describe the structure and function of a sensory neurone and a motor neurone and state that intermediate neurones connect sensory neurones and motor neurones

Control and Coordination in Mammals – Sensory, Interneurone and Motor Neurones

Learning Objective (AO1)

Describe the structure and function of a sensory neurone and a motor neurone, and state that interneurones (intermediate neurones) connect sensory neurones with motor neurones. In addition, explain how action potentials are generated and conducted, how synaptic transmission occurs, how neuronal pathways are integrated centrally, and how they interact with hormonal control and feedback mechanisms (AO2–AO3).

1. Syllabus Overview (Cambridge AS & A‑Level Biology 9700 – Topic 15)

• Basic structure of neurones (dendrites, soma, axon, myelin, terminals).
• Generation and propagation of action potentials (resting potential, threshold, all‑or‑none, refractory periods, saltatory conduction).
• Myelination: Schwann cells (PNS) vs. oligodendrocytes (CNS) and quantitative effect on speed.
• Synaptic transmission – vesicle cycle, neurotransmitters, re‑uptake, enzymatic degradation.
• Interneuronal circuits – excitatory & inhibitory interneurones.
• Simple reflex arcs, complex reflexes, central pattern generators (CPGs).
• Central integration pathways – ascending and descending tracts, thalamic relay, cortical motor areas.
• Hormonal control (adrenaline, insulin, calcium) and feedback loops (negative & positive) with explicit examples.
• Interaction between the nervous and endocrine systems (e.g., stress response, glucose regulation).

2. Sensory Neurone (Afferent Neurone)

Transmits information from specialised peripheral receptors toward the central nervous system (CNS).

• Dendrites / Sensory endings: free nerve endings, Meissner’s corpuscles, Pacinian corpuscles, hair‑cell bundles, photoreceptors – each tuned to a specific stimulus (mechanical, thermal, chemical, photic).
• Cell body (soma): located in a dorsal‑root ganglion (spinal) or a cranial‑nerve ganglion; contains nucleus, Nissl bodies, mitochondria and the Na⁺/K⁺‑ATPase that maintains the resting membrane potential (≈ ‑65 to ‑75 mV).
• Axon: usually a single, long fibre that enters the dorsal root and projects centrally into the spinal cord or brainstem.
• Myelin sheath: many sensory fibres are myelinated by Schwann cells. Myelination raises conduction velocity to ≈ 120 m s⁻¹ (vs. ≤ 2 m s⁻¹ for unmyelinated fibres) via saltatory conduction.
• Synaptic terminals: terminate on interneurones in the dorsal horn; release the excitatory neurotransmitter glutamate (most common) and, in some nociceptive fibres, substance P or CGRP.

3. Motor Neurone (Efferent Neurone)

Conveys impulses from the CNS to effectors (skeletal muscle fibres, cardiac muscle, smooth muscle or glands) to produce a response.

• Cell body (soma): situated in the ventral grey matter of the spinal cord or in cranial‑nerve nuclei (inside the CNS).
• Dendrites: receive synaptic input from interneurones and other motor neurones.
• Axon: a single, often very long fibre that exits the CNS via the ventral root, becomes myelinated by oligodendrocytes in the CNS and by Schwann cells after it leaves the CNS.
• Myelin sheath: heavy myelination enables rapid transmission (≈ 100–120 m s⁻¹). Nodes of Ranvier are spaced ≈ 1 mm apart.
• Synaptic terminals (neuromuscular junction): release the neurotransmitter acetylcholine (ACh), which binds to nicotinic receptors on the muscle fibre, causing depolarisation and contraction.

4. Interneurone (Intermediate Neurone)

Located entirely within the CNS, interneurones form the essential bridge between sensory and motor pathways.

• Location: dorsal and ventral horns of the spinal cord, brainstem nuclei, cerebellum, basal ganglia and cerebral cortex.
• Types:

  • Excitatory – release glutamate; amplify incoming signals.
  • Inhibitory – release GABA or glycine; dampen or shape the response.

