Reflex Arcs and the CNS / PNS

This lesson is mapped to AQA 7402 Section 3.6.2 — reflex arcs, CNS / PNS organisation, somatic and autonomic nervous systems (refer to the official AQA specification document for exact wording). The reflex arc is the simplest unit of integrated nervous behaviour — the shortest path from stimulus to response. It is also, paradoxically, one of the most powerful: reflexes coordinate breathing, balance, swallowing, blinking, pupillary diameter, blood pressure, and the protective withdrawal of limbs from damage. Without reflex arcs, organisms could not survive long enough to use their higher cognitive faculties.

This lesson uses the architectural framework established in lesson 0 (CNS, PNS, somatic, autonomic) and the cellular machinery established in lessons 1–3 (action potentials, synapses, muscle) to explain how the components fit together at the circuit level. We treat the monosynaptic knee-jerk reflex, the polysynaptic withdrawal reflex (with crossed extensor for postural balance), the somatic / autonomic split, and the sympathetic / parasympathetic ("fight or flight" vs "rest and digest") partition of the autonomic system, paraphrasing Cannon's mid-twentieth-century framework. We close with an A*-grade discussion of how cortical processes can override reflex behaviour.

Key Definition: A reflex arc is a neural pathway that produces a rapid, automatic, stereotyped response to a specific stimulus without conscious thought. It consists, in its simplest form, of a receptor, a sensory neurone, the spinal cord (sometimes with a relay neurone), a motor neurone, and an effector.

---

Why Reflexes Matter — The Survival Advantage

Reflex arcs confer several adaptive advantages:

1. Speed. The withdrawal reflex from a hot surface completes in approximately 200 ms. The brain takes ~300–500 ms to produce a deliberate voluntary action. The reflex therefore arrives first — by the time you consciously feel the pain, your hand has already moved.
2. No conscious processing required. The pathway is genetically wired and does not depend on attention or learning, freeing higher cognitive resources for non-routine decisions.
3. Stereotyped output. The same stimulus produces the same response every time, reducing the variability that would arise from voluntary control.
4. Protective. Many reflexes minimise damage — withdrawal from heat or pain, blink to corneal stimulation, vomiting to ingested toxins, sneezing and coughing to airway irritants.
5. Postural maintenance. Stretch reflexes (knee-jerk type) continuously adjust skeletal muscle tone to maintain upright posture without conscious effort.

Exam Tip: Mark schemes routinely award 1 mark for naming a survival advantage of reflexes. "Rapid response prevents tissue damage" is a higher-scoring answer than "it is fast".

---

The Generic Reflex-Arc Pathway

Stimulus → Receptor → Sensory (afferent) neurone → CNS (relay neurone, optional) → Motor (efferent) neurone → Effector → Response

graph LR
    A["Stimulus"] --> B["Receptor"]
    B --> C["Sensory neurone<br/>afferent"]
    C --> D["Spinal cord<br/>relay neurone (if polysynaptic)"]
    D --> E["Motor neurone<br/>efferent"]
    E --> F["Effector<br/>(muscle / gland)"]
    F --> G["Response"]
    D -.->|ascending tract| H["Brain<br/>conscious awareness"]

    style D fill:#27ae60,color:#fff
    style F fill:#e74c3c,color:#fff

The classification of a reflex by the number of synapses it contains is examinable:

• Monosynaptic reflex — one synapse only (sensory neurone synapses directly with motor neurone). The knee-jerk reflex is the canonical example.
• Polysynaptic reflex — two or more synapses, including at least one relay (interneurone). The withdrawal reflex is the canonical example.

Each additional synapse adds ~0.5–1 ms of delay, so monosynaptic reflexes are slightly faster than polysynaptic ones.

---

The Knee-Jerk (Stretch) Reflex — Monosynaptic

The knee-jerk reflex tests the L3/L4 spinal cord and the femoral nerve. Sherrington's framework of reflex circuitry (paraphrased from his early-twentieth-century synaptic-integration concept) is the conceptual basis.

1. The doctor strikes the patellar tendon with a tendon hammer.
2. The tendon stretch is transmitted to the quadriceps muscle, mechanically stretching the muscle spindle stretch receptors embedded within it.
3. Stretching the muscle spindle opens stretch-mediated Na⁺ channels in the sensory neurone ending (see lesson 4); a generator potential is produced.
4. Action potentials propagate along the Ia afferent sensory neurone (one of the fastest myelinated axons in the body) toward the spinal cord.
5. In the spinal cord, the sensory neurone synapses directly with a motor neurone in the ventral horn — no relay neurone is involved.
6. The motor neurone fires, contracting the quadriceps, which extends the knee — the visible kick.
7. Simultaneously, reciprocal inhibition via a parallel inhibitory pathway relaxes the antagonist (hamstrings) so that the kick is not opposed.

