VPB 121: Veterinary Physiology - II (2+1)

NEURONS

• These are basic building blocks for communicating information using both chemical and electrical signals. Neurons are designed with specialized properties to receive process and transmit the information. Neurons like any cell have a cell membrane boundary, cellular organelles and extensions which perform conduction of information and reception.
• Cell membrane is electrically excitable. Neuronal shape and size vary depending upon the function. But they have common functions to change incoming information into electrical potential. This process is known as impulse generation.

Neuronal anatomy

• Soma (cell body) (perikaryon) responsive of metabolic maintenance of the cell.
• Processes to receive and conduct information. They are of two types
  • Dendrites
  • Axons
• Most of the neurons posses multiple dendrites and a single axon. These anatomical structures are arranged in specific zones and play a different role in neural signaling.

Neuronal soma 

• Body of the neuron consists a nucleus surrounded by cytoplasm.
• It contains neurofibrils,nissil bodies ( basophilic granules) or tigroid bodies, golgi apparatus and mitochondria. The nucleus is large, circular with large spherical nucleolus. Nucleolus has a ribonucleic acid (RNA) and no centrosome. Absence of this attributes to the failure of normal regeneration, it looses the cell division.

Neurofibrills

• Fine filamentous meshwork in cytoplasm connecting dendrites and axons.

Nissil granules

• Clumped basophilic granules in the cytoplasm concerned with the protein synthesis & have the ribonucleic proteins. These granules are not present in the areas of origin of axon. Their size, number and shape vary.      

Dendrites

• They are fine branching extensions of the neuron originate from cell body and serve as the receptive surface.
• These show heavy branching and are responsible to sense and gather incoming signals. As they receive,  are converted  into an electrical signal. This in turn is propagated as change in membrane potential.  
• The dendrite with extensive arborizatrion receives many inputs.

Axons

• This is otherwise known as nerve fibers. This is the long extension from soma to conduct information away from cell body. Nerves are general term referring bundles of axons running through the tissues. Axon has neurofibril and mitochondria. Axon arises from an area in the caudal soma known as axon hillock which is devoid of nissil granules. This area is said to be trigger zone where action potential are initiated. Axon terminates into numerous branches called TELODENDRIC or TERMINAL BUTTONS which allows the signals to pass on simultaneously to many structures. Telodendria contain vesicles to store synaptic transmitters.
• Both axon and dendrite are nourished by the cytoplasm of the cell. They transfer nutrients to the terminal axons. Cytoplasmic flow depicts ambeoid movement of the phagocytic cell. The flow in axon is popularly known as axoplasmic flow.