Sensory Information Course Materials

Sensory information   Involves: o Localisation – where (hand, arm, leg, torso, etc.). o Characterisation – what (e.g. touch, pain, warmth, cold, etc.). o Proprioception – sense of movement/relation of parts. All sensations depend on intact/functional afferent nerves. Afferent nerves  Vary by size and myelination, depending on their purpose.  Large diameter myelinated afferent fibres (groups I and II) are associated with tactile and kinaesthetic sensibility.  Small diameter unmyelinated afferent fibres (groups III and IV) are associated with thermal and pain sensations. Primary afferent nerves  At the periphery, the information is carried across a single neuron.  They are bipolar, with a central soma and two axonal arms o Primary afferent terminal is at the skin and acts as a detector of stimulus. o The soma contains the DNA material, and is the site for protein synthesis. o Central termination at the spinal cord acts to synapse with spinal cord.  Primary afferent terminal detects a stimuli because of an action potential produced that propagates along the neuronal pathway. Clinical corner: axonal blockade  Compression block. o Progressive axonal block (Aβ  C fibres) with prolonged pressure. o Chronologically, there is a loss of tactile sensation (Aβ), fast pain (Aδ), burning/slow pain (C) and then motor command. o Recovery occurs in the opposite direction (except for motor). o Susceptible nerves are close to the surface (e.g. ulnar nerve, radial nerve) or where they pass through small, bony tunnels (e.g. medial nerve).  Anaesthetic block. o Progressive axonal block (C  Aβ) with the use of an anaesthetic. o Chronologically, there is a loss of burning/slow pain, fast pain, tactile sensation and then motor command. o Efficacy of anaesthetic is examined by testing sensory function dependent of dose. Classification of sensory receptors  According to the form of energy constituting the effective stimulus: o Mechanoreceptor: cutaneous receptors for touch and pressure; joint receptors; stretch receptors of muscles and tendons. o Photoreceptor: rods and cones of retina for light. o Thermoreceptor: cutaneous receptors for warmth and cold; central hypothalamic receptors. o Chemoreceptor: taste and olfactory receptors; carotid body receptors; osmoreceptors.  According to the location