• Functions:

  • Integration of multiple sensory inputs.
  • Generation of patterned motor output (central pattern generators).
  • Modulation of reflex strength (reciprocal inhibition, presynaptic inhibition).
  • Higher‑order processing in cortical circuits (planning, decision‑making).

5. Action Potential Generation & Conduction (AO2)

• Resting membrane potential: ≈ ‑70 mV (range ‑65 to ‑75 mV) maintained by the Na⁺/K⁺‑ATPase (3 Na⁺ out, 2 K⁺ in).
• Threshold: depolarisation to about ‑55 mV opens voltage‑gated Na⁺ channels.
• Depolarisation (upstroke): rapid Na⁺ influx; membrane potential rises to ≈ +30 mV (≈ 100 mV amplitude).
• Repolarisation: Na⁺ channels inactivate, voltage‑gated K⁺ channels open, K⁺ efflux restores negative interior.
• Hyper‑polarisation: K⁺ channels remain open briefly, membrane potential falls below resting level.
• All‑or‑none principle: once threshold is reached, an action potential of fixed amplitude travels the whole axon.
• Refractory periods:

  • Absolute refractory period ≈ 1 ms – no new AP can be generated.
  • Relative refractory period ≈ 2–4 ms – a stronger stimulus can trigger another AP.

• Myelinated conduction: impulse jumps between nodes of Ranvier (saltatory conduction) → speeds up to 120 m s⁻¹.
• Unmyelinated conduction: continuous wave of depolarisation → ≤ 2 m s⁻¹.

6. Synaptic Transmission (Neurotransmission)

1. Arrival of an action potential at the presynaptic terminal opens voltage‑gated Ca²⁺ channels.
2. Calcium influx triggers the SNARE‑mediated fusion of synaptic vesicles with the presynaptic membrane.
3. Neurotransmitter release into the synaptic cleft (e.g., glutamate, ACh, GABA, norepinephrine, dopamine).
4. Binding to postsynaptic receptors:

   • Ionotropic receptors → fast excitatory (EPSP) or inhibitory (IPSP) postsynaptic potentials.
   • Metabotropic receptors → slower, modulatory effects via second messengers.

5. Termination of the signal:

   • Re‑uptake (e.g., choline transporter for ACh, Na⁺/Cl⁻ cotransporter for GABA).
   • Enzymatic degradation (acetylcholinesterase, monoamine oxidase, catechol‑O‑methyltransferase).
   • Diffusion away from the cleft.

7. Central Integration Pathways (AO2)

• Ascending tracts:

  • Dorsal column‑medial lemniscal pathway – fine touch, vibration, proprioception; synapse in the dorsal column nuclei → thalamus → primary somatosensory cortex.
  • Spinothalamic tract – pain and temperature; synapse in the dorsal horn → contralateral thalamus → somatosensory cortex.

• Descending tracts:

  • Corticospinal (pyramidal) tract – voluntary motor control; terminates on α‑motor neurones in the ventral horn.
  • Reticulospinal and vestibulospinal tracts – posture and balance.

• Thalamic relay: the ventral posterior nucleus acts as a hub, forwarding sensory information to the appropriate cortical area.
• Cortical motor areas:

  • Primary motor cortex (M1) – initiates voluntary movement.
  • Premotor and supplementary motor areas – planning and sequencing.
  • Somatosensory cortex – provides feedback for fine‑tuned movements.

• Central Pattern Generators (CPGs): networks of interneurones in the spinal cord that produce rhythmic outputs (e.g., walking, breathing) without supraspinal input.

8. Reflexes

8.1 Simple (Monosynaptic) Reflex – Knee‑Jerk

1. Muscle‑spindle receptor detects stretch of the quadriceps.
2. Sensory neurone carries the impulse to the dorsal horn.
3. Direct excitatory interneurone (single synapse) in the ventral horn.
4. Motor neurone exits via the ventral root to the quadriceps.
5. Result: rapid contraction (≈ 30 ms latency).

