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Chapter 3: Body Senses
• The body senses provide information about surfaces in direct contact with the skin (touch,
mechanoreception), about the position and movement of body parts (proprioception and kinesthesis)
and about the position and movement of the body relative to the outside world (balance).
• The vestibular system deals with balance, and the somatosensory system with the rest.
• The system helps us:
o Control grip force when grasping an object; an object slipping out of our hands generates
o Identifying objects by: texture, temperature, shape and weight
o Avoiding damaging situations, such as pain. Miss C was born with analgesia, insensitivity to
pain; she could not sneeze, cough, gag or blink reflexively. She often suffered injuries such as
biting her tongue when chewing, and died at 29 from infections that may have been preventable if
she sensed pain.
o Social functions such as pleasant touch
o Controlling body posture and movement
The Somatosensory System
• Proprioception and
kinesthesis seem largely invisible, because they are unconscious. In the case of Ian Waterman, who lost
proprioception after a flu-like illness, he lost all ability to control his movements. This was due to the loss
destruction of sensory nerves that supply the brain with touch and proprioceptive information.
• Ian required 18 months in a rehabilitation hospital, and now can make controlled bodily movements with
intense concentration, visual vigilance, and imagery. He must constantly monitor his movements visually,
and study unfamiliar environments beforehand in order to navigate them.
Awareness, volitional control Automatic control
Vestibular system included Vestibular system not included
• Both also depend on
muscle spindles, joint
receptors, and Golgi tendon
Physiology of Somatosensation
• The somatosensory
receptors include 8
receptors, and 2 pathways to
the primary somatosensory
cortex. Page 2 of 11
• Sensory nerve fibers are usually pseudounipolar, having one extension which splits into a dendrite-
like part, and an axon-like part. Action potentials are generated at the dendritic parts, versus usually only
having graded potentials and action potentials being generated at the axon hillock.
• The extensions can be either myelinated or not; myelin increases transmission speed with potentials
jumping from node to node.
• Sensory nerve fibres are classified by : transmission speed, adaptation pattern, receptive field size, and
morphology. Total name may be Aα SA-I.
• 1) Speed: Fast speed is determined by myelination and large fibre size (lower resistance). C fibers are
the slowest, thin and no myelination at 0.5-2 m/s. Aα, Aβ and Aδ fibres are faster; Aα transmits at 70-120
m/s, almost instantaneously in the human
o This can be measured by using an
electrode to stimulate the nerve, and
measuring time need for the signal to
• 2) Adaptation Pattern: Tested by looking at
response with increasing current, maintained
current level, and decreasing current.
o Fast-adapting FAI and FAII fibres
respond only to change, while slow-
adapting SAI and SAII fibres respond during the whole phase of stimulation, including when it is
• 3) Receptive Field Size: Category I receptors have small receptive fields with sharply defined borders;
category II receptors have large receptive fields with obscure borders.
• 4) Morphology: Nerve fibres can be free-nerve endings with no special structure (simple receptors),
have encapsulated receptors (complex neural receptors) with a special connective tissue capsule, or
have an accessory structure (special senses receptors) where a specialized receptor cell synapses
onto the sensory neuron.
Receptor Location Sensory Function Sensory Fiber
Free Nerve Skin, superficial Pain, temperature, tickle
Meissner’s Glabrous skin Light, dynamic touch FAI: Fast-adapting, small
Corpuscles (hair-free), receptive field
Merkel’s Disks Skin, superficial Static pressure SAI: Slow-adapting, small
Pacinian Skin, deep Pressure, vibration FAII: Fast-adapting, large
Corpuscles receptive field Page 3 of 11
Ruffini’s Skin, deep Skin stretch SAII: Slow-adapting, large
Corpuscles receptive field
Muscle Spindles Muscles Muscle length
Golgi Tendon Tendons Muscle tension
Joint Receptors Joints Joint position
• Touch Receptors: Five different types. Free nerve endings do not have a structural specialization for
transducing stimuli. The other four mechanoreceptors have specialized structure:
o The three corpuscles are complex neurons where the receptor structure is part of the neuron;
the Merkel disks are special sense receptors, with the sensory structure separate and synapsing
onto the neuron.
o Panician: Onion-like capsules in which layers of membrane are separated by fluid. Mechanical
stimulation deforms the structure, leading to receptor response. Panician corpuscles can respond
at 250-350Hz to dynamic stimulation, such as high-frequency vibration caused by movement
across a fine-textured surface.
o Meissner: Intermediate temporal response of 30-50Hz for moderate dynamic stimulation.
o Merkel + Ruffini: Sluggish temporal response, suited to signal relatively stable, unchanging
• Proprioceptors: Three types of receptors found in and around the limbs, either around the tendons
that attach muscles to bone, muscles themselves, or the joints.
o Muscle spindles consist of 4-8 specialized muscle fibers surrounded by a capsule of connective
tissue. The axons of sensory nerves encircle the fibers in the capsule to provide information on
o Large muscles for coarse movements have few muscle spindles, while muscles used for fine
and accurate movements, like those in the hands and around the eyes, have many.
• The cell bodies of mechanoreceptors are in the dorsal root ganglion of the spinal column, with their
peripheral axons ending in various sensory specializations and their central axons projecting towards the
• Bell-Magendie Law: Sensory information enters dorsa rootl, motor information leaves ventral root.
• There are 31 spinal nerves, which can
divide the body into regions on a dermatomal
map – each part of the body surface
connects to a specific pair of nerves, e.g.
nipples to T4.
