Somatosensory Physiology Flashcards
What are the components of the somatosensory system?
it is part of the sensory system concerned with the conscious perception of touch, pressure, pain, temperature, position, movement and vibration which arise from the muscles, joints, skin and fascia
What are the main components of the somatosensory system?
- consists of primary, secondary and tertiary neurones
- sensory receptors housed in the dorsal root ganglia project to secondary neurones of the spinal cord that decussate and project to the thalamus or cerebellum
- tertiary neurones project to the postcentral gyrus of the parietal lobe, forming a sensory homunculus
- a sensory homunculus maps sub-regions of the cortical postcentral gyrus to certain parts of the body
What is the main role of the somatosensory system?
it provides a crucial line of communication between us and the outside world
Where are the sensory receptors located in the somatosensory system?
it is an unusual sensory system as receptors are distributed throughout the body including skin, musculoskeletal system and internal organs
this provides a wide variety of information
What are the main receptors of the somatosensory system?
What can damage to the peripheral or central components result in?
the main receptors are tactile (innocuous), nociceptive, proprioceptive and thermal
damage to peripheral or central components results in conditions such as neuropathy, neuralgia, phantom pain
What are the 4 main classes of somatosensory receptors?
- tactile (innocuous) sensations
- proprioception
- thermal sensations
- nociceptive (painful) sensations
How are tactile (innocuous) sensations mediated?
these are sensations which are not harmful
mediated by low threshold mechanoreceptors
there are Merkel, Ruffini, Meissner and Pacinian types
How is proprioception mediated?
mediated by muscle (spindle) and joint (Golgi tendon) receptors
there are some inputs from cutaneous mechnoreceptors
How are thermal sensations mediated?
mediated by thermoreceptors
these are localised to discrete zones that exhibit hot and cold sensitivity
How are nociceptive (painful) sensations mediated?
mediated by mechanical, thermal and polymodal nociceptors
What is meant by a polymodal nociceptor?
most nociceptors respond to:
- noxious mechanical stimuli - pressure, squeezing, cuts
- noxious thermal stimuli - heat or extreme cold
- noxious chemical stimuli
when they respond to all 3 of these categories, they are polymodal
What is the primary event in somatosensation?
generation of an action potential in an afferent fibre ending
What are the 5 diffferent sensory receptors in the skin?
- Meissner corpuscle
- Pacinian corpuscle
- Ruffini’s corpuscles
- Merkel’s disks
- free nerve endings
What is glabrous skin?
an epidermal covering that is totally or relatively devoid of hairs
glabrous skin is smooth and hairless
Where are Meissner corpuscles and Merkel’s disks located?
they are both found just beneath the epidermis
Meissner corpuscles sit between the dermal pupillae
Merkel’s disks are aligned with the pupillae
Where are Pacinian and Ruffini’s corpuscles located?
pacinian corpuscles are large encapsulated endings located in the subcutaenous tissue
Ruffini’s corpuscles are located deep in the dermis
the long axis of the corpuscle is orientated parallel to the skin
What is the morphology of Meissner corpuscles like?
looping axonal terminals that inter-twine supporting cells
What is the morphology of Merkel’s discs like?
a dome structure atop axon terminals
What is the morphology of Pacinian corpuscles like?
sensory axon surrounded by a fluid-filled capsule
it has an onion-shaped apperance
What is the morphology of Ruffini endings like?
nerve terminals intertwined with collagen fibrils
What is the morphology of nociceptors like?
free nerve endings that penetrate epithelial cells
there is no morphological specialisation
Which types of sensory receptors in the skin detect touch?
What is the receptive field like?
- Meissner corpuscle
- Merkel cells
- Pacinian corpuscle
- Ruffini endings
What is meant by the receptive field of a neurone?
the place on a sensory surface (i.e. skin) that a stimulus must reach in order to activate that neurone
What sensations are detected by the different mechanoreceptors in the skin?
Ruffini corpuscle:
- skin stretch
Meissner’s corpuscle:
- changes in texture
- slow vibrations
Pacinian corpuscle:
- deep pressure
- fast vibrations
Merkel’s disc:
- sustained touch and pressure
How are the receptive fields for the cutaneous sensory receptors tested?
a microelectrode is introduced into the median nerve of the arm
the receptive fields of sensory receptors are tested using a stimulus probe on the hand
action potentials from a single median nerve axon are recorded
What receptive fields of cutaneous sensory receptors are shown below?
What is the spatial and temporal resolution of the somatosensory system like?
somatosensory system has a high spatial ( < 0.5 mm) and temporal (100 characters per minute) resolution
What are the following sensory receptors that encode Braille inputs in a blind subject?
How do human 2-point discrimination thresholds vary across the body surface?
sensitivity is correlated with the density of sensory innervation
fingertips are face are areas of high sensitivity
torso and limbs are areas of low sensitivity
What are the 2 types of proprioceptors in muscle and joints?