8.2 Complex (Polysynaptic) Reflex – Withdrawal Reflex

• Stimulus: painful pinch to the foot.
• Multiple interneurones: excitatory to flexor motor neurones, inhibitory to extensor motor neurones (reciprocal inhibition).
• Integration in the spinal cord allows a coordinated withdrawal movement.

8.3 Central Pattern Generators (CPGs)

Networks of interneurones in the lumbar spinal cord generate the alternating flexor‑extensor pattern for locomotion. Supraspinal centres (e.g., mesencephalic locomotor region) can modulate the rhythm but are not required for its basic generation.

9. Hormonal Control & Feedback Loops (AO2)

Interaction example – Stress response: A stressful stimulus activates the hypothalamus → releases CRH → pituitary ACTH → adrenal cortex cortisol. Simultaneously, the hypothalamus stimulates the sympathetic nervous system → adrenal medulla releases adrenaline. The combined neuronal and hormonal actions raise blood glucose, increase cardiac output and sharpen alertness. Cortisol provides a longer‑term negative feedback on CRH and ACTH release.

10. Comparison of Sensory and Motor Neurones

11. Practical Activities (AO3)

11.1 Reflex‑Latency Experiment (Monosynaptic Reflex)

1. Materials: ruler (30 cm), stopwatch, safety goggles.
2. Procedure:

   1. Student A holds the ruler vertically, hand relaxed.
   2. Student B releases the ruler without warning; Student A catches it as quickly as possible.
   3. Record the distance (d) the ruler falls before being caught.
   4. Calculate the reaction time using \(t = \sqrt{2d/g}\) (g ≈ 9.8 m s⁻²).
   5. Repeat three trials, compute the mean and standard deviation.

3. Link to syllabus: use the calculated time to estimate conduction velocity (distance travelled by the impulse = length of the afferent + efferent pathways). Discuss how myelination, temperature and fatigue would affect the result (AO2).
4. Safety & error analysis:

   • Ensure the ruler is dropped vertically to avoid lateral motion.
   • Possible errors: reaction‑time delay of the catcher, inaccurate distance measurement, air resistance (negligible), and individual variation in synaptic delay.

11.2 Neurotransmitter Release vs. Calcium Concentration (Data‑Interpretation)

Students are given a table showing the amount of glutamate released from a cultured hippocampal synapse at different extracellular Ca²⁺ concentrations (0.5 mM, 1 mM, 2 mM, 4 mM). They must:

• Plot Ca²⁺ concentration (x‑axis) against amount of neurotransmitter released (y‑axis).
• Explain the relationship using the role of voltage‑gated Ca²⁺ channels in the vesicle‑fusion process (AO2).
• Predict how a Ca²⁺ channel blocker (e.g., verapamil) would alter the graph and discuss the physiological implications (AO3).

12. Sample Assessment Questions (AO1–AO3)

1. Labelled diagram (AO1) – Draw a reflex arc showing a sensory neurone, an interneurone and a motor neurone. Label dendrites, soma, axon, myelin, synaptic terminals and state the function of each part.
2. Data interpretation (AO2) – A table shows conduction velocities for myelinated (≈ 120 m s⁻¹) and unmyelinated (≈ 1 m s⁻¹) fibres. Explain the difference in terms of myelin structure and saltatory conduction.
3. Experimental design (AO3) – “Design an experiment to investigate the effect of ambient temperature on reflex latency.” Include hypothesis, variables, method, safety considerations and expected outcome.
4. Integrative question (AO2) – Explain how the release of adrenaline during a stressful situation complements the neuronal ‘fight‑or‑flight’ response, linking sympathetic activation, hormone action and feedback regulation.
5. Evaluation (AO3) – Given the data from the calcium‑dependence experiment, evaluate the reliability of the results and suggest two ways to improve the experimental design.

13. Suggested Diagram (Figure)