In this article we will discuss about:- 1. Origin of Neural Tissue 2. Properties of Neural Tissue 3. Components 4. Functions.
Origin of Neural Tissue:
The neural tissue in general develops from the ectoderm of the embryo, but the microgliocytes (to be described ahead) arise from the mesoderm of the embryo.
Special Properties of Neural Tissue:
The special properties of the cells of the nervous tissue are, excitability and conductivity. Excitability is the ability to initiate nerve impulse in response to stimuli (changes outside and inside the body).
Conductivity means the ability to transmit a nerve impulse (potential change in membrane of a nerve cell). The reaction is called response. The response may be sensation, such as pain or some activity such as muscle contraction or glandular secretion.
Components of Neural Tissue:
1. Neurons:
Neurons are structural and functional units of neural system. Each typical neuron (multipolar) consists of the cell body called cyton or perikaryon or soma and nerve fibre (axon).
(i) Cyton (Cell Body):
The cyton contains cytoplasm (neuroplasm), prominent spherical nucleus, mitochondria, Golgi bodies, endoplasmic reticulum, ribosomes, lysosomes, fat globules, Nissl’s granules and neurofibrils. Nissl’s granules are comparatively large and irregular masses of ribosomes and rough endoplasmic reticulum. They probably synthesize proteins in the cell.
(ii) Processes of Neuron:
The processes of neurons are called neurites. The latter are of two types— dendrites (Dendron’s) and an axon or axis cylinder (neuraxon). Dendrites may be one to several but axon is always one. The dendrites are usually shorter and tapering processes. Axon is usually long process of uniform thickness. The part of cyton from where the axon arises is called axon hillock. Most sensitive part of neuron is axon hillock.
The axon ends in a group of branches, the terminal arborizations (axon endings or presynaptic knob). The latter end on other neuron, muscle fibres or gland cells. Certain axons also give rise to side branches, called collateral fibres.
The neuroplasm of axon contains abundant neurofibrils and mitochondria but Nissl’s granules, Golgi bodies, ribosomes and fat globules are absent. The plasma membrane and neuroplasm of axon are respectively called axolemma and axoplasm.
Synapse:
A synapse is a site of junction between terminal arborizations of axon of one neuron and the dendrites of another neuron. The fibres, however, do not meet, their cell membranes remain separated by a microscopic gap of about 200 A. Each neuron receives an impulse through its dendrites and passes it on to the next neuron through synapse.
A fresh impulse is set up in the dendrites at the synapse with the help of chemicals called neurotransmitters, such as acetylcholine produced by the secretory vesicles of the synaptic knobs. Acetylcholine is the first neurotransmitter to be discovered.
Synapse between an axon and a dendrite is called axodendritic synapse and when it is between an axon and a cell body it is known as axosomatic synapse. Mostly there are present axodendrite synapses.
Types of Neurons:
The neurons are classified on the basis of their structure and function.
(A) On the basis of the structure, the neurons are of five types (Fig. 7.36):
(i) Nonpolar or Un-polarized Neurons:
Each neuron bears several branched processes. There is no functional difference between dendrites and axon. Each process can bring an impulse to the cyton, or can take it away from the cyton. These neurons are rare in vertebrates but occur in cnidarians (= coelenterates) such as Hydra.
(ii) Unipolar Neurons:
Such neuron has a single process (projection), which arises from cyton. True unipolar neurons with an axon and no dendrite are found in early embryos of invertebrates and vertebrates.
(iii) Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into an axon and a dendrite. Such neurons are termed pseudo unipolar neurons. In adult vertebrates, the dorsal root ganglia of spinal nerves possess pseudo unipolar neurons.
(iv) Bipolar Neurons:
These neurons have only two processes, an axon at one end and a dendrite at another end. Bipolar neurons are found in the retina of eyes, olfactory epithelium and cochlear and vestibular ganglia (cochlea and vestibule are the parts of membranous labyrinth of internal ear).
(v) Multipolar Neurons:
These neurons have several dendrites and an axon. Motor neurons and interneurons are multipolar. They are the most common type of neurons. Multipolar neurons occur in the grey matter of the brain and spinal cord.
(B) On the basis of function, neurons are of three types:
(i) Sensory (= Receptor or Afferent) Neurons:
They connect sense organs with the central nervous system (brain and spinal cord). They bring sensory impulse from sense organs to the central nervous system.
(ii) Motor (= Effector or Efferent) Neurons:
They connect the central nervous system to the effectors (muscles and glands). They carry motor impulses from the central nervous system to the effectors.
(iii) Interneurons (= Connector, Relaying or Adjustor Neurons):
They are present in the central nervous system and occur between the sensory and motor neurons for distant transmission of impulses. They are neither sensory nor motor, but are meant for integrating and analysing the input of information and distributing it to other parts of nervous system.
