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Nervous system and the neuron

The nervous system coordinates and regulates bodily functions.

The motion of your leg muscles when you walk is coordinated by the nervous system.

Animals need a fast way for different parts of the body to communicate with each other. The nervous system uses electrical signals to transmit information.

The nervous system can be split into two parts:

  • The central nervous system (CNS) is made up of the brain and the spinal cord. It sends out instructions to other parts of the body.
  • When you are hungry, the CNS will determine that you need food, recognise a biscuit as a food source and coordinate movements to grab it!

  • The peripheral nervous system (PNS) detects stimuli from inside and outside the body. This information is sent to the CNS.

    Touch-sensitive receptors in the skin provide the CNS with information about what is around you.

    The PNS also conveys commands generated by the CNS to muscles and glands. It includes any nerve fibres not within the CNS.

Neurons (nerve cells) transmit electrical impulses around the nervous system.

Neurons function in a similar way to electrical wires.

Most neurons share the same basic features:

  • Cell body (soma) : Contains the nucleus.
  • Dendrites : Receive impulses from other neurons.
  • Axon : Transmits impulses from the cell body to a target.
  • Myelin sheath : A fatty material that wraps around the axon. It electrically insulates the axon, speeding up impulse travel.

A synapse is a junction between two nerve cells that allows them to communicate with each other. The synapse allows an impulse travelling down an axon to be passed on to another neuron.

Every dendrite is connected to another neuron by a synapse. The brain alone contains around 100 trillion of these connections!

A nerve is a bundle of neurons in the peripheral nervous system or spinal cord. The brain does not contain nerves, but it does contain neurons.

Structure of the neuron.
Structure of the neuron.

Neurons can be grouped by their function. There are three important types of neuron:

Types of neuron can be identified by their different shapes.
Types of neuron can be identified by their different shapes.
  • Sensory neurons transmit signals from receptors (such as the eye) to relay neurons. Impulses in sensory neurons travel towards the central nervous system (CNS).
  • Relay neurons transmit signals from one neuron to another. Impulses in relay neurons travel within the CNS. They normally have short axons.

    Most brain cells are relay neurons.

  • Motor neurons transmit signals from relay neurons to effectors (muscles and glands). Impulses in motor neurons travel away from the CNS.
  • Motor neuron activity can cause a gland to release hormones into the blood.

The nervous system allows organisms to detect and respond to stimuli in the environment that cause a physiological response.

When someone is talking to you, you detect the stimulus (their voice) and can respond (your reply).

The nervous system is organised for stimulus-response action:

  • Stimulus: A change in the external or internal environment.
  • Receptor: Detects the stimulus. The nervous system contains many receptors that detect different types of stimuli.
  • Effector: Responds to the stimulus. These are either muscles or glands.

    Effectors produce effects. A muscle can contract to produce movement. A gland can release hormones to alter bodily functions.

The nervous system is very fast. A stimulus-response such as moving your hand away from a hot plate takes only 30 milliseconds!

Descartes (16th century) recognised the stimulus-response role of the nervous system.
Descartes (16th century) recognised the stimulus-response role of the nervous system.

A reflex arc is a small group of connected neurons that produces reflex responses.

Reflexes are involuntary movements; we cannot control them. They normally have a protective role.

Being prodded by a pin initiates a movement away from the pin.

The reflex arc involves the following steps:

  1. Receptor receives a stimulus and converts it into an electrical impulse.
  2. Sensory neuron transmits the impulse from the receptor to a relay neuron in the spinal cord.
  3. Relay neuron transmits the impulse from the sensory neuron to a motor neuron.
  4. Motor neuron sends the signal from the spinal cord to an effector muscle.
Sensory and motor neurons have different paths to and from the spinal cord.
Sensory and motor neurons have different paths to and from the spinal cord.

The brain is not involved in the reflex arc. This allows the reflex to be much faster as impulses do not have to travel all the way to the brain and back.

An average reflex takes only 30 to 50 milliseconds!