Coordination between various organs and systems of the body is crucial to homeostasis. The two systems that maintain homeostasis are the neural system and the endocrine system. The neural system is for quick coordination of nerve impulses. The endocrine system provides chemical integration through hormones. All animals have neurons, these cells can perceive, receive and transmit different kinds of stimuli. Neural organisation varies from lower invertebrates to vertebrates.
The human neural system is divided into two parts – the central neural system and the peripheral neural system. The brain and the spinal cord form the central neural system. It is the site of information-processing and control. The peripheral neural system comprises cranial and spinal nerves. It controls the voluntary functions of the body.
The nerve fibres of peripheral nerves are of two types – afferent and efferent nerve fibres. Afferent fibres conduct nerve impulses from the sense organs to the central nervous system. Efferent fibres conduct nerve impulses from the central nervous system to the involuntary organs.
The peripheral neural system is divided into the somatic neural system and the autonomic neural system. The somatic neural system relay impulses from the central neural system to the skeletal muscles. The autonomic neural system controls involuntary functions. It is further classified into the sympathetic neural system comprises of thoracic and lumbar regions and parasympathetic neural system comprises of the cranial and sacral neurons.
A neuron is a branched nerve cell and is the longest cell in the body consists of the cyton and nerve processes. Cyton has spherical nucleus, Nissl’s granules which help in protein synthesis, mitochondria present in it provide high energy for the conduction of impulses.
Nerve processes are of two types – dendrons and axons. Dendrons transmit impulses towards the cell body. An axon is always single, covered by plasma membrane and contains axoplasm.The distal end of an axon forms a synaptic knob which contains neurotransmitters. On the basis of the number of axons and dendrites, neurons are divided into three types – multipolar, bipolar and unipolar.
Axons are of two types – myelinated and non-myelinated. Myelinated nerve fibres are enclosed by Schwann cells. The gaps between two adjacent myelin sheaths are called the Nodes of Ranvier. These fibres are found in cranial and spinal nerves.
On the basis of the functions of neurons, they can be divided into three types –sensory, motor and association neurons. Sensory neurons are found in the sense organs, receive impulses from receptors and direct them towards the central nervous system. Motor neurons are found in the central nervous system, they carry impulses from the central nervous system to organs such as the muscles and glands. Association neurons, which are also called inter neurons or mixed neurons, are found in the central nervous system, for example, the dorsal horn of the spinal cord.