Show the model answerAttempt it first — that is what makes it stick
Define pain
What earns the marks
| The whole definition | Both qualities (sensory and emotional) and both kinds of damage (actual or potential) |
|---|---|
| The distinction | Pain is an experience; nociception is a neural process |
| Definition | Occurs without the other | |
|---|---|---|
| Nociception | The neural process of encoding noxious stimuli. | Yes: it continues in an anaesthetised patient who feels nothing |
| Pain | The conscious experience | Yes: phantom limb pain, with no nociceptors to activate |
(b) The pain pathway
What earns the marks
| Receptor and transduction | Free nerve endings; mediators open transducer channels; action potentials generated |
|---|---|
| Fibres | Aδ first pain, C second pain |
| First-order neurone | Cell body in the dorsal root ganglion; synapse in the dorsal horn |
| Second-order neurone | Crosses within one or two segments; contralateral spinothalamic tract |
| Third-order neurone | Thalamus to somatosensory cortex, and medially to cingulate and insula |
| A diagram | The fastest way to earn the structure marks |
Read the question: “Pain pathway” is the ascending pathway from receptor to cortex. Modulation is a sentence here at most, and part (c) is where the reflex response belongs.
The ascending pathway, in three neurons
Read it from the bottom. The first-order neuron has its cell body in the dorsal root ganglion, outside the cord, and synapses in the dorsal horn. The second-order neuron starts in the dorsal horn, crosses in the cord and ascends on the opposite side. At the brainstem the projection divides: the lateral, neospinothalamic route relays in the ventral posterolateral nucleus (VPL) and reaches the primary somatosensory cortex, which is where and how intense; the medial, paleospinothalamic route passes through the reticular formation and periaqueductal grey to the intralaminar nuclei and on to the cingulate and insula, which is how unpleasant. The third-order neuron runs from thalamus to cortex. From the face the plan is the same, with the first-order cell body in the trigeminal ganglion, the second in the spinal trigeminal nucleus and the third in the ventral posteromedial nucleus.
| Step | Structure | Detail |
|---|---|---|
| Transduction | Nociceptor: free nerve endings in skin, muscle, joints and viscera | Tissue damage releases K⁺, H⁺, ATP, bradykinin, serotonin, histamine and prostaglandins; TRPV1, ASIC and P2X3 open; the generator potential fires action potentials through voltage-gated Na⁺ channels |
| Fibres | Aδ and C | Aδ: thinly myelinated, 12 to 30 m/s, sharp and localised first pain. C: unmyelinated, 0.5 to 2 m/s, dull, diffuse second pain |
| First-order neurone | Cell body in the dorsal root ganglion | Enters the dorsal root, divides in Lissauer's tract over 2 to 3 segments, synapses in the dorsal horn: Aδ in laminae I and V, C in lamina II. Glutamate and substance P |
| Second-order neurone | Cell body in the dorsal horn | Nociceptive-specific or wide dynamic range. Crosses in the anterior white commissure within one or two segments; ascends in the contralateral anterolateral quadrant as the spinothalamic tract |
| Third-order neurone | Cell body in the thalamus | Lateral route: ventral posterolateral nucleus to primary and secondary somatosensory cortex (location, intensity). Medial route: via reticular formation and periaqueductal grey to intralaminar nuclei, anterior cingulate and insula (unpleasantness, arousal, autonomic response) |
The medial stream matters for part (c): its targets in the reticular formation sit beside the regions that control blood pressure, and the insula carries the pain-related autonomic response. That is where the cardiovascular response starts.
(c) The cardiovascular response to pain
What earns the marks
| Afferent | Nociceptive input via the spinoreticular, spinolimbic and paleospinothalamic routes |
|---|---|
| Integration | Medullary cardiovascular centres and hypothalamus; defence and alerting responses |
| Efferent | Sympathetic outflow and adrenal catecholamines up; vagal tone down |
| Effects | Heart rate, contractility, cardiac output, vascular resistance and blood pressure up |
| Consequences | Myocardial oxygen demand up while supply may fall; salt and water retention; hypercoagulability |
| The exception | Severe visceral pain: hypotension and bradycardia |
Read the question: “Discuss” asks for mechanism and consequence. A list of “tachycardia, hypertension” without the pathway that produces them and what they cost the heart is a description, not a discussion.
From nociceptive input to the cardiovascular response
Integration
Efferent
| Variable | Change | Mechanism | Why it matters |
|---|---|---|---|
| Heart rate | Rises | β1 stimulation of the sinoatrial node; vagal withdrawal | Shortens diastole, the time for coronary flow to the left ventricle |
| Contractility | Rises | β1 stimulation; circulating adrenaline | More myocardial work |
| Cardiac output | Rises | Rate and contractility together | |
| Systemic vascular resistance | Rises | α1 arteriolar vasoconstriction; angiotensin II | Raises afterload |
| Arterial pressure | Rises | Cardiac output and resistance both up | Under anaesthesia, hypertension and tachycardia are the usual signs of nociception |
| Myocardial oxygen balance | Demand up; supply may fall | Rate, contractility and afterload raise demand; sympathetic coronary vasoconstriction and a blunted metabolic vasodilatation reduce supply | Myocardial ischaemia and infarction in a patient with coronary disease |
| Circulating volume | Expands | Renin, angiotensin II, aldosterone and ADH: sodium and water retention | More preload |
| Coagulation | Hypercoagulable | Fewer natural anticoagulants, more procoagulants, inhibited fibrinolysis, more reactive platelets, higher viscosity | Deep venous thrombosis, vascular graft failure, myocardial ischaemia |
- The size of the neuroendocrine and sympathetic response is proportional to the degree of surgical trauma, and is reduced by reducing the nociceptive input.
- That is the cardiovascular argument for effective analgesia, regional block and multimodal regimens: less input, less sympathetic outflow, less myocardial oxygen demand.