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The Nervous System


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Title: The Nervous System

The Nervous System
  • the major controlling, regulatory, and
    communicating system in the body.
  • the center of all mental activity including
    thought, learning, and memory.
  • together with the endocrine system, the nervous
    system is responsible for regulating and
    maintaining homeostasis.
  • Through its receptors, the nervous system keeps
    us in touch with our environment, both external
    and internal.

  • The activities of the nervous system can be
    grouped together as three general, overlapping
  • Sensory
  • Integrative
  • Motor

  • Sensory
  • millions of sensory receptors detect changes,
    called stimuli, which occur inside and outside
    the body.
  • They monitor such things as temperature, light,
    and sound from the external environment.
  • Inside the body (the internal environment),
    receptors detect variations in pressure, pH,
    carbon dioxide concentration, and the levels of
    various electrolytes.
  • All of this information is called sensory input.

  • Integrative
  • Sensory input is converted into electrical
    signals called nerve impulses that are
    transmitted to the brain.
  • There the signals are brought together to create
    sensations, to produce thoughts, or to add to
  • Decisions are made each moment based on the
    sensory input. This is integration.

  • Motor
  • Based on the sensory input and integration, the
    nervous system responds by sending signals to
    muscles, causing them to contract, or to glands,
    causing them to produce secretions.
  • muscles and glands are called effectors because
    they cause an effect in response to directions
    from the nervous system. This is the motor output
    or response

Structure of the Nervous System
  • The Nervous system has two major divisions
  • 1. The Central Nervous System (CNS)
  • consist of the Brain and the Spinal Cord.
  • The average adult human brain weighs 1.3 to 1.4
    kg .The brain contains about 100 billion nerve
    cells,called Neurons and trillons of "support
    cells" called glia.
  • The spinal cord is about 43 cm long in adult
    women and 45 cm long in adult men and weighs
    about 35-40 grams.

  • 2. The Peripheral Nervous System (PNS)
  • consists of the neurons NOT Included in the Brain
    and Spinal Cord.
  • Is divided into two divisions
  • Somatic nerves ( voluntary)
  • Autonomic nerves (involuntary) 

  • The somatic nerves
  • Controls the skeletal muscles, bones and skin
  • Some neurons collect information from the Body
    and transmit it TOWARD the CNS.  These are called
  • Other neurons transmit information AWAY from the
    CNS.  These are called EFFERENT NEURONS.

  • The autonomic nerves
  • Control the internal organs of the body
  • Regulatory system that works with the endocrine
    system to maintain homeostasis
  • Consist of motor neurons that operate without
    conscious control.
  • Organized into the sympathetic nervous system and
    the parasympathetic nervous system
  • Autonomic regulation involves constant interplay
    of balance between sympathetic and

  • The sympathetic nervous system
  • Prepares the body for stress increases heart
    rate, increases the release of glucose, dilates
    the pupils, increases blood flow to the skin,
    causes release of epinephrine

  • The parasympathetic nervous system
  • Restores normal balance decreases heart rate,
    stores glucose, constricts pupils, decreases
    blood flow to the skin.

  • Activity of autonomic system is regulated by
    neurons in brain and spinal cord
  • brainstem contains centers necessary for control
    of heart rate, blood pressure, and body
  • control of brainstem regions exerted by
  • hypothalamus, in turn, influenced by limbic
    system structures
  • emotional responses accompanied by extensive
    changes in autonomic function

The Neuron
The Neuron
The Neuron
  • is the functional unit of the nervous system.
  • Humans have about 100 billion neurons in their
    brain alone!
  • While variable in size and shape, all neurons
    have three parts
  • Dendrites
  • The cell body
  • The axon

  • Dendrites
  • Bring information to the cell body (incoming)
  • Rough Surface (dendritic spines)
  • Usually many dendrites per cell ( can be up to a
  • No myelin sheath
  • Branch near the cell body

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  • The Cell Body (Soma)
  • contains the nucleus, mitochondria and other
    organelles typical of eukaryotic cells.
  • Is the metabolic control center of the neuron and
    its manufacturing and recycling center it is in
    the cell body that neuronal proteins are

  • The Axon
  • Take information away from the cell body
  • Smooth Surface
  • Generally only 1 axon per cell
  • Can have myelin
  • Branch further from the cell body

Types of Neurons
  • Three types of neurons
  • Sensory neurons
  • Interneurons
  • Motor neurons

Sensory Neurons
  • typically have a long dendrite and short axon
  • carry messages from sensory receptors to the
  • The cell bodies of the sensory neurons leading to
    the spinal cord are located in clusters, called
    ganglia, next to the spinal cord.
  • The axons usually terminate at interneurons.

-Interneurons are found only in the central
nervous system where they connect neuron to
neuron. -They are stimulated by signals reaching
them from sensory neurons, other interneurons or
both. -are also called association neurons. -It
is estimated that the human brain contains 100
billion (1011) interneurons averaging 1000
synapses on each or some 1014 connections
Motor Neurons
  • Typically have a long axon and short dendrites
  • Transmit messages from the central nervous system
    to the muscles (or to glands).
  • The axons connecting your spinal cord to your
    foot can be as much as 1 m long (although only a
    few micrometers in diameter).

