Chapter 14 - Coordination and Response : Nervous System 

Nervous System

• The human %%nervous system%% consists of:
  (a) %%Central nervous system%% (CNS) consisting of the %%brain%% and %%spinal cord.%%

  (b) %%Peripheral nervous system%% (PNS) consisting of %%nerves%% connecting the central nervous system and the rest of the body. The function of the PNS is to conduct %%sensory and motor signals%% between the CNS and the %%limbs%% and %%organs%% (%%receptors and effectors%%).

• A %%stimulus%% is a change in the environment that causes an organism to react. Stimuli are detected by %%sensory receptors%%.

• A response is a change in the body as a result of the stimulus. %%Effector cells%% are muscle cells or gland cells, which carry out the response to stimuli.

• Bodily functions are classified into %%voluntary actions%% and %%involuntary actions%%.

• Involuntary actions are actions that cannot be consciously controlled, such as heartbeat, peristalsis, vasoconstriction and reflex actions.

• Voluntary actions are actions that are consciously controlled.

Nervous tissue

• %%Nerve impulses%% are transmitted by nerves, which are bundles of %%neurones%% wrapped in connective tissue.

• A %%neurone%% is a nerve cell.

• There are three main types of neurones:
  (a) %%Sensory neurones%% – Respond to stimuli affecting cells of the sensory organ they are found in and relay signals to the CNS
  (b) %%Intermediate neurones (relay neurones)%% – Transmit nerve impulses from the sensory neurones to the motor neurones; found within the CNS
  (c) %%Motor neurones%% – Transmit nerve impulses from the CNS to the effector muscle cells or gland cells

• Neurones share common characteristics:

  (a) A relatively large cell body containing the nucleus and organelles.

  (b) %%Nerve fibres%% that increase the distance over which nerve impulses can be transmitted. There are two types of nerve fibres.

  • %%Axons%% are long, that conduct nerve impulses away from the cell body of the neurone.
  • %%Dendrons%% are branched projections that conduct nerve impulses towards the cell body.
  • At the terminal ends of axons and dendrons, the nerve fibre branches. These branches are known as dendrites. Where the axon is connected to muscles, these branches are also known as %%motor end plates%%.

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Sensory Neurone

• The sensory neurone has a smooth and rounded cell body, a single long dendrite and a short axon. The dendron is structurally similar to an axon and is myelinated.

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Motor Neurone

• The motor neurone consists of a cell body and a long thin axon covered by a %%myelin sheath%%.
• Around the cell body are branching dendrites that receive nerve impulses from other neurones and conduct them towards the cell body.
• The axon conducts signals away from the cell body towards the effector cells.

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Synapses

• A %%synapse%% is a junction between two neurones or between a neurone and an effector.
• At a synapse, impulses from the axon of one neurone are transmitted to the dendrites of another neurone or to effector cells.

Reflex actions

• %%Reflex actions%% are involuntary responses to a specific stimulus. They cannot be consciously controlled.

• The pathway by which nerve impulses travel during reflex actions is called a %%reflex arc%%.

• It consists of:

  (a) %%Receptor%%

  (b) %%Sensory neurone%%

  (c) %%Intermediate neurone%% / relay neurone (located in CNS)

  (d) %%Motor neurone%%

  (e) %%Effector%%

• Receptors in the skin detect the stimulus.

• Nerve impulses are produced which are transmitted by the sensory neurone to the %%spinal cord%%.

• In the spinal cord, the nerve impulses are transmitted across a synapse to an intermediate neurone and then across another synapse to the motor neurone. Nerve impulses are also transmitted to the %%brain%%.

• Nerve impulses travel along the motor neurone to the motor end plate.

• The nerve impulses stimulate the motor end plate and cause the muscle to contract.

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