Chapter 9 & 10 - The somatosensory system
We perceive touch, proprioception, temperature, pain with the somatosensory system.
● Exteroception: direct interaction with external world (touch, pain, temperature)
○ Pain has the thinnest axon, slowest conduction (poorly myelinated).
● Proprioception: posture and movement of the body (knowing where our limbs are,
how we move), with receptors in muscles, tendons and joints
○ Thickest axon, fastest conduction (myelinated).
● Interoception: internal organs in body cavities
○ Conscious: sensations of pain
○ Unconscious: for example chemoreceptors that measure pH and blood gases
The dorsal-column medial lemniscus pathway
Stimuli generate a graded depolarization. When this is high enough there is an action
potential in the afferent fibre. Sensory information from afferent fibres/first-order neurons
enters via basal ganglia the dorsal horn of the spinal cord via the dorsal root. From there,
the neurons stay ipsilateral (same side of the body), and the information goes to the dorsal
columns into the medulla, to the ventral posterolateral nucleus of the thalamus and then
into the primary somatosensory cortex.
The dorsal columns of the spinal cord have fibres:
● From the lower limbs, medial (gracile tract)
● From the upper limbs, trunk and neck, lateral (cuneate tract).
In the medulla, the tract from the lower body ends in the gracile nucleus and the upper
body tract ends in the cuneate nucleus. Information is crossed to the other side of the body
and passed on to second-order neurons: the medial lemniscus tract (from medulla to the
thalamus), that consists of internal arcuate fibres, with those from the lower limbs ventrally
and from upper limbs dorsally.
After ascending through the pons and midbrain the medial lemniscus rotates, so the fibres
of the upper limbs become medial/ventrally and lower limbs lateral/dorsally. The axons of the
medial lemniscus synapse with thalamic third-order neurons in the ventral posterolateral
nucleus (VPL). Upper body information arrives in the ventral posterior medial nucleus
(VPM). The thalamus acts as a relay station.
The third order-neurons in the VPL send their
axons to the primary somatosensory cortex, via
the internal capsule that terminates in the
postcentral gyrus of the cerebral cortex. The
PSC lies posterior to the central sulcus.
Information can go to the secondary
somatosensory cortex that lies in the upper
lateral sulcus and from there to the amygdala and
hippocampus.
A homunculus is the representation of the body in the somatosensory cortex. The hands
and face are more sensitive and are bigger on the map.
Stereognosis is recognising an object using touch.
,All first-order neurons are mechanoreceptors, reacting to mechanical pressure. As a result,
they stretch, which opens ion-channels. They can vary in location, axon diameter, sensitivity,
peripheral adaptation and receptive fields:
● Merkel cell:
○ In the epidermis
○ Slowly adapting
○ 25% of the mechanoreceptors are in the hand
○ Small receptive field (high spatial resolution)
○ Sensitive for edges, points, curves, texture and shape
○ A point leads to a faster depolarization than a flat surface.
● Meissner corpuscle:
○ In the dermis
○ Rapidly adapting
○ 40% of the mechanoreceptors are in the hand
○ Large receptive field (low spatial resolution)
○ Contains axons with collagen and myelin
○ Sensitive for low frequency vibrations when an object goes along the skin and
grip control
● Ruffini corpuscle:
○ In the dermis
○ Slowly adapting
○ 20% of the mechanoreceptors are in the hand
○ Large receptive field (low spatial resolution)
○ Not very sensitive
○ Corpuscle endings lie longitudinal
○ Sensitive for proprioception and stretching of the skin
● Pacinian corpuscle:
○ In the subcutaneous layer
○ Rapidly adapting
○ 15% of the mechanoreceptors are in the hand
○ Large receptive field (low spatial resolution)
○ Very sensitive for high-frequency vibrations
The peripheral adaptation of the receptors is the adaptation due to prolonged stimulation:
● Slowly adapting neurons: continuously respond to a stimulus, registering intensity
and duration (Merkel cell-neurite complexes, Ruffini corpuscles). These are important
for spatial information such as size, shape and proprioception.
● Rapidly adapting neurons: respond only at the onset and offset of stimulation,
sensing changes in the stimulation (Meissner corpuscles, Pacinian corpuscles).
These are important for movement and vibration.
