Homeostasis and the Nervous System

Homeostasis is the maintenance of a constant internal environment despite changes in external conditions. The nervous system provides rapid, short-lived communication essential for coordinating responses to stimuli. Together, homeostatic mechanisms and the nervous system ensure that conditions within the body remain within the narrow limits necessary for enzyme function, cell survival, and overall health.

Key Definition: Homeostasis is the maintenance of a relatively stable internal environment within narrow limits, by means of physiological processes involving receptors, coordinators, and effectors operating through feedback mechanisms.

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Principles of Homeostasis

Negative Feedback

The primary mechanism for homeostasis is negative feedback:

1. A receptor (sensor) detects a deviation from the set point (the normal/optimal value).
2. A coordinator (control centre, e.g., the hypothalamus) processes the information and determines the appropriate response.
3. An effector (muscle or gland) produces a response that reverses the change and restores the set point.
4. Once the set point is restored, the receptor detects this and the corrective response is reduced or stopped.

A described diagram of negative feedback would show a circular pathway: a stimulus causes a deviation from the set point → the receptor detects the change → the coordinator processes the information → the effector produces a response that opposes the stimulus → the variable returns to the set point → the response is reduced. Arrows would show the loop, with an indication that the effector's response opposes the original change.

flowchart TD
    Stim["Stimulus
(deviation from set point)"] --> Rec["Receptor
(detects change)"]
    Rec --> Coord["Coordinator
(e.g., hypothalamus)
Processes information"]
    Coord --> Eff["Effector
(muscle or gland)
Produces response"]
    Eff -->|"Response OPPOSES
original change"| Restore["Variable returns
to set point"]
    Restore -->|"Receptor detects
restoration"| Reduce["Corrective response
reduced or stopped"]
    Reduce -.->|"Cycle repeats if
deviation recurs"| Stim

Examples of negative feedback:

• Body temperature regulation (thermoregulation)
• Blood glucose concentration
• Blood water potential (osmoregulation)
• Blood CO₂ concentration and pH

Positive Feedback

• The response amplifies the original change, moving the variable further from the set point.
• Relatively rare but important in specific contexts:
  • Oxytocin release during labour — uterine contractions stimulate more oxytocin release, intensifying contractions until birth occurs.
  • Blood clotting cascade — activated clotting factors activate more clotting factors, rapidly sealing a wound.
  • Depolarisation phase of an action potential — Na⁺ influx causes further voltage-gated Na⁺ channels to open, leading to a rapid, explosive depolarisation.

Exam Tip: When explaining negative feedback, always describe the full loop: stimulus → receptor → coordinator → effector → response that opposes the original stimulus. Many students describe only half the loop and lose marks. Also make clear that negative feedback is a continuous process, not a one-off event.

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Thermoregulation

Thermoregulation is the maintenance of a stable core body temperature (approximately 37 °C in humans), which is essential for optimal enzyme activity.

The Role of the Hypothalamus

The hypothalamus is the thermoregulatory centre in the brain. It contains thermoreceptors that monitor the temperature of the blood flowing through it (core temperature). The hypothalamus also receives nerve impulses from peripheral thermoreceptors in the skin, which detect external temperature changes.

Response to Overheating (Core Temperature Rises Above 37 °C)

Effector Response	Mechanism
Vasodilation	Arterioles in the skin dilate, increasing blood flow to surface capillaries. More heat is lost by radiation from the skin surface. The skin appears flushed/red.
Sweating	Sweat glands secrete sweat (mainly water and NaCl) onto the skin surface. Evaporation of sweat absorbs latent heat from the skin, cooling the body.
Hair erector muscles relax	Hairs lie flat against the skin, reducing the insulating layer of trapped air.
Behavioural responses	Seeking shade, removing clothing, reducing activity.
Decreased metabolic rate	Less heat generated by metabolic reactions (longer-term response).

