Neural System — Revision Notes

⚡ 30-Second Revision

Neuron: — Functional unit. Cell body, dendrites (receive), axon (transmit).
Glial Cells: — Support neurons (myelin, nutrients).
Resting Potential (RMP): — $\approx -70,\text{mV}$. Maintained by $\text{Na}^+/\text{K}^+$ pump ($3,\text{Na}^+$ out, $2,\text{K}^+$ in) and $\text{K}^+$ leak channels.
Action Potential: — All-or-none.

- Depolarization: $\text{Na}^+$ influx (voltage-gated $\text{Na}^+$ channels open). - Repolarization: $\text{K}^+$ efflux (voltage-gated $\text{K}^+$ channels open, $\text{Na}^+$ channels inactivate). - Hyperpolarization: Brief undershoot (slow $\text{K}^+$ channel closure).

Conduction: — Saltatory (myelinated, faster) vs. Continuous (unmyelinated).
Synapse: — Junction for signal transfer.

- Chemical: Neurotransmitters, synaptic cleft, $\text{Ca}^{2+}$ influx triggers release. - Electrical: Gap junctions, direct ion flow, faster.

Neurotransmitters: — Chemical messengers (e.g., Acetylcholine, GABA, Norepinephrine).
CNS: — Brain + Spinal Cord (processing).
PNS: — Nerves outside CNS (relay).

- Somatic: Voluntary (skeletal muscle). - Autonomic: Involuntary (viscera). - Sympathetic: Fight/Flight ($\uparrow$ HR, dilate pupils, $\downarrow$ digestion). - Parasympathetic: Rest/Digest ($\downarrow$ HR, constrict pupils, $\uparrow$ digestion).

Reflex Arc: — Receptor $\rightarrow$ Afferent $\rightarrow$ Interneuron $\rightarrow$ Efferent $\rightarrow$ Effector.

2-Minute Revision

The neural system is your body's rapid communication network, built from neurons and glial cells. Neurons transmit electrochemical signals called action potentials. At rest, a neuron maintains a negative charge inside (resting membrane potential, $\approx -70,\text{mV}$) due to the $\text{Na}^+/\text{K}^+$ pump and $\text{K}^+$ leak channels.

When stimulated, it depolarizes ($\text{Na}^+$ influx), then repolarizes ($\text{K}^+$ efflux), and briefly hyperpolarizes before returning to rest. This impulse travels along the axon, faster in myelinated axons (saltatory conduction).

Signals are passed between neurons at synapses. Chemical synapses use neurotransmitters, released via $\text{Ca}^{2+}$ influx, to bind to receptors on the postsynaptic neuron. The system is divided into the Central Neural System (CNS - brain, spinal cord) for processing, and the Peripheral Neural System (PNS - nerves) for relay.

The PNS includes the Somatic (voluntary) and Autonomic (involuntary) systems. The Autonomic system has sympathetic (fight/flight) and parasympathetic (rest/digest) branches, which often have opposing effects.

Reflex arcs provide rapid, involuntary responses.

5-Minute Revision

The neural system, the body's master control and communication network, is composed of neurons and supporting glial cells. Neurons are specialized for transmitting electrical signals. Each neuron has dendrites (receive signals), a cell body (integrates signals), and an axon (transmits signals). The axon is often insulated by a myelin sheath, which speeds up impulse conduction via saltatory conduction.

Nerve impulses, or action potentials, are generated when a neuron's membrane potential reaches a threshold. The resting membrane potential (RMP) of about $-70,\text{mV}$ is maintained by the $\text{Na}^+/\text{K}^+$ pump (pumps $3,\text{Na}^+$ out, $2,\text{K}^+$ in) and differential membrane permeability to ions.

An action potential involves three phases: depolarization (rapid $\text{Na}^+$ influx through voltage-gated $\text{Na}^+$ channels, making the inside positive), repolarization ($\text{K}^+$ efflux through voltage-gated $\text{K}^+$ channels, restoring negative potential), and a brief hyperpolarization (undershoot due to slow $\text{K}^+$ channel closure).

Signals are transmitted between neurons at synapses. Chemical synapses are most common: an action potential at the presynaptic terminal opens voltage-gated $\text{Ca}^{2+}$ channels, leading to $\text{Ca}^{2+}$ influx, which triggers the release of neurotransmitters into the synaptic cleft.

These neurotransmitters bind to receptors on the postsynaptic membrane, causing either excitation (EPSP) or inhibition (IPSP). Electrical synapses involve direct ion flow through gap junctions, offering faster but less modifiable transmission.

