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The nervous system is the body's primary internal communication system, responsible for detecting stimuli, coordinating responses, and enabling complex behaviours such as thought, emotion, and language. At A-Level, understanding the structure and function of the nervous system is essential for explaining both normal behaviour and the physiological basis of stress.
Key Definition: The nervous system is a complex network of neurons and supporting cells that transmits electrical and chemical signals throughout the body, enabling rapid communication between different organs and systems.
The human nervous system can be divided into two major subdivisions:
graph TD
A[Nervous System] --> B[Central Nervous System - CNS]
A --> C[Peripheral Nervous System - PNS]
B --> D[Brain]
B --> E[Spinal Cord]
C --> F[Somatic Nervous System]
C --> G[Autonomic Nervous System - ANS]
G --> H[Sympathetic Division]
G --> I[Parasympathetic Division]
Exam Tip: When drawing diagrams of the nervous system, always show the full hierarchy from the nervous system down to the sympathetic and parasympathetic divisions. Examiners reward completeness and accurate labelling.
The CNS comprises the brain and the spinal cord. It is the body's processing centre — receiving incoming information from sensory receptors, integrating and interpreting that information, and sending outgoing commands to effectors (muscles and glands).
The brain is the most complex organ in the body, containing approximately 86 billion neurons (Azevedo et al., 2009). It is divided into several major regions:
| Brain Region | Primary Functions |
|---|---|
| Cerebrum (cerebral cortex) | Higher-order thinking, voluntary movement, sensation, language, memory |
| Cerebellum | Coordination of movement, balance, fine motor skills |
| Medulla oblongata | Vital autonomic functions: heart rate, breathing rate, blood pressure |
| Hypothalamus | Homeostasis, temperature regulation, hunger, thirst, endocrine control via the pituitary gland |
| Thalamus | Relay station — routes sensory information to the appropriate cortical areas |
The cerebral cortex is highly folded (gyri and sulci), increasing the surface area available for higher cognitive processing. It is divided into four lobes: frontal, parietal, temporal, and occipital.
The spinal cord extends from the base of the brain (medulla oblongata) down through the vertebral column. It serves two critical functions:
Key Definition: A reflex arc is a neural pathway that controls a reflex action. The typical sequence is: stimulus → receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector → response.
In a spinal reflex, the response occurs before conscious awareness because the signal does not need to travel to the brain for processing. This makes reflexes extremely fast (typically under 0.5 seconds) and protective.
The PNS consists of all the nerves and ganglia outside the brain and spinal cord. It connects the CNS to the rest of the body and is subdivided into:
Key Definition: The autonomic nervous system (ANS) is the division of the peripheral nervous system that governs involuntary physiological processes, including heart rate, digestion, and respiratory rate.
The two branches of the ANS work in opposition — they are antagonistic — to maintain a state of balance (homeostasis).
| Function | Sympathetic Division | Parasympathetic Division |
|---|---|---|
| Heart rate | Increases | Decreases |
| Breathing rate | Increases | Decreases |
| Pupil size | Dilates | Constricts |
| Digestion | Inhibits (blood diverted to muscles) | Stimulates |
| Blood pressure | Increases | Decreases |
| Saliva production | Inhibits (dry mouth) | Stimulates |
| Bladder | Relaxes | Contracts |
| Adrenaline secretion | Stimulates | No direct effect |
| Overall state | Arousal — "fight or flight" | Rest — "rest and digest" |
The sympathetic division dominates in situations of stress, danger, or excitement. It prepares the body for action. The parasympathetic division dominates during relaxation and recovery, returning the body to its resting state.
Exam Tip: Remember the parasympathetic division as "rest and digest" and the sympathetic division as "fight or flight." Examiners often ask you to predict the effect of sympathetic or parasympathetic activation on a particular body function — always refer to the table above.
The fight-or-flight response is an acute stress reaction first described by Walter Cannon (1932). It represents the body's rapid physiological preparation for dealing with a perceived threat — either by confronting it (fight) or escaping from it (flight).
Key Definition: Adrenaline (epinephrine) is a hormone and neurotransmitter secreted by the adrenal medulla in response to sympathetic nervous system activation. It acts on multiple organ systems to prepare the body for rapid physical action.
The hypothalamus is the critical link between the nervous system and the endocrine (hormonal) system. In the fight-or-flight response, it serves as the "command centre," detecting the threat via neural input and activating the sympathetic division. The pathway is often called the sympathomedullary pathway (SAM pathway):
Hypothalamus → Sympathetic nervous system → Adrenal medulla → Adrenaline release
This response is extremely rapid — adrenaline can be detected in the blood within seconds of perceiving a threat. Once the threat has passed, the parasympathetic division gradually restores the body to its resting state.
Exam Tip: When evaluating the fight-or-flight response, always include the Taylor et al. (2000) tend-and-befriend alternative. This demonstrates critical evaluation (AO3) and awareness of gender bias in psychological research.
The nervous system is divided into the CNS (brain and spinal cord) and the PNS (somatic and autonomic nervous systems). The autonomic nervous system has two antagonistic branches: the sympathetic division (arousal) and the parasympathetic division (relaxation). The fight-or-flight response, first described by Cannon (1932), involves the SAM pathway (hypothalamus → sympathetic NS → adrenal medulla → adrenaline) and prepares the body for rapid action. While evolutionarily adaptive, this model has been criticised for gender bias (Taylor et al., 2000) and oversimplification (Gray, 1988).
| Psychologist | Date | Contribution |
|---|---|---|
| Walter Cannon | 1932 | Described the fight-or-flight response |
| Shelley Taylor et al. | 2000 | Proposed the tend-and-befriend alternative in females |
| Jeffrey Gray | 1988 | Identified the freeze response as a third option |