The Nervous System

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.

Overview of the Nervous System

The human nervous system can be divided into two major subdivisions:

Central Nervous System (CNS) — the brain and spinal cord.
Peripheral Nervous System (PNS) — all nerves outside the CNS.

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 Central Nervous System (CNS)

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

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

The spinal cord extends from the base of the brain (medulla oblongata) down through the vertebral column. It serves two critical functions:

Relay function — transmitting sensory information from the body to the brain and motor commands from the brain to effectors.
Reflex arcs — coordinating rapid, involuntary responses to potentially harmful stimuli without involving the brain (e.g., withdrawing a hand from a hot surface).

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 Peripheral Nervous System (PNS)

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:

The Somatic Nervous System (SNS)

Controls voluntary movements by transmitting motor commands from the CNS to skeletal muscles.
Also carries sensory information from receptors (e.g., skin, eyes, ears) back to the CNS.
Generally under conscious control — you decide to pick up a pen, turn your head, or walk across a room.

The Autonomic Nervous System (ANS)

Controls involuntary functions such as heart rate, digestion, respiration rate, pupil dilation, and glandular secretion.
Operates largely below conscious awareness, maintaining homeostasis.
Divided into two antagonistic branches: the sympathetic and parasympathetic divisions.

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 Sympathetic and Parasympathetic Divisions

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

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).

The Physiological Mechanism

A threat is perceived by the sensory organs and the information is relayed to the hypothalamus.
The hypothalamus activates the sympathetic branch of the ANS.
The sympathetic nervous system stimulates the adrenal medulla (the inner part of the adrenal glands, located on top of the kidneys).
The adrenal medulla releases adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream.
These hormones act on target organs to produce the characteristic fight-or-flight changes.

Physiological Changes During Fight-or-Flight

Increased heart rate and blood pressure — more blood is pumped to the muscles, delivering oxygen and glucose for energy.
Increased breathing rate — more oxygen is taken in and more carbon dioxide expelled.
Pupil dilation — improves visual acuity and peripheral vision.
Glycogenolysis in the liver — glycogen is broken down to glucose, raising blood sugar levels to supply energy.
Blood diverted from the digestive system to skeletal muscles — digestion is non-essential during an emergency.
Sweating increases — cools the body in anticipation of physical exertion.
Inhibition of the immune system and reproductive functions — non-essential processes are suppressed.

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 Role of the Hypothalamus and Adrenal Medulla

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.

Evaluation of the Fight-or-Flight Response

Strengths (AO3)

Evolutionary value — the response has clear survival benefits. Cannon (1932) argued that animals (including humans) who could rapidly mobilise energy for fight or flight were more likely to survive and reproduce, making this response adaptive.
Physiological evidence — the role of adrenaline and the SAM pathway is well-established through both animal experiments and human studies (e.g., measurement of adrenaline levels during stress).
Universal — the basic fight-or-flight response is observed across a wide range of species, suggesting a deep evolutionary origin.

Limitations (AO3)

Gender bias — Taylor et al. (2000) proposed that the fight-or-flight model was developed primarily from research on males. Females may be more likely to respond to threat with a "tend-and-befriend" response — nurturing offspring (tend) and forming alliances with other females (befriend), mediated by the hormone oxytocin rather than adrenaline alone.
Modern relevance — the fight-or-flight response evolved for acute physical threats (e.g., predators). In modern life, stressors are often chronic and psychological (e.g., work deadlines, financial worries). Prolonged activation of the stress response can lead to health problems such as hypertension, immune suppression, and cardiovascular disease.
Oversimplification — the response is not purely "fight or flight." Gray (1988) identified a third option — freeze — in which an organism becomes immobile in the face of threat. This is observed in prey animals and has been documented in humans experiencing extreme fear.

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.

Summary

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).

Key Studies

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