Im currently studying in first year mbbs and and for my physiology finals they may ask a question as how is touch transmitted. how many marks is this question usually for
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I now have enough from Guyton & Hall to give you a thorough answer. Here is the full picture:
How is Touch Transmitted? - 1st Year MBBS Physiology
About the marks
In most 1st year MBBS physiology exams (India and internationally), this question appears as:
- 5-mark short answer question (SAQ) - the most common format
- Sometimes as a 10-mark long answer if combined with pain/temperature pathways
- Occasionally as 3-mark short notes if only asking about receptors OR only the pathway
A 5-mark answer requires: receptors + nerve fibers + spinal pathways + higher centers. That is exactly what is covered below.
Complete Answer: Transmission of Touch
Step 1 - Receptors in the Skin
Touch, pressure, and vibration are all detected by mechanoreceptors. There are six main types:
| Receptor | Location | Adapts | Detects |
|---|---|---|---|
| Meissner corpuscles | Non-hairy skin (fingertips, lips) | Rapidly | Movement over skin, low-frequency vibration |
| Merkel discs | Basal epidermis (hairy + non-hairy skin) | Slowly | Sustained touch, steady-state pressure |
| Ruffini endings | Deeper dermis, joint capsules | Very slowly | Continuous deformation, heavy prolonged touch |
| Pacinian corpuscles | Deep dermis and fascial tissues | Very rapidly (hundredths of a second) | Rapid vibration (30-800 Hz), rapid compression |
| Hair end-organs | Base of hair follicles | Readily | Initial contact, object movement |
| Free nerve endings | Everywhere in skin | - | Crude touch, tickle, itch |
(Guyton and Hall Textbook of Medical Physiology)
Step 2 - Peripheral Nerve Fibers
- Specialized receptors (Meissner, Merkel, Pacinian, Ruffini, hair end-organs) - transmit via large myelinated Aβ fibers (30-70 m/s) - carries fine/discriminative touch
- Free nerve endings - transmit via small myelinated Aδ fibers (5-30 m/s) and unmyelinated C fibers (<2 m/s) - carry crude touch, tickle, itch
Step 3 - Entry into Spinal Cord
All somatic sensory signals enter via the dorsal roots of spinal nerves. From here, touch is carried by two separate pathways depending on the type:
Step 4a - Dorsal Column-Medial Lemniscal Pathway (Fine/Discriminative Touch)
This carries:
- Fine/precise touch (high localization)
- Fine gradations of intensity
- Vibration
- Proprioception and joint position sense
Route:
- First-order neuron: Enters dorsal horn → travels ipsilaterally up the dorsal columns (fasciculus gracilis for lower limb/trunk; fasciculus cuneatus for upper limb)
- Synapses in dorsal column nuclei (nucleus gracilis / nucleus cuneatus) in the medulla
- Second-order neuron: Decussates (crosses midline) as internal arcuate fibers → forms the medial lemniscus → ascends to the VPL (ventral posterolateral) nucleus of the thalamus
- Third-order neuron: From thalamus → primary somatosensory cortex (postcentral gyrus, areas 3,1,2)
Step 4b - Anterolateral (Spinothalamic) Pathway (Crude Touch)
This carries:
- Crude (poorly localized) touch
- Pain, temperature, tickle
Route:
- First-order neuron: Enters dorsal horn → synapses in the dorsal horn immediately
- Second-order neuron: Crosses to the opposite side of the cord at the same level → ascends in the anterior and lateral white columns (anterior spinothalamic tract for crude touch)
- Terminates in the thalamus (VPL nucleus) and lower brainstem
- Third-order neuron: Thalamus → somatosensory cortex
Key Differences to Remember
| Feature | Dorsal Column | Anterolateral |
|---|---|---|
| Type of touch | Fine, discriminative | Crude |
| Fiber size | Large myelinated (Aβ) | Small myelinated/unmyelinated |
| Speed | 30-110 m/s | Few to 40 m/s |
| Where it crosses | Medulla | Spinal cord (same level) |
| Spatial orientation | High | Low |
Quick Diagram (for memory)
SKIN RECEPTORS
↓ (Aβ / Aδ / C fibers)
DORSAL ROOT → enters spinal cord
Fine touch: Crude touch:
Dorsal columns Crosses at cord level
↓ (ipsilateral) ↓ (contralateral)
Medulla nuclei Ant. spinothalamic tract
↓ (decussate) ↓
Medial lemniscus Thalamus (VPL)
↓ ↓
Thalamus (VPL) Somatosensory cortex
↓
Somatosensory cortex
(postcentral gyrus)
For a 5-mark question, write: receptors (2 marks) + pathways with where they cross and where they go (3 marks). Keep it structured with headings - examiners reward clarity.
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