Nervous Control


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Nervous Organisation

The central nervous system, is made up of the brain and spinal cord

The peripheral nervous system, is made up of pairs of nerves that originate from either the brain or the spinal cord

The peripheral nervous system is divided into:
Sensory neurones, which carry nerve impulses from receptors towards the nervous system.

Motor neurons, which carry nerve impulses away from the central nervous system to effectors. 

The motor nervous system can be further subvided as follows:
voluntary nervous system which carries nerve impulses to body muscles ad under voluntary (concious) control.  

autonomic nervous system which carried nerve impulses to galnds, smooth muscle and cardiac muscle and is not under voluntary control that is, it is involuntary (subconcious).  

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The spinal cord

The spinal cord is a column of nervous tissue that runs along the back and lies inside the vertebral column for protection. Emerging at intervals along the spinal cord are pairs of nerves.


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A reflex arc

The response of a reflex arc is involuntary. you will immediatley withdraw your hand if its placed on a high heat or sharp object, By the time the brain has received nerve impulses from the receptors in the hand, the muscles in the arm are already pulled the hand clear of danger.  This involuntary responses is called a reflex. the pathway of neurones is known as a reflex arc. 

1. The stimulus - heat from the object
2. receptor - temperature receptors in the skin create a nerve impulse in a sensory neurone.
3. a sensory neurone - passes the nerve impulse to the spinal cord
4. an intermediate neurone - links the sensory neurone to the motor neurone in the spinal cord.
5. a motor neurone - carries the nerve impulse fromt he spinal cord to a muscle in the upper arm.
6. an effector - the muscle in the upper arm, which is stimulated to contract.
7. the response - pulling the hand away from the hot object.   

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A reflex arc cont...

Any action that makes survival more likely is clearly of value.
reflexes are involuntary, the actions they control do not need to be considered because there is only one obvious course of actino, that is, to remoe the hand from the hot object.  Reflex actinos are important for the following reasons.

  • They are involuntary and therefore do not require the decision making powers of the brain, thus leaving it more free to carry out more complex responses. in this way, the brain is no overloaded with situations in which the response is always the same. some impulses are nevertheless sent to the brian, so that it is informed of what is happening and can sometimes over-ride the reflex if necessary.
  • They protect the body from harmful stimuli. They are effective from birth and do no have to be learned.
  • they are fast, because the neurone pathway is short with very few, typically one or two synapses (which are the slowest link in a neurone pathway). this is important in withdrawal reflexes.  
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