The Nervous System


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Purpose of the Nervous System

  1. It allows animals to sense and respond to changing conditions in their environment.
  2. It provides communication between and coordination of all organs and organ systems.
  3. It establishes homeostasis by monitoring the internal environment of the organism
  4. It works in close association with the endocrine system.

 


Cellular Basis of the Nervous System

 

The nervous system is based on specialized cells called neurons. The structure of these cells are adapted to transmit signals similar to an on-off switch. Hundreds or thousands of these signals can be integrated together just as the circuitry of a computer can be fashioned into programs to accomplish specific functions. But unlike the two dimensions of the circuit board the interconnections of neurons are three dimensional.

Once the sensory input (stimulus) has been received by the network of nerve cells the raw data (signals) are added to and integrated with existing information about external and internal conditions. The nervous system is then capable of selecting an appropriate muscle or gland that will produce a suitable response.

How information is stored (learning) is not entirely understood, but "memory" is likely a combination of neural activity between cells, new connections, and other permanent changes.

 


Types of neurons

There are three types of neurons two of which are illustrated below:
  1. Sensory neurons - designed to detect or receive stimuli
  2. Interneurons - found mostly in the central nervous system and form vast networks to interpret the senses and control varous effectors.
  3. Motor neurons - sent impulses to activate muscles or glands.
Motor Neuron
Sensory Neuron
Nerve cells make up less than half of the volume of the brain. The rest is made of neuroglia, cells which provide support, protection, and assistance to the neurons.

 

Neurons have 3 functional zones which are:

  1. the cell body (receives input and triggers the nerve impulse)
  2. dendrites (input stimuli to the cell body)
  3. axon (the long entension of the cell body which may have a number of branches for output)

 


The Neuron at Rest

The resting potential is maintained or reestablished by sodium/potassium pumps located in the plasma membrane of the axon.

 

A neuron can create an electrical potential between the interior of the axon and the fluid surrounding it. The value of this potential is -70 millivolts. Therefore each neuron is much like a minature rechargable battery.

Various proteins in the membrane of the axon create and maintain the resting potential. One protein, the sodium-potassium ion pump is able to move 3 sodium ions (Na+) out of the cell for every 2 potassium ions (K+) into the cell.

 

 

The Na/K pump requires ATP because it moves these ions against their concentration gradients.

Other proteins act as one-way gates which open or close depending on the voltage they experience. One of these gates allows Na+ to diffuse back through the membrane to allow depolarization during the action potential (sse below). A second gate protein prevents K+ from escaping until the action potential begins.

 


The Action Potential:

Threshold Level. Not all stimuli are strong enough to result in an action potential. Each neuron's activity will depend on the number of dendrites it has and how many receptors are in its membrane.

Each stimulus results in a slight change in voltage which produces a graded, local signal.

Once the stimulus or collection of stimuli reach the required level or threshold the action potential will begin

It is a rapid, short-lived reversal in the ion gradient or resting potential which involves the following:

  1. it is an all or none event
  2. its magnitude is independent of the stimulus
  3. it has 4 phases which are:
    1. resting
    2. depolarization
    3. repolarization
    4. refractory period
     

     

  4. is controlled by voltage-sensitive gated channels
  5. is an "explosive" example of positive feedback, where depolarization to threshold potential causes further depolarization.
  6. can not occur continuously because the hyperpolarization during the refractory period prevents further depolarization.
  7. can only travel in one direction since the refractory period inhibits depolarization in the opposite direction.

 

 
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