Control & Coordination in Mammals – Full Chapter
Objective (AO1)
Outline the role of sensory receptor cells in detecting stimuli and stimulating the transmission of impulses in sensory neurones, and describe how these signals are integrated with the nervous and endocrine systems to produce coordinated responses.
Checklist – Cambridge 9700 (AS + A‑Level) Coverage
| Syllabus Section | Topics Covered | Status |
|---|
| Control & Coordination (Topic 15) | - Structure of the nervous system (CNS & PNS)
- Impulse generation & propagation
- Synaptic transmission
- Reflex arcs & voluntary motor pathways
- Sensory receptor cells – transduction & impulse initiation
- Endocrine system – hormones, signal‑transduction
- Neuro‑endocrine integration & feedback control (blood‑glucose, Ca²⁺, thermoregulation)
| ✓ – fully revised below |
| Practical Skills (Paper 3 & 5) | Receptor‑potential recordings, stimulus‑response curves, electrophysiology, hormone assays, feedback experiments | ✓ – Practical Skills box included |
| Mathematical/Quantitative Skills | Logarithmic stimulus‑response relationships, conduction‑velocity calculations, hormone‑concentration curves | ✓ – Math Note sidebar included |
1. The Nervous System – Overview
1.1 Major Divisions
- Central nervous system (CNS): brain and spinal cord; processes information and generates commands.
- Peripheral nervous system (PNS): sensory (afferent) neurones, motor (efferent) neurones, and autonomic fibres.
1.2 Basic Neurone Structure
| Region | Function |
|---|
| Dendrites | Receive synaptic inputs. |
| Soma (cell body) | Contains nucleus; integrates inputs. |
| Axon | Conducts action potentials away from soma. |
| Myelin sheath | Insulates axon; speeds conduction (saltatory). |
| Nodes of Ranvier | Sites of voltage‑gated Na⁺ channel clustering; regeneration of the impulse. |
| Terminal boutons | Release neurotransmitters onto postsynaptic cells. |
1.3 Conduction Velocity
- Myelinated A‑fibres: 10–120 m s⁻¹ (fast, precise).
- Unmyelinated C‑fibres: 0.5–2 m s⁻¹ (slow, dull pain).
- Velocity ∝ √(axon diameter) for unmyelinated fibres; ∝ diameter for myelinated fibres.
2. Sensory Receptor Cells – Detecting Stimuli
2.1 Definition
Specialised cells that convert a specific type of environmental energy (mechanical, thermal, chemical, or electromagnetic) into an electrical signal called a receptor potential. This process is known as sensory transduction (AO1).
2.2 Main Types of Receptors
| Receptor Type | Stimulus | Typical Ion(s) | Location |
|---|
| Mechanoreceptor | Deformation (touch, pressure, vibration) | Na⁺, Ca²⁺ (influx) | Skin, tendons, muscle spindles |
| Thermoreceptor | Temperature change | K⁺ (outward) & Na⁺ (inward) | Skin, hypothalamus |
| Photoreceptor (rod/cone) | Light | Na⁺/Ca²⁺ channels closed → hyperpolarisation | Retina |
| Chemoreceptor (taste/olfactory) | Soluble chemicals | Na⁺, Ca²⁺ (via GPCR pathways) | Taste buds, olfactory epithelium |
| Nociceptor | Potentially damaging mechanical/thermal/chemical stimuli | Na⁺, Ca²⁺ (influx) | Skin, muscles, viscera |
2.3 Structure of a Typical Receptor Cell
- Stimulus‑sensitive membrane region – contains specialised ion channels or receptor proteins that interact directly with the stimulus.
- Transduction machinery – stimulus‑gated ion channels (or GPCRs) that change membrane permeability, producing a graded receptor potential.
- Link to a sensory neurone – either a true synapse (e.g., photoreceptor → bipolar cell) or a direct continuation of the plasma membrane (e.g., dorsal‑root ganglion neurone).
2.4 From Receptor Potential to Action Potential (AO2)
- Stimulus opens stimulus‑gated channels → Na⁺/Ca²⁺ influx (or K⁺ efflux) → receptor potential (graded).
- If the depolarisation reaches the threshold at the trigger zone of the attached sensory neurone, voltage‑gated Na⁺ channels open.
- An all‑or‑none action potential is generated, usually at the first node of Ranvier.
- The action potential travels along the sensory neurone to the CNS for processing.
2.5 Example Pathways (AO2/AO3)
- Touch (Meissner’s corpuscle): Pressure → stretch‑sensitive Na⁺/Ca²⁺ channels → receptor potential → dorsal‑root‑ganglion neurone → dorsal‑column‑medial lemniscal pathway → primary somatosensory cortex.
- Vision (Rod): Photon → rhodopsin conformational change → closure of cGMP‑gated Na⁺ channels → hyperpolarisation → reduced glutamate release → bipolar cell modulation → optic nerve → thalamus → visual cortex.