Response to Cooling (Core Temperature Falls Below 37 °C)

Effector Response	Mechanism
Vasoconstriction	Arterioles in the skin constrict, reducing blood flow to surface capillaries. Less heat is lost by radiation. The skin appears pale.
Shivering	Involuntary, rapid contraction and relaxation of skeletal muscles generates heat from increased respiration.
Hair erector muscles contract (piloerection)	Hairs stand erect, trapping a thicker layer of insulating air (more significant in furry mammals than in humans).
Behavioural responses	Seeking shelter, adding clothing, curling up, huddling.
Increased metabolic rate	Thyroid hormones and adrenaline can increase basal metabolic rate, generating more heat (longer-term response).
Reduced sweating	Sweat glands produce less sweat.

Key Definition: Vasoconstriction is the narrowing of arterioles, reducing blood flow to the skin surface and decreasing heat loss. Vasodilation is the widening of arterioles, increasing blood flow to the skin surface and increasing heat loss. Note: it is the arterioles that constrict or dilate, not the capillaries themselves.

Exam Tip: A common error is to write that "blood vessels move closer to or further from the skin surface." This is incorrect. The blood vessels do not move — instead, the arterioles constrict or dilate, which controls how much blood flows through the surface capillary beds.

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The Nervous System — Organisation

Division	Components
Central nervous system (CNS)	Brain and spinal cord
Peripheral nervous system (PNS)	Sensory neurones (afferent) and motor neurones (efferent)
Somatic nervous system	Voluntary control of skeletal muscles
Autonomic nervous system (ANS)	Involuntary control of smooth muscle, cardiac muscle, and glands
Sympathetic division (of ANS)	"Fight or flight" — increases heart rate, dilates pupils, diverts blood to muscles
Parasympathetic division (of ANS)	"Rest and digest" — decreases heart rate, promotes digestion, constricts pupils

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Brain Structure

The brain is the main coordinator of the nervous system. Key regions include:

Brain Region	Location	Function
Cerebrum (cerebral cortex)	Largest part of the brain; two hemispheres with a highly folded surface	Higher cognitive functions: conscious thought, reasoning, memory, language, sensory processing, voluntary movement
Cerebellum	Posterior, below the cerebrum	Coordination of movement, posture, balance, fine motor control; does NOT initiate movement
Medulla oblongata	Base of the brain, continuous with the spinal cord	Controls vital involuntary functions: heart rate, breathing rate, blood pressure; contains the cardiovascular and respiratory centres
Hypothalamus	Below the thalamus, above the pituitary	Links the nervous and endocrine systems; controls body temperature, hunger, thirst, osmoregulation; regulates the pituitary gland

Exam Tip: Questions may ask you to describe how scientists have determined the function of each brain area. Methods include: studying patients with brain damage (e.g., Phineas Gage for the frontal lobe), functional MRI (fMRI) scanning, electrical stimulation during surgery, and the effects of stroke on specific regions.

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Neurones

Types of Neurone

• Sensory neurones — transmit impulses from receptors to the CNS; long dendron, cell body in the dorsal root ganglion (located outside the spinal cord).
• Motor neurones — transmit impulses from the CNS to effectors; long axon, cell body in the ventral horn of the spinal cord (or in the brain).
• Relay neurones (interneurones) — connect sensory and motor neurones within the CNS; short axon, many dendrites.

Neurone Structure

• Cell body (soma) — contains the nucleus, many mitochondria (for ATP to maintain the resting potential), rough ER, and Nissl granules (dense rough ER for protein synthesis).
• Dendrites — receive impulses from other neurones or receptors; highly branched to increase surface area for synaptic connections.
• Axon — transmits impulses away from the cell body; can be very long (up to 1 m in motor neurones).
• Myelin sheath — insulating layer of Schwann cells wrapped around the axon; increases speed of impulse transmission by enabling saltatory conduction.
• Nodes of Ranvier — gaps (~1 µm wide) in the myelin sheath where the axon membrane is exposed and depolarisation occurs, enabling saltatory conduction (the impulse "jumps" between nodes, greatly increasing speed from ~2 m/s to ~120 m/s).
• Axon terminal (synaptic knob) — contains many mitochondria and vesicles of neurotransmitter.