The whole reflex completes in ~20–40 ms. Its physiological role outside the doctor's surgery is to maintain muscle length — when an unexpected load stretches the quadriceps (e.g. you stumble), the reflex contracts the muscle to compensate and restores posture before you fall.

Clinical Relevance

• Absent knee-jerk — suggests damage to L3/L4 sensory or motor pathway, peripheral neuropathy (e.g. diabetes), or lower motor neurone disease.
• Hyperactive knee-jerk — suggests upper motor neurone lesion (loss of cortical inhibition on the spinal reflex) — typical of stroke or spinal cord injury.

---

The Withdrawal Reflex — Polysynaptic, with Crossed Extensor

The withdrawal reflex protects against damage. If you step on a sharp object:

1. Pain receptors (nociceptors) in the foot detect the noxious stimulus.
2. Action potentials propagate along the afferent sensory neurone to the spinal cord.
3. In the spinal cord, the sensory neurone synapses with a relay (inter)neurone — note this extra synapse compared with the monosynaptic knee-jerk.
4. The relay neurone synapses with a motor neurone that drives the ipsilateral flexor muscles (biceps femoris, hamstrings).
5. The flexor contracts and withdraws the limb from the noxious stimulus.
6. Crossed extensor reflex (parallel pathway): the relay neurone also activates a crossed inhibitory interneurone and a contralateral motor neurone, which contracts the contralateral extensor (the opposite leg's quadriceps). This extends the opposite leg to support the body weight that the withdrawing leg can no longer carry — preventing a fall.
7. Ascending afferents from the relay neurone send the pain signal up the spinothalamic tract to the brain, where conscious pain perception emerges hundreds of milliseconds later.

The complete withdrawal-plus-crossed-extensor reflex is a beautiful example of how the nervous system coordinates multiple muscles in opposite directions on opposite sides of the body through a single reflex circuit, without any conscious supervision.

graph TD
    A["Noxious stimulus<br/>(e.g. sharp object)"] --> B["Nociceptor"]
    B --> C["Afferent neurone<br/>to spinal cord"]
    C --> D["Relay neurone<br/>(in spinal grey matter)"]
    D --> E["Ipsilateral flexor MN<br/>withdraws limb"]
    D --> F["Contralateral extensor MN<br/>supports body weight"]
    D --> G["Ascending tract<br/>brain → pain perception"]

    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#3498db,color:#fff

---

CNS / PNS — The Anatomy

Central Nervous System (CNS)

The CNS is the brain + spinal cord. It contains:

• Most cell bodies of relay neurones (interneurones) — forming the "grey matter" (so called because cell bodies are unmyelinated and appear pinkish-grey).
• The myelinated tracts of ascending and descending axons — forming the "white matter".
• The motor neurone cell bodies whose axons exit to the PNS.

Peripheral Nervous System (PNS)

The PNS is everything outside the CNS — the cranial nerves, spinal nerves, and their branches. It is functionally divided into the somatic and autonomic systems:

Feature	Somatic	Autonomic
Target	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Control	Voluntary (conscious + reflex)	Involuntary
Number of neurones per pathway	One (CNS → effector)	Two (CNS → ganglion → effector)
Neurotransmitter at effector	ACh (nicotinic)	ACh or noradrenaline (see below)
Examples	Walking, writing, talking	Heart rate, gut peristalsis, sweating, pupil diameter

The Autonomic Split — Sympathetic vs Parasympathetic

The autonomic system is itself subdivided into two antagonistic divisions, paraphrasing Walter Cannon's framework of the "fight or flight" vs "rest and digest" balance (no verbatim quotation — his original 1929 framing is well-known but should not be quoted directly):

Feature	Sympathetic	Parasympathetic
Activated during	Stress / exercise ("fight or flight")	Rest / digestion ("rest and digest")
Ganglion location	Close to CNS (sympathetic chain)	Close to effector
Pre-ganglionic neurotransmitter	ACh	ACh
Post-ganglionic neurotransmitter	Noradrenaline (mostly)	ACh
Heart rate	Increases	Decreases
Pupil diameter	Dilates	Constricts
Airway diameter	Dilates (bronchodilation)	Constricts
Salivary secretion	Decreases (dry mouth)	Increases
Gut peristalsis	Decreases	Increases
Sweat glands	Activates (cholinergic exception in sympathetic)	No effect
Adrenal medulla	Stimulates release of adrenaline	No direct effect

Most visceral organs receive dual innervation — both sympathetic and parasympathetic input. The instantaneous state of an organ reflects the balance between the two divisions, not the activity of either alone. This dual-control logic is examined synoptically with course 7 — homeostasis: the heart rate during exercise is the sympathetic accelerator minus the parasympathetic brake.