- Taste (Sweet): Sucrose binds to GPCR on taste‑bud cell → IP₃/DAG cascade → Ca²⁺‑dependent channel opening → depolarisation → gustatory neurone (VII, IX, X) → gustatory cortex.
- Smell: Odourant binds to olfactory GPCR → ↑cAMP → opening of Na⁺/Ca²⁺ channels → depolarisation → olfactory neurone → olfactory bulb → piriform cortex.
- Pain (TRPV1): Heat >43 °C activates TRPV1 → Na⁺/Ca²⁺ influx → strong depolarisation → A‑δ/C‑fibre action potential → spinothalamic tract → somatosensory & limbic cortices.
3. Synaptic Transmission – From Neurone to Neurone
3.1 Chemical Synapse Components
- Presynaptic terminal: vesicles containing neurotransmitter (e.g., acetylcholine, glutamate).
- Synaptic cleft: ~20 nm gap.
- Postsynaptic membrane: ligand‑gated ion channels (excitatory) or G‑protein‑coupled receptors (modulatory).
3.2 Sequence of Events (AO2)
- Action potential arrives → voltage‑gated Ca²⁺ channels open.
- Ca²⁺ influx triggers vesicle fusion (SNARE proteins) → neurotransmitter released.
- Neurotransmitter binds to postsynaptic receptors → opening of ion channels.
- Resulting postsynaptic potential:
- Excitatory postsynaptic potential (EPSP): Na⁺ influx → depolarisation.
- Inhibitory postsynaptic potential (IPSP): Cl⁻ influx or K⁺ efflux → hyperpolarisation.
- If the summed EPSPs reach threshold at the axon hillock, a new action potential is generated.
3.3 Neurotransmitter Examples (AO1)
| Neurotransmitter | Typical Receptor | Effect |
|---|
| Acetylcholine (ACh) | Nicotine (ionotropic) / Muscarinic (GPCR) | Excitatory at neuromuscular junction; modulatory in CNS. |
| Glutamate | AMPA, NMDA (ionotropic) | Principal excitatory transmitter in CNS. |
| GABA | GABAA (ionotropic), GABAB (GPCR) | Major inhibitory transmitter. |
| Dopamine | D1‑like, D2‑like GPCRs | Modulatory; involved in reward, motor control. |
4. Reflex Arcs & Voluntary Motor Pathways
4.1 Simple Reflex (e.g., Patellar Reflex)
- Stretch receptors (muscle spindles) detect sudden lengthening.
- Afferent sensory neurone carries impulse to spinal cord.
- Synapse directly onto an efferent motor neurone (monosynaptic).
- Motor neurone activates the quadriceps → knee‑jerk.
Key features: short latency, no cortical involvement, protective.
4.2 Voluntary Motor Pathway (Corticospinal Tract)
- Upper motor neurone in primary motor cortex generates an action potential.
- Impulse travels down the internal capsule → brainstem → pyramidal decussation.
- Descends in the lateral corticospinal tract to spinal anterior horn.
- Synapse with lower motor neurone → skeletal muscle fibre → contraction.
4.3 Autonomic Motor Pathways
- Two‑neuron chain: pre‑ganglionic (CNS) → post‑ganglionic (target organ).
- Neurotransmitters: ACh at both synapses (parasympathetic) or ACh then norepinephrine (sympathetic).
5. The Endocrine System – Hormonal Coordination
5.1 General Features (AO1)
- Glands release hormones directly into the bloodstream.
- Hormones travel to distant target cells that possess specific receptors.
- Responses are generally slower but longer‑lasting than neural signals.
5.2 Major Glands & Representative Hormones
| Gland | Hormone(s) | Target/Function |
|---|
| Pituitary (anterior) | GH, ACTH, TSH, LH, FSH, Prolactin | Growth, metabolism, thyroid, reproduction. |
| Adrenal cortex | Cortisol, Aldosterone | Stress response, Na⁺/K⁺ balance. |
| Pancreas (β‑cells) | Insulin | Lowers blood glucose. |
| Pancreas (α‑cells) | Glucagon | Raises blood glucose. |
| Thyroid | Thyroxine (T₄), Triiodothyronine (T₃) | Regulates basal metabolic rate. |
| Hypothalamus | CRH, TRH, GnRH, ADH, Oxytocin | Controls pituitary release; water balance; uterine contraction. |
5.3 Hormone Signal‑Transduction (AO2)
- Peptide hormones → bind cell‑surface receptors → activate G‑proteins → second messengers (cAMP, IP₃/DAG) → intracellular response.
- Steroid hormones → diffuse through membrane → bind intracellular receptors → act as transcription factors → change gene expression (slow response).
6. Integration of Nervous & Endocrine Systems
6.1 Neuro‑endocrine Control (Hypothalamic‑Pituitary Axis)
- Hypothalamic neurosecretory cells release releasing or inhibiting hormones into the hypophyseal portal vessels.
- Anterior pituitary responds by secreting target hormones (e.g., ACTH → adrenal cortex).
