Somatosensory systems Flashcards
Sensory receptors generate a receptor potential, which is…
a change in their membrane potential in response to appropriate stimulation, this process is called transduction and is different in different receptors.
How is the sensory receptor different for somatosensory systems vs other sensory systems?
For somatosensory systems the sensory receptor is the modified ending of the primary afferent neuron, and is depolarised directly by the stimulus. In other systems, the sensory receptor is a specialised cell type which forms synaptic connections with the first afferent neuron. Alterations in membrane potential alters sensory cell neurotransmitter release, with effects on the primary afferent.
In vertebrates, all sensory receptors, except which receptors, depolarise when stimulated.
Photoreceptors- hyperpolarised by light.
What are the properties of receptor potentials?
They are small amplitude, graded in size depending on stimulus strength, passively conducted over the receptor cell surface or along neurites, delay with time and distance and can be summated. A receptor potential will trigger APs for as long as it remains beyond the firing threshold, the frequency of firing will be higher the greater its amplitude. Sensory receptors demonstrate adaptation.
What are the three types of receptors in the somatosensory system?
Mechanoreceptors, found in skin, muscles, joints and viscera, thermoreceptors, confined to the skin and nociceptors, found almost every where except the brain.
How are skin mechanoreceptors classified?
As slowly or rapidly adapting and separately as being of two types, type I and type II, distinguished by their location and receptive fields.
What are the properties of the Ruffini organ?
Slowly adapting, afferent has a frequency of firing that is directly proportional to the extent to which overlying skin is indented by mechanical force. It codes skin position/ stretch.
What are the properties of Meissner’s corpuscle?
Rapidly adapting, afferent only fires when skin displacement is changing with time. It codes the velocity with which skin is displaced.
What are the properties of the pacinian corpuscle?
Adapts so rapidly that its afferents respond to skin acceleration. It is responsible for the sensation of vibration.
Where are type I mechanoreceptors located and what are examples of them?
They are superficial, lying at the boundary of the epidermis and dermis and have small RFs with well-defined boundaries. they include meissner’s corpuscles and merkels disks.
Where are type II mechanoreceptors located and what are examples?
Type II are deep in the dermis and have large RFs with poorly defined edges, and include ruffini corpuscles and pacinian corpuscles.
What is the difference between type I and II receptors in terms of sensation perceived?
Type I receptors are more concerned with form and texture perception than type II.
How do the densities of type I receptors differ across the body surface?
Highest in the fingertips, lips and tongue and lowest in the trunk. Areas with higher density have a proportionally greater representation in the somatotropic maps.
Pacinian corpuscles can respond to indentation as little as 1um, what is the process of transduction?
The force is transmitted through the corpuscle to deform the neurite within –>opening of stretch-sensitive NA channels in the membrane–> brief depolarisation–> membrane potential returns to normal very fast because the receptor adapts by individual connective tissue layers of the corpuscle sliding over each other which relieves the neurite deformation
How does innervation of hairy skin differ from that of glabrous (non-hairy) skin?
Hairy skin has a lower density of merkel’s disk and it possesses two additional types of mechanoreceptors closely associated with hairs (lanceolate and pilo-ruffini endings and hair follicle receptor)
What are the properties of skin thermoreceptors?
They are the naked terminals of small diameter afferents. They are slowly adapting and tonically active. Thermoreceptor afferents have just three to four terminals and have very small RFs, although infrared radiation is poorly localised.
There are two types of thermoreceptor, warm and cold, which fire over different temperature ranges, how do these perceive temperature?
They do not respond to noxious temperatures. Skin temperature is perceived by comparing the relative activities of warm and cold receptors. Thermoreceptors signal the direction in which temperature changes. Skin cooling briefly silences warm receptors and causes cold receptor firing rates to rise. Similarly, skin warming silences the cold receptors and boosts warm receptor firing
What are nociceptors?
The bare-endings of small diameter afferents that are receptors for noxious (tissue-damaging), pain-producing stimuli.
What are the properties of nociceptors?
They are high threshold (require intense stimulation to excite them). They have no background firing. they are classified by what excites them.
What are the four types of nociceptors?
Mechanical, thermal, polymodal, itch
What sensation does mechanical nociceptors in the skin give rise to?
Sharp, pricking pain.
What are the properties of mechanical nociceptor afferent?
Each nociceptor is one of 5-20 branches of A(delta) afferent with low conduction velocities
What do mechanoreceptors in the visceral peritoneum respond to?
Excessive distension
What are the afferents of thermal nociceptors?
A(delta) and C fibers
What do polymodal receptors respond to?
Puncture, temperatures in excess of 48 degrees, and to a wide variety of molecules liberated as a result of tissue damage (K, H, bradykinin, prostaglandins, serotonin and histamine)
Afferents of polymodal nociceptors are C fibers (conduct at less then 1 ms) what is the effect of this?
Because conduction is slow, the burning or aching pain they produce arrives last after a blow. They are responsible for visceral and muscle pain and toothache
What are the afferents of itch receptors and what do they respond to?
Belong to a separate class of C fiber that respond to histamine release from mast cells
What is a dermatome?
The region of skin innervated by a dorsal root. These are numbered for the spinal cord segment served by the dorsal root
Cutaneous low threshold mechanoreceptor primary afferents relaying skin mechanoreceptor and proprioceptor enter the dorsal roots and synapse with which neurons?
