or at rest.

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Organization of the Nervous System
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Types of Neuroglial Cells and Their Functions

• Microglia
• Spider-like phagocytes
• Dispose of debris
• Ependymal cells
• Line cavities of the brain and spinal cord
• Circulate cerebrospinal fluid

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Types of Neuroglial Cells and Their Functions

• Oligodendrocytes
• Produce myelin sheath around nerve fibers in the
  central nervous system
• Satellite cells
• Protect neuron cell bodies
• Schwann cells
• Form myelin sheath in the peripheral nervous
  system

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Neuroglial Schwann Cells

• Schwann cells produce myelin sheaths in
  jelly-roll like fashion
• Nodes of Ranvier gaps in myelin sheath along
  the axon

Figure 7.5
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Neurons

• Neurons nerve cells
• Cells specialized to transmit messages
• Major regions of neurons
• Cell body nucleus and metabolic center of the
  cell
• Processes fibers that extend from the cell body
  (dendrites and axon)
• Classification of Neurons as it relates to their
  functions
• Sensory (afferent) neurons
• Carry impulses from the sensory receptors
• Cutaneous sense organs
• Proprioceptors detect stretch or tension
• Motor (efferent) neurons
• Carry impulses from the central nervous system
• Interneurons (association neurons)
• Found in neural pathways in the central nervous
  system
• Connect sensory and motor neurons

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Neurons
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Neuron Structure

• Cell body (Soma)
• Nucleus
• Large nucleolus
• _ Nissl substance specialized rough
  endoplasmic reticulum
• Neurofibrils intermediate cytoskeleton that
  maintains cell shape

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Neuron Structure

• Extensions outside the cell body
• Dendrites conduct impulses toward the cell body
• Axons conduct impulses away from the cell body
• Axons end in axonal terminals
• Axonal terminals contain vesicles with
  neurotransmitters
• Axonal terminals are separated from the next
  neuron by a gap
• Synaptic cleft gap between adjacent neurons
• Synapse junction between nerves

The black arrows indicate the direction of the
impulse movement along the neuron.
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Nerve Impulses

• Irritability ability to respond to stimuli
• Conductivity ability to transmit an impulse
• The plasma membrane at rest is polarized
• Fewer positive ions are inside the cell than
  outside the cell. The resting potential
  difference is -70 millivolts between the outside
  and inside of the cell. The inside of the cells
  axon contains K ions and Na ions are found
  outside of the axons membrane.

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Neuron Depolarization and Action Potentials

• Depolarization a stimulus depolarizes the
  neurons membrane
• A deploarized membrane allows sodium (Na) to
  flow inside the membrane through special proteins
  in the membrane called sodium channel proteins.
• The movement of the ions initiates an action
  potential in the neuron due to the increase in
  voltage from -70 millivolts up to 30 millivolts
  within the axon
• The action potential travels down the axon like a
  wave.

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Neuron Depolarization and Action Potentials

• If the action potential (nerve impulse) starts,
  it is propagated over the entire axon (all or
  none response)
• When the axons internal charge reaches 40
  millivolts. The Na channels close and the K
  channels open and potassium ions rush out of the
  neuron after sodium ions have entered. The K
  ions move out until a negative charge of -70
  millivolts is reestablished in the axon. Then
  the K channel proteins close. This repolarizes
  the axons membrane. However the Na and K
  ions are in opposite locations of where they were
  before the neuron depolarized.
• The sodium-potassium pump restores the original
  configuration by pumping Na ions out and K
  ions back into the axon.
• This action requires ATP

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Action Potentials and Neural Synapse

• The impulse continues to move toward the cell
  body of the next neuron in the pathway.
• Impulses are able to cross the synapse to another
  nerve
• Neurotransmitter is released from a nerves axon
  terminal
• The dendrite of the next neuron has receptors
  that are stimulated by the neurotransmitter
• An action potential is started in the dendrite

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Neural Synapse
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Reflex Response

• Reflex rapid, predictable, and involuntary
  responses to stimuli
• Reflex arc direct route from a sensory neuron,
  to an interneuron, to an effector

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Central Nervous System

• CNS develops from the embryonic neural tube
  (ectoderm)
• The neural tube becomes the brain and spinal cord
• The opening of the neural tube becomes the
  ventricles
• Four chambers within the brain
• Filled with cerebrospinal fluid which nourishes
  and cushions the brain

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CNS The Brain
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CNS Brain Cerebrum

• Paired (left and right) superior parts of the
  brain. Right hemisphere controls the left side of
  the body and the left hemisphere controls the
  right side of the body.
• Include more than half of the brain mass
• The surface is made of ridges (gyri) and grooves
  (sulci)
• Fissures (deep grooves) divide the cerebrum into
  lobes
• Surface lobes of the cerebrum
• Frontal lobe
• Parietal lobe
• Occipital lobe
• Temporal lobe