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Receptors — The Pacinian Corpuscle

Key Definition: A receptor is a cell or group of cells that detects a stimulus by converting the energy of the stimulus into a nerve impulse (electrical signal). This conversion is called transduction.

The Pacinian corpuscle is a pressure receptor found in the skin, joints, and some internal organs. It is an excellent example of a transducer.

Structure: A described diagram would show a single sensory neurone ending surrounded by concentric layers (lamellae) of connective tissue, resembling the layers of an onion. The sensory nerve ending in the centre contains stretch-mediated sodium channels.

How it works:

1. When pressure is applied, the lamellae are deformed and press on the nerve ending.
2. The deformation stretches the membrane of the nerve ending, opening stretch-mediated Na⁺ channels.
3. Na⁺ ions flow into the nerve ending, causing depolarisation — this is the generator potential (or receptor potential).
4. If the generator potential reaches the threshold, an action potential is triggered in the sensory neurone.
5. The greater the pressure, the more Na⁺ channels open, and the larger the generator potential — so stronger stimuli are more likely to reach the threshold and produce more frequent action potentials.

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Rod and Cone Cells — Photoreceptors in the Retina

Feature	Rod Cells	Cone Cells
Sensitivity	Very sensitive to light; function in dim light (scotopic vision)	Less sensitive; require bright light (photopic vision)
Visual acuity	Low acuity — many rods converge onto one bipolar neurone (retinal convergence/summation)	High acuity — usually one cone per bipolar neurone; fine detail and sharp images
Colour vision	Cannot distinguish colours; contain one pigment (rhodopsin)	Three types (red, green, blue), each with a different iodopsin pigment; allow colour vision
Distribution	Concentrated at the periphery of the retina	Concentrated at the fovea (centre of the retina)
Pigment	Rhodopsin (broken down by light into retinal + opsin; bleaching)	Three types of iodopsin (red-sensitive, green-sensitive, blue-sensitive)

Exam Tip: Rod cells provide high sensitivity but low acuity because of convergence (many rods synapse onto one bipolar neurone, so the signals are summed — spatial summation — but the brain cannot distinguish exactly which rod was stimulated). Cone cells provide high acuity because each cone has its own bipolar neurone, so the brain can identify the exact point of stimulation.

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Reflex Arcs

A reflex arc is the pathway taken by a nerve impulse during a rapid, involuntary response (a reflex). Reflexes protect the body from harm.

A described diagram of a spinal reflex arc (e.g., the withdrawal reflex when touching a hot object) would show:

1. Receptor in the skin (e.g., pain receptor/nociceptor in the finger) detects the stimulus.
2. Sensory neurone transmits the impulse along a long dendron, past the cell body in the dorsal root ganglion, and into the spinal cord via the dorsal root.
3. Relay neurone in the grey matter of the spinal cord receives the impulse and transmits it to a motor neurone. The synapse between sensory and relay neurones is in the dorsal horn.
4. Motor neurone transmits the impulse from the ventral horn of the spinal cord, out via the ventral root, along a long axon to the effector.
5. Effector (e.g., biceps muscle in the arm) contracts, pulling the hand away from the hot object.

flowchart LR
    R["Receptor
(pain receptor
in skin)"] -->|"Nerve impulse"| SN["Sensory Neurone
(via dorsal root
ganglion)"]
    SN -->|"Impulse enters
spinal cord"| RN["Relay Neurone
(in grey matter
of spinal cord)"]
    RN --> MN["Motor Neurone
(exits via
ventral root)"]
    MN -->|"Impulse to effector"| E["Effector
(e.g., biceps
muscle contracts)"]

Features of Reflex Arcs

• Rapid — few synapses mean a short pathway and fast response.
• Involuntary — does not require conscious thought (though the brain is informed after the reflex has occurred).
• Protective — prevents damage before conscious processing can occur.
• The spinal cord acts as the coordinator, not the brain (for spinal reflexes).

