Neural Tissue

A neuron is a specialised cell that forms the structural and functional unit of the nervous system and is responsible for transmitting nerve impulses.

Neurolemma is the outermost thin protective sheath covering the myelin sheath of a nerve fibre.

Nodes of Ranvier are gaps present at intervals in the myelin sheath that help in faster transmission of nerve impulses.

Axon terminals are closely placed near the dendrites of another one or more neurons but are not connected. Such gaps in between are called synaptic clefts.

A stimulus is any sudden change in the internal or external environment that brings about a response in an organism.

A response is the reaction or change in an organism produced as a result of a stimulus.

A nerve impulse is an electrical disturbance that travels along a neuron in response to a stimulus.

Receptors are specialised sensory cells that receive stimuli and convert them into nerve impulses.

Effectors are muscles or glands that produce a response by contracting or secreting substances when stimulated by nerve impulses.

The structural and functional unit of nervous system is called neuron.

• Neuron is the structural & functional unit of the nervous system. Three parts — Cyton (cell body with nucleus, no centrosome so cannot divide), Dendrites (receive impulses toward cell body), Axon (transmits impulses away from cell body).
• Axon is covered by myelin sheath interrupted by Nodes of Ranvier. Ends in terminal bulbs that release neurotransmitters across a synaptic cleft to the next neuron's dendrite.
• Key properties — Excitability (perceive stimulus) | Conductivity (transmit excitation).
• Stimulus types — Threshold (minimum effective) | Subliminal (no effect) | Supraliminal (strong, same as threshold) | Summation effect (weak stimuli added together produce an impulse).
• All or None Law — impulse conducted fully or not at all. Refractory period — the nerve cannot respond to a second stimulus. Synaptic delay — 0.3–0.5 ms to cross a synapse. Synaptic fatigue — halt due to neurotransmitter exhaustion.
• Velocity is higher in long, thick, myelinated nerves; higher in homeotherms; faster in voluntary fibres than autonomic nerves.