The Nerve Impulse.
The Neuron at Rest
  • The plasma membrane of neurons contains many
    active Na-K-ATPase pumps.
  • These pumps shuttle Na out of the neuron and K
    into the neuron when ATP is hydrolyzed.
  • Three Na are pumped out of the neuron at a time
    and two K ions are pumped in

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  • This creates a concentration gradient for Na.
    As Na accumulates on the outside of the neuron,
    it tends to leak back in.
  • Na must pass through proteins channels to leak
    back through the hydrophobic plasma membrane.
    These channels restrict the amount of Na that
    can leak back in.
  • This maintains a strong positive charge on the
    outside of the neuron

  • The K inside the neuron also tends to follow its
    concentration gradient and leak out of the cell.
  • The protein channels allow K to leak out of the
    cell more easily.
  • As a result of this movement in Na and K ions,
    a net positive charge builds up outside the
    neuron and a net negative charge builds up

  • This difference in charge between the outside and
    the inside of the neuron is called the Resting
  • The resting potential in most neurons is
  • 70 mV.
  • When the neuron is at rest, it is polarized

Initiation of the Action Potential
  • A change in the environment ( pressure,
    heat,sound, light) is detected by the receptor
    and changes the shape of the channel proteins in
    part of the neuron usually the dendrites.
  • The Na channels open completely and Na ions
    flood into the neuron. The K channel close
    completely at the same time and K ions can no
    longer leak out of the neuron in that particular

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  • The interior of the neuron in that area becomes
    positive relative to the outside of the neuron.
  • This depolarization causes the electrical
    potential to change from 70 mV to 40 mV
  • The Na channels remain open for about 0.5
    milliseconds then they close as the proteins
    enter an inactive state.
  • The total change between the resting state (-70
    mV) and the peak positive voltage ( 40mV) is the
    action potential ( about 110 mV)

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  • The spike in voltage causes the K pumps to open
    completely and K ions rush out of the neuron.
    The inside becomes negative again. This is
  • So many K ions get out that the charge goes
    below the resting potential. While the neuron is
    in this state it cannot react to additional
  • The Refractory period lasts from 0.5 to 2
  • During this time, the Na-K-ATPase pump
    reestablishes the resting potential.

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Transmission of the impulse
  • The stimulus induces depolarization in a very
    small part of the neuron, at the dendrites.
  • The sequence of depolarization and repolarization
    generates a small electrical current in this
    localized area.
  • The current affects the nearby protein channels
    for Na and causes them to open.

  • When the adjacent channels open, Naions flood
    into that area of the neuron and an action
    potential occurs. This in turn will affect the
    areas next to it and the impulse passes along the
    entire neuron.
  • The electric current passes outward over the
    membrane in all directions BUT the area to one
    side is still in the refractory period and is not
    sensitive to the current. Therefore the impulse
    moves from the dendrites toward the axon.

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Threshold stimulus
  • Action potentials occur only when the membrane in
    stimulated (depolarized) enough so that sodium
    channels open completely.
  • The minimum stimulus needed to achieve an action
    potential is called the threshold stimulus.
  • If the membrane potential reaches the threshold
    potential (generally 5 - 15 mV less negative than
    the resting potential), the voltage-regulated
    sodium channels all open. Sodium ions rapidly
    diffuse inward, depolarization occurs.

All-or-None Law
  • Action Potentials occur maximally or not at all.
  • In other words, there's no such thing as a
    partial or weak action potential. Either the
    threshold potential is reached and an action
    potential occurs, or it isn't reached and no
    action potential occurs.
  • However, different neurons have different
    densities of Na channels and therefore have
    different APs

  • The AP remains constant as it travels down the
    neuron. Its amplitude is always the same because
    it corresponds to wide open Na channels.
  • The frequency of the AP can change.

Conduction Velocity
  • impulses typically travel along neurons at a
    speed of anywhere from 1 to 120 meters per second
  • the speed of conduction can be influenced by
  • The diameter of a fiber. Velocity increases as
    diameter increases.
  • Temperature. As temperature increases, the
    velocity increases. Axons of birds and mammals
    can be very small because of the high body
  • the presence or absence of myelin.

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  • Neurons with myelin (or myelinated neurons)
    conduct impulses much faster than those without
  • Because fat (myelin) acts as an insulator,
    membrane coated with myelin will not conduct an
  • So, in a myelinated neuron, action potentials
    only occur along the nodes and, therefore,
    impulses 'jump' over the areas of myelin - going
    from node to node in a process called saltatory
    conduction (the word saltatory means 'jumping')

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  • The Action Potential, or nerve impulse is an
    electrochemical event involving the rapid
    depolarization and repolarization of the nerve
    cell membrane.
  • The axon terminals of one neuron do not touch the
    dendrites of other neurons. What happens when the
    impulse reaches the axon terminal?