14.2 - The Nervous System & Reflexes

The nervous system and how it coordinates a response

• The nervous system lets the body react to its surroundings and coordinate its response.
• It's built from nerve cells called neurones, which run to all parts of the body.
• The central nervous system (CNS) is made up of the brain and the spinal cord.
• The peripheral nervous system (PNS) is the nerves outside the brain and spinal cord, connecting the CNS to the rest of the body.

Stimuli, receptors and effectors

• Stimulus - A change in the environment inside or outside the body.
• Receptors - Cells that detect a particular kind of change. Different receptors detect different stimuli, and many are grouped together in sense organs:
  • Light receptors in the eyes
  • Sound receptors in the ears
  • Touch and temperature receptors in the skin
  • Chemical receptors in the tongue and nose
• Effectors - The parts that carry out the response. They are either:
  • Muscles - Respond by contracting
  • Glands - Respond by secreting a substance such as a hormone
• When a receptor detects a stimulus, the information is carried as an electrical impulse.

The coordination pathway

1. Information passes from the receptor along a sensory neurone to the CNS.
2. The CNS is the coordinator - it receives the information, decides the response, and links the right receptor to the right effector.
3. It sends an electrical impulse out along a motor neurone to an effector.
4. The effector produces the response.

The whole pathway runs: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.

The neurones and the nerve impulse

• The nervous system is made of nerve cells called neurones.
• A nerve impulse is a small electrical charge that passes along the neurone.

The three types of neurone

1. Sensory neurones - Carry impulses from the receptors to the central nervous system
2. Relay neurones (also called interneurones, connector neurones, or association neurones) - Carry impulses through the central nervous system, linking sensory neurones to motor neurones
3. Motor neurones - Carry impulses from the central nervous system out to the effectors

The structure of a neurone

• A neurone has a cell body containing the nucleus, with thin processes extending from it.
• Dendrites - Short branched processes that carry impulses towards the cell body
• Dendron - A single long process in a sensory neurone that carries impulses towards the cell body
• Axon - A long process that carries the impulse away from the cell body
• The branched endings let one neurone connect to many others.

Two features make the impulse travel quickly

1. Myelin sheath - A fatty layer wrapping many axons; it acts as an electrical insulator and speeds the impulse up.
2. Length - A neurone is very long, and one long neurone carries an impulse faster than a row of short ones joined end to end, because passing the impulse from one neurone to the next slows it down.

Together these adaptations let the nervous system carry information rapidly, so the body can respond quickly.

Transmission across a synapse

• Where the end of one neurone meets the next, the two are separated by a tiny gap. This junction is called a synapse.
• At the end of the first neurone, the signal-carrying chemical - a neurotransmitter - is stored in small membrane-bound sacs called vesicles.
• The membrane of the next neurone, on the far side of the gap, carries receptor proteins that the neurotransmitter can attach to.

How the signal crosses

1. An electrical impulse reaches the end of the first neurone.
2. This triggers the vesicles to release their neurotransmitter into the gap.
3. The neurotransmitter molecules diffuse across the gap.
4. They bind to the receptor proteins on the membrane of the next neurone.
5. This binding sets off a new electrical impulse in that neurone, and the signal carries on.

The signal travels one way only

• The neurotransmitter is released only from the first neurone, and the receptor proteins are only on the membrane of the second.
• So the signal can cross the synapse in one direction only.
• This is why impulses always travel the same way through the nervous system and cannot run backwards across a synapse.

The reflex arc and reflex action

• A reflex action is an automatic, rapid response to a stimulus that happens without any conscious thought.
• Because the body doesn't spend time deciding what to do, a reflex is faster than an ordinary, considered response.
• Reflexes are often protective - they help prevent injury. Examples:
  • Pulling your hand away from a hot object before you've thought about it
  • The pupil of the eye getting smaller in bright light to protect the eye

The reflex arc

• A reflex arc is the pathway the information follows in a reflex, from the receptor that detects the stimulus to the effector that responds.
• The neurones pass through the spinal cord or an unconscious part of the brain, rather than the conscious part - which is why the response is so quick.

The pathway of a reflex arc

1. A receptor detects the stimulus.
2. The receptor sends an impulse along a sensory neurone to the central nervous system.
3. The sensory neurone passes the impulse across a synapse to a relay neurone.
4. The relay neurone passes it across another synapse to a motor neurone.
5. The motor neurone carries the impulse out to the effector - a muscle or a gland - which produces the response.

Example - Touching something sharp

1. A receptor in the skin detects the stimulus.
2. The impulse travels in along the sensory neurone to a relay neurone in the spinal cord.
3. The impulse passes out along the motor neurone.
4. The muscle contracts to pull the hand away.