Source: A Brain-Mind Odyssey, Ch. 16
Tags: somatosensory, mechanoreceptor, TRP receptor, dorsal root ganglion, receptive field, somatosensory cortex S1, Wilder Penfield, homunculus, body map, neglect syndrome, somatosensory agnosia, phantom limb, Ramachandran, mirror therapy, motor cortex M1, neuromuscular junction, apraxia, mirror neurons, anosognosia, mouse whisker barrels, neuroplasticity
Touch, temperature, and pain signals originate at receptor endings in the skin, travel through dorsal root ganglia and the spinal cord to the primary somatosensory cortex (S1) in the parietal lobe, which contains a topographic body map (homunculus). The motor cortex (M1) in the frontal lobe sends commands via the spinal cord to muscles. Both maps are distorted, with the largest representations for the most sensitive or dexterous body parts. Damage to these areas produces specific deficits, from numbness to paralysis to phantom limb sensations, each of which reveals how the brain constructs bodily experience.
Mechanoreceptor
A sensory receptor activated by physical stimuli such as touch or pressure, via mechanically gated ion channels.
Dorsal root ganglion (DRG)
Clusters of sensory neuron cell bodies near the spinal cord. DRG fibres innervating the skin are contiguous with axons sending signals into the CNS.
Receptive field (somatosensory)
The region of skin within which a physical stimulus elicits activity in a specified neuron.
Somatosensory homunculus
The distorted body map on the cortical surface of S1, in which body parts are sized according to sensory sensitivity rather than physical size. Fingers and lips have the largest representations.
Phantom limb
The continued experience of a limb's presence after amputation. Caused by cortical reorganisation in which adjacent body map regions form new connections with the deafferented area.
Anosognosia
A lack of awareness of one's own disease or disability. Patients may confabulate explanations for why they cannot move paralysed limbs.
Mirror neurons
Cells in the premotor cortex that fire both when performing a movement and when observing the same movement in another person.
Apraxia
A disorder of the organisation of movement, typically caused by lesions in premotor areas. The person can move individual muscles but cannot coordinate purposeful actions.
Dendrites of somatosensory neurons terminate in the upper layers of the skin
Their membranes contain receptor proteins that respond to:
Touch and pressure (mechanically gated ion channels)
Temperature changes (TRP receptors)
Painful or noxious stimuli
Cell bodies for somatosensory fibres are located in clusters near the spinal cord called dorsal root ganglia (DRG)
DRG nerve fibres innervating the skin are contiguous with the axons that carry signals into the CNS
Unusual: the action potential travels toward the cell body (opposite to the typical direction in most neurons)
Each somatosensory neuron responds to stimulation within a specific patch of skin (its receptive field)
Smaller receptive fields correspond to higher spatial resolution (fingertips have tiny receptive fields; the back has large ones)
Located in the anterior parietal lobe, immediately posterior to the central sulcus, along the postcentral gyrus
Contains a topographic body map (somatosensory homunculus)
Receives signals from the contralateral side of the body
The map is roughly arranged in anatomical order, but with notable distortions:
Fingers and lips have disproportionately large representations (most sensitive regions)
Some placements do not match physical anatomy (e.g., genitals represented near the foot rather than the centre of the body)
A lesion in S1 produces loss of sensation in the corresponding body region
Electrically stimulated regions of the cerebral cortex in patients undergoing brain surgery (patients were awake)
By listening to patients describe their experiences during stimulation, Penfield mapped the somatosensory cortex in the parietal lobe and the motor cortex in the frontal lobe
Secondary, supplementary, and association somatosensory areas
Lesions here produce neglect syndromes and somatosensory agnosia rather than outright loss of sensation
Neglect: touch sensation still works, but stimuli are usually ignored unless attention is specifically drawn to them
Agnosia: touch sensations may feel strange or confusing
Mouse whiskers have the most robust representation in the somatosensory cortex
Appear as distinct columnar cell clusters called "barrels," each receiving input from a single specific whisker
Cutting a whisker causes the adjacent barrels to become more sensitive, forming new neural connections with neighbouring regions
This involves new axon and dendrite growth plus strengthening of connections
A clear example of neuroplasticity
A person continues to experience the felt presence of an amputated limb
The region of S1 that normally receives input from the missing limb is deafferented (no longer receiving signals)
Adjacent regions of the body map form new connections with the deafferented area
Result: touching the shoulder or face may elicit sensations felt in the phantom limb
V.S. Ramachandran
Mapped an entire representation of a phantom arm onto a patient's shoulder and face
Developed mirror therapy: placing the intact hand in a box with a mirror so that its reflection appears where the phantom limb would be
Opening and closing the intact hand creates the visual illusion of the phantom hand moving
This visual feedback can reduce phantom limb pain
Demonstrates the power of the brain's multimodal integration
Located immediately anterior to the central sulcus, in the frontal lobe
When M1 neurons fire, signals travel via the spinal cord to neuromuscular junctions (NMJs)
At the NMJ, acetylcholine is released and triggers contraction of skeletal muscle fibres
Contralateral connection: left M1 controls right side of body and vice versa
A lesion in M1 produces an inability to move muscles in the corresponding part of the body map (partial paralysis)
The body map of motor neurons that initiate skeletal muscle contraction
Like the somatosensory homunculus, it is distorted: areas requiring fine motor control (hands, face) have larger representations
Premotor cortex sits anterior to M1
Apraxia: a lesion in premotor areas can disrupt the organisation and sequencing of movements, even though individual muscles can still contract
Found in premotor areas of the frontal lobes
Active both when performing a movement and when observing the same movement performed by another person
Thought to play a role in understanding others' actions and possibly in imitation learning
A lack of knowledge or awareness of one's own disease or disability
Classic example ("The Man Who Mistook His Wife for a Hat"): a man with left leg paralysis did not know why he was in the hospital and experienced his leg as an alien object
Patients may confabulate elaborate stories and excuses for why they cannot move affected body parts
Retinal ganglion cells (and other sensory neurons) have centre-surround organisation
Centre: stimulation produces excitation (increased action potentials)
Surround: stimulation produces inhibition (decreased action potentials)
The visual grid illusion (illusory dark spots at intersections) results from this lateral inhibition property
⚠️ S1 is in the parietal lobe (postcentral gyrus); M1 is in the frontal lobe (precentral gyrus). Both are contralateral. Know the anatomical distinction.
⚠️ The somatosensory homunculus is distorted by sensitivity, not physical size. Lips and fingertips dominate.
⚠️ Phantom limb sensations arise from cortical reorganisation (neuroplasticity), not from the missing limb itself. Ramachandran's mirror therapy is a classic exam topic.
⚠️ Distinguish neglect (posterior somatosensory lesion: sensation works but is ignored) from numbness (S1 lesion: sensation lost) from agnosia (sensation is present but feels wrong).
⚠️ Mouse whisker barrels demonstrate neuroplasticity: removing one whisker's input causes neighbouring barrels to expand their connections.
⚠️ Anosognosia involves lack of awareness of one's own deficit. Patients confabulate rather than acknowledge paralysis.
⚠️ Mirror neurons fire both when performing and observing an action.
Q: Where is the primary somatosensory cortex located, and what does a lesion there produce?
A: S1 is in the anterior parietal lobe, along the postcentral gyrus. A lesion produces loss of sensation in the corresponding contralateral body region.
Q: Why are the fingers and lips so large on the somatosensory homunculus?
A: The size of each body part's representation on the homunculus corresponds to sensory sensitivity, not physical size. The fingertips and lips have the highest density of receptors and smallest receptive fields, giving them the most detailed tactile discrimination and therefore the largest cortical representation.
Q: How does phantom limb pain arise, and how does mirror therapy address it?
A: After amputation, the cortical region that used to receive input from the missing limb is deafferented. Adjacent body map regions (e.g., face, shoulder) form new connections with this area, producing referred sensations and sometimes pain. Mirror therapy uses a mirror to create a visual illusion of the phantom limb moving normally. This visual feedback can reduce pain, demonstrating the brain's reliance on multisensory integration to construct bodily experience.
Q: What do mouse whisker barrels demonstrate about neuroplasticity?
A: Each cortical barrel receives input from a single whisker. When a whisker is cut, its barrel loses input, and adjacent barrels form new connections with it, becoming more sensitive. This involves new axon and dendrite growth, showing that the somatosensory cortex physically rewires in response to changes in sensory input.
Q: What is the difference between apraxia and paralysis?
A: Paralysis (from an M1 lesion) is the inability to move specific muscles. Apraxia (from a premotor lesion) is a disorder of movement organisation: individual muscles work, but the person cannot coordinate them into purposeful, sequenced actions.
somatosensory cortex S1 postcentral gyrus, mechanoreceptor, TRP receptor temperature, dorsal root ganglion DRG, receptive field, somatosensory homunculus body map, Wilder Penfield cortical stimulation, neglect syndrome, somatosensory agnosia, mouse whisker barrels neuroplasticity, phantom limb, Ramachandran mirror therapy, motor cortex M1 precentral gyrus, neuromuscular junction acetylcholine, motor body map, apraxia premotor, mirror neurons, anosognosia confabulation, centre-surround receptive field lateral inhibition, visual grid illusion, contralateral connectivity