The Nervous System

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The Nervous System
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Function

• 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.

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

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• 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.
  

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• 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
  memory.
• Decisions are made each moment based on the
  sensory input. This is integration.

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• 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

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Structure of the Nervous System
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• 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.

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• 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) 

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• 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
  AFFERENT NEURONS.
• Other neurons transmit information AWAY from the
  CNS.  These are called EFFERENT NEURONS.

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• 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
  parasympathetic

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• 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

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

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• 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
  temperature
• control of brainstem regions exerted by
  hypothalamus
• hypothalamus, in turn, influenced by limbic
  system structures
• emotional responses accompanied by extensive
  changes in autonomic function

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The Neuron
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The Neuron
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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

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• Dendrites
• Bring information to the cell body (incoming)
• Rough Surface (dendritic spines)
• Usually many dendrites per cell ( can be up to a
  thousand)
• 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
  synthesized

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• 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

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Types of Neurons

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

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Sensory Neurons

• typically have a long dendrite and short axon
• carry messages from sensory receptors to the
  CNS.
• 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.

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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
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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).

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The Nerve Impulse.
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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

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• 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
  inside.

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

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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
  area.

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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
  repolarization.
• 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
  stimuli.
• The Refractory period lasts from 0.5 to 2
  milliseconds.
• 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.

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• 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.

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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

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• 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.

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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
  temperature.
• the presence or absence of myelin.

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• Neurons with myelin (or myelinated neurons)
  conduct impulses much faster than those without
  myelin.
• Because fat (myelin) acts as an insulator,
  membrane coated with myelin will not conduct an
  impulse.
• 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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Summary

• 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?