Nerve Fibres:
Axon or dendrite of a nerve cell covered with one, two or three sheaths is called nerve fibre. Dendrites are surrounded only by one sheath. An axon may be surrounded by two or three sheaths.
(A) On the basis of structure, the nerve fibres are of two types:
Medullated or myelinated and non medullated or non-myelinated.
(i) Medullated (= Myelinated) Nerve fibres:
They consist of the following parts:
(a) Axis Cylinder:
It is simply the axon or dendrite of a nerve cell. Its neuroplasm (= axoplasm) contains longitudinal neurofibrils and mitochondria. The plasma membrane surrounding the axis cylinder is called axolemma. Axolemma conducts the nerve impulses.
(b) Medullary Sheath (= Myelin Sheath):
The medullary sheath is composed of substance called myelin. Myelin contains lipids, proteins and water. Thus myelin resembles the plasma membranes of a cell. The medullary sheath serves as an insulating layer, preventing loss of energy of the nerve impulse during its passage along the fibre.
It works much in the manner of the coating of an electric wire. The medullary sheath is continuous around the nerve fibres in the central nervous system but in the nerve fibres of the peripheral nerve fibres it is absent at certain points called the nodes of Ranvier. The part of a nerve fibre between two successive nodes of Ranvier is called inter node. Each segment of the myelin sheath is formed by one Schwann cell.
(c) Neurilemma (= Schwann cell Sheath):
Outside the myelin sheath there is a layer of Schwann cell cytoplasm which is called neurilemma. A single large and flat nucleus is present in the cytoplasm. The neurilemma is continuous over the nodes of Ranvier. As stated above Schwann’s cells produce myelin sheath.
A thin layer of connective (areolar) tissue, called the endoneurium, covers the neurilemma. The medullated nerve fibres of the brain and spinal cord lack neurilemma. Due to lack of neurilemma the nerve fibres of the brain and spinal cord do not regenerate after injury.
These fibres have an incomplete covering of neuroglia cells which produce the myelin sheath. Neurilemma present round the peripheral nerve fibres (nerve fibres of the cranial and spinal nerves) enables them to regenerate after injury.
Certain axons give out lateral branches, called collateral fibres. The latter arise from nodes of Ranvier at right angles. The medullated nerve fibres are found in the white matter of the brain and spinal cord and in the cranial and spinal nerves.
When an impulse travels along a medullated nerve fibre, it does not proceed uniformly along the length of the axis cylinder, but jumps from one node of Ranvier to the next. This is called the saltatory conduction of impulses. Thus, the conduction of impulses is faster in medullated nerve fibres.
(ii) Non-medullated (= Non-myelinated) Nerve Fibres:
A non-medullated nerve fibre consists of an axis cylinder surrounded by neurilemma and connective tissue. The nuclei cause swelling along the fibre. Under electron microscope, a trace of myelin is seen around the axon. The non-medullated nerve fibres form grey matter.
These fibres do not have the nodes of Ranvier and intemodes. They also lack collateral fibres. The non-medullated nerve fibres are found in the autonomic nerves. They conduct nerve impulses much slower than the medullated nerve fibres.
(B) On the basis of Function, the nerve fibres are again of two types:
(i) Afferent (= Sensory) Nerve Fibres:
They carry nerve impulses from the sense organs to the central nervous system (brain and spinal cord).
(ii) Efferent (= Motor) Nerve Fibres:
They carry nerve impulses from the central nervous system to the effector organs (muscles and glands).
Neuron Polarity:
Nerve fibres carry impulses in one direction only from dendrites to cyton and hence to the axon. Thus one end of a neuron receives an impulse and the other end discharges it. This property of neuron is called neuron polarity.
It can be explained with the examples of afferent (sensory) nerve fibres and efferent (motor) nerve fibres. The former carry nerve impulse from the sense organs to the central nervous system and the latter carry the nerve impulses from the central nervous system to the effector organs.
Grey and White Matter:
The nervous tissue which forms the brain and spinal cord is of two types: grey matter and white matter. The grey matter consists of cell bodies of neurons, their dendrites and proximal ends of their axons. Most of the fibres within the grey matter are non-medullated.
Grey matter looks grey in fresh state. The white matter looks white in fresh state which consists predominantly of medullated nerve fibres. Neuroglia (to be described ahead) and blood vessels are present in both grey and white matter.
The isolated masses of grey matter present anywhere in the central nervous system are called nuclei. In-fact, nuclei are groups of cell bodies of neurons. Aggregations of the cell bodies of neurons may also be found outside the central nervous system. Such aggregations are called ganglia. In general, the white matter carries impulses from one part of the body to another and the grey matter integrates the impulses.