The neural system is organized into the Central Neural System (CNS), comprising the brain and spinal cord, which is the command and integration center. The Peripheral Neural System (PNS) consists of all nerves outside the CNS, relaying information.

The PNS is further divided into the Somatic Neural System (controls voluntary skeletal muscles) and the Autonomic Neural System (ANS) (controls involuntary visceral functions). The ANS has two antagonistic branches: the Sympathetic Neural System (prepares for 'fight or flight' – e.

g., increases heart rate, dilates pupils) and the Parasympathetic Neural System (promotes 'rest and digest' – e.g., decreases heart rate, constricts pupils). Rapid, involuntary responses are mediated by reflex arcs, involving a receptor, afferent neuron, interneuron, efferent neuron, and effector.

Prelims Revision Notes

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Neuron Structure: — Cell body (soma), dendrites (receive signals), axon (transmit signals). Axon hillock is where action potential initiates. Axon terminals release neurotransmitters. Myelin sheath (Schwann cells in PNS, oligodendrocytes in CNS) insulates axon, increasing speed. Nodes of Ranvier are gaps in myelin.
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Glial Cells: — Support neurons. Astrocytes (CNS, blood-brain barrier), Oligodendrocytes (CNS myelin), Schwann cells (PNS myelin), Microglia (immune), Ependymal cells (CSF production).
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Resting Membrane Potential (RMP): — $\approx -70,\text{mV}$. Maintained by: (a) $\text{Na}^+/\text{K}^+$ pump (active, $3,\text{Na}^+$ out, $2,\text{K}^+$ in), (b) more $\text{K}^+$ leak channels than $\text{Na}^+$ leak channels, (c) large intracellular anions.
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Action Potential (Nerve Impulse): — All-or-none event.

* Threshold Stimulus: Required to initiate (typically $\approx -55,\text{mV}$). * Depolarization: Voltage-gated $\text{Na}^+$ channels open, rapid $\text{Na}^+$ influx, membrane potential becomes positive (e.

g., $+30,\text{mV}$). * Repolarization: Voltage-gated $\text{Na}^+$ channels inactivate, voltage-gated $\text{K}^+$ channels open, rapid $\text{K}^+$ efflux, membrane potential becomes negative again.

* Hyperpolarization (Undershoot): Slow closure of $\text{K}^+$ channels leads to brief potential more negative than RMP.

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Conduction:

* Continuous: In unmyelinated axons, slower. * Saltatory: In myelinated axons, impulse 'jumps' between Nodes of Ranvier, faster and energy-efficient.

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Synaptic Transmission:

* Chemical Synapse: Most common. Action potential $\rightarrow$ voltage-gated $\text{Ca}^{2+}$ channels open $\rightarrow$ $\text{Ca}^{2+}$ influx $\rightarrow$ neurotransmitter release (exocytosis) into synaptic cleft $\rightarrow$ neurotransmitter binds to postsynaptic receptors $\rightarrow$ ion channels open $\rightarrow$ Postsynaptic Potential (EPSP or IPSP).

Neurotransmitters removed by degradation/reuptake. * Electrical Synapse: Gap junctions, direct ion flow, faster, bidirectional, less common.

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Neurotransmitters: — Acetylcholine (ACh), Norepinephrine (NE), Dopamine, Serotonin, GABA (inhibitory), Glutamate (excitatory).
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Divisions of Neural System:

* CNS: Brain (forebrain, midbrain, hindbrain) + Spinal Cord. * PNS: Nerves extending from CNS. * Somatic Neural System (SNS): Voluntary control of skeletal muscles. * Autonomic Neural System (ANS): Involuntary control of viscera. * Sympathetic: 'Fight or Flight' (e.g., $\uparrow$ HR, dilate pupils, $\downarrow$ digestion). * Parasympathetic: 'Rest and Digest' (e.g., $\downarrow$ HR, constrict pupils, $\uparrow$ digestion).

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Reflex Arc: — Receptor $\rightarrow$ Afferent neuron $\rightarrow$ (Interneuron) $\rightarrow$ Efferent neuron $\rightarrow$ Effector. Rapid, involuntary response.

Vyyuha Quick Recall

To remember the sequence of action potential phases: Don't Really Hate Resting. Depolarization (Na+ in) Repolarization (K+ out) Hyperpolarization (K+ slow close) Resting state (Na+/K+ pump)