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NERVOUS SYSTEM. Two major divisions 1. Central Nervous System (CNS) 2. Peripheral Nervous System (PNS) Three specific functions 1. Receives sensory input 2. Performs integration of all input 3. Generates motor output . Nervous Tissue. Contains two types of cells

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NERVOUS SYSTEM

  • Two major divisions

    1. Central Nervous System (CNS)

    2. Peripheral Nervous System (PNS)

  • Three specific functions

    1. Receives sensory input

    2. Performs integration of all input

    3. Generates motor output


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Nervous Tissue

  • Contains two types of cells

    1. Neuroglial cells(make up more than half the brain)

    2. Neurons

  • Neuroglial cell function

    1. Structural and protective

    2. Support and nourish neurons

    3. Insulation and development guide

    (myelin sheath)


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Nervous Tissue (cont’)

  • Neurons

    1. Three types of neurons

    a. Sensory neurons – transmit nerve impulses from stimulus to the CNS

    b. Interneurons – in central nervous system

    c. Motor neurons – instructions away from to muscles or glands.

    2. Parts of a neuron: dendrites, cell body- nucleus, axon, myelin sheath & nodes of Ranvier


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Myelin Sheath

  • In the PNS, the neuroglial cells responsible for myelination are called Schwann cells

  • In the CNS, Schwann cells are absent, therefore myelin is provided by oligodendrocytes

  • Speeds signal conduction

  • Made of lipids and phospholipids

  • Guides nerve regeneration

  • Attacked by immune system in multiple sclerosis


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The Nerve Impulse

  • All cells have a membrane potential or

    voltage (electrical charge) across their plasma membrane

  • At rest, when no nerve impulse is being received, the potential is about - 65mV

    1. Potential due to difference in Na+ and K+ concentration across the axon membrane

    2. K+ able to cross axon membrane, Na+not


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The Nerve Impulse cont’

3. Build up of neg. charge results inside

axon and more pos. charge outside, hence membrane potential = -65mv

  • When the axon receives an impulse, the membrane potential changes

    1. When the membrane potential reaches – 40Mv (threshold level), Na+ channels open

    2. Influx of Na+ causes depolarization of the membrane to occur

    3. When the membrane potential reaches +40mV,

    K+ channels open – repolarization occurs

    4. Inside of axon reverts to pre-dominant neg chg.


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The Nerve Impulse cont’

  • When an impulse has passed a section of an axon, a refractory period begins (period following an action potential when a neuron can’t conduct another impulse) as the sodium channels are unable to open.

  • This keeps the impulse running down the axon in the same direction


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The Synapse

  • Synapses separate one neuron form another

  • Neurons physically do not touch one another

  • A synapse is a gap that separates neurons

  • Neurotransmitters released from the terminal bulb enter into the synapse and move across the space (called the synaptic cleft) by diffusion

  • Neurotransmitters leave the presynaptic neuron and diffuse toward the postsynaptic neuron, where they settle on receptors and initiate a reaction


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The Central Nervous System

  • The brain has four main parts

    • Brain stem + midbrain

    • Diencephalon

    • Cerebellum

    • Cerebrum

    • The entire brain is involved in the integration of sensory input and motor responses.

    • Each section has a slightly different role


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Brain cont’

  • Brain stem – medulla oblongata and pons and midbrain

    • Regulates heart rate, breathing and blood pressure

    • Portion of the brain (anatomically and physiologically) closest to the spinal cord

  • Cerebellum

    • Maintains muscle tone, posture and balance, and fine-tuning conscious and unconscious movement

    • Portion of the brain posterior to the brain stem


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Brain cont’

  • Diencephalon

    • Functions mainly as a relay center for sensory information from the body and motor responses from the cerebrum (including centers for visual and auditory startle reflexes)

    • The thalamus and hypothalamus are located in the diencephalon


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Brain cont’

  • Cerebrum

    • Largest portion of the brain, integrates information and generates appropriate responses

    • The center of the brain responsible for learning, remembering and planning activities


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Spinal cord

  • Extends from the brain into the vertebral column

  • It is the main route of communication between the brain and the body

  • The spinal cord connects to almost every organ


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Protection and Support

  • Meninges:3 layers around brain and spinal cord

    • Three connective tissue membranes

    • Inflammation is meningitis

  • Cerebrospinal fluid

    • Absorbs shocks and nourishes neurons

    • There are four ventricles (cavities) in the brain filled with CSF – circulates around spinal cord

    • Reabsorbed in brain; if not working, hydrocephaly


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Protection and Support

  • The axial skeleton provides bony protection for the CNS

  • What separates the CNS from the axial skeleton?

  • Meninges and Cerebrospinal fluid


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CNS - review

  • The brain (skull) and spinal cord (vertebral column) are encased in the skeleton and are covered by meninges

  • It is the main integration center of the body

  • Sensory information enters the CNS, where it is analyzed and an appropriate

    response is generated

  • Motor information leaves the CNS and brings about the body’s response.


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Peripheral Nervous System

  • The neurons of the PNS are arranged in bundles called nerves

  • Nerves can be motor, sensory or mixed depending on the neurons they contain

  • Information traveling to and from the CNS must go through the PNS


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  • The PNS picks this information by:

    • Special senses receptors: enable us to see, hear, taste, and smell

    • General sensory receptors: found in our skin and inform us about temperature, light touch, pressure, and pain

    • Visceral receptors: monitor proprioception (help control movement and awareness of limb position) and organ functioning


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The Autonomic Nervous System

  • This division of the PNS governs our body’s responses to subtle changes in homeostasis

  • There are 2 subdivisions

    • Sympathetic – includes the nerves that control the body when it is actively moving and burning energy – also called the “Fight, Flight, Fright” system.


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