- Feedback: circulating hormones (cortisol, thyroid hormones) inhibit hypothalamic and pituitary release (negative feedback).
6.2 Homeostatic Feedback Loops (AO3)
| Variable | Sensor (Receptor) | Control Centre | Effector | Response |
|---|
| Blood Glucose | Pancreatic β‑cells (high) & α‑cells (low) | Pancreas (endocrine) & hypothalamus (neural) | Insulin‑responsive tissues (muscle, adipose) / Liver | Insulin ↓ glucose; Glucagon ↑ glucose. |
| Plasma Ca²⁺ | Parathyroid chief cells (low Ca²⁺) | Parathyroid gland | Bone (resorption), kidney (reabsorption), intestine (via vitamin D) | Parathyroid hormone ↑ plasma Ca²⁺. |
| Core Body Temperature | Thermoreceptors in skin & hypothalamus | Hypothalamic pre‑optic area | Vasculature (vasodilation/constriction), skeletal muscle (shivering), sweat glands | Heat loss or heat production to restore set‑point. |
6.3 Example: Stress Response
- Perceived threat → hypothalamus activates sympathetic nervous system → adrenal medulla releases adrenaline (fast).
- Simultaneously, hypothalamus releases CRH → pituitary secretes ACTH → adrenal cortex releases cortisol (slow, prolongs glucose mobilisation).
- Negative feedback by cortisol on hypothalamus and pituitary terminates the response.
7. Summary Table – Key Points (AO1‑AO2)
| Process | Key Feature | Typical Example | Relevance to Exam |
|---|
| Sensory transduction | Stimulus‑gated ion channels → receptor potential (graded) | Mechano‑stretch channels in Pacinian corpuscle | AO1: define; AO2: describe mechanism. |
| Action potential generation | Threshold → voltage‑gated Na⁺ channels → all‑or‑none spike | Trigger zone at first node of Ranvier | AO2: explain conversion from graded to all‑or‑none. |
| Synaptic transmission | Ca²⁺‑dependent vesicle release → EPSP/IPSP | Glutamate at CNS excitatory synapse | AO2: outline steps; AO3: analyse drug effects. |
| Reflex arc | Monosynaptic (stretch) or polysynaptic (withdrawal) | Patellar reflex | AO1: name components; AO2: describe pathway. |
| Hormonal feedback | Negative feedback via circulating hormone | Thyroid‑hypothalamic‑pituitary axis | AO3: evaluate why feedback stabilises. |
8. Practical Skills Box (AO3)
Typical investigations for Cambridge Paper 3 & 5
- Record receptor potentials from isolated mechanoreceptors while varying pressure; plot stimulus‑response curve.
- Use specific ion‑channel blockers (e.g., tetrodotoxin, capsazepine) to demonstrate their role in transduction.
- Measure conduction velocity of sensory fibres by stimulating at two points and timing the arrival of the action potential at a recording electrode.
- Assess hormone release (e.g., insulin) from pancreatic islets in response to graded glucose concentrations; construct dose‑response curve.
- Investigate negative feedback by adding cortisol to cultured pituitary cells and measuring ACTH secretion.
9. Math Note Sidebar (AO2 – Quantitative Skills)
Stimulus‑Response Relationship (Weber‑Fechner)
For many receptors the receptor potential (Vr) varies logarithmically with stimulus intensity (I):
Vr = k · log10(I/I0)
where k is a constant and I0 is the threshold intensity. This explains why a large increase in stimulus is required for a noticeable change at high intensities.
In endocrine feedback loops, hormone concentration (C) often follows a first‑order decay:
C(t) = C₀ e‑kt
Use this equation to calculate half‑life (t½ = ln 2 / k) of hormones such as cortisol.
10. Quick Revision Checklist
- Define sensory receptor cell, receptor potential, and transduction.
- List the five major receptor types and the ions involved.
- Explain how a graded receptor potential becomes an all‑or‑none action potential.
- Describe the main steps of chemical synaptic transmission.
- Outline a simple reflex arc and a voluntary motor pathway.
- Identify the major endocrine glands and one key hormone each.
- Explain negative feedback using at least two homeostatic examples.
- Be able to sketch and label a diagram of a mechanoreceptor linked to a dorsal‑root neurone.
- Know the equations for stimulus‑response (logarithmic) and hormone decay (exponential).
Suggested Diagrams (to be added to study notes)
- Cross‑section of a mechanoreceptor (e.g., Pacinian corpuscle) showing lamellae, stimulus‑gated channels, and the attached sensory neurone.
- Action potential propagation along a myelinated axon with nodes of Ranvier highlighted.
- Chemical synapse – vesicle fusion, neurotransmitter release, postsynaptic receptor.
- Monosynaptic reflex arc (patellar reflex) with labelled sensory and motor neurones.
- Hypothalamic‑pituitary‑target gland axis illustrating releasing hormone, pituitary hormone, target organ, and feedback.