Dorsal horn cells- interneurons in Rexed laminae III-VI. They modify or mediate spinal reflexes. Each afferent sends a collateral up the dorsal columns to synapse with neurons in the dorsal column nuclei in the medulla
The dorsal column nuclei consist of the nucleus gracilis and nucleus cuneatus, what is the difference in their inputs?
The cuneate nucleus receives input from C1-8 and the gracile nucleus get its input from T7-12, lumbar and sacral spinal segments. Lateral inhibition in the DCN shapes these inputs.
Where do axons leaving the dorsal column nuclei project to?
They cross the midline to ascend on the opposite side as the medial lemniscus, terminating in the ventralposterolateral division of the ventro-basal thalamus.
What do the Ventroposterolateral neurons give rise to?
thalamo-cortical axons which project to the primary somatosensory cortex SI (brodman’s areas 1, 2, 3a and 3b) situated over the post central gyrus. SI neurons project to SII
What are the general properties of the DCML system?
-Great strength of synaptic connections
-Properties of its neuron are matched to the sensory receptors supplying them (features of stimuli transmitted with high fidelity)
-Somatotropic mapping preserves localisation at every stage.
Neurons in SI are organised into columns aligned at right angles to the brain surface, what is the significance of this?
Each column gets input from a single type of receptor, and from a specific location. Adjacent locations are represented in adjacent columns in a somatotropic manner. Extensive neural connections exist within a column, connections between columns are sparse.
What is each area in SI responsible for?
3b is important for tactile discrimination, area 1 is concerned with analysis of texture, area 2 with stereogenesis (the ability to perceive the 3D shape of an object by touch). Area 2 gets input from muscles and joints and has reciprocal connections with the motor cortex. It may inform the motor system of the sensory consequences of moving.
Where does the secondary somatosensory cortex get input from?
From the thalamus and SI. Many neurons have bilateral RFs (stimuli in corresponding regions on both side of the body will evoke a response). Inputs from the contralateral body surface arise as a consequence of the decussation of the medial lemniscus. Inputs from the ipsilateral side of the body enter SII from the contralateral side via the corpus callosum
What is the function of the SII?
By integrating information from both sides of the body, SII is the first stage in forming a whole body perceptual experience. It enables tactile discrimination learned with one hand, being easily performed with the other. It is important in controlling movement via connections with the motor cortex. It has inputs to the limbic cortex enabling tactile learning.
What makes up the descending pathway of the somatosensory cortex?
It is made by the corticospinal (pyramidal) tract either directly or via its connections with the brainstem reticular nuclei.
What is the function of the somatosensory descending connections?
They are probably the vehicle by which somatosensory input can be selectively filtered as an attention mechanism.
what is the definition of pain?
It is the unpleasant sensory and emotional experience associated with noxious stimuli, those which can cause tissue damage. Nociceptive pain is felt only in the presence of acute injury and inflammation. Pathological (clinical) pain is associated with chronic inflammatory disease or to damage or dysfunction of the nervous system and has no obvious purpose.
What sensations do the anterolateral pathways convey?
temperature and pain sensations, and poorly discriminated touch sensation
What are the primary afferents of the anterolateral pathways?
Small diameter high threshold dorsal root ganglion cells driven by nociceptors and low threshold dorsal root ganglion cells excited by thermoreceptor or mechanoreceptor input
What is the route the primary afferents of the anterolateral pathway?
They are situated laterally in the dorsal roots and enter Lissauer’s tract, where they divide into ascending and descending branches that enter the dorsal horn within within a couple of segments.
Where do the primary afferents of the anterolateral pathways synapse?
Nociceptive Alpha(delta) afferents synapse with projection neurons in laminae I and II, and with the distal dendrites of lamina V cells, although visceral and muscle nociceptors do not project to lamina II. Nociceptive C fibers synapse with other laminae I and II neurons.
Where do the primary afferents of the anterolateral pathways synapse?
Nociceptive Alpha(delta) afferents synapse with projection neurons in laminae I and II, and with the distal dendrites of lamina V cells, although visceral and muscle nociceptors do not project to lamina II. Nociceptive C fibers synapse with other laminae I and II neurons. Large diameter fibers from the mechanoreceptors enter the dorsal horn medially and synapse with neurons in deeper Rexed laminae (IV-VIII)
What are the three pain pathways?
Spinothalamic tract, Spinoreticular tract and spinoparabrachial tract
What does the spinothalamic tract consist of?
It arises from neurons in laminae I, II, IV and V, axons of which cross over and ascend on the contralateral side to the posterior (I and II) and ventroposterolateral (IV and V) nuclei of the thalamus. Thalamic neurons project to the primary somatosensory cortex.
What does the spinothalamic tract mediate?
Fast, well-localised and well-characterised pain.
The largest number of spinothalamic tract cells are found in lamina V. Where do they receive input from?
Their dendrites extend into lamina I and receive inputs from A(delta) (fast pain) afferents. They also receive connections from large diameter A(beta) mechanoreceptor afferents and are said to be wide dynamic range cells - get inputs from low and high threshold afferents.
Where do lamina IV cells get input from?
Only from large diameter A(beta) mechanoreceptor afferents and not from nociceptor afferents.
Where does the spinoreticular tract arise from?
Lamina V-VIII and makes bilateral connections with reticular nuclei in the brainstem–> second order neurons that project to reticular (intralaminar) nuclei of the thalamus–> SI, insula, anterior cingulate cortex and prefrontal cortex