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The Nerve Impulse

Resting Potential

• At rest, the neurone has a potential difference of approximately −70 mV across the membrane (inside negative relative to outside).
• Maintained by the sodium-potassium pump (Na⁺/K⁺-ATPase: 3 Na⁺ pumped out for every 2 K⁺ pumped in — this is active transport requiring ATP).
• Potassium leak channels allow K⁺ to diffuse out down its concentration gradient, making the inside more negative (K⁺ is more permeable at rest than Na⁺).
• The result is a polarised membrane with a net negative charge inside.

Action Potential

Phase	Events	Membrane Potential
Resting	Na⁺/K⁺ pump maintains −70 mV; K⁺ leak channels open	−70 mV
Depolarisation	Stimulus reaches threshold (~−55 mV); voltage-gated Na⁺ channels open rapidly; Na⁺ rushes in down its electrochemical gradient (positive feedback)	Rises to approximately +40 mV
Repolarisation	Voltage-gated Na⁺ channels close (inactivate); voltage-gated K⁺ channels open (slightly delayed); K⁺ rushes out	Falls back towards −70 mV
Hyperpolarisation	K⁺ channels are slow to close; K⁺ continues to leave; potential briefly overshoots the resting value	Briefly below −70 mV (e.g., −80 mV)
Recovery	Na⁺/K⁺ pump restores ion distribution; K⁺ channels close fully	Returns to −70 mV

Key Principles

• All-or-nothing principle — an action potential either fires fully (+40 mV peak) or not at all. There is no "partial" impulse. Stimulus intensity is coded by frequency of action potentials, not amplitude.
• Threshold — the minimum depolarisation required to trigger an action potential (~−55 mV). Sub-threshold stimuli produce only local, graded depolarisations that decay with distance.
• Refractory period — during the absolute refractory period, no stimulus (however strong) can trigger another action potential because Na⁺ channels are inactivated. During the relative refractory period, only a very strong stimulus can trigger an action potential. The refractory period ensures (a) unidirectional propagation (the region behind the action potential is refractory), and (b) limits the maximum frequency of impulses.

Worked Example 1 — Speed of Transmission

A sensory neurone is 0.8 m long and transmits an impulse in 8 ms. What is the speed of conduction?

Speed = distance / time = 0.8 m / 0.008 s = 100 m s⁻¹

This is consistent with a large-diameter, myelinated neurone (saltatory conduction).

Factors Affecting Speed of Transmission

Factor	Effect	Explanation
Myelination	Myelinated neurones conduct much faster (~120 m/s vs ~2 m/s)	Saltatory conduction — impulse jumps between nodes of Ranvier
Axon diameter	Wider axons conduct faster	Less electrical resistance to ion flow along the axon
Temperature	Higher temperature increases speed (up to ~40 °C)	Ions diffuse faster; enzyme/channel activity increases; above ~40 °C, proteins denature

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Synapses

A synapse is the junction between two neurones (or a neurone and an effector). Most synapses are chemical synapses with a synaptic cleft approximately 20 nm wide.

Key Definition: A synapse is the junction between two neurones, consisting of the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane. Transmission across the synapse is typically chemical (via neurotransmitters).

Synaptic Transmission (using acetylcholine as the example neurotransmitter)

1. An action potential arrives at the presynaptic terminal (synaptic knob).
2. Depolarisation of the presynaptic membrane causes voltage-gated Ca²⁺ channels to open; Ca²⁺ ions flow into the presynaptic terminal down their concentration gradient.
3. The influx of Ca²⁺ causes synaptic vesicles (containing the neurotransmitter acetylcholine, ACh) to move to and fuse with the presynaptic membrane, releasing ACh into the synaptic cleft by exocytosis.
4. ACh diffuses across the synaptic cleft (~20 nm — a very short distance, so diffusion is rapid).
5. ACh binds to specific complementary receptors (nicotinic or muscarinic receptors) on the postsynaptic membrane. These receptors are ligand-gated ion channels.
6. Binding causes Na⁺ channels to open, and Na⁺ flows into the postsynaptic neurone, causing depolarisation. If the depolarisation reaches the threshold, a new action potential is generated.
7. ACh is rapidly broken down by the enzyme acetylcholinesterase (AChE) in the synaptic cleft. The products (choline and ethanoic acid) are reabsorbed into the presynaptic terminal and used to resynthesise ACh, using ATP from the many mitochondria present.