- the muscle spindle
- golgi tendon organ
What is the role of the muscle spindle?
muscle spindles provide sensory feedback from muscle fibres on body position and movement
What is the role of the Golgi tendon organ?
golgi tendon organs regulate muscle tension or force of contraction
they prevent muscle overloading
What are the mammalian mechnosensors?
Where are they expressed?
Piezo channels
these are trimers that are shaped like a propeller with three blades organised around a central pore
they are expressed in the lungs, bladder and skin, where mechanosensation has important biological roles
What are the molecular sensors of temperature?
thermally activated TRP channels are the molecular sensors for temperature detection
this is mainly TRPV1 and TRPA1
What is detected by TRPV1?
- vaniloids (capsaicin)
- heat
- protons (acidity)
- endocanabinoids
What are the main roles of TRPV1?
- detection and regulation of body temperature
- provides a sensation of scalding heat and pain (nociception)
- cooperates with TRPA1 (chemical irritant receptor) to detect noxious environmental stimuli
What it detected by TRPA1?
- mustard oil
- cold
- airborne irritants, allergens
- cigarette smoke
What is the role of TRPA1?
it is a sensor for environmental irritants giving rise to somatosensory modalities
e.g. cold, pain, itch
In general, what are low diameter and slow diameter sensory afferents innervating somatosensory receptors associated with?
large diameter, rapidly conducting afferents are associated with low threshold mechnoreceptors
small diameter, slow conducting afferents are associated with nociceptors and thermoreceptors
How is conduction velocity related to axon diameter?
conduction velocity is positively correlated with axon diameter
i.e. large diameter - rapidly conducting afferent
Label the segment of the spinal cord
How is the body divided into dermatomes?
What does each dermatome represent?
the body surface is subdivided into numbered dermatomes
each dermatome represents that area of skin innervated by the left and right dorsal roots of a single spinal segment
What do dorsal roots contain that supply each dermatome?
each dermatome represents that area of skin innervated by the left and right dorsal roots of a single spinal segment
dorsal roots contain sensory afferents innervating receptors of peripheral targets
e.g. in skin or muscle
How are the boundaries of dermatomes defined?
the boundaries are not absolute and there is overlap between adjacent dermatomes
What types of sensations are carried by the 2 ascending spinal pathways?
dorsal column medial lemniscus pathway:
- innocuous sensitivities
- touch, vibration, two-point discrimination, pressure
spinothalamic pathway:
- noxious and thermal sensitivity
- pain, temperature, some touch
What are the 3 cortical somatosensory areas?
- primary somatosensory cortex (s1)
- secondary somatosensory cortex (S2)
- association areas
Where is the primary somatosensory cortex located?
What is its role?
localised to the postcentral gyrus and flanked by the central sulcus and postcentral sulcus
receives strong somatosensory inputs from the thalamus
areas are specialised
e.g. 3b processes information from the skin on texture, shape and size
where is the secondary somatosensory cortex located?
What is its role?
lies adjacent to S1 along the lateral sulcus
has a key role in sensory and motor integration
receives corpus callosum inputs to form “joined up” body image
builds info from multiple body areas forming body image
Where are the association areas located?
What is their role?
localised to posterior parietal cortex
plays a role in integration of multinodal senses
What is shown by the homunculus man?
a map is produced that is a cross-section through the postcentral gyrus
lower torso is represented medially and upper torso laterally
the area of the cortex given over to a body region is distorted
the size of the central cortical area is correlated with peripheral sensory innervation
What is the importance of sensory integration?
sensory integration allows us to combine different modalities and submodalities
e.g. identification of an object by touch alone
combining touch and vision
What is the importance of sensory integration and the role of the association areas?
sensory information is kept segregated en route to the primary sensory cortex
eventually these sensory inputs come together in the association areas in the posterior parietal cortex
What 2 main disorders can damage to the posterior parietal cortex produce?
- astereoagnosia
- neglect syndrome
What is meant by agnosia and astereoagnosia?
agnosia:
- the inability to recognise objects despite normal sensory functioning
astereoagnosia:
- cannot recognise objects by feeling them
- despite normal sense of touch
- no problem recognising the object by sight or sound
What is meant by neglect syndrome?
a body part or visual field is disregarded
e.g. they may deny existence of a leg or only eat food on one half of a plate
What components of somatosensory processing occur in the cortex?
- limbic system - learning and memory
- motor system
- polysensory association areas
These arise from the posterior parietal lobe (S2) and postcentral gyrus (S1)
What components of somatosensory processing are located in the thalamus and medulla?
mechanoreceptors feed into the spinal cord
this runs into the dorsal column in the medulla
the ventrobasal complex is located in the thalamus
