Nervous Tissue | Biology for Grade 11 PDF Download

Introduction

Nervous tissues originate from ectoderm of embryo.

Special Properties

• The special properties of the cells of nervous tissues are excitability and conductivity.
• The cells of nervous tissues are specialized for receiving stimuli and transmitting message.

These tissue forms nervous system of the body and include the following parts:

Composition of Nervous Tissue

It is formed of four types of cells –

• Neurons
• Neuroglia
• Neurosecretory cells
• Ependymal cells

1. Neurons

• A neuron is a nerve cell with all its branches, Neuron is formed from neuroblast.
• It is structural and functional unit of nervous system.

Neuron is the longest cell of the body.

Structure

(a) Cyton  

(b) Nerve Processes

(a) Cyton

• Cyton is also called cell body or soma, its shape is variable.
• Its cytoplasm is granular called neuroplasm, within neuroplasm has a prominent spherical nucleus golgi bodies, endoplasmic reticulum lysosome, fat globules, Nissl’s granules and neurofibril is found.
• Nissl's granules are comparatively large and irregular masses of ribosomes and rough endoplasmic reticulum. It is believed that Nissl’s granules synthesize protein in the cell. Nissl's granules are made up of m-RNA, ER, Ribosomes and has affinity for basic dyes.
• A mature neuron cannot divide.

(b) Nerve Processes

The nerve processes are also called neurites. Nerve processes can be divided into two parts-

(i) Dendrites  

(ii) Axon

(i) Dendrites

Dendrites may be one or several. It is branched structure.

(ii) Axon

• It is single, long and cylindrical process whose main function is to conduct the nerve impulses away from the cyton, so, axon is efferent in nature.
• It is the longest nerve process of a neuron.

Note:

• Giant squid (loligo) has axon of about 1500 mm in diameter.
• Neurons with very long axon is called golgi type I.
• Axon possess only neuro-fibrils. (Nissls Granule, Golgi body, Ribosome, fat globules are absent). The part of cyton from where the axon arises is called axon hillock. Some axon also gives rise to side branches called collateral fibres.
• The plasmalemma of axon is called axolemma whereas cytoplasm is called axoplasm.
• The axon ends in a group of branched, the terminal arborization, ends of terminal arborization possess knob like structure called synaptic knob or synaptic buttons.

Synapse

• Synaptic knobs comes to lie very close to the dendrons of next neuron to form the synapses. There is a microscopic gap of about 200 Å called synaptic cleft.
• The nerve impulses are transmitted from axon to dendron with the help of chemical called neurotransmitters which is either acetylcholine or adrenalin (epinephrine) Acetylcholine or adrenaline is produced by the secretory vesicles of the synaptic knobs.

Types of Neurons

On the basis of number of dendron and axon, neurons are of 3 types:

• Unipolar neuron: The neuron having a single process the axon are called unipolar neuron. e.g.: Unipolar nervous system occurs in embryo. 
• Bipolar neuron: The neuron having one dendron and an axon at the opposite pole of the cell are known as bipolar neuron. e.g. Bipolar neuron occur in retina of eye, Olfactory epithelium, Organ of Corti, Taste bonds. 
• Multipolar neuron: The neuron which have many dendrons and one axon are termed as multipolar neuron. e.g. 

Multipolar neuron occur in nervous system of adults.

On the basis of functions, neurons are of 3 types:  

• Sensory or Afferent Neuron: They connect Sensory organs with central nervous system and brings sensory impulses into it.    
• Motor or Efferent Neuron: They connect central nervous system with the effectors. (muscles and glands) and carry motor impulses to them. 
• Interneurons or Adjustor Neuron: They are present in the central nervous system (Brain and spinal cord) and connect two or more neurons for distant transmission of impulses.

Nerve Fibres

An axon of a neuron is covered with one or two sheaths.

On the basis of presence or absence of sheaths on nerve fibre, nerve fibres are of two types 

• Medullated or Myelinated Nerve Fibre
• Non Medullary Nerve Fibre

Medullated or Myelinated Nerve Fibre

• In these nerve fibres around the nerve a sheath of fatty substance is formed which is termed as medullary sheath or myelin sheath.
• The medullary sheath is not continuous and point of absence of medullary sheath is called nodes of Ranvier. The part of medullated nerve fibre between two adjacent nodes is called an internode. Medullary sheath forms an insulating coat and prevents loss of energy during conduction of nerve impulse.
• The medullary sheath and node of ranvier are surrounded by a transparent cellular outer covering known as neurolemma of Schwann cell.
• Just beneath the neurolemma a thin layer of cytoplasm lies, which contains nuclei to form Schwann cells (Sheath cells) at intervals. These nuclei are termed nuclei of Schwann cell.
• Medullated nerve fibre are found in brain spinal cord, cranial and spinal nerves. In the central nervous system, medullated nerve fibres form white matter.

Non-Medullated (Non Myelinated) Nerve Fibre

• In these nerve fibre no medullary sheath is found. The axon is surrounded by neurilemma and just below neurilemma a layer of cytoplasm containing nuclei at intervals is present. The node of Ranvier and internode are not present.
• The non medullated nerve fibres exists in autonomic nervous system.
• In central nervous system non medullated nerve fibre are found in grey matter.

Functionally the nerve fibres are of two types:

• Afferent or Sensory Fibre: Afferent fibre carries the sensory impulse from the receptor organs to the central nervous system (brain and spinal cord).
• Efferent or Motor Fibre: Efferent fibre carries impulses from the central nervous system to the various effector organs (muscles and glands).

2. Neuroglia or Glial Cells

• These are non nervous cells which lie between the neurons of CNS, ganglia and retina of the eye.
• These are many times (10 times approx) more numerous than neurons.

Types of Neuroglial Cells

Neuroglia cells are of following types

• Microglia Cell: These are spindle shaped small cells.
• Astrocytes: These are highly branched.
• Oligodendrocytes: These have few branched processes which resemble dendrons of the neuron.

Functions

• These act as packing cells between  neurons.
• These provide nutrition to neurons
• These act as phagocyte and consume micro organism.
• These help in memory process
• These insulate the adjoining neurons.

3. Ependymal Cells

These cells form an epithelium called ependyma that lines the ventricles of brain and the central canal of the spinal cord. The cells are generally ciliated.

4. Neurosecretory Cells