10. Animal Tissue - part 05 - Nervous Tissue

 


10. Animal Tissue - part 05 - Nervous Tissue


Nervous Tissue :
  • Nervous tissue is composed of nerve cells or neurons and neuroglia. 
  • Neuron is the structural and functional unit of nervous system. 
  • Neuroglial cell are non-nervous supporting cells that fill in the interneuronal space. 
  • The neuroglial cells are capable of regeneration and division but neurons are not capable of regeneration because of lack of centriole. 
  • Intercellular matrix is absent in the neural tissue. 
  • Neuron is an impulse generating and impulse conducting unit. They bring about quick communication within the body. 
  • Neurons change action potential of their membrane on receiving any external stimulus. This property of neuron is called excitability. 
  • They also carry a wave of electric impulse from dendron to axon, the processes of neuron. This is called conductivity.
  • A neuron is made up of cyton or cell body. It contains granular cytoplasm called neuroplasm and centrally placed nucleus.
  • Neuroplasm contains - 
  1. Mitochondria
  2. Golgi apparatus
  3. RER and 
  4. granules called Nissl’s granules. 
  • They are made up of RNA. 
  • Cell body gives out two types of processes namely - 
  1. dendron and 
  2. axon. 
Dendrons  :
  • are short, branched, processes. 
  • The fine branches of dendron are called dendrites. 
  • They carry impulse towards cyton.

Axon :
  •  is single, elongated, cylindrical process. 
  • Axon is bounded by axolemma. 
  • The protoplasm of the axon is axoplasm. 
  • It contains large number of mitochondria and neurofibrils.
  • Axon is enclosed in a fatty sheath called myelin sheath. 
  • Outer covering of myelin sheath is neurilemma. 
  • Myelin sheath and neurilemma are parts of another cell called Schwann's cell.
  • Schwann cell shows nucleus at periphery. 
  • The myelin sheath is absent at intervals along the axon and the place is called Node of Ranvier.
  • The terminal arborization of an axon is called telodendron.

Types of Neurons [Based on their functions] :
  1. Afferent Neuron 
  2. Efferent Neuron
  3. Interneuron or association neuron


1. Afferent Neuron : 
  • It carries impulses from sense organ to central nervous system (CNS). Hence it is also called sensory neuron. 
  • It is found in dorsal root of spinal cord.

2. Efferent Neuron : 
  • It carries impulses from CNS to effector organ. Hence it is also called motor neuron. 
  • It is found in ventral root of spinal cord.

3. Interneuron or association neuron : 
  • These are located between sensory and motor neurons. 
  • These perform processing, integration of sensory impulses and activate appropritate motor neuron to generate motor impulse.

Classification of Neurons [Depending on the presence or absence of myelin sheath]
  1. Myelinated nerve fibre
  2. Non-myelinated nerve fibre

Myelinated or medullated nerve fibres :
  • have a insulating fatty layer called myelin sheath around the axon. 
  • This makes the fibre appear white in colour. 
  • This sheath is secreted by Schwann cells. 
  • The sheath is not continuous. It is interrupted at nodes of Ranvier. 
  • Neurilemma surrounds the axon.
  • The impulse is conducted at a faster rate in such nerve fibres because it jumps from one node to the next. Such transmission of impulse is called saltatory conduction. 
  • Myelin sheath prevents the loss of the impulse during conduction. 
  • Cranial nerves of vertebrates are myelinated. 

Non-myelinated nerve fibre :
  • Schwann cell of a non-medullated nerve fibre does not secrete myelin sheath.
  • These fibres are grey in colour due to absence of fatty layer. 
  • Conduction of impulse in a nonmyelinated fibre is slower as compared to myelinated nerve fibre. 
  • Nerves of autonomous nervous system are non-myelinated. 
  • Functional contact between axonal ends and dendrites of adjacent neurons is called a synapse. 
Types oF Neurons [Based on the number of processes given out from cyton] :

  1. Unipolar/Monopolar Neuron
  2. Bipolar Neuron 
  3. Multipolar Neuron
1. Unipolar/Monopolar Neuron : 

  • It has a single process originating from cyton. 
  • Both axon and dendron arise from cyton at one point. 
  • They conduct impulses to central nervous system. 
  • Ex. Neurons of dorsal root ganglion of spinal nerve.

2. Bipolar Neuron : 

  • It has two processes. A single dendron and an axon are given off from opposite poles of the cyton. 
  • They bring about transmission of special senses like sight, smell, taste, hearing etc. 
  • Ex. Neurons of retina of eye, olfactory epithelium.

3. Multipolar Neuron : 

  • Cyton is star shaped and gives out more than two processes.
  • There is only one axon and remaining are dendrons. 
  • Axon initiates from a funnel shaped area called axon-hillock.

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