Nervous tissue

Nervous tissue: Neuron and Neuroglia

• Nervous tissue contains densely packed nerve cell ( neuron),which are specialized for nerve impulse conduction.
• Origin- ectoderm
• Nervous tissue consists of

1. Neuron or nerve cell (functional unit of Nervous system)
2. Neuroglia (glial cell)

Neuron:

• About 100 billions of neurons are present in nervous system.
• They are Specialised type of cell, they vary in shape and size, all neurons contains three principle parts- cell body, dendrites and an axon

Cell body

• Has a large nucleus, which contain prominent nucleolus, as well as other several structures (Nissl bodies, ER,lysosome, mitochondria, neuroflament), responsible for metabolism, growth and repair of neuron
• Nissl bodies- made up of RNA, RER and free ribosome, help in protein synthesis
• Neurofilament and neurotubules are thread like protein, runs parallel to long process
• Neurofilament- semisolid structure that provide skeletal framework to axon
• Neurotubules- transport intracellular proteins between cell body and the processes

Dendrites-

• Many thread cytoplasmic extension arises from cell body called dendrites
• It conducts nerve impulse toward the cell body
• They are myelinated and have Nissl’s granule and neurofibril

Axon-

• Usually one of the cytoplasmic extension is long and unbranched called axon.
• It is covered by lipid sheath called myelin sheath
• Myelin sheath is formed by specialized non-neural cell called schwann cell (neurolemmocytes) in PNS and by Oligodendrocytes in CNS. The outer sheath of these cell is known as neurolemma
• It conduct nerve impulse away from cell body
• It lacks nissl’s granules

Types of neuron:

I. Types of neuron based on structure-

1. Unipolar- have single processes, very common sensory neuron in PNS,
2. Bipolar- two processes- a dendrires and an axon, eg. Retina, cochlea, smell receptor
3. Multi polar-many processes- many dendrites but one axon eg. Brain and spinal cord

II. Types of neuron based on function-

1. General somatic afferent (sensory)- carry sensory impulse from skin, skeletal muscles, joints and connective tissue to CNS
2. General visceral afferent- impulse from visceral organ to CNS
3. General somatic efferent(motor)- CNS to skeletal muscles
4. General visceral efferent- CNS to visceral organs
5. Special visceral efferent- brain to muscles of jaws, pharynx, facial expression, larynx
6. Special afferent- receptor cell (olfactory, optics, auditory, vestibule, gustation) to CNS

2. Neuroglia

• Glial cells are non conducting cells that protect and nurture as well as support cells of nervous tissue.
• There are 4 types of neuroglia cells

i) Astrocytes–

• largest,most numerous glial cell, with long star like processes, help form the blood –brain barrier.
• Function: structural support, transport of substance between blood vessels and neurons, mop up excess ions (k) and neurotransmitters.

ii) Oligodendrocytes-

• relatively small, with several branching processes,found in grey and white matter of CNS,
• function: produce myelin sheath

iii) Microglial cell–

• smallest glial cell, cuboidal or columnar shaped, it is a macrophage, engulf damaged neuron

iv) Ependymal cell-

• elongated cell, arranged in single layer in inner lining of spinal cord and ventricle of brain.