Nervous tissue:
It forms nervous system. It is responsible for control / coordination of body processes. It consists of nerve cells or neurons, neuroglia or glial cells, ependymal cells and neurosecretory cells. It is ectodermal in origin exception is microglial cells (mesodermal). Two important basic features of nerve cell are excitability and conductivity.
Nerve cell or neuron:
Neuron is unit of nervous system. It is the longest cell in body. It consists of two parts which are cell body or cyton or soma and cell processes or cytoplasmic extensions.
Cell body or Cyton or soma contains centrally placed nucleus, mitochondria, Golgi bodies, endoplasmic reticulum and neurofibrils. Cytoplasm also contains prominent granules called as Nissl’s granules which are rich in RNA and are concerned with protein synthesis. Nissl’s granules are absent in axon.
Cell processes arises from cell body and are of two types which are dendrons and axon. Dendrons are numerous, small, wide and short processes that break up into numerous fine branches which carry impulses towards the cell body. Thus, they are afferent in nature. Axon also called as nerve fiber is single long and unbranched process. No Nissl’s granules are found in it. It carries impulses away from cell body. Thus, It is efferent or motor in nature. At terminal end forms branches called terminal aborization and each branch has swollen end called as synaptic knob or terminal buttons or axon terminalis which secrets neurosecretory substances like acetylcholine, adrenaline etc. It remains near to dendrites of another neuron with microscopic gap called syneptic cleft where nerve impulse is transmitted by neurochemicals or neurotransmitters like acetylcholine or adrenaline. Axon remains covered by axolemma and its cytoplasm is called as axoplasm which contains neurofibril and mitochondria. The part of cell body from where axon arises is called axon hillock.
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| Neuron |
Mature nerve cell has no centriole. Thus,
it cannot divide.
Neurosecretory cells: Special type of nerve cells which are secretary in nature. They are present in hypothalamus. They produce hormone releasing factors which passes into blood and by hypophysial portal vein reaches into pituitary and stimulates pituitary to produce hormones.
Ependymal cells: They are present as lining in the cavities of brain and spinal cord. The epithelium (cuboidal-ciliated) is called ependyma.
Neuroglia or Glial cells: The non-nervous cells present in between neurons of CNS, ganglia and retina of eye are called as glial cells/ neuroglia. They are about 10 times more than nerve cells. They provide support and are responsible for protection. They are of two main type microglial cells or microglyocytes and macroglial cells or macroglyocytes.
The microglyocytes are small sized and spindle shaped cells. They are phagocytic in nature. The Macroglyocytes are large sized and they are of two types; Astrocytes and Oligodendrocytes.
Astrocytes: They are large sized with a number of branched processes and are star shaped. They form blood brain barrier.
Oligodendrocytes: They are with large sized cell body with few branched processes and looks like spikey ball. The most important function of oligodendrocytes is to form myelin, insulating sheath around nerve fibres, axon in central nervous system A oligodendrocytes form myelenaation around upto 50 nerve fibres. Mylenation in peripheral nervous system is done by Schwann cells. A Schwann cell can wrap around one axon.
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| Glial cells |
Functions of glial cells:
-They are packing cells and help to keep nerve cells in their position.
-They provide nutrients to nerve cells.
-They insulate adjoining neurons and prevent lateral transmission of nerve impulses in CNS.
-The Phagocytic cells engulf microorganisms.
-They are capable of division so are responsible for repair wear/tear in nervous system.
- They help in memory process.
Types of nerve fibre:
·
On the
basis of structure: on the basis of presence or absence of myelin sheath
nerve fibres are of two types;
# medullated or myellinated: are fibres with myelin (phospholipid) sheath. It is white coloured. They are found in white matter of brain and spinal cord, cranial and spinal nerves. The unmyelinated constrictions or gaps in axon are called nodes of Ranvier. Myelin forms insulating layer and helps in rapid conduction of nerve impulse; Saltatory conduction.
# Non-medullated or non-myelinnated: are without myelin sheath. They are found in grey matter of brain and spinal cord. It is grey coloured. Nodes of Ranvier are absent. They help in integration of nerve impulse.
· On the basis of function: on the basis of function nerve fibres may be of following types:
# Sensory neuron: Carry information from sense organs upto central nervous system. They are also called as afferent nerve.
#Association neuron: they interconnect sensory and motor neuron in brain and spinal cord.
#Motor neuron: Carry information from brain and spinal cord upto effector parts ( muscles and glands). They are also called efferent nerve.
Types of neuron on the basis on number of nerve processes: Neuron may be of following types on the basis of number of nerve processes;
# Unipolar:- Neuron with only one nerve process called as neurite extending from cell body. The process serves both functions, receive and send information. Are found in dorsal root ganglia of spinal cord of adult vertebrates.
# pseudounipolar:- Neuron with one process which immediately divides to form one dendrite and one axon. Are also found in dorsal root ganglia.
# Bipolar:- neuron has single dendrite and axon at opposite poles of the cell body. Are found in retina, inner ear and olfactory epithelium.
# Multipolar:- Neoron has several dendrites and one axon. Most neurons in CNS are of this type.
*Arrangement of neuron: Neuron may lie end to end forming chain. Dendrites of one neuron remain very close to axon terminalis of another neuron with microscopic gap called synaptic cleft/synapse where nerve impulses are transmitted by neurochemicals or neurotransmitters like acetyle choline or adrenaline.
Neurons may get aggregated to form mass in brain and spinal cord called as nucleus while outside CNS is called ganglion.
* Nerve: Each nerve fibre ( axon) is surrounded by fine fibrous connective tissue called endoneurium. Parrel bundles of nerve fibres are called fasciculi which remain covered by perineurium. Bundle of fasciculi covered by epineurium form a nerve.