Nerve Fibres and Nerves

The long axons of neurons along with the associated structures are called the nerve fibres. The fibres may be enclosed within sheaths called as myelin sheath.

These are called the myelinated fibres. Those that are not covered by the myelin sheath are called the non-myelinated fibres.

However, the action potential is not generated in the areas where there is a sheath over the fibre. Along the fibres there are regions where the myelin sheath is absent. These regions are called the nodes of Ranvier. The action potential jumps from one node to the other.

Many nerve fibres are bunched together to form a nerve. The bundles of fibres are enclosed within connective tissue called the epineurium.

Types of nerves

Nerves are of three types based on the types of neurons they carry. They are:

Sensory nerves or the receptor nerves

They are made up of only sensory neurons. For e.g., the cranial nerves that conduct impulses from the organs to the central nervous system.

Motor nerves or the effector nerves

They are made up of only motor neurons. For e.g., the cranial nerves that conduct impulses from the central nervous system to the muscles and glands.

Mixed nerves

The nerves that are made up of both sensory and motor neurons.

For e.g., all spinal nerves.

Receptors and Effectors

Receptors

are structures at the ends of the nerve fibres that collect the information to be conducted by the nerves. These receptors may be specialised sense organs like the Meissner's corpuscles of the skin, specialised nerve endings like the Pacinian corpuscles of skin or the specialised organs, the sense organs.

There are certain receptors that are simple nerve terminals without any specializations. These are called free nerve endings and are found in the skin.

The receptors receive the stimuli and convert them to electrical impulses. These are then transmitted by the neurons and the nerves as nerve impulses.

Different sensations like temperature, pressure, touch, light, sound, pain, osmoconcentration, stretch and chemical substances are received by different types of receptors.

The information collected by the receptors is conducted by the nerve fibres to the central nervous system.

This in turn is conducted by the nerves to the effectorsThe nerves to the effectors conduct the message from the brain.

Effectors

Effectors are muscles or glands which work in response to the stimulus received from the motor nerves.