Nervous System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The nervous system is the body’s rapid communication network, enabling organisms to respond to environmental changes and coordinate body functions. It works alongside the endocrine system (which uses hormones for slower, longer-lasting responses). The fundamental unit is the neurone (nerve cell), which transmits electrical impulses called action potentials.

Neurone Structure:

A neurone consists of:

• Cell body (soma): Contains the nucleus and most organelles; biosynthetic centre
• Dendrites: Multiple short, branching extensions; receive incoming signals from other neurones
• Axon: Single long extension; carries impulses away from the cell body; may be myelinated
• Axon terminals (synaptic boutons): End of axon; contain neurotransmitters for passing signals to next neurone

Types of Neurones:

Type	Function	Structural Feature
Sensory (afferent)	Transmit sensory input from receptors to CNS	Pseudounipolar (cell body to one side)
Motor (efferent)	Transmit commands from CNS to effectors (muscles/glands)	Multipolar (one axon, many dendrites)
Relay/Interneurone	Connect sensory to motor neurones within CNS	Multipolar

The Nerve Impulse — Action Potential:

At rest, the neurone membrane is polarised (~-70 mV, negative inside). This resting potential is maintained by the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and K⁺ leak channels.

When stimulated sufficiently (reaches threshold: ~-55 mV):

1. Depolarisation: Na⁺ channels open (voltage-gated); Na⁺ rushes IN; membrane potential rises to ~+30 mV
2. Repolarisation: Na⁺ channels close; K⁺ channels open; K⁺ rushes OUT; membrane potential falls back
3. Hyperpolarisation: K⁺ channels remain open slightly longer; membrane potential briefly more negative than rest (~-90 mV)
4. Resting potential restored by Na⁺/K⁺ pump

The action potential is all-or-none: if threshold is reached, it fires at full strength; below threshold = no response. Frequency of firing encodes signal intensity.

Saltatory Conduction: In myelinated axons, the myelin sheath (from oligodendrocytes in CNS, Schwann cells in PNS) allows the impulse to “jump” between Nodes of Ranvier — this speeds up conduction from ~1 m/s (unmyelinated) to ~100 m/s (myelinated).

⚡ Exam Tip (MDCAT): The refractory period (absolute and relative) is crucial — during the absolute refractory period (when Na⁺ channels are inactivated), no new impulse can be generated regardless of stimulus strength. This prevents the impulse from travelling backwards and ensures one-way conduction along the axon.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of neural communication.

The Synapse:

The synapse is the junction between two neurones (or between a neurone and an effector). The presynaptic neurone releases neurotransmitter; the postsynaptic neurone has receptor sites for it.

Synaptic Transmission Steps:

1. Action potential arrives at axon terminal → depolarisation
2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters terminal
3. Ca²⁺ triggers vesicle fusion with presynaptic membrane → neurotransmitter released via exocytosis
4. Neurotransmitter diffuses across synaptic cleft (20–40 nm wide)
5. Binds to receptors on postsynaptic membrane
6. Postsynaptic potential generated (EPSP or IPSP — see below)
7. Neurotransmitter broken down or reabsorbed

Excitatory vs Inhibitory Postsynaptic Potentials (EPSPs/IPSPs):

• EPSP: Neurotransmitter opens Na⁺ or Ca²⁺ channels → Na⁺ enters → depolarisation → makes firing more likely. Example neurotransmitter: glutamate
• IPSP: Neurotransmitter opens K⁺ or Cl⁻ channels → hyperpolarisation → makes firing less likely. Example neurotransmitter: GABA (major inhibitory neurotransmitter in CNS)

Summation:

• Temporal summation: Multiple impulses from the same presynaptic neurone arrive in rapid succession
• Spatial summation: Simultaneous EPSPs from multiple presynaptic neurones add up
• One EPSP is usually sub-threshold; summation of several EPSPs is needed to reach threshold and generate an action potential

Neurotransmitters:

Neurotransmitter	Primary Effect	Location/Notes
Acetylcholine (ACh)	Excitatory (muscles), varied (brain)	Neuromuscular junction; Alzheimer’s disease involves ACh loss
Noradrenaline (norepinephrine)	Excitatory/inhibitory depending on receptor	Sympathetic nervous system; mood regulation
Dopamine	Inhibitory/excitatory	Reward pathway; Parkinson’s (dopamine deficiency)
Serotonin	Generally inhibitory	Mood, sleep, appetite; SSRI antidepressants
GABA	Major inhibitory in CNS	Reduces neuronal excitability
Glutamate	Major excitatory in CNS	Involved in learning and memory; overexcitement → neurotoxicity
Endorphins	Inhibitory (pain relief)	Natural painkillers; opium-like effects