Nerves:
A nerve consists of several bundles of nerve fibres called fasciculi. Each fasciculum (sing, of fasciculi) is surrounded by a layer of connective tissue called the perineurium. The perineurium is made up of flattened cells and collagen fibres (white fibres).
Each nerve fibre is surrounded by a layer of connective tissue called the endoneurium. The endoneurium contains collagen fibres, fibroblasts, Schwann cells, endothelial cells and macrophages.
A dense layer of connective tissue that surrounds the entire nerve is called the epineurium. The epineurium contains fat which cushions nerve fibres. Loss of this fat can lead to pressure on nerve fibres and paralysis.
Types of Nerves:
The nerves are of three types according to the nature of the fibres they are composed of:
(i) Sensory (= Afferent Nerves):
They contain sensory fibres. Olfactory, optic and auditory cranial nerves are sensory nerves.
(ii) Motor (= Efferent) Nerves:
They contain motor nerve fibres. Oculomotor, pathetic, abducens, spinal accessory and hypoglossal cranial nerves are motor nerves.
(iii) Mixed Nerves:
They contain both sensory and motor nerve fibres. Trigeminal, facial, glossopharyngeal and vagus cranial nerves are mixed nerves. Most of spinal nerves are mixed nerves.
2. Neuroglia (= or Neuroglial Cells or Glial Cells:
Neuroglia (neuro = nerve; glia = glue) or neuroglial cells are specialised cells found in the brain and spinal cord supporting the neurons and their fibres. About 50 percent of all brain cells are neuroglial cells. They may be divided into two major catagories: Macroglia (= largeglial cells) and microglia (= small glial cells).
(a) Macroglia (= Large glial cells):
They are formed from the ectoderm of the embryo and are of two types: astrocytes and oligocytes.
(i) Astrocytes (astro = Star; cyte = cell):
These cells are star shaped that give off a number of processes. They may be subdivided into fibrous and ptotoplasmic astrocytes.
Fibrous Astrocytes:
These are mainly seen in white matter of CNS. Their processes are thin and are asymmetrical.
Protoplasmic Astrocytes:
These are mainly seen in grey matter of CNS. Their processes are thicker than those of fibrous astrocytes and are symmetrical. The astrocytes are thought to separate and provide mechanical support to the neurons.
They also insulate adjacent neurons so that impulses pass from one neuron to the next only over the synapse where packing cells are missing. Astrocytes communicate С with one another through calcium channels. Astrocytes play a role in maintenance of the blood brain barrier. They are also responsible for repair of damaged areas of nervous tissue.
(ii) Oligodendrocytes (Oligo = few; dendro = tree):
These cells have rounded nucleus. The cytoplasm is rich in mitochondria, microtubules and glycogen. They have fewer and shorter cell processes. They occur in two distinct areas, near the medullated nerve fibres and near the surfaces of the somata (pi. of soma) of neurons.
Oligodendrocytes form myelin sheaths around the axons that lie with the central nervous system (brain and spinal cord).
(b) Microglia (= Microglial cells or Small glial cells):
Microglia (micro- small; glia = glue) or microglial cells are formed from the mesoderm of the embryo. They are probably derived from the monocytes that invade the brain during foetal life. These are the smallest neuroglial cells.
They are more numerous in grey matter than in white matter. Microglial cells have short and fine processes. They engulf and destroy microbes and cellular debris. Thus they are phagocytic and also act as scavengers. They may migrate to area of injured nervous tissue and function as small macrophages.
3. Ependymal Cells (= Ependyma):
These cells are arranged as an epithelial layer, one cell thick, which lines the ventricles (cavities of the brain) and the central canal of the spinal cord. The cells vary from squamous to columnar according to their location. Their free surface bears numerous microvilli and cilia.
The microvilli help in the absorption of cerebrospinal fluid. The movements of the cilia contribute the flow of the cerebrospinal fluid. The ependymal cells possess one or more long processes towards opposite side which penetrate the nervous tissue.
4. Neurosecretory Cells:
These specialized nerve cells function as endocrine organs. They release chemicals from their axons into the blood instead of synaptic cleft. Neurosecretory cells of the hypothalamus of the vertebrate brain secrete neurohormones (= releasing factors Fig. 7.42).
These neurohormones are carried from the hypothalamus to the anterior lobe of the pituitary gland where they regulate the secretion of pituitary hormones such as ACTH, TSH, GH, LH, FSH and prolactin.
Functions of Neural Tissue:
The neural tissue is meant for reception, interpretation and transmission of information. The sensory cells receive stimuli. The sensory neurons carry sensory nerve impulses to the intemeurons which send motor impulse through motor neurons to the effectors (muscles and glands). The neural tissue co-ordinates and integrates the activities of various body parts. This tissue is also seat of experiences, memories, etc..