27. Nervous tissue. Histogenesis, General characteristics, Classification.

Nerve tissue is made of 2 types of cells; neurons and glial cells and has ectodermal origin. Its main function is to sense stimuli and transmit signals to and from different parts of the body.

The nervous system is divided into the central nervous system (CNS) which consists of the brain and spinal cord, and the peripheral nervous system which consists of the nerves outside of the CNS which connect the brain and spinal cord to the organs and muscles of the body. 

All nerve fibres (axons and dendrites) is split into the automatic and somatic nervous system.                                    

The CNS is made up of grey matter and white matter. It has unmyelinated neurones and glial cells, and myelinated nerve fibres which dominate the white matter, whereas the grey matter consists mainly of neuron bodies. 

The CNS has a rich vascular supply which is more abundant in grey matter. 

A nerve is made up of many neurons bound together by connective tissue. A sheath of dense connective tissue called the epineurium surrounds each nerve. The epineurium penetrates the nerve to form the perineurium which surrounds bundles of nerve fibres. Blood vessels can be seen in the epineurium. 

The endoneurium, consists of a thin layer of loose connective tissue, surrounds the individual nerve fibres.

Neurons are the main cells of the nerve tissue and derive from ectoblast. They collect stimuli and transform them into a nervous impulse by synthesising and releasing neurotransmitters. 

A neuron consists of 3 parts; a body, axon and dendrites. 

Nerve impulses are transmitted in one direction, i.e. into the cell body through the dendrites and away from the cell body through the axon. 

The cell body is enclosed by a plasma membrane and has a central nucleus. 

A prominent Golgi complex is found next to the nucleus and is used to package secretory products and produce lysosomes. 

Nissl bodies are found in the cytoplasm of the cell body which functions to synthesise proteins. 

The axon is surrounded by myelin sheath. 

In the peripheral NS the myelin sheath is made by the Scwann cells and in the CNS it is made by the oligodendrocytes. 

The myelin sheath is interrupted at intervals by the nodes of Ranvier. This increases the speed of electrical impulses as it “jumps” from node to node. As a result the speed of transmission is increased in myelinated nerve fibres. This is known as saltatory conduction.

Nerve tissue is classified according to the axon’s length, according to the shape of the body, according to the number of processes and according to their function. 

According to the axons length, nerve tissue can be golgi type 1 containing a long axon and golgi type 2 containing a shorter axon. 

According to the shape of the body of the neutrons they are star-shaped, spindle-shaped, pear-shaped and pyramidal-shaped. 

According to the number of processes it contains they are mulitpolar neurons, bipolar neurons and unipolar neurons.

According to their function there are three types of neurons: sensory, relay and motor neurons.

Sensory neurons conduct nerve impulses from the receptors to the central nervous system. 

Relay neurons, relay impulses from the sensory neuron to the motor neuron. 

Motor neuron, transmits the impulse from the central nervous system to an effector, resulting in a response. 

The central types of neuroglial cells are astrocytes, oligodendrocytes, ependyma and microglia. Neurons and macroglia originate from ectoblast, whilst microglia originate from mesenchyme.

They provide mechanical support to neurons. They are non-conducting and act as insulators between the neurons and prevent neuronal impulses from spreading in unwanted directions. 

They can remove the foreign material and cell debris by phagocytosis. They can repair damaged areas of nervous tissue by proliferation, gliosis.

Glial cells can store neurotransmitters.

The oligodendrocytes produce myelin sheath in the central nervous system. 

Ependymal cells secrete cerebral spinal fluid (CSF), they line the ventricles of the brain and central canal of the spinal cord.