CNS vs PNS:

• CNS (Central Nervous System): Brain + spinal cord; processing and integration centre
• PNS (Peripheral Nervous System): All nerves outside CNS; subdivided into:
  • Somatic (voluntary control of skeletal muscles)
  • Autonomic (involuntary; further divided into sympathetic and parasympathetic)

Autonomic Nervous System:

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1–L2)	Craniosacral (CN III, VII, IX, X; S2–S4)
Neurotransmitter at effector	Noradrenaline (usually)	Acetylcholine (always)
“Fight or Flight” effects	Increases heart rate, BP; dilates pupils; inhibits digestion	Decreases heart rate, BP; constricts pupils; promotes digestion
Bronchi	Dilates	Constricts
Bladder	Relaxes	Contracts

⚡ Common MDCAT Error: Students confuse the roles of sympathetic and parasympathetic systems. Use the mnemonic “S for Speed and Stress” for sympathetic (fight or flight) and “P for Peace and Rest” for parasympathetic. Remember: the vagus nerve (CN X) is the major parasympathetic nerve of the body.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

The Spinal Cord and Reflex Arcs:

The spinal cord extends from the foramen magnum to L1–L2 vertebra. It contains:

• Grey matter (butterfly shape — neuronal cell bodies, dendrites, synapses) in the centre
• White matter (myelinated axons in tracts) on the outside

A Reflex Arc: The simplest neural circuit for a rapid, automatic response to a stimulus — does not require conscious brain involvement.

Components: Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector (muscle/gland)

Example: Patellar reflex (knee-jerk):

1. Hammer taps patellar ligament → stretch of quadriceps muscle
2. Muscle spindles (receptors) detect stretch → sensory neurone fires
3. Sensory neurone synapses directly with motor neurone in spinal cord (monosynaptic reflex)
4. Motor neurone fires → quadriceps contracts → leg extends
5. Simultaneously, inhibitory interneurone inhibits hamstring (antagonist) → no opposition

This is a monosynaptic reflex — only one synapse between sensory and motor neurone. Most reflexes are polysynaptic (involve at least one interneurone).

The Brain — Major Regions:

Region	Key Functions
Cerebrum (cerebral cortex)	Higher functions: thinking, planning, voluntary movement; divided into frontal, parietal, temporal, occipital lobes
Cerebellum	Coordination of movement, balance, motor learning
Thalamus	Relay station for sensory information (except smell) to cortex
Hypothalamus	Homeostatic control: hunger, thirst, temperature regulation; links nervous and endocrine systems
Brainstem	Vital functions: breathing, heart rate, consciousness; contains medulla, pons, midbrain
Hippocampus	Formation of new memories; spatial navigation
Amygdala	Emotional processing, particularly fear

Long-Term Potentiation (LTP) — The Neural Basis of Learning:

LTP is a long-lasting increase in synaptic strength following high-frequency stimulation. Key mechanism:

• glutamate release → postsynaptic NMDA receptors (a type of glutamate receptor) activated
• NMDA receptors allow Ca²⁺ entry (requires both glutamate binding AND depolarisation to remove Mg²⁺ block)
• Ca²⁺ influx triggers: (1) insertion of more AMPA receptors into postsynaptic membrane, (2) activation of kinases that strengthen the synapse
• Result: stronger, more responsive synapse — the cellular basis of memory formation

Neurodegenerative Diseases:

• Alzheimer’s disease: Amyloid plaques and neurofibrillary tangles (tau protein); acetylcholine deficiency; memory loss
• Parkinson’s disease: Death of dopaminergic neurones in substantia nigra (basal ganglia); dopamine deficiency; tremors, rigidity
• Multiple sclerosis: Autoimmune demyelination of CNS axons; impaired saltatory conduction

⚡ MDCAT Exam Pattern: Questions frequently test action potential phases, synapse transmission steps, the difference between EPSP/IPSP, autonomic nervous system effects, and reflex arc components. A common MDCAT question: identifying which part of the brain controls a specific function, or tracing the pathway of a reflex. The sympathetic system always uses noradrenaline at the effector (except sweat glands, which use ACh), while the parasympathetic system always uses acetylcholine.

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