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DrX Whiz Niraj
Nervous System: Electrophysiology & CNS
Comprehensive exam notes on Organization of Nervous System, Action Potentials, Synapses, and the Anatomy of the Brain and Spinal Cord.
The Nervous System is the master controlling and communicating system of the body. It consists of two main divisions:
- Central Nervous System (CNS): Brain and Spinal Cord. It is the integration and command center.
- Peripheral Nervous System (PNS): Cranial nerves and Spinal nerves. It links the body to the CNS.
A. Neurons (Nerve Cells)
Neurons are the basic functional units. They possess electrical excitability. Parts include:
- Dendrites: Receive signals.
- Cell Body (Soma): Contains nucleus.
- Axon: Conducts impulses away.
B. Neuroglia (Glial Cells)
Supportive cells that nourish and protect neurons. They do not generate action potentials.
- CNS Glia: Astrocytes (Blood-Brain Barrier), Oligodendrocytes (Myelin sheath), Microglia (Phagocytosis), Ependymal cells (Produce CSF).
- PNS Glia: Schwann cells (Myelin sheath), Satellite cells.
Neurons communicate via electrical signals called Action Potentials (Nerve Impulses). The resting membrane potential of a neuron is -70 mV (inside is negative relative to outside).
Steps of Action Potential:
- 1. Depolarization: A stimulus opens Sodium (Na⁺) channels. Na⁺ rushes INTO the cell, making the inside positive (+30 mV).
- 2. Repolarization: Na⁺ channels close and Potassium (K⁺) channels open. K⁺ rushes OUT of the cell, restoring the negative resting potential.
- 3. Hyperpolarization: K⁺ channels are slow to close, causing the potential to drop slightly below -70 mV before returning to normal via the Na⁺/K⁺ pump.
A Synapse is the junction between two neurons or between a neuron and an effector (muscle/gland). Signals are transmitted across the synaptic cleft using chemical messengers called Neurotransmitters.
| Neurotransmitter | Type / Function | Clinical Significance |
|---|---|---|
| Acetylcholine (ACh) | Excitatory at NMJ, Inhibitory in heart. | Deficiency linked to Alzheimer's disease. |
| Dopamine | Emotional responses, addictive behaviors, movement. | Deficiency causes Parkinson's; Excess linked to Schizophrenia. |
| Serotonin (5-HT) | Mood, appetite, sleep induction. | Low levels linked to severe Depression. |
| GABA | Main Inhibitory neurotransmitter in CNS. | Target for anti-anxiety drugs (Benzodiazepines). |
Meninges (Protection)
Three connective tissue layers covering the brain and spinal cord:
- Dura Mater: Outermost, tough layer.
- Arachnoid Mater: Middle, spider-web like.
- Pia Mater: Innermost, delicate layer hugging the brain.
Note: Cerebrospinal fluid (CSF) flows in the Subarachnoid space (between arachnoid and pia mater).
Major Parts of the Brain
- Cerebrum: Largest part. Seat of intelligence, reasoning, and sensory perception.
- Cerebellum: Regulates posture, balance, and coordinates complex muscle movements.
- Brain Stem: Consists of Midbrain, Pons, and Medulla Oblongata (Controls vital functions like heart rate and breathing).
Spinal Cord & Reflex Activity
The spinal cord conducts sensory nerve impulses to the brain via Ascending (Afferent) tracts and motor impulses from the brain to muscles via Descending (Efferent) tracts.
Clinical Alert: Meningitis & CSF
Meningitis is a dangerous inflammation of the meninges, usually caused by bacterial or viral infection. To diagnose it, doctors perform a Lumbar Puncture (Spinal Tap). They insert a needle between the L3 and L4 vertebrae to collect a sample of Cerebrospinal Fluid (CSF) from the subarachnoid space without damaging the spinal cord (which ends at L2).