Neural Tissue

All living organisms are made of cells. A unicellular organism has a single cell in its body, i.e., a single cell performs all basic life activities. However, in multicellular organisms, there are millions of cells. Most of these cells are specialized to carry out only a few functions efficiently. These functions are taken up by a distinct group of cells. Thus, it can be said that there is a division of labor in multicellular organisms.

A tissue is a group of cells that are similar in structure, origin, and function. The term ’tissue’ was given by Bichat. The branch of science dealing with the tissues is called ‘histology’. Mayer introduced the term histology. Malpighi is considered the “founder of histology. Broadly animal tissues are classified into four types: epithelial, connective, muscular, and neural.

Neural Tissue

• Origin. Neural tissue develops from the ectoderm of the embryo. Except for microglia which develop from mesoderm.
• Special properties. The special properties of the cells of the neural tissues are excitability and conductivity. The cells of neural tissue are specialized for receiving stimuli and transmitting messages. Neural tissue consists of neurons and neuroglia.

Neurons (Nerve cells)

Structural and functional units of neural tissue are called neurons. A neuron is differentiated into two parts: cyton and neurites.

1. Cyton (Cell body, Soma, Perikaryon): The cyton contains cytoplasm (neuroplasm), nucleus mitochondria, Golgi bodies, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, etc. Besides these, the presence of Nissl granules and neurofibrils is characteristic of all neurons. Nissl granules are irregular masses of ribosomes and rough endoplasmic reticulum. They probably synthesize proteins in the cell. Neurons lack a centrosome.
2. Neurites: The processes of neurons are called neurites. The latter are of two types: dendrons or dendrites and an axon or axis cylinder. The part of cyton from where the axon arises is called the axon hillock. The axon ends in a group of branches, the terminal arborizations (axon endings). Certain axons also give rise to side branches, called collateral fibers. The neuroplasm of the axon contains abundant neurofibrils and mitochondria, but Nissl granules, Golgi bodies, ribosomes, and fat globules are absent. 

Synapse

The region of the union of the terminal arborization of the axon of one neuron with the dendrites of another neuron is called a synapse. Impulses are transferred from one neuron to the next neuron through synapses.

Types of Neurons

On the basis of the structure, the neurons are of three types:  

1. Unipolar neurons: They occur in the neural system of embryos.
2. Bipolar neurons: They occur in the retina of the eye.
3. Multipolar neurons: They occur in the neural system of adults.

Neurons can be classified according to their functions as follows:

1. Sensory or afferent Neurons
2. Motor or efferent Neurons
3. Interneurons or Adjustor neurons. They are present in the central neural system (brain and spinal cord).

Nerve Fibres

On the basis of structure, the nerve fibers are of two types:

1. Medullated (myelinated) nerve fibers: These are found in the brain, spinal cord, and cranial and spinal nerves. They have myelin sheath and nodes of Ranvier and Schwann cells. In the central neural system, these fibers form white matter. Saltatory conduction occurs in medullated nerve fibers.
2. Non-medullated (nonmyelinated) nerve fibers: These are found in the autonomic neural system. In the central neural system, these fibers create grey matter.

Functionally, the nerve fibers are again of two types:

1. Afferent or sensory fibers
2. Efferent or motor fibers

Nerve impulses pass between neurons through the synapse with the help of chemicals which are called neurotransmitters such as acetylcholine.

A nerve is a bundle of numerous nerve fibers which are separated into several small bundles, termed fasciculi. The latter are bound together by vascular connective tissue, the epineurium. The epineurium also creates a sheath around the nerve itself. Each fascicule is surrounded by another connective tissue called the perineurium. Inside the fascicule, its fibers are bound together by another connective tissue called the endoneurium.

Neuroglia (or neuroglial cells) 

These are specialized cells found in the brain and spinal cord, supporting the body. They are divided into two major categories: Macroglia (= large glial cells) and microglia (= the neurons and their fibers. Neuroglia is more than one-half the volume of neural tissue in human small glial cells). Macroglia are of two types:

1. Astrocytes. There are two types of astrocytes: Fibrous astrocytes are found chiefly in white matter, and Protoplasmic astrocytes occur chiefly in grey matter. The astrocytes provide the repair mechanism and replace lost tissue.
2. Oligodendrocytes. They are found in close association with large nerve cells, and in rows between bundles of fibers in the white matter. They play a metabolic role in the formation and preservation of the myelin sheath of the nerve fibers in the central neural system.

Microglia are found diffusely through both grey and white matter. Microglia are mesodermal in origin. These cells are the scavengers of the neural system and become phagocytic in case of injury and death of the other elements.  Ependymal cells and neurosecretory are also part of neural tissue.

