AIPMT-NEET Biology Aspirants, read out the next AIPMT-NEET Biology Study material/ Notes of Neuron. In this we will learn about Neuron structure and Myelination important for AIPMT-NEET Biology. Free online notes for AIPMT-NEET.
Nervous system has two types of cells: Neurons and Neuroglia. Neurons are structural and functional unit of Nervous system (Don’t get it confused with Nephron which is the functional unit of kidney), while neuroglia support, nourish and protect neurons.
Neuron has 3 main parts: 1) Cell body, 2) Dendrites and 3) Axon.
CELL BODY: Also known as Perikaryon or soma.
- Cell body contains nucleus surrounded by cytoplasm which has cell organelles like lysosomes, mitochondria and golgi complex.
- Clusters of rough endoplasmic reticulum in cell body appear like granular bodies are termed as Nissl’s granules or Nissl bodies.
- Neurofibrils in cell body help to maintain its shape and also provide support.
DENDRITES: These are short fibres projecting out of the cell body.
- Appear like short trees as they branch repeatedly. These are short, tapered and highly branched.
- These are receiving or input portions of neurons. They take signals from other cells and transmit these impulses towards cell body.
- Dendrites also contain Nissl’s Body.
AXON: Axon is a long, thin, cylindrical projection or fiber, which joins cell body at one side and other side of which is branched.
- Axon joins cell body at Axon-hillock, which is a cone shaped elevation.
- Axon has mitochondria, neurofibrils but no rough E.R. so no protein synthesis in it.
- Cytoplasm of axon is called as Axoplasm which is surrounded by plasma membrane of axon called as Axolemma.
- Axon Collateral: Side Branches from axon are called as axon collaterals.
- Axon terminals: Axon and axon collaterals end by dividing into fine processes called as axon terminals.
- Tips of axon terminals swell into bulb-like structures called as Synaptic end bulb or Synaptic knob.
- Synaptic knob contains many tiny membrane-enclosed sacs called as Synaptic vesicles, which stores chemical called as Neurotransmitter. Neurotransmitter is a chemical released from synaptic vesicle which excites or inhibits another neuron.
- Axon transmits nerve impulses away from the cell body to synapse or to neuro-muscular junction.
Note: Synapse is the junction between two neurons through which nerve impulse gets transmitted from one neuron to another. It is the site of communication between two neurons.
CLASSIFICATION OF NEURONS
On the basis of no. of processes (axon and dendrites) extending from the cell body, neurons are of 3 types: Multipolar neurons , Bipolar neurons and Unipolar neurons.
- Multipolar neurons have 1 axon and many dendrites extending from the cell body. Mostly found in brain (cerebral cortex) and spinal cord. Mostly neurons are multipolar neurons in body.
Direction of flow of information in multipolar neurons:
Dendrites to Cell body to Axon to Axon terminals
- Bipolar neurons have 1 axon and 1 dendrite extending from the cell body. Found in retina of eye, inner ear and olfactory region of brain.
- Unipolar neurons have only one process extending from the cell body which divides into 1 axon and dendrites. Usually found in embryonic stage.
Myelinated and Unmyelinated neurons
Myelin sheath : It is a lipid and protein multilayer covering found around some axons. It helps in 2 ways: 1) it electrically insulates axon of neurons, and 2) it increases the speed of nerve impulse conduction.
On the basis of myelin sheath, there are 2 types of axons: Myelinated axons and Unmyelinated axons:
- Axons with covering of myelin sheath are called as Myelinated axons and neuron called as Myelinated neuron/nerve fiber. These are found in spinal and cranial nerves.
- Axons without covering of myelin sheath are called as Unmyelinated axons and neuron called as Unmyelinated neuron/nerve fiber. These are found in Autonomic nervous system and somatic nervous system.
Myelinated nerve fibers
Process of myelin sheath formation around axons is called as myelination. Two types of cells are responsible for myelination: Schwann cells (in the PNS) and Oligodendrocytes (in the CNS). Both Schwann cells and oligodendrocytes are neuroglia.
Myelinated nerve fibers are enveloped with Schwann cells (SEE Figure: Structure of Neuron). Schwann cells start forming myelin sheath around axon during fetal development. Myelin sheath form around axon spirally in multiple layer and the outermost layer is called as neurolemma, which encloses schwann cell’s nucleus and cytoplasm. Neurolemma help in axon regeneration during injury in PNS only.
Gap between two adjacent myelin sheaths which appear at intervals along the axon is called as Nodes of Ranvier. (SEE Figure: Structure of Neuron)
Unmyelinated nerve fibres
These fibres are also enclosed by a one layer of Schwann cell but that does not form a myelin sheath around the axon.
Amount of myelin around axon increases from birth to maturity which increases the speed of nerve impulse conduction. Infants still have myelination going on, so their response to stimuli is slow and less coordinated than adults.
NEUROGLIA : Neuroglia (glia= glue) are smaller than neurons but outnumber neurons. Neuroglia help to protect, support and nourish neurons. They do not help in nerve impulse conduction. Role of neuroglia comes at the time of injury. They fill the injured spaces by multiplying themselves and replacing neurons.
- There are 6 types of neuroglia present, 4 found in CNS and 2 in PNS: Astrocytes, Oligodendrocytes, Ependymal cells, Microglia (in CNS) and Schwann cells, Satellite cells (in PNS).
- Oligodendrocytes and Schwann cells help in myelin sheath formation.
- Microglia function as phagocytes.
OTHER IMPORTANT TERMS
Nerve – Bundle of axons located in PNS.
Tract –Bundle of axons located in CNS.
Ganglia /Ganglion – Cluster of neuronal cell bodies located in PNS. Appears like swelling or knot.
Nucleus – Cluster of neuronal cell bodies located in CNS.
White matter – Present in brain and spinal cord. Composed of Myelinated axons. Myelin gives white color to it.
Gray matter – Present in brain and spinal cord. Composed of cell bodies, neuroglia, dendrites, unmyelinated axons. Nissl bodies give gray appearance to it.