Neural Tissue — Revision Notes

⚡ 30-Second Revision

Neurons: — Functional units, transmit impulses. Parts: Dendrites (receive), Cell Body (metabolic center), Axon (transmit).
Neuroglia: — Support cells. CNS: Astrocytes (support, BBB), Oligodendrocytes (myelin), Microglia (phagocytosis), Ependymal (CSF). PNS: Schwann cells (myelin), Satellite cells (support).
Resting Potential: — $-70,\text{mV}$, inside negative. Maintained by $Na^+/K^+$ pump (3 $Na^+$ out, 2 $K^+$ in) and $K^+$ leak channels.
Action Potential: — All-or-none. Depolarization ($Na^+$ influx via voltage-gated $Na^+$ channels). Repolarization ($K^+$ efflux via voltage-gated $K^+$ channels). Hyperpolarization (slow $K^+$ channel closure).
Conduction: — Continuous (unmyelinated), Saltatory (myelinated, faster, jumps between Nodes of Ranvier).
Synapse: — Junction for signal transmission. Chemical: Action potential $ightarrow$ $Ca^{2+}$ influx $ightarrow$ Neurotransmitter release $ightarrow$ Receptor binding $ightarrow$ Postsynaptic potential (EPSP/IPSP).

2-Minute Revision

Neural tissue is the body's communication network, composed of neurons and glial cells. Neurons are specialized for transmitting electrical signals (action potentials) using their dendrites, cell body, and axon.

Glial cells provide essential support, insulation (myelin sheath by Schwann cells in PNS and oligodendrocytes in CNS), and protection. The resting membrane potential, typically $-70,\text{mV}$, is maintained by the $Na^+/K^+$ pump and differential ion permeability.

An action potential is an 'all-or-none' event involving rapid depolarization (voltage-gated $Na^+$ influx) followed by repolarization (voltage-gated $K^+$ efflux). Myelin dramatically speeds up conduction via saltatory conduction.

Synaptic transmission, primarily chemical, involves the release of neurotransmitters from the presynaptic terminal (triggered by $Ca^{2+}$ influx) that bind to receptors on the postsynaptic membrane, generating either excitatory (EPSP) or inhibitory (IPSP) potentials.

Understanding these cellular and molecular mechanisms is key for NEET.

5-Minute Revision

For a comprehensive revision of neural tissue for NEET, focus on these interconnected concepts. Start with the fundamental components: the neuron and its parts (dendrites, cell body with Nissl granules, axon, axon hillock, axon terminals).

Understand the functional significance of each part, especially how dendrites receive and axons transmit signals. Next, delve into the diverse roles of glial cells: astrocytes for structural support and the blood-brain barrier, oligodendrocytes (CNS) and Schwann cells (PNS) for myelination, microglia for immune defense, and ependymal cells for CSF production.

Crucially, master the mechanism of nerve impulse generation. This involves the resting membrane potential (negative inside, maintained by $Na^+/K^+$ pump and $K^+$ leak channels), the threshold potential, and the phases of the action potential: rapid depolarization due to voltage-gated $Na^+$ channel opening and $Na^+$ influx, followed by repolarization due to voltage-gated $K^+$ channel opening and $K^+$ efflux, and a brief hyperpolarization.

Remember the 'all-or-none' principle. Conduction velocity is enhanced by myelination, leading to saltatory conduction at the Nodes of Ranvier. Finally, understand synaptic transmission: the arrival of an action potential at the presynaptic terminal, $Ca^{2+}$ influx, neurotransmitter release into the synaptic cleft, binding to postsynaptic receptors, and the generation of either excitatory (EPSP) or inhibitory (IPSP) postsynaptic potentials.

Also, recall the different types of neurons based on structure (multipolar, bipolar, unipolar) and function (sensory, motor, interneuron). Practice drawing and labeling diagrams of neurons and synapses to reinforce structural and functional relationships.

For example, trace the path of a signal through a simple reflex arc, identifying each neuron type and synapse involved.