Ependymal Cells

These cells form a cellular membrane that lines the ventricles (cavities of the brain) and the central canal of the spinal cord. Their free surface bears numerous microvilli and cilia. The microvilli help in the absorption of cerebrospinal fluid. The movements of cilia contribute to the flow of cerebrospinal fluid.

Neurosecretory Cells (Neurosecretory neurons)

These occur in the neural system. The hypothalamic neuro-secretory cells are present in the hypothalamus (part of the brain). They release neurohormones (releasing factors) into the bloodstream. These hormones are carried to the anterior lobe of the pituitary gland where they regulate the secretion of hormones such as ACTH, TSH, GH, LH, FSH, and prolactin.

Nervous Tissue Location

• The nerve tissue is located in the Peripheral nervous system (PNS) and also the Central nervous system (CNS) formed by the spinal cord and the brain.
• It is responsible for controlling and performing various body activities.
• The nervous tissue includes the nerve cells or the neurons. 

Characteristics of Nervous Tissue

• It is composed of various nerve cells.
• The brain is a vital organ.
• The transmission of signals is across neurotransmitters.
• Nervous tissue comprises the CNS and PNS of the nervous system.
• The contains dendrites, cell body, axons, and nerve endings.
• It contains two different cells—neurons and glial cells.
• Nerve cells live long, and cannot be split and replaced (except memory cells).
• The presence of specialization at axonal terminals is called synapsis.

Functions Of Nervous Tissue

• Carries messages from other neurons to the cell body.
• Nervous tissue responds to stimuli.
• It stimulates the contraction of muscles and plays a crucial role in emotions, memory, and seasoning.
• It carries out communication and integration.
• Provides electrical insulations to nerve cells and removes debris.
• Neurons create and carry out nerve impulses. These are responsible for the production of electrical signals that are transmitted across distances, they do so by secreting chemical neurotransmitters.

Types of Nerves

Nerves can be classified into four types based on their functionality. These are motor nerves, sensory nerves, autonomic nerves, and cranial nerves.

Motor Nerves

• Motor nerves are responsible to send signals from the spinal cord and brain to all the muscles of the body.
• The signals allow humans to perform activities such as talking, walking, drinking water, blinking eyes, sitting, etc.
• Any injury to the motor nerve can lead to the weakening or shrinking of the muscles.

Sensory Nerves

• The sensory nerves are responsible to generate signals in conflicting directions from another type of nerve known as motor nerves.
• These nerves gather information such as pressure, pain, temperature, etc from the sensors that are located in the muscles, skin, and other internal organs which move it back to the brain and spinal cord.
• Its injury causes numbness, pain, tingling, sensation, and hypersensitivity.

Autonomic Nerves

• These nerves control the actions of the muscles of the heart.
• The autonomic nerves manage the functions that are not under control, that is involuntary.

Cranial Nerves

There are 12 pairs of cranial nerves that appear from the lower side of the brain. Cranial nerves help to make facial expressions, movement of eye, and process smells. Hereunder, is the list of cranial nerves from front to back and their major functions:

1. Olfactory – a sense of smell
2. Optic – vision
3. Oculomotor – Eye movement; pupillary constriction and accommodation
4. Trochlear – Eye movements (intorsion, downward gaze)
5. Trigeminal – Somatic sensation from face, mouth, cornea
6. Abducens – Eye movements(abduction or lateral movement).
7. Facial – Controls the muscles of facial expression, etc.
8. Vestibulocochlear – Hearing, sense of balance
9. Glossopharyngeal – salivary gland
10. Vagus – muscles of the vocal cord.
11. Spinal accessory – shoulder and neck muscles
12. Hypoglossal nerves – the movement of the tongue.

FAQs on Neural Tissue

Question 1: What is a neuron?

Answer:

Nervous tissue contains highly specialized unit cells called nerve cells or neurons.

Question 2: What is the function of dendrites and axons?

Answer:

The dendrites receive impulses and the axon takes impulses away from the cell body.

Question 3: What does a neuron look like?

Answer:

Neurons or nerve cells are a unit of nervous tissue which serves to transmit messages in our body. It consists of a cell body with a nucleus and cytoplasm, from which long thin hair-like parts arise. Usually, each neuron has a single long part called the axon, and many short branched processes(parts) called dendrites.

Question 4: Do neurons carry impulses toward the brain?

Answer:

Sensory neurons carry impulses toward the brain.

Question 5: What is a synapse?

Answer:

The region of the union of the terminal arborization of the axon of one neuron with the dendrites of another neuron is called a synapse.

Question 6: What are CNS and PNS?

Answer:

The Central Nervous System consists of the brain and spinal cord. The Peripheral Nervous System includes all peripheral nerves.

Question 7: What is the function of Cranial nerves?

Answer:

They play a major role in vision, hearing, and movement of muscles.