The Nervous System Carries Electrical Impulses

1. A neurone is a cell specialised to carry an electrical impulse rapidly from one part of the body to another.
2. Bundles of neurones are wrapped in connective tissue to form the nerves that run through the body.
3. The pathway of any nervous response follows the same order: stimulus, receptor, sensory neurone, CNS, motor neurone, effector, response.
4. Nervous responses are fast and short lived, which is what distinguishes them from the slower, longer lasting effects of hormones.

Receptors Detect the Stimulus

1. A receptor detects a stimulus and converts that stimulus into an electrical impulse.
2. Receptors are usually grouped inside a sense organ, so light receptors sit in the retina of the eye and sound receptors sit in the cochlea of the ear.
3. Each type of receptor responds to only one type of stimulus, which is why the eye cannot detect sound.
4. The skin contains separate receptors for touch, pressure, temperature and pain.
5. Receptors inside the body monitor conditions such as blood glucose concentration, blood pressure and the position of the muscles.

Sensory Neurones

1. A sensory neurone carries the impulse from a receptor to the central nervous system.
2. It has a long dendron that carries the impulse towards the cell body, and a shorter axon that carries it onwards into the CNS.
3. The cell body sits partway along the side of the neurone rather than at one end, which is the easiest way to recognise it in a diagram.

Relay Neurones

1. A relay neurone lies entirely within the central nervous system, in the brain or the spinal cord.
2. It carries the impulse from a sensory neurone to a motor neurone.
3. It is short, with a cell body in the middle and many short branches at both ends.
4. Those branches connect to large numbers of other neurones, which is how the CNS links incoming information to a coordinated response.

Motor Neurones

1. A motor neurone carries the impulse from the central nervous system to an effector.
2. Its cell body sits at one end, inside the CNS, with many short dendrites branching from it.
3. A single very long axon then runs all the way to the effector, and in an adult a motor neurone reaching the foot can be about a metre long.
4. The axon ends in branches that spread across the muscle so that many muscle fibres are stimulated at once.

The Axon and the Dendron

1. The axon carries the impulse away from the cell body, and the dendron carries it towards the cell body.
2. Both are long and thin, which allows a single cell to span a large distance without the impulse being passed between cells.
3. Passing an impulse along one continuous fibre is far faster than passing it from cell to cell, because every junction adds a delay.
4. A sensory neurone has both a dendron and an axon, while a motor neurone has dendrites and one long axon.

The Myelin Sheath

1. Many axons are wrapped in a myelin sheath, a fatty layer made by cells that coil around the fibre.
2. The sheath acts as an electrical insulator, so the impulse cannot leak out through the membrane.
3. The sheath is broken by regular gaps, and the impulse jumps from one gap to the next instead of travelling along the whole membrane.
4. This makes the impulse travel much faster, and a myelinated neurone can conduct at over 100 metres per second while an unmyelinated one manages about 1 metre per second.
5. Myelin also stops impulses crossing between neighbouring axons in the same nerve, so signals stay separate.

Effectors Carry Out the Response

1. An effector is a muscle or a gland.
2. A muscle responds by contracting, which produces movement.
3. A gland responds by secreting a substance such as a hormone, sweat or an enzyme.
4. The impulse itself does not do anything useful until it reaches an effector, so the effector is what converts the signal into a response.

Synapses

1. Neurones do not physically touch, so there is a tiny gap between the end of one neurone and the start of the next.
2. The junction, including the gap, is called a synapse.
3. An electrical impulse cannot jump the gap, so the signal must be carried across in a different way.
4. The signal is passed across by a chemical, which means transmission is electrical along the neurone and chemical at the synapse.

How an Impulse Crosses a Synapse

1. The electrical impulse arrives at the end of the first neurone.
2. This causes tiny sacs called vesicles to move to the membrane and release a neurotransmitter into the gap.
3. The neurotransmitter diffuses across the gap, which is fast because the gap is extremely small.
4. It binds to receptor molecules on the membrane of the next neurone, and the receptors have a shape complementary to the neurotransmitter.
5. Binding triggers a new electrical impulse in the second neurone, and the impulse continues on its way.
6. The neurotransmitter is then broken down or reabsorbed, so the second neurone is not stimulated continuously.

Why Synapses Are Useful

1. Vesicles are only present on one side of the synapse, so the impulse can travel in one direction only.
2. One neurone can form synapses with many others, which allows a single input to trigger several different responses.
3. Several weak inputs can be added together at a synapse, so a response happens only when the stimulus is strong enough to matter.
4. The cost is a slight delay, because diffusion takes longer than conduction along an axon, which is why a pathway with fewer synapses is faster.

Measuring the Speed of a Nervous Response