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Spec Mapping — OCR H420 Module 5.1.5 — Plant and animal responses, content statements covering the organisation of the mammalian nervous system (central + peripheral, somatic + autonomic, sympathetic + parasympathetic), the major regions of the brain (cerebrum, cerebellum, medulla oblongata, hypothalamus, pituitary), and the role of the autonomic nervous system in controlling heart rate (refer to the official OCR H420 specification document for exact wording). This lesson knits together the cellular-level neurone material with the whole-organism control architecture.
Having looked at individual neurones, synapses and hormones, we now zoom out to consider how they are organised into the mammalian nervous system. This lesson covers the central and peripheral nervous systems, the autonomic divisions, the major regions of the brain, and reflex actions.
The functional organisation of the autonomic nervous system was characterised by Walter Cannon (paraphrase, 1929) in his work on the "fight-or-flight" response — he coined the term and identified the sympathetic-adrenal-medullary axis as the rapid effector of acute stress responses, complementing Hans Selye's later work (1936, paraphrase) on the longer-term HPA-axis "stress response". The functional segregation of brain regions — motor cortex in frontal lobes, sensory cortex in parietal lobes, vision in occipital, hearing in temporal — was established by Korbinian Brodmann (1909, paraphrase) and confirmed by lesion studies and later by functional imaging. Wilder Penfield (1937, paraphrase) constructed the famous sensory and motor homunculi by direct cortical stimulation during epilepsy surgery, showing that the cortical representation of each body part is proportional to its sensory and motor importance, not its physical size. Modern functional MRI confirms and extends these maps with non-invasive resolution. The cerebellum's role in motor learning was characterised classically by lesion experiments and now in molecular detail through long-term depression at parallel-fibre to Purkinje-cell synapses — a fascinating example of how cellular plasticity supports whole-organism skill acquisition.
Key Definitions:
- Central nervous system (CNS) — the brain and spinal cord; the "control centre".
- Peripheral nervous system (PNS) — all nerves outside the CNS; transmits information to and from it.
- Somatic nervous system — the voluntary part of the PNS; controls skeletal muscle.
- Autonomic nervous system — the involuntary part of the PNS; controls smooth muscle, cardiac muscle and glands.
- Reflex — a rapid, automatic, involuntary response to a stimulus that bypasses conscious processing.
The mammalian nervous system can be subdivided neatly:
flowchart TB
NS[Nervous system] --> CNS["Central nervous system<br/>Brain + spinal cord"]
NS --> PNS[Peripheral nervous system]
PNS --> SOM["Somatic<br/>Voluntary skeletal muscle"]
PNS --> AUT["Autonomic<br/>Involuntary"]
AUT --> SYM["Sympathetic<br/>Fight or flight"]
AUT --> PAR["Parasympathetic<br/>Rest and digest"]
CNS vs PNS — the CNS is the processing centre; the PNS is the wiring that connects it to receptors and effectors.
Somatic vs autonomic — somatic is under conscious control (e.g. lifting a cup); autonomic is not (e.g. peristalsis).
Sympathetic vs parasympathetic — two branches of the autonomic system with opposing effects.
The somatic nervous system (SoNS) carries motor commands from the CNS to skeletal muscles. Its defining features:
The sensory half of the SoNS, carrying information from touch, pressure, temperature, pain and proprioceptors, is also considered part of the somatic system.
The autonomic nervous system (ANS) controls the internal organs, running essentially below the level of conscious awareness. Its defining features:
The ANS has two divisions with generally opposing effects: the sympathetic (fight or flight) and the parasympathetic (rest and digest). Most organs receive both, and their activity is set by the balance between the two.
Effects (remember as preparing for action):
Effects:
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| When active | Stress, exercise, fear | Rest, digestion |
| Origin | Thoracic and lumbar spinal cord | Brainstem and sacral spinal cord |
| Pre-ganglionic length | Short | Long |
| Ganglia | Sympathetic chain, near CNS | Close to / within target organ |
| Post-ganglionic transmitter | Noradrenaline (mostly) | Acetylcholine |
| Heart | Speeds up | Slows down |
| Bronchi | Dilate | Constrict |
| Pupils | Dilate | Constrict |
| Gut | Inhibits | Stimulates |
The mammalian brain is a staggeringly complex structure, but OCR requires you to know the location and function of five key regions.
The cerebrum is the largest part of the brain, divided into left and right hemispheres connected by the corpus callosum. Its outer layer is the cerebral cortex, a highly folded sheet of grey matter (cell bodies) covering white matter (myelinated axons). Key functions:
The highly folded cortex (gyri and sulci) increases its surface area, accommodating more neurones than a smooth cortex of the same volume — the folding gives roughly a threefold increase in surface area. Humans have about 86 billion neurones overall, most of them in the cerebral cortex. The motor and sensory cortices are organised somatotopically, producing the famous motor and sensory homunculi in which body parts are represented in proportion to their motor or sensory importance — disproportionately large hands and lips, small back and torso.
The cerebellum (Latin: "little brain") sits below and behind the cerebrum. Although small relative to the cerebrum, it contains more neurones than the rest of the brain combined (~50–80 billion granule cells alone). It coordinates:
Damage to the cerebellum causes ataxia — jerky, uncoordinated movements, intention tremor (worsening as the target is approached), and slurred speech (dysarthria). Alcohol, which selectively affects the cerebellum at intoxication-relevant blood concentrations, causes the staggering gait, dysmetria, and dysarthria of intoxication. The cerebellum is also implicated in some cognitive and emotional processing, though its non-motor roles remain an active research area.
The medulla oblongata lies at the base of the brainstem, between the pons (above) and the spinal cord (below), and controls automatic functions vital for life:
Damage to the medulla is usually fatal because basic life support systems collapse — this is why severe brainstem injury produces brain death even when the cerebrum is intact, and is the anatomical basis of the clinical definition of death.
The hypothalamus is a small region below the thalamus, weighing only about 4 g but disproportionately important. Despite its size, it is crucial for homeostasis:
The hypothalamus is the bridge between the nervous and endocrine systems — it receives neural inputs from cortex, limbic system, brainstem and viscera, and converts these into hormonal outputs that coordinate whole-body physiology.
The pituitary gland (hypophysis) hangs below the hypothalamus on a stalk (the infundibulum), sitting in a small bony cavity in the sphenoid bone called the sella turcica. It has two parts of completely different embryological origin:
Because it controls many other endocrine glands, the pituitary is often called the "master gland" — though the hypothalamus is the real master. Pituitary tumours can produce dramatic clinical syndromes: GH-secreting tumours cause acromegaly (excess growth in adults) or gigantism (in children, before growth-plate fusion); ACTH-secreting tumours cause Cushing's disease (excess cortisol via HPA axis); prolactin-secreting tumours cause galactorrhoea and amenorrhoea.
The medullary cardiovascular centre receives afferent input from two sets of sensors:
The cardiovascular centre integrates these inputs and sends efferent commands through two outflows:
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