14.1 Mammalian nervous system

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central nervous system (CNS) – the brain and the spinal cord
peripheral nervous system (PNS) – all of the nerves in the body.
It allows us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions.
It works by sending electrical impulses along the nerves.
Electrical impulses are electrical signals that pass along nerve cells known as neurons.
A bundle of neurons is known as a nerve.
A nerve is easy to see. It is white, tough and stringy and consists of hundreds of microscopic nerve fibres bundled together. Most nerves contain a mixture of sensory and motor fibres, so a nerve can carry many different impulses.
The nerves also carry impulses back to the CNS from receptors in the sense organs of the body.
These impulses make us aware of changes in our surroundings or ourselves.
Nerve impulses from the sense organs to the CNS are called sensory impulses.
Those from the CNS to the effectors, resulting in a response, are called motor impulses.
Glands and muscles are called effectors because they act when they receive nerve impulses or hormones.

Each neuron has a cell body consisting of a nucleus surrounded by cytoplasm.
Dendrites are fibres which branch from the cell body to make contact with other neurons. A long filament of cytoplasm, surrounded by an insulating sheath, runs from the cell body of the neuron. This filament is called a nerve fibre[axon].
The axon is insulated by a fatty sheath [myelin sheath] with small uninsulated sections along it [nodes]. This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next

There are three main types of neurons: sensory, relay and motor.

Neurons never touch each other.
A synapse is a junction between two neurons.
When an impulse arrives at the synapse, vesicles in the cytoplasm are stimulated to release tiny amounts of the neurotransmitter molecules.
The molecules rapidly diffuse across the synaptic gap and bind with neurotransmitter receptor proteins in the membrane of the neuron on the other side of the synapse.
This then stimulates a new impulse in the neuron
The neurotransmitters are destroyed to prevent continued stimulation of the second neuron, which would cause repeated impulses to be sent.
Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions.
They slow down the speed of nerve impulses slightly because of the time taken for the chemical to diffuse across the synaptic gap.
As this is the only part of the nervous system where messages are chemical opposed to electrical, it is the only place where drugs can act to affect the nervous system – this is where heroin works

A reflex action is a rapid and automatic response to a stimulus
A stimulus is changing the external or internal environment of an organism.
An involuntary (or reflex)response does not involve the brain as the coordinator of the reaction.
This is an automatic and rapid response to a stimulus such as touching something sharp or hot.
As it does not involve the brain, the reflex response is quicker than any other type of nervous response.
This helps to minimise the damage to the body.

Reflex actions are:

1. Automatic

2. Fast

3. Protective

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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