Coordination and Response

• Electrical impulses travel along neurones.
• The role of the nervous system is the coordination and regulation of body functions.

Mammalian nervous system

• The mammalian nervous system consists of the CNS and PNS.
  • The central nervous system consists of the brain and spinal cord. It acts as the body’s primary center for processing info and directing body functions. It is connected to different parts that make up the peripheral nervous system.
  • The peripheral nervous system consists of the nerves outside the brain and spinal cord. The cranial nerves link the brain to all organs in the head and some in the thorax and abdomen. The spinal nerves leave the spinal cord in pairs through gaps in between the vertebrae.

Reflex actions

• A reflex action is automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).
• The sequence of events is:
  • stimulus $\rightarrow$ receptor $\rightarrow$ coordinator $\rightarrow$ effector $\rightarrow$ response
  • A stimulus is a change in the environment that is detected by a sense organ.
  • A receptor is a cell that detects stimuli.
  • A coordinator is a part of the body connects info about the stimulus to the effector.
  • An effector is an organ that brings about responses.
  • A response is the reaction of an organism to a particular stimulus.

Neurones and reflex arcs

• Neurones are nerve cells. They are highly specialized. Their structure allows them to transmit information as nerve impulses over long distances.

• Sensory neurones transmit impulses from sense organs to the brain and spinal cord.
• Motor neurones transmit impulses away from the brain and spinal cord to effector organs (muscles and glands)
• Relay neurones connect the sensory neurone to the motor neurone inside the brain and spinal cord to facilitate communication.

Structure of neurones

• Sensory and motor neurones are surrounded by myelin.
  • Myelin is formed by separate cells that grow around the neurones to form a layer rich in fat. This insulation makes impulses travel very quickly.

Reflex arcs

• A reflex arc is the arrangement of neurones that control a reflex action.

Synapses

• A synapse is a junction between two neurones.
• Synapses ensure that impulses travel in one direction only.

Structure

• The gap is called the synaptic gap.
• The presynaptic neurone is the neurone that carries the impulse to the synapse. The postsynaptic neurone is the neurone that carries the impulse away from the synapse.
• The synaptic bulbs are swellings at the axons of neurones.
• The presynaptic membrane is the surface of the synaptic bulb. It is separated by the synaptic gap from the postsynaptic membrane of the cell body or axon of the next neurone.
  • The postsynaptic membrane has a large number of protein receptors for the neurotransmitter substance.
• There are vesicles in the synaptic bulb that contain many mitochondria and neurotransmitter molecules.

Synaptic transmission

1. When an impulse arrives at the synaptic bulb it causes vesicles containing the neurotransmitter to move towards the presynaptic membrane.
2. The vesicles fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic gap.
3. The neurotransmitter diffuses across the synaptic gap and attaches to specific receptor sites on the postsynaptic membrane. These receptor sites ahve a complementary shape to the neurotransmitter, but the binding is only temporary.
4. The binding of the neurotransmitter triggers an impulse in the postsynaptic neurone. Once this has happened the neurotransmitter is broken down by an enzyme in the synaptic gap.
5. The mitochondria provide energy to reform the transmitter.