VCE Psychology (Year 11) · Unit 1 — The Brain, Behaviour and Mental Processes · 12m read

Nervous System — Structure

Ever wonder how your brain tells your muscles to move in a split second? Your nervous system is like your body's communication highway, split into two main parts: your central nervous system (the brain and spinal cord, which are the command centre) and your peripheral nervous system (the nerves branching out to the rest of your body). Understanding how these parts work together is crucial for VCE biology and explains everything from why you jump at a loud noise to how you learn and remember.

Learning objective: Describe the structure and divisions of the nervous system.

By the end of this lesson you can:

• I distinguish CNS and PNS.
• I describe somatic vs autonomic divisions.
• I outline neuron structure and function.

The nervous system is a communications network. The CNS is headquarters; the PNS is the cabling that carries messages to and from every part of the body.

Central and peripheral nervous systems

TWO MAIN DIVISIONS:
• CENTRAL NERVOUS SYSTEM (CNS) — brain + spinal cord.
• PERIPHERAL NERVOUS SYSTEM (PNS) — all nerves outside the CNS.

CNS FUNCTIONS:
• BRAIN processes and integrates information.
• SPINAL CORD relays signals between brain and body, controls reflexes.

PNS DIVISIONS:
• SOMATIC — voluntary muscles + sensory input.
• AUTONOMIC — involuntary functions (heart, digestion).

AUTONOMIC SUBDIVISIONS:
• SYMPATHETIC — "fight or flight" — arouses body in threats.
• PARASYMPATHETIC — "rest and digest" — conserves energy.
• ENTERIC — independent gut nervous system (in VCE from 2023).

NEURON STRUCTURE:
• DENDRITES — receive signals.
• CELL BODY (soma) — integrates.
• AXON — transmits to next neuron.
• MYELIN SHEATH — insulates axon, speeds conduction.
• AXON TERMINAL — releases neurotransmitters.

NEURON TYPES:
• SENSORY (afferent) — sense organs → CNS.
• MOTOR (efferent) — CNS → muscles/glands.
• INTERNEURONS — CNS connectors.

SYNAPSE:
• Gap between neurons.
• Signal crosses via neurotransmitters (chemicals).

ACTION POTENTIAL:
• Electrical impulse down axon.
• All-or-nothing event.

GLIAL CELLS:
• Support neurons (not signalling).
• Form myelin, clean debris, maintain environment.

SPINAL REFLEX:
• Quick response bypassing brain.
• Sensory neuron → interneuron → motor neuron.
• Example: hand on hot stove.

COMMON MISCONCEPTIONS:
• We use more than 10% of our brain.
• Nerves and neurons aren't interchangeable (nerves = bundles of neurons).

CONTEXT:
• Disorders (MS: myelin damage).
• Drugs work by altering neurotransmitter activity.

Quick map

• CNS: Brain + spinal cord.
• PNS: Everything else.
• Autonomic: Involuntary: SNS vs PNS.

Quick reference

| Division | Role |
|---|---|
| CNS | Brain and spinal cord |
| Somatic NS | Voluntary movement, sensory |
| Autonomic NS | Involuntary functions |
| Sympathetic | Fight-flight-freeze |
| Parasympathetic | Rest and digest |

Worked example (Easy)

Describe the path of a signal when you step on a pin.

1. Sensory neuron. Pain receptor → sensory neuron → spinal cord.
2. Spinal reflex. Interneuron in spinal cord activates motor neuron immediately.
3. Motor output. Motor neuron → leg muscles → lifts foot. Simultaneously, signal travels up to brain for conscious awareness.

Answer: Sensory neuron → spinal cord interneuron → motor neuron → leg muscle. Brain is informed shortly after.

Worked example (Hard)

A person with multiple sclerosis (MS) experiences numbness, slowed reactions, and muscle weakness. Explain these symptoms using your knowledge of neuron structure.

1. Identify the damage. MS attacks the MYELIN SHEATH on axons in the CNS, causing demyelination.
2. Link to symptoms. Myelin speeds signal conduction via saltatory conduction. Without it, signals travel slowly or are blocked entirely.
3. Explain each symptom. Numbness: sensory signals don't reach the brain efficiently. Slowed reactions: signal transmission delay. Muscle weakness: motor signals are incomplete or slow reaching muscles.

Answer: MS damages myelin, slowing or blocking neural transmission, causing sensory loss, delayed reactions, and motor impairment.

Common mistakes

• Confusing the somatic and autonomic nervous systems Students mix up voluntary (somatic) and involuntary (autonomic) control Fix: Somatic = voluntary muscle movement + conscious sensory input. Autonomic = involuntary functions (heart rate, digestion, pupil dilation).
• Saying the sympathetic and parasympathetic systems do the same thing Students forget they have OPPOSITE roles Fix: Sympathetic = "fight or flight" (arousal). Parasympathetic = "rest and digest" (calming). They work antagonistically to maintain homeostasis.
• Using "nerves" and "neurons" interchangeably In everyday language these seem synonymous Fix: A neuron is a single nerve cell. A nerve is a bundle of many neuron axons wrapped together. VCAA distinguishes these precisely.

The synapse: where chemistry meets electricity

Neural communication involves both electrical and chemical processes. Within a neuron, signals travel as electrical action potentials — rapid all-or-nothing voltage changes that propagate along the axon. But between neurons, the signal must cross a tiny gap called the synapse, and this is where chemistry takes over.

When an action potential reaches the axon terminal, it triggers the release of neurotransmitters — chemical messengers stored in vesicles. These molecules cross the synaptic cleft (gap of ~20 nm) and bind to receptors on the post-synaptic neuron's dendrites. Depending on the neurotransmitter and receptor type, the effect can be excitatory (increasing the chance of an action potential) or inhibitory (decreasing it).

Key neurotransmitters include: dopamine (reward, motivation), serotonin (mood, sleep), acetylcholine (memory, muscle activation), GABA (inhibition, calming), and glutamate (excitation, learning). Many psychiatric medications and recreational drugs work by altering neurotransmitter activity — blocking receptors, preventing reuptake, or mimicking the neurotransmitter's shape.

The myelin sheath dramatically speeds up electrical conduction by insulating the axon and enabling saltatory conduction — the signal "jumps" between gaps in the myelin (nodes of Ranvier). Myelinated neurons can transmit at up to 120 m/s, compared to ~2 m/s for unmyelinated ones. Diseases like multiple sclerosis attack myelin, devastating neural communication.

Practice questions

1. The sympathetic nervous system primarily:

   1. Calms the body
   2. Digests food
   3. Arouses body for threats
   4. Repairs tissues

   (1 marks)

2. Explain how the parasympathetic and sympathetic nervous systems interact to maintain balance.

   (3 marks)

Summary

• Distinguish CNS and PNS.
• Describe somatic vs autonomic divisions.
• Outline neuron structure and function.