Structure of the Nervous System
Central Nervous System (CNS): brain + spinal cord - processes information and coordinates responses.
Peripheral Nervous System (PNS): all nerves outside the CNS - sensory nerves carry impulses to the CNS, motor nerves carry impulses from the CNS.
A nerve is a bundle of nerve cells (neurons) grouped together, similar to individual wires in electrical cabling. Nerve cells do not undergo mitosis - if damaged, they cannot be replaced.
Neuron Structure
Each neuron has three main parts:
- Dendrites: short branching fibres that convert chemical messages to electrical impulses and direct them toward the cell body. Branched to increase surface area for receiving impulses.
- Cell body (soma): contains the nucleus and organelles. Produces neurotransmitter chemicals.
- Axon: long fibre that conducts electrical impulses away from the cell body.
- Schwann cells: wrap around the axon to form the myelin sheath.
- Myelin sheath: white fatty insulating layer that protects and electrically insulates the axon, speeding up conduction of impulses.
- Nodes of Ranvier: gaps in the myelin sheath where ions move in/out, enabling saltatory conduction (impulse 'jumps' between nodes).
- Synaptic knob: terminal endpoint of the axon containing neurotransmitter vesicles that store chemical messengers.
Types of Neurons
- Sensory neurons: carry impulses from receptors to the CNS. Cell body is on a side-branch off the axon.
- Motor neurons: carry impulses from the CNS to effectors (muscles/glands). Cell body at the end of the axon.
- Interneurons (relay neurons): connect sensory and motor neurons within the CNS. Cell body located midway along the axon.
The Brain - Structure and Function
The brain is protected by the skull. It is surrounded by three membranes called the meninges and cerebrospinal fluid (a clear liquid that protects against shock and provides nutrients).
The CNS has two regions: white matter (axons - carries impulses) and grey matter (cell bodies - processes information). In the brain, grey matter is on the outside; in the spinal cord, grey matter forms a butterfly shape in the centre.
| Brain Structure | Function |
|---|
| Cerebral cortex (cerebrum) | Receives and processes impulses from sense organs. Controls memory, language, voluntary muscle movement, and intelligence. |
| Cerebellum | Controls unconscious motor functions: muscle coordination and balance. |
| Brain stem | Controls involuntary actions: heart rate, breathing rate, blood pressure. |
| Thalamus | Relay station - all motor and sensory information passes through before being directed to the cerebral cortex for interpretation. |
| Epithalamus | Contains the pineal gland which secretes melatonin to control circadian (24-hour) rhythms. Regulates motor pathways and emotions. |
| Hypothalamus | Controls homeostasis - body temperature, fluid balance, metabolism. Links nervous and endocrine systems by controlling the pituitary gland. |
The Spinal Cord
Controls most reflex actions. 31 pairs of spinal nerves connect the spinal cord to specific body regions. Each spinal nerve branches into:
- Dorsal root (back): carries sensory neurons to the spinal cord. Cell bodies lie in the dorsal root ganglion.
- Ventral root (front): carries motor neurons from the spinal cord.
A central canal filled with cerebrospinal fluid runs through the centre.
Neurotransmission - Mechanism of Impulse Transfer
Nerve impulses are electro-chemical: electrical along the axon, chemical across synapses.
- At rest, the neuron membrane has a charge difference: inside negative, outside positive.
- A threshold stimulus (minimum strength required) must be reached for an impulse to fire. Below threshold → no impulse.
- All-or-nothing rule: once threshold is reached, the same impulse is carried regardless of stimulus strength.
- Movement of ions reverses the charge (inside becomes positive) - this change travels along the axon to the synaptic knob.
- At the synapse: impulse triggers neurotransmitter vesicles to release chemicals (e.g. acetylcholine) into the synaptic cleft.
- Neurotransmitters diffuse across the cleft and lock into receptors on the post-synaptic neuron.
- This causes an inrush of ions → new impulse generated in the next neuron.
- Enzymes in the synaptic cleft digest the neurotransmitters, stopping the response. Products are recycled by the pre-synaptic neuron.
- The neuron returns to its resting charges. The refractory period is the minimum time required before the neuron can carry another impulse.
The Reflex Arc
A reflex action is a rapid, involuntary, automatic response. The impulse bypasses the brain (goes only to the spinal cord) for faster protection:
- Sensory receptor detects a stimulus (e.g. pain receptor in skin)
- Sensory neuron carries impulse to spinal cord via dorsal root
- Sensory neuron synapses with an interneuron in the CNS
- Interneuron synapses with a motor neuron
- Motor neuron carries impulse out via ventral root to an effector muscle
- Muscle contracts → response (e.g. hand pulled away from hot object)
The brain can sometimes override a reflex (e.g. holding a hot plate to avoid dropping food) - but this must be learned.
Enzyme Inhibition at Synapses
Some medicines inhibit enzymes that break down neurotransmitters, increasing their levels and prolonging the response:
- Alzheimer's treatment: enzyme inhibitors enhance acetylcholine levels, improving nerve cell communication.
- SSRIs (for depression): block serotonin reuptake, keeping serotonin active longer.
- Nicotine: inhibits enzymes that break down neurotransmitters → overstimulation of receptors. Also raises dopamine → addiction.
Dopamine & Endorphins
Endorphins: neurotransmitters released by the pituitary gland and hypothalamus during pleasurable activities (eating, laughing, exercise). Known as the body's natural painkillers - they block nerves receiving pain signals, allowing the body to function in stressful situations.
Dopamine: the 'feel-good' neurotransmitter released following pleasurable activities. Creates a temporary 'dopamine rush' - explains why sugary foods are addictive. When the brain doesn't produce enough dopamine → Parkinson's disease (involuntary shaking), treated with L-dopa (converted to dopamine in the body).
Dopamine and endorphins work together: e.g. a runner experiences muscle pain → endorphins released to block pain → followed by a dopamine surge → 'runner's high' → encourages running again.
Addictive drugs (e.g. cocaine) interfere with normal dopamine recycling, allowing excessive dopamine to remain active.
Lifestyle factors: exercise (↑ both), dietary choices, sleep quality, social interaction all affect dopamine and endorphin levels.
Homeostasis
Homeostasis = maintaining a stable internal environment (e.g. body temperature, blood pH, water content, CO₂ levels). Uses negative feedback: change detected → response opposes change → returns to normal.
Components: receptors (detect change) → control centre (CNS/hypothalamus) → effectors (muscles/glands carry out response).
