Mammalian Nervous System and Brain

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. It addresses OCR A-Level Biology A specification 5.1.5(e)–(g).

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.

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The Big Picture

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.

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Somatic Nervous System

The somatic nervous system (SoNS) carries motor commands from the CNS to skeletal muscles. Its defining features:

• Voluntary — under conscious control.
• Single motor neurone from CNS to muscle.
• Myelinated for speed.
• Neurotransmitter: acetylcholine at the neuromuscular junction.
• Effect: always excitatory (muscle contraction). No inhibitory somatic motor output exists.

The sensory half of the SoNS, carrying information from touch, pressure, temperature, pain and proprioceptors, is also considered part of the somatic system.

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Autonomic Nervous System

The autonomic nervous system (ANS) controls the internal organs, running essentially below the level of conscious awareness. Its defining features:

• Involuntary.
• Two neurones from CNS to effector, connected at a peripheral ganglion.
• Targets: smooth muscle, cardiac muscle, glands.
• Effects: can be either excitatory or inhibitory depending on receptor type.

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.

Sympathetic Nervous System

• Dominant during stress, exercise, fright, flight or fight.
• Origin: thoracic and lumbar spinal cord (thoracolumbar outflow).
• Short pre-ganglionic fibres, long post-ganglionic fibres.
• Ganglia lie close to the spinal cord in the sympathetic chain.
• Pre-ganglionic neurotransmitter: acetylcholine.
• Post-ganglionic neurotransmitter: noradrenaline (except sweat glands, where ACh is used).

Effects (remember as preparing for action):

• Heart rate and contractility increase.
• Vasoconstriction in gut and skin; vasodilation in skeletal muscle.
• Bronchodilation.
• Pupil dilation.
• Sweat production.
• Glycogenolysis in liver; glucose release.
• Inhibition of gut motility and digestion.
• Adrenaline release from adrenal medulla (which is essentially part of the sympathetic system).

Parasympathetic Nervous System

• Dominant at rest, during digestion and sleep.
• Origin: brainstem (cranial nerves, especially the vagus) and sacral spinal cord (craniosacral outflow).
• Long pre-ganglionic fibres, short post-ganglionic fibres.
• Ganglia lie close to or in the target organs.
• Pre- and post-ganglionic neurotransmitter: acetylcholine at both synapses.

Effects:

• Heart rate and contractility decrease.
• Bronchoconstriction.
• Pupil constriction.
• Salivation and gastric secretion.
• Increased gut motility.
• Bladder contraction (urination).

Comparison Table

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

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The Brain

The mammalian brain is a staggeringly complex structure, but OCR requires you to know the location and function of five key regions.

1. Cerebrum

The cerebrum is the largest part of the brain, divided into left and right hemispheres. Its outer layer is the cerebral cortex, a highly folded sheet of grey matter (cell bodies) covering white matter (myelinated axons). Key functions:

• Higher mental functions — conscious thought, reasoning, language, memory, personality.
• Voluntary movement — motor cortex in the precentral gyrus (frontal lobe) sends commands to skeletal muscles.
• Sensory perception — somatosensory cortex in the postcentral gyrus (parietal lobe) receives information from touch, pressure, pain and temperature receptors.
• Specialised areas for vision (occipital lobe), hearing (temporal lobe), smell and taste.

The highly folded cortex increases its surface area, accommodating more neurones than a smooth cortex of the same volume. Humans have about 86 billion neurones overall, most of them in the cerebral cortex.

2. Cerebellum

The cerebellum sits below and behind the cerebrum. It coordinates:

• Balance — integrating information from the vestibular system.
• Fine motor control — smoothing movements and making them precise.
• Motor learning — riding a bike, playing an instrument, any skilled movement.