Prelims Revision Notes

I. Neural Tissue Components:

Neurons: — Structural & functional units. Excitable, conductive.

* Cell Body (Soma): Nucleus, Nissl granules (RER + ribosomes, protein synthesis), mitochondria. * Dendrites: Short, branched, receive signals, conduct towards soma. * Axon: Long, single, transmits signals away from soma. Originates from axon hillock (no Nissl granules). Ends in axon terminals with synaptic vesicles.

Neuroglia (Glial Cells): — Support cells, non-conductive.

* CNS Glia: * Astrocytes: Star-shaped, most abundant. Structural support, regulate chemical environment, blood-brain barrier. * Oligodendrocytes: Form myelin sheath in CNS (insulation, faster conduction). * Microglia: Phagocytic, immune defense. * Ependymal cells: Line ventricles, produce & circulate CSF. * PNS Glia: * Schwann cells: Form myelin sheath in PNS. * Satellite cells: Support neuron cell bodies in ganglia.

II. Nerve Impulse (Action Potential):

Resting Membrane Potential (RMP): — $-70,\text{mV}$ (inside negative).

* Maintained by: $Na^+/K^+$ pump (3 $Na^+$ out, 2 $K^+$ in) and greater permeability to $K^+$ (leak channels).

Action Potential (AP): — All-or-none, rapid, transient reversal of RMP.

* Threshold Potential: $-55,\text{mV}$ (approx.). * Depolarization: Stimulus $ightarrow$ membrane reaches threshold $ightarrow$ voltage-gated $Na^+$ channels open rapidly $ightarrow$ $Na^+$ influx $ightarrow$ inside becomes positive (up to $+30,\text{mV}$).

* Repolarization: Voltage-gated $Na^+$ channels inactivate $ightarrow$ voltage-gated $K^+$ channels open $ightarrow$ $K^+$ efflux $ightarrow$ inside becomes negative again. * Hyperpolarization (Undershoot): $K^+$ channels close slowly, brief period of more negative potential.

* Refractory Period: Prevents immediate re-firing, ensures unidirectional propagation.

Conduction:

* Continuous: Unmyelinated axons, slower, regeneration along entire length. * Saltatory: Myelinated axons, faster, impulse 'jumps' between Nodes of Ranvier (gaps in myelin).

III. Synaptic Transmission (Chemical Synapse):

Presynaptic Terminal: — Contains synaptic vesicles with neurotransmitters.
Synaptic Cleft: — Gap between neurons.
Postsynaptic Membrane: — Contains receptors.
Steps:

1. AP arrives at presynaptic terminal. 2. Voltage-gated $Ca^{2+}$ channels open $ightarrow$ $Ca^{2+}$ influx. 3. $Ca^{2+}$ triggers fusion of vesicles with presynaptic membrane $ightarrow$ Neurotransmitter release into cleft.

4. Neurotransmitter binds to receptors on postsynaptic membrane. 5. Ion channels open on postsynaptic membrane $ightarrow$ Postsynaptic Potential (PSP). * EPSP (Excitatory PSP): Depolarization, increases likelihood of AP.

* IPSP (Inhibitory PSP): Hyperpolarization, decreases likelihood of AP. 6. Neurotransmitter removed from cleft (enzymatic degradation, reuptake, diffusion).

IV. Neuron Classification:

Structural: — Multipolar (most common), Bipolar (retina), Unipolar (sensory ganglia).
Functional: — Sensory/Afferent (receptors to CNS), Motor/Efferent (CNS to effectors), Interneurons (within CNS, connect sensory & motor).

Vyyuha Quick Recall

Neuroglia Always Support Neurons Communicating All Potentials.

Neuroglia: Support cells (Astrocytes, Oligodendrocytes, Schwann cells, Microglia, Ependymal cells).
Always Support: Their primary role is support and protection.
Neurons: The main communicators.
Communicating: Transmit signals.
All Potentials: Resting Potential, Action Potential, Postsynaptic Potentials.