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CNS Brain Cerebrum
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CNS Brain Diencephalon

• Sits on top of the brain stem
• Enclosed by the cerebral hemispheres
• Made of three parts
• Thalamus
• Hypothalamus
• Epithalamus

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Diencephalon Thalamus

• The Thalamus Surrounds the third ventricle The
  relay station for sensory impulses. Transfers
  impulses to the correct part of the cortex for
  localization and interpretation

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Diencephalon Hypothalamus

• Located under the thalamus and is important
  autonomic nervous system center. It helps
  regulate body temperature, controls water
  balance, and regulates metabolism. It is also an
  important part of the limbic system (emotions)
  and the pituitary gland is attached to the
  hypothalamus.

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Diencephalon Epithalamus

• It forms the roof of the third ventricle. It
  houses the pineal body (an endocrine gland). It
  includes the choroid plexus which forms
  cerebrospinal fluid.

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CNS Brain Brain Stem

• Attaches to the spinal cord
• Parts of the brain stem
• Midbrain
• Pons
• Medulla oblongata

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Brainstem Midbrain

• It is mostly composed of tracts of nerve fibers.
  It has two bulging fiber tracts called cerebral
  peduncles. It has four rounded protrusions
  called corpora quadrigemina. These function as
  reflex centers for vision and hearing.

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Brainstem Pons

• It is the bulging center part of the brain stem
  and is mostly composed of fiber tracts. It
  contains nuclei involved in the control of
  breathing.

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Brainstem Medulla oblongata

• It is the lowest part of the brain stem and
  merges into the spinal cord. It includes
  important fiber tracts and contains important
  control centers such as heart rate control,
  blood pressure regulation, breathing ,
  swallowing, and vomiting.

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CNS Brain Cerebellum

• It is composed of two hemispheres with convoluted
  surfaces. It provides involuntary coordination
  of body movements.

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Meninges

• Below the bone surrounding the brain and spinal
  cords are specialized membranes called the
  meninges.
• Dura mater (outer most layer) tough mother
• Double-layered external covering
• Periosteum attached to surface of the skull
• Meningeal layer outer covering of the brain
• Folds inward in several areas
• Arachnoid layer
• Middle layer
• Web-like
• Pia mater gentle mother
• Internal layer
• Clings to the surface of the brain

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CSF Cerebrospinal Fluid

• Similar to blood plasma composition
• Formed by the choroid plexus
• Forms a watery cushion to protect the brain
• Circulated in arachnoid space, ventricles, and
  central canal of the spinal cord

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Location and Circulation of CSF
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Spinal Cord Structure
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Nerve Anatomy

• Nerve bundle of neuron fibers
• Neuron fibers are bundled by connective tissue
• Endoneurium surrounds each fiber
• Groups of fibers are bound into fascicles by
  perineurium
• Fascicles are bound together by epineurium
• Mixed nerves both sensory and motor fibers

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Diseases and Disorders of the Nervous System

• Cerebrovascular Accident CVA
• Commonly called a stroke
• The result of a ruptured blood vessel supplying
  (cerebral hemorrhage) a region of the brain or a
  vessel is obstructed by a clot.
• Brain tissue supplied with oxygen from that blood
  source dies, swelling occurs in the brain due to
  leaking of blood from vessels.
• Loss of some functions or death may result
• This is due often to elevated blood pressure or
  hypertension.

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Diseases and Disorders of the Nervous System

• Epilepsy This disease is due to random,
  mis-firing of neurons within the brain affecting
  sensory and motor regions of the brain. Ranging
  in effects from sleep-like state of consciousness
  (narcolepsy), muscle paralysis and spasms (Petit
  mal and Grand mal seizures). Still not
  understood why this disease occurs. However in
  some cases it can result from brain trauma or
  injury.

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Diseases and Disorders of the Nervous System

• Concussion
• Slight brain injury
• No permanent brain damage
• Contusion
• Nervous tissue destruction occurs
• Nervous tissue does not regenerate
• Cerebral edema
• Swelling from the inflammatory response or injury
• May compress and kill brain tissue
• May becaused by infectious agents such as viruses
  (encephalitis) or bacteria which cross the blood
  brain barrier or infect the meninges or CSF
  surrounding the brain (meningitis)

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Diseases and Disorders of the Nervous System

• Alzheimers Disease
• Progressive degenerative brain disease
• Mostly seen in the elderly, but may begin in
  middle age
• Structural changes in the brain include abnormal
  protein deposits and twisted fibers within
  neurons called plaques
• Can only be truly diagnosed by autopsy and study
  of brain tissue at death
• Victims experience memory loss, irritability,
  confusion and ultimately, hallucinations and
  death