• The spinothalamic pathway carries
responses from free-nerve endings, and
terminated in spinal cord Rexed’s laminae I
and II laminae IV to VI thalamus
o Crosses in the spinal segment Page 4 of 11
o Axons are lightly or unmyelinated, and have relatively slow conduction velocities of 2 m/s
• The lemniscal pathway carries responses from mechanoreceptors to the dorsal column nuclei in the
medulla of the brainstem thalamus
o Crosses in the medulla
o Axons are myelinated, and have relatively fast conduction velocities of 20 m/s
• Neurons in the thalamus send axons to the primary somatosensory cortex.
• Branching projections of interneurons and motor
neurons in the spinal cord are responsible for reflexes,
such as for withdrawal from painful stimuli and the
o When hit on the knee, muscle spindles and
tendon receptors send excitatory signals to
sensory neurons. The sensory neuron synapses
onto both motor neurons in the quadriceps, and in
o It excites the quadriceps for leg extension, and
inhibits the hamstrings to prevent leg retrieval.
• The reflex is mediated by a feedback control loop: the muscle spindles sense the current position, the
controller in the spinal cord decide if there is a difference to the reference knee position, and if so it
signals the muscle effectors to move into the desired position.
• Spinal processing also mediates the pain withdrawal reflex, and the gating of pain from top-down
• Microneurography: Microelectrode is
inserted to peripheral afferent nerves to
obtain direct electrophysiological
• Each afferent nerve produces a
response only when stimulation fell in a
specific area of the hand, which is the
fiber’s receptive field. These fields are
roughly circular, and purely excitatory (not centre-surround). Receptive fields differ depending on
location, being much smaller on finger tips than on the body trunk.
Cortical Representation of Somatosensation
• The primary somatosensory cortex occupies a long, thin strip of cortical surface running from ear to
ear, in the parietal cortex on the postcentral gyrus behind the central sulcus. Neurons project
• Each primary somatosensory cortex cell receives input from only one type of receptor (e.g. only
• All receptors projecting to an individual cortical neuron are located in a small area of the body, forming
the cell’s receptive field. Receptive fields are arranged in the antagonistic center-surround manner, with
an excitatory centre and inhibitory
o This center-surround lateral
inhibition allows the cell to
respond best to relatively small
stimuli that fill the centre but not the
surround, for sensitivity to very
small changes in stimulus. Page 5 of11
o This also enhances differences, like the contrast between the edges of two stimuli, one more
intense than the other.
• Vertical Organization: The cortex can be divided into six layers on the basis of cell number, density,
o All cells within a vertical cortical column connect to the
same type of sensory receptor, representing one unique
modality: labeled line code.
o Cells in neighbouring columns all connect to a different
o Neighbouring columns also have receptive fields that
largely overlap on the body surface = hypercolumns
together encode everything from one area on the body
• Horizontal Organization: Orderly progression of part of body
covered by receptive fields along the surface. This was first
described by Wilder Penfield, who discovered patients reported
tactile sensations on specific parts of the body based on electrical
stimulation of different areas of the cortex.
o The body is mapped out in a homunculus topographic map
across a cross-sectional view of the cortex.
o Different areas of the body have different areas of cortical surface
devoted to them, according to cortical magnification. In humans,
large areas are devoted to the hands and lips, and small areas to the
back or the leg; this differs between species, reflecting survival
important of that body part.
Cortical Representation of Pain
• Thalamic neurons from the spinothalamic tract conveying information
about pain projects to the cortex, but its representation is not well
understood; removal of this area does not necessarily alleviate chronic
• Pain also projects to the reticular formation, pons, and midbrain.
• Neurons in the spinal cord convey both information about pain from the body surface, and from internal
organs. Pain from internal organs can therefore be “referred” to a more superficial area, like pain in the
heart muscle being referred to the chest wall and left arm.
• Different receptor and fiber types respond over
different ranges of stimulus, such as vibration
• Each fiber constitutes a specialized channel for
conveying information about the particular frequency
range to which it is most responsive, among the four
afferent fiber types (FAI, FAII, SAI, SAII).
• Duplex Theory: Two different minima of absolute
threshold, at two frequency ranges, are due to two Page 6 of11
sensors that respond to different frequency spectra. Meissner FA-I responds best to 20Hz, while Pacini
FA-II responds best to over 200 Hz.
• Our ability to discriminate fine touches is measured using two-point discrimination/acuity/limen. A
pair of calipers is placed on the skin of the subject, and the subject must report whether they feel a single
point, or two points, at various distances between the caliper pins.
• Acuity varies markedly in different regions of the body: best on tongue and hands (2 to 3mm), worst on
back and neck (50mm). This reflects the variation in receptive field size, and area of cortical
• This is also related to point localization error: when an area is stimulated, and then again, subject
must determine if it was at the same place or at a different place. The distance needed to sense
difference is measured.
• Low voltage pulses delivered to each of the four fibres (FAI, FAII, SAI, SAII) by intraneural
microstimulation evoke sensations. SAI evokes sustained pressure, while FAI and FAII fibers evoke
“flutter”, “vibration”, or “tingling” depending on the frequency of stimulation; SAII does not evoke any
• This confirms the link between mechanoreceptor activity and perception of touch.
• A natural mechanical stimulus will elicit activity in several types of afferent fiber, and the resulting
sensation reflects the combined activity of the whole population of fibres.
• Free nerve endings are found in all parts of the body, except in the brain and bones. They signal pain in
response to tissue damage, to protect against further injury.
• Pain does not disappear once the external stimulus has been removed, but persists to promote healing.
• Gate-Control Theory of Pain: A “gate” in the spinal cord, consisting of lamina II cells, regulates
incoming signals from receptors. When the gate is open, pain is felt; when it is closed, there is no pain.
This is controlled by descending projections from the brain, reflecting cognitive and emotional factors like