sequenceDiagram
    participant Pre as Presynaptic neurone
    participant Cleft as Synaptic cleft
    participant Post as Postsynaptic neurone
    Pre->>Pre: Action potential arrives
    Pre->>Pre: Ca²⁺ channels open, Ca²⁺ enters
    Pre->>Pre: Vesicles fuse with membrane (exocytosis)
    Pre->>Cleft: ACh released into cleft
    Cleft->>Post: ACh binds to receptors
    Post->>Post: Na⁺ channels open → depolarisation
    Post->>Post: If threshold reached → action potential
    Cleft->>Cleft: AChE breaks down ACh
    Cleft->>Pre: Choline reabsorbed & ACh resynthesised

Roles of Synapses

• Ensure unidirectional transmission (vesicles and neurotransmitter are only in the presynaptic terminal; receptors are only on the postsynaptic membrane).
• Allow summation:
  • Temporal summation — rapid successive impulses from one presynaptic neurone release enough neurotransmitter to reach the threshold.
  • Spatial summation — impulses arriving simultaneously from multiple presynaptic neurones combine to reach the threshold.
• Enable integration of signals — some synapses are excitatory (depolarise the postsynaptic membrane, e.g., by opening Na⁺ channels) and others are inhibitory (hyperpolarise the postsynaptic membrane, e.g., by opening Cl⁻ channels). The postsynaptic neurone "sums" all inputs and only fires if the net effect reaches the threshold.

Worked Example 2 — Temporal Summation

A single presynaptic impulse releases enough neurotransmitter to cause a 10 mV depolarisation of the postsynaptic membrane, but the threshold requires a 15 mV depolarisation. If two impulses arrive in rapid succession (before the first depolarisation has decayed), the combined effect is 10 + 10 = 20 mV, which exceeds the threshold (15 mV), and an action potential is triggered.

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Drugs and Synapses

Many drugs and toxins exert their effects by interfering with synaptic transmission:

Drug/Toxin	Mechanism	Effect
Nicotine	Mimics acetylcholine; binds to nicotinic ACh receptors on the postsynaptic membrane	Stimulates the postsynaptic neurone; produces feelings of alertness; addictive because it stimulates dopamine release in reward pathways
Curare	Blocks nicotinic ACh receptors on the postsynaptic membrane (competitive antagonist)	Prevents ACh from binding; blocks neuromuscular transmission; causes muscle paralysis
Nerve agents (e.g., sarin, organophosphates)	Inhibit acetylcholinesterase (irreversible inhibition)	ACh is not broken down; it accumulates in the synaptic cleft; continuous stimulation of the postsynaptic membrane; causes uncontrolled muscle contraction, paralysis, and death
SSRIs (e.g., fluoxetine/Prozac)	Block reuptake of serotonin from the synaptic cleft back into the presynaptic neurone	Serotonin remains in the cleft for longer, prolonging its stimulatory effect on the postsynaptic neurone; used to treat depression and anxiety
Atropine	Blocks muscarinic ACh receptors (competitive antagonist)	Used to dilate pupils (mydriasis) during eye examinations; reduces secretions
Botulinum toxin (Botox)	Prevents fusion of synaptic vesicles with the presynaptic membrane	No neurotransmitter is released; muscle paralysis; used medically to treat muscle spasms and cosmetically to reduce wrinkles

Key Definition: An agonist is a substance that binds to a receptor and mimics the effect of the natural neurotransmitter (e.g., nicotine). An antagonist is a substance that binds to a receptor and blocks the natural neurotransmitter from binding, preventing its effect (e.g., curare).

Exam Tip: When explaining how a drug affects synaptic transmission, always state: (1) what the drug binds to or inhibits, (2) the effect on the neurotransmitter or receptor, and (3) the overall effect on the postsynaptic neurone (more or less stimulation). Link this to the observable effect (e.g., muscle paralysis, mood changes).