Biology B1 (Nerves and hormones)

The nervous system allows the body to respond to changes in the environment in a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli; this process also involves the nervous system but bypasses the brain.

Receptors

Receptors are groups of specialised cells that can detect changes in the environment called stimuli. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

• Receptors found in the eyes are sensitive to light
• Receptors found in the ears are sensitive to sound
• Receptors found in the tongue are sensitive to chemucals in food
• Receptors found in the nose are sensitive to chemicals found in the air
• Receptors found in the skin are sensitive to touch, pressure, pain and temperature.

Cells

Most animal cells have a nucleus, cytoplasm and cell membrane. Light receptors have these cell components too.

The human central nervous system (CNS) consists of the brain and spinal cord. When a receptor is stimulated it sends a signal along the nerve cells, also called neurones, to the brain. The brain then coordinates the response.

Neurones

Neurones are nerve cells that carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:

• sensory neurones carry signals from receptors to the spinal cord and brain
• relay neurones carry messages from one part of the CNS to another
• motor neurones carry signals from the CNS to effectors.

This diagram shows a typical neurone (a motor neurone), which has tiny branches at each end and a long fibre that carries the signals.

A motor neurone

Synapses

Where two neurones meet there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.

Reflex actions

When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates the response, but sometimes a very quick response is needed, one that does not involve the brain: this is a reflex action.

Sequence

In a simple reflex action:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector

An effector is any part of the body that produces the response. Examples of effectors:

• a muscle contracting
• a gland releasing (secreting) a hormone or other chemical.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it.

This is what happens:

1. receptor detects a stimulus - a change in the environment
2. sensory neurone sends impulses to relay neurone
3. motor neurone sends impulses to effector
4. effector produces a response.

Hormones are chemical substances that regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the bloodstream. Several hormones are involved in the female menstrual cycle. Hormones can be…