An all-or-none signal
that still carries how strong the stimulus was.

The minimum high-yield framework

1. Three classifications — by modality (mechano-, thermo-, noci-, chemo-, photoreceptor), by location (extero-, proprio-, interoceptor), and by adaptation (rapidly or slowly adapting).
2. Transduction converts stimulus energy into a graded receptor potential, usually by increasing sodium conductance. Where the receptor is a specialised ending on a sensory nerve, the same potential is called a generator potential.
3. Graded, then all-or-none. The generator potential is proportional to stimulus strength and decays with distance; once it reaches threshold at the first node, it triggers all-or-none action potentials.
4. Four codes. Modality by labelled line; intensity by frequency and recruitment; location by which fibres and by receptive-field size; duration by adaptation rate.
5. Adaptation. Rapidly adapting receptors report change; slowly adapting receptors report a maintained state. Nociceptors are the exception — they do not adapt, they sensitise.
6. Two pathways, three neurons each. Dorsal column-medial lemniscus crosses in the medulla; spinothalamic crosses in the cord. Both relay in the thalamus and end in the postcentral gyrus.
7. Receptive field size sets acuity. Small fields, dense innervation and large cortical representation go together — hence the homunculus.

1. The stimulus reaches the receptor. Mechanical deformation, a temperature change, a chemical, or light.
2. Ion channels open. In most somatic receptors this is a mechanically-gated or thermally-gated cation channel, and the dominant effect is an increase in sodium conductance.
3. A graded depolarisation results — the receptor potential, or, in a specialised ending on a sensory nerve fibre, the generator potential. Its amplitude is proportional to stimulus intensity, and it spreads electrotonically, decaying with distance.
4. If it reaches threshold at the first node of Ranvier, all-or-none action potentials are generated and propagate to the central nervous system. Their frequency is proportional to the amplitude of the generator potential.

The relationship between stimulus intensity and firing frequency is not necessarily linear. Over much of the range it follows a logarithmic relationship — the Weber-Fechner law: the perceived increase in intensity is proportional to the fractional change in the stimulus, not the absolute one. This is what allows one sensory system to cover an enormous dynamic range without saturating, and it is why a given increment feels large against a small background and negligible against a large one.

Adaptation has two mechanisms, and both are worth naming. The first is a fall in the generator potential despite a continuing stimulus — in the Pacinian corpuscle this is largely mechanical, because the fluid between the lamellae redistributes and stops transmitting a static force to the core. The second is a reduction in the excitability of the ending: increased potassium conductance, inactivation of sodium channels, or increased activity of the electrogenic Na⁺/K⁺-ATPase.

Two-point discrimination is the minimum separation at which two simultaneous stimuli are felt as two rather than one, and it varies enormously across the body — of the order of a few millimetres on the fingertip and several centimetres on the back. Three things vary together and explain it:

• Receptive field size. Small on the fingertips and lips, large on the trunk.
• Innervation density. More receptors per unit area where fields are small.
• Cortical representation. The area of postcentral gyrus devoted to a body part is proportional to its innervation density, not to its physical size — which is what the sensory homunculus depicts. The hand, lips and tongue are disproportionately large; the trunk and legs small.

The same principle holds in the motor cortex, where representation is proportional to the precision of control rather than to muscle bulk.

Sensation from the face does not use either: it travels in the trigeminal nerve, entering the brainstem at mid-pontine level. Its fibres segregate into two streams that mirror the spinal arrangement — one for discriminative touch, vibration and proprioception, terminating in the principal sensory nucleus, and one for pain, temperature and crude touch, descending in the spinal trigeminal tract. Both then cross and ascend to the ventral posteromedial nucleus of the thalamus, rather than the posterolateral nucleus that serves the body.

The primary somatosensory cortex (S1) occupies the postcentral gyrus and is somatotopically organised as the sensory homunculus. It is where the modality, intensity and location of a stimulus become conscious. The secondary somatosensory cortex and the posterior parietal association areas integrate across modalities and across the two sides of the body, and they are what allow an object to be recognised by touch alone (stereognosis) and the body to be located in space.

Two clinical consequences follow. A lesion of S1 produces loss of discriminative sensation — the ability to localise, to judge intensity, to discriminate two points — while crude awareness of touch and pain may survive, because the thalamus and the older paleospinothalamic projections retain some capacity for it. And a lesion of the non-dominant parietal association cortex produces neglect: the sensation arrives, but the patient does not attend to that half of the world.

Everything about assessing a regional block is applied sensory physiology. Cold sensation is the most sensitive test of block height because it is carried by small unmyelinated C and thinly myelinated Aδ fibres, which are blocked earliest; pinprick, also Aδ, is next; light touch, carried by large myelinated Aβ fibres, persists longest and its loss marks the most conservative estimate of block height. The three levels are typically two or more segments apart, and the sympathetic (B fibre) level is higher still.

The same logic reads the other way in a returning block: touch returns before pinprick, and motor function returns last of all. And it explains why a patient can accurately report being touched during surgery under an adequate spinal without that indicating a failed block — the modality they are reporting is carried by the fibres least affected.

Testing a dermatome tests a spinal segment, and the useful landmarks are worth carrying: T4 at the nipple line, T6 at the xiphisternum, T10 at the umbilicus and L1 at the inguinal ligament. A block to T4 is required for caesarean section because peritoneal traction is referred through afferents entering at that level and above.