Biology: Nerves and hormones

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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 also involves the nervous system but bypasses the brain.

Receptors are groups of specialised cells that detect changes in the environment called stimuli. Receptors are often located in the sense organs. Each organ has receptors specialised to particular kinds of stimulus. 

  • Eyes: light
  • Ears: sound and position of the head
  • Tongue: chemicals in food
  • Nose: chemicals in the air
  • Skin: touch, pressure, pain and temperature

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

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

Neurons are nerve cells which carry information as tiny electrical signals. There are three types of neuron:

  • Sensory neurons carry signals from the receptors to the spinal cord and brain
  • Relay neurons carry messages from one part of the CNS to another
  • Motor neurons carry signals from the CNS to effectors

When two neurons meet there is a gap called a synapse. Signals cross the gap using chemicals. One neuron releases chemicals into the gap. The chemical diffuses across the gap and makes the next neuron transmit an electrical signal. 

When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates a response, but sometimes a very quick response is needed which does not include the brain. This is called a reflex action.

A simple reflex action:

Stimulus --> Receptor --> Sensory neuron --> Relay neuron --> Motor neuron --> Effector (any part of the body which produces the response, e.g. muscle/gland secreting a hormone or other chemical)

Reflex actions are quick and happen without us thinking. 

  • Receptor detects a change a stimulus
  • Sensory neuron sends impulses to relay neuron
  • Motor neuron sends impulses to effector
  • Effector produces a response

Control in the human body

Hormones are chemical substances that regulate processes in the human body. They are secreted by glands and travel through the bloodstream. They can be used to control human fertility and have advantages and disadvantages.

The internal environment of the body is controlled by hormones and the nervous system. The maintenance of a constant internal environment is called homoeostasis. 

Water in the body has to be controlled to protect cells from either too much water entering or leaving them. Water content is controlled by water loss from:

  • The lungs - when we exhale
  • The skin - through sweating
  • Passing urine - produced by the kidneys

Ion (salt) levels in the body are controlled to protect cells from too much water entering or leaving them. Ion levels are controlled by the loss of ions from:


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