The receptive field is the area of the sensory space where a stimulus can lead to an action
potential in a neuron. Fingers have a small receptive field and a high density of neurons: In
● Small receptive fields: provide high spatial resolution (Merkel cell-neurite complexes,
Meissner corpuscles).
● Large receptive fields: Provide lower spatial resolution (Pacinian corpuscles, Ruffini
corpuscles).
,The soma of the first-order neurons are found in the dorsal root ganglia next to the spinal
cord. These neurons are pseudounipolar, meaning that they have their dendrites and axon
connected without having the cell body in between. Similar pseudo-unipolar neurons can
also be found in the cranial nerve ganglia associated with the sensory cranial nerves. All
pseudounipolar neurons are sensory neurons, and mainly carry information about touch,
vibration, proprioception, pain and temperature.
Proprioception has the fastest and thickest axon (Ia, II), then touch (Aβ), then pain and
temperature (Aδ, (C)). All first-order touch neurons have the same axon type (Aβ).
Dermatome: an area of skin that is mainly supplied by the afferent nerve fibers of a single
spinal nerve.
Two point discrimination: the minimal distance between two stimuli to perceive the stimuli
as distinct points. Hands have very good discrimination, arms have a larger distance,
because there are less neurons present. The two-point discrimination depends on:
● Convergence, meaning that multiple first-order neurons stimulate 1 second-order
neuron. The higher the convergence, the bigger the receptive field is (so lower spatial
resolution). So a low convergence leads to the smallest receptive field and low two
point-discrimination.
● Lateral inhibition (a way to improve the acuity of neurons). It occurs when an
excited second-order interneuron inhibits the activity of its neighbours. This process
increases the contrast in sensory input, making the perception of stimuli more distinct
(higher spatial resolution). So when a tactile stimulus activates a neuron, it can
activate inhibitory interneurons, which inhibit the activity of neighbouring neurons.
This decreases the response of these neurons, and sharpens the sensory signal (the
contrast between the stimulated neuron and inhibited neurons is larger).
, The trigeminal pathway
Mechanoreceptor information from the face is conveyed by a separate set of first-order
neurons in the trigeminal ganglion. There are ophthalmic, maxillary and mandibular
branches, that each innervate a territory on the face. The trigeminal ganglion neurons form
the sensory roots of the trigeminal nerve.
The neurons enter the brainstem at the pons and terminate on other neurons in the
trigeminal brainstem complex. This has a principal nucleus (containing most afferents
conveying information from low-threshold receptors) and spinal nucleus (receiving input
from collaterals of mechanoreceptors). The second-order neurons of the trigeminal
brainstem nuclei have axons that cross the midline and ascend to the ventral posterior
medial nucleus (VPM) of the thalamus via the trigeminal lemniscus tract. Neurons in the
VPM send their axons to the primary and secondary somatosensory cortex.
In the primary somatosensory cortex there are many Brodmann areas: areas with different
functions. For example, hands and lips have a larger Brodmann area than arms.
● 3a: receives a bit of proprioceptive information, and
sends it to secondary somatosensory cortex → to
amygdala and hippocampus
● 3b: receives most information, and sends it to 1 and 2
and to the secondary somatosensory cortex → to
amygdala and hippocampus
● 1: receives a bit of proprioceptive information and a
lot from 3b, and sends it to secondary somatosensory
cortex → to amygdala and hippocampus
● 2: receives a bit of proprioceptive information and a
lot from 3b, and sends it to secondary somatosensory
cortex → to amygdala and hippocampus or to parietal
areas 5 and 7 → motor and premotor cortical areas
Pain is perceived with unencapsulated free-nerve endings called nociceptors:
● Unimodal: myelinated Aδ fibers
○ Mechanical: sensitive for pushing/bumping etc.
i. Mechanoreceptors: these are not encapsulated, so less sensitive for
stretch than the touch receptors.
○ Thermal: sensitive for temperature
i. Transient receptor potential channel V1 (TRPV1): warmth makes
the membrane more fluid, so a lipid can be lost, opening the channel.
This also occurs when eating spicy food.
● Polymodal: unmyelinated C fibers
○ Sensitive for extreme temperatures, mechanical stimulation and chemical
stimulation
■ An example of a chemical receptor is acid-sensing ion channel 3
(ASIC3), found near muscles.
The first pain is sharp and short (unimodal, goes through myelinated axons / Aδ fibers).
The second pain is duller and longer (polymodal, goes through unmyelinated axons / C
fibers).
