14.1) Nervous control in humans
Co-ordination is the way all the organs and systems of the body are made to work efficiently together.
A nerve impulse is an electrical signal that passes along nerve cells called neurons.
The human nervous system consists of:
- the central nervous system (CNS) – the brain and spinal cord
- the peripheral nervous system – nerve cells that carry information to or from the CNS
Nerve cells are also called neurons. They are adapted to carry electrical impulses from one place to another:
- The axon is an extended cymiddlelasm thread along which electrical impulses travel.
- Axons are coated by a layer of myelin called myelin sheath, this is an electrically insulating layer which is essential for the proper functioning of the nervous system.
- Dendrite’s function is to pick up electrical impulses from other cells.
- Motor end plate passes the electrical impulses from the neurone to the muscle fibres.
Sensory Neurones: carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.
Motor Neurone: Transmits electrical impulses from the Central nervous system to the effectors.
Relay Neurone: Relay neurones are located in the CNS. Their job is to pass electrical impulses from the sensory neurone onto the motor neurone, so it acts like a diversion.
The reflex arc:
A reflex action is the means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors. (muscles and glands)
A well-known reflex is the knee-jerk reflex.
- Receptor in the skin detects a stimulus (the change in temperature).
- Sensory neurone sends impulses to relay neurone.
- Motor neurone sends impulses to effector.
- Effector produces a response (muscle contracts to move hand away).
Voluntary and involuntary actions:
The reflex arc is a reflex action. Reflex means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some impulses are carried by other neurons to the brain, and some impulses are passed onto the motor neurone to the effector muscle and the response takes place. The electrical impulses going to your brain are much slower that the ones going to the effector muscle directly. This is why the reflex action takes place before you realise it, it is uncontrollable.
Reflex actions are said to be involuntary actions. Involuntary actions start at the sense organ heading to the effector. They are extremely quick.
Voluntary actions are the ones that you make the choice to do. Like picking up a bag from the floor for example. Your brain sends electrical impulses to the effector muscles ordering them to contract so you could pick the bag up. Voluntary actions are slower than involuntary actions and they start at the brain.
Synapse: is a junction between two neurones.
- When an impulse arrives at the synapse, vesicles in the cytoplasm release a tiny amount of the neurotransmitter
- It rapidly diffuses across the gap (aka synaptic cleft) and binds with neurotransmitter receptor molecules in the membrane of the neuron on the other side of the synapse.
- This then sets off an impulse in the neurone.
- Sometimes several impulses have to arrive at the synapse before enough transmitter substance is released to cause an impulse to be fired off in the next eurone.
- Synapses control the direction of impulses because neurotransmitter substances are only synthesised on one side of the synapse, while receptor molecules are only present on the other side.
- They slow down the speed of nerve impulses slightly because of the time taken for the chemical to diffuse across the synaptic gap.
- Many drugs produce their effects by interacting with receptor molecules at synapses.
Heroin, stimulates receptor molecules in synapses in the brain, triggering the release of dopamine (a neurotransmitter), which gives a short-lived ‘high’.