STRUCTURE, FUNCTION & CLASSIFICATION
OBJECTIVES
By the end of the lecture, you should be able to:
Define nerve/neuron.
Draw, label and identify different parts of a neuron.
Classify neurons on the basis of function and structure.
Differentiate b/w afferent & efferent neurons.
Explain the process of myelination
Nervous System
What makes up the brain, the spinal cord or your peripheral nerves?
THE NEURONS
What is a Neuron?
It is the basic structural and functional unit of the nervous system.
It is a highly differentiated and specialized excitable tissue.
•The Human NS contains 100 billion neurons.
(Nerve cell and neuron can be used interchangeably.)
FUNCTIONS OF THE NEURON:
•Reception of the stimulus
•Generation of the nerve Impulse
•Transmission of the nerve Impulse
Layers surrounding the nerve fibers
•Endoneurium: Fine reticular tissue lying just next to neurilemma. It surrounds individual fibers separating them from each other.
It also forms the Endoneurial tube.
•Perineurium: Several nerves surrounded by layer of connective tissue.
•Epineurium: Nerve trunk itself surrounded by a loose layer of elastic tissue and CT.
Structure of A Typical Neuron
A typical neuron has the following parts:
1.Somaor Cell body
2.Dendrites
3.Axons(with the axon terminals)
SOMA (Cell body)
•Different shapes
–Fusiform, stellate, oval, rounded, pyramidal.
•Different sizes
–5 to 135 micrometers
•Nucleus
-typically large
–one nucleolus (usually)
•Cytoplasmwhich has:
–Nisslbodies
–Neurofibrils
•All organelles
–mitochondria, ribosomes, endoplasmic reticulum, lysosomes & Golgi apparatus
Soma (Cell Body)
•Nissl bodies
–also known asNissl substance,
–Scattered granules closely related to ER with rosettes of free ribosomes.
–Synthesize proteins.
–Stained with basic dyes.
–Tigroid substance (due to striped appearance)
–Are NOT present in the axon.
–If nerve is damaged, they dissolve & disappear through the process of chromatolysis(deprivation of oxygen, injury, trauma, poisoning etc)
•Neurofibrils
–Formed by clumping of neurotubules & neurofilaments.
–Delicate threads running from cytoplasm of the nerve cell body into the axon.
Functions:
1.Neuronal microtubules transport substances from the cell body to the distal cell processes.
2.Neurofibrils give support and shape to the neuron.
Point to remember:
Nerve cell body is the most vital part-
if it is damaged the entire neuron dies!
Dendrites
•Short, tree-like, highly branched tapering processesof the nerve cell.
•Receive incoming signals and then carry these signals to the cell body.
•Small knob-like projections called dendritic spines may be seen.
•Have all the components of the cell body
•Considered part of the neuronal receptor membrane
Axon
•Also called AXIS CYLINDER or NERVE FIBER.
•It is the longest process of the neuron.
•A single axon arises from a cone-shaped area of the neuronal cell body called the axon hillock.
•Axon hillock & first 100 μm of axon (no myelin sheath) is called Initial segment.
•Trigger zone: is the name given to the axon hillock & initial segment. It is an area showing high excitability as it has the max. no. of voltage-gated sodium channels.
Axon
AXON IS MADE UP OF:
•Jelly-like semi fluid substance called Axoplasm.
•Plasma membrane called Axolemma.
•Mitochondria and ER.
•NO Nissl granules so Does NOT synthesize proteins.
AXON ENDS IN:
Terminal Buttons (Synaptic knob or Bouton Terminaux)
–Axon break up into no. of terminal branches called Telodendria or Terminal filaments.
–At their end is a small swelling called Terminal knob.
–These knobs contain granules or vesicles with neurotransmitter substance.
VIVA/ MCQ QUESTIONS:
WHAT ARE AFFERENT AND EFFERENT FIBERS?
•Afferent
–When nerve fiber carries impulses from the periphery towards the CNS, it is called an Afferent nerve fiber.
•Efferent
–When the nerve fiber carries impulses from the CNS to the periphery, it is called an Efferent nerve fiber.
What is Anterograde and Retrograde flow?
•Anterograde flow
–Flow of axoplasm from the soma/ cell body to the axon.
–Cell body continuously synthesizes new material which is carried to the axon!
–It usually occurs along the neurotubules.
–Energy is provided by ATP.
–400 mm/day to 0.5 mm/day.
–Enzymes for neurotransmitter (NT) synthesis, Ca, synthesized NT
•Retrograde flow
–Occurs in the reverse direction.
–From the axon terminals
(peripheral regions) to the soma.
–Proteins, NGF, neurotropic proteins, herpes virus and other viruses as Polio virus, rabies, even some used up synaptic vesicles for recycling.
Axonal Transport in Neurons
There are 2 main types of classifications of neurons based on the structure and function.
CLASSIFICATION OF NEURONS
ANATOMICAL CLASSIFICATION
This classification is based on structure.
Depending on the number of processes, the neuron is divided into:
1.Unipolar
2.Bi-polar
3.Multi-polar
PHYSIOLOGICAL CLASSIFICATION
This classification is based on function of the neurons.
It is also called FUNCTIONAL CLASSIFICATION.
On the basis of the function, the neurons are divided into:
1.Motor neurons
2.Sensory neurons
3.Interneurons
Myelin is an electrically insulating material that forms a layer, the myelin sheath, usually around only the axon of a neuron.
It is essential for the proper functioning of the nervous system. It is formed by either a Schwann cells or anoligodendrocytes, both of which are types of glial cells.
The process of formation of the myelin sheath is called myelination.
MYELINATION
HOW DOES THE PROCESS OF MYELINATION TAKE PLACE?
Myelination is carried out by myelin forming cells that wrap themselves around the axons in jelly-roll fashion.
These myelin-forming cells are:
•Schwann cells in the PNS (peripheral nervous system) and,
•Oligodendrocytesin the CNS (brain & the spinal cord).
Outside the CNS: Schwann cells
•Myelination is not part of the neuron but is done by the schwann cells in the peripheral nervous system.
•As the diagram shows, the nerve cell invaginates the schwann cell.
•The schwann cell wraps around the axon in concentric spirals.
•Collectively, the various layers form the myelin sheath (a patch of myelin might be made of up to 300 layers of wrapped lipid bilayers)
Nodes of Ranvier
•In myelinated nerve fiber, the myelin sheath is not a continuous sheath, but is deficient at regular intervals.
•Between the myelinated regions, at the NODES OF RANVIER, the axonal membrane is bare and exposed to the ECF.
•It is here that a maximum conc. of voltage gated Sodium channels are present.
Inside the CNS: OLIGODENDROCYTES
Myelination