---

Reflex Inhibition Under Cortical Control

An A-Level-depth point: reflex arcs are not absolutely automatic. Cortical inhibition can override many reflexes if the conscious decision to do so is made before the stimulus, or in the brief window before the reflex fires. Examples:

• A doctor about to take a hot pan from a child can suppress the withdrawal reflex briefly enough to grip the pan and place it down safely — at the cost of mild burns.
• A boxer can suppress the blink reflex to maintain visual targeting through an opponent's feint.
• Trained meditators can modulate autonomic functions (heart rate, blood pressure) that are usually outside voluntary control.

The neural substrate is descending inhibitory pathways from the motor cortex and limbic system onto spinal interneurones. The cortex cannot abolish the reflex arc but can reduce its gain. This is the substrate of the broader principle that the nervous system is hierarchical — higher centres modulate lower ones, and a reflex is the lowest level of the hierarchy.

---

A-Level Deep Dive

Spec mapping

This content sits in AQA 7402 Section 3.6.2 — reflex arc structure and survival role; CNS / PNS; somatic and autonomic nervous systems (refer to the official AQA specification document for exact wording).

Synoptic links

This lesson connects to:

1. AQA 7402 Section 3.6.2 — neurone physiology (lessons 0–2 this course): every step of a reflex uses the cellular machinery established earlier. The sensory neurone is the cell type defined in lesson 0; the AP is the event defined in lesson 1; the synapse is the integration step defined in lesson 2.
2. AQA 7402 Section 3.6.3 — homeostasis (course 7 — homeostasis): the autonomic nervous system is the efferent arm of most homeostatic control loops. Sympathetic-driven thermoregulatory vasoconstriction, parasympathetic-driven salivary secretion, and sympathetic-driven adrenaline release during hypoglycaemia all involve the structures of this lesson.
3. AQA 7402 Section 3.6.2 — muscle contraction (lesson 3 this course): the somatic motor neurone of every voluntary reflex synapses at a neuromuscular junction and triggers the actin-myosin cycle covered in lesson 3.

Mark-scheme literacy

Reflex-arc questions split AO marks predictably:

AO	Typical share	Earned by
AO1 (knowledge)	50–60%	Naming the components in order, distinguishing CNS / PNS / somatic / autonomic
AO2 (application)	30–40%	Explaining mono- vs polysynaptic; linking sympathetic / parasympathetic effects to context
AO3 (analysis / evaluation)	10–20%	Evaluating survival advantages; explaining cortical override

Reward language includes "the sensory neurone synapses directly with the motor neurone in the spinal cord — this is a monosynaptic reflex", "the relay neurone in the spinal grey matter allows information to be sent simultaneously to the brain and to the contralateral side", "sympathetic activity dilates the pupil while parasympathetic activity constricts it". Mark-loss patterns include omitting the sensory-motor synapse in the spinal cord, calling all reflexes monosynaptic, and confusing sympathetic and parasympathetic effects on specific organs (heart rate accelerates with sympathetic, decelerates with parasympathetic).

Specimen question modelled on the AQA paper format

Question (6 marks): A person accidentally touches a hot pan. Describe the reflex arc that causes them to withdraw their hand, and explain how this reflex differs from a voluntary movement.

Mark scheme decomposition:

Mark	AO	Awarded for
1	AO1	Pain receptors / nociceptors / thermoreceptors in the skin detect the stimulus
2	AO1	Impulses pass along a sensory (afferent) neurone to the spinal cord
3	AO1	The sensory neurone synapses with a relay (inter)neurone, which synapses with a motor neurone
4	AO2	The motor neurone stimulates the biceps / flexor muscle, which contracts and withdraws the hand
5	AO2	The reflex is involuntary / does not require conscious thought / does not involve the cerebral cortex initially
6	AO3	Survival advantage: rapid response (~200 ms vs ~500 ms for voluntary) reduces tissue damage; parallel ascending pathway informs the brain after the response

Split: AO1 = 3, AO2 = 2, AO3 = 1.