Introduction to the Nervous System

The Nervous System consists of the brain, spinal cord and thousands of nerves.

The Brain and Spinal Cord are collectively known as the Central Nervous System.

The Peripheral Nervous System is the collective term for all nerves outside the brain and spinal cord.

Functions of the Nervous System

The general function of the nervous system is to allow the organism to respond to stimuli.

A Stimulus is any change in the environment of the organism. The nervous system will then induce a response, which may be movement, speech, blinking etc.

Advanced organisms have specialised sense organs, which are designed to react to stimuli.These are often called receptors and are sensitive to one stimuli (e.g. Skin to touch, eyes to light patterns)

Reacting to Stimuli

When a receptor is exposed to a stimulus, an electronic impulse is sent to the central nervous system via nerves.The brain then analyses the message and based on experiences / memory, the brain sends out a suitable response back to an effector (muscle) via nerve fibres. When the nerve impulse reaches the effector muscle it contracts accordingly.

Neurons or Nerve Cells

There are three types of nerve cells of which two are important to us.

Sensory Neurons move from the receptor to the Central Nervous System.

Motor Neurons move from the Central Nervous System to the effector muscle.

Movement of the message in neurons is based on Electro-chemical conduction. This is similar to electricity in wires but much slower.

Inside nerve cells the impulse moves quite fast but the message must also move out of one cell and into another. This is slow and is based on charged ions (Atoms with extra or missing electrons).

This is why elements like Chlorine, Magnesium, Sodium and Potassium are important in the diet of animals. When these elements are not available, the effector muscles will not react as required and will cramp.

For example, low Mg with cause muscle spasms.

The Reflex Arc

A reflex is the simplest form of response to a stimulus. The most common reflex is the knee jerk. In a reflex reaction, the response occurs before the message reaches the brain, or the message may not reach the brain at all.

This is because of the reflex arc!

The receptor cell sends an impulse through a sensory neuron to the spinal cord. The message doesn’t go directly to the brain but to the cell body of a motor neuron and then to the muscle. The muscle contracts.

The brain may get the message soon after. In reflexes, the impulse is not related to stored information in the brain before the response.