Nervous System: Study Notes

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Nervous System

Nervous tissue consists of

1. Neuron
2. Neuroglia

NEURON

• Neurons are highly specialized cells responsible for the conduction of impulses.
• Neurons communicate using both electrical and chemical forms of communication.
• Electrical communication occurs via ion exchange and the generation of membrane potentials down the length of the axon.
• Chemical communication occurs via neurotransmitter release from the presynaptic cell and the binding of these neurotransmitters to the postsynaptic cell.

Cell Body

The cell body contains the nucleus, cytoplasm, cell surface membrane, and organelles. The cell body of a motor neuron is irregular in shape.

Dendron

The nerve fibers that transmit nerve impulses towards the cell body are called dendrons. A motor neuron has many dendrons. The end branches of dendrons are called dendrites. The dendrites of a dendron in a motor neuron receive nerve impulses from other neurons.

Axon

The nerve fibers that transmit nerve impulses away from the cell body are called axons. Axons in motor neurons are usually long.

Myelin Sheath

The layer of fatty substances enclosing many nerve fibers is called the myelin sheath. It insulates the axon, similar to how a rubber sheath insulates an electricity-conducting wire.

Node of Ranvier

The regions where the myelin sheath is absent are called nodes of Ranvier. Nerve impulses cannot be transmitted through the myelin sheath, so they 'jump' from one node to the next. This process helps speed up the transmission of impulses along the nerve fiber.

Axon Terminals

The axon terminals of a motor neuron transmit nerve impulses to the effector, such as a muscle or gland.

Motor End Plate

The junction between the axon terminal and the muscle fiber is called the motor end plate. Nerve impulses are transmitted across the motor end plate by chemicals that stimulate the muscles.

Myelin

• Many axons are coated in myelin, an insulating substance that prevents signal loss.
• Myelin is created by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.
• Myelin prevents dissipation of the neural impulse and crossing of neural impulses from adjacent neurons.

Types of Nephron

On the Basis of Structure:

Anaxonic neuron: Anaxonic neurons have more than two processes, but axons cannot be distinguished from dendrites.

Bipolar neuron: Bipolar neurons have two processes separated by the cell body.

Pseudounipolar neuron: Pseudounipolar neurons have a single elongate process with the cell body situated to one side.

Multipolar neuron: Multipolar neurons have more than two processes; there is a single axon and multiple dendrites.

On the Basis of Function:

Sensory neurone: (Receptor neurone) Transmits nerve impulses from the sense organs or receptors to the central nervous system.

Relay neurone: (Intermediate neurone) Transmits nerve impulses from the sensory neurone to the motor neurone. They are found within the central nervous system.

Motor neurone: (Effector neurone) Transmits nerve impulses from the central nervous system to the effectors (e.g., muscles or glands).

NEUROGLIA

Neuroglia or glial cells are other cells within the nervous system in addition to neurons.

• Astrocytes nourish neurons and form the blood–brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue.
• Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.
• Microglia are phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.
• Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons.

NERVE IMPULSE

The electrochemical signal developed by a neuron is called nerve impulse.

It has following 4 phases:

Resting Membrane Potential (RMP):

• Neurons exhibit a resting membrane potential of approximately –70 mV.
• Maintained by selective ion permeability and the Na+/K+ ATPase pump (3 Na+ out, 2 K+ in).

Action Potential:

• Begins with depolarization, increasing membrane voltage beyond threshold (~ -55 mV).
• Na+ channels open, allowing sodium ions to rush in, increasing membrane potential to +40 mV.

Repolarization:

• Na+ channels self-inactivate.
• K+ channels open, allowing potassium ions to exit the cell, decreasing the membrane potential.

Undershoot:

• K+ channels stay open longer, causing the membrane potential to dip below resting potential.
• Eventually, K+ channels close, restoring RMP.

Signal Transmission:

• Depolarization spreads to neighboring patches, triggering their own action potentials.
• The action potential travels in one direction due to the refractory period.

Refractory Period:

• Inactivated Na+ channels prevent the action potential from traveling backward.
• Ensures unidirectional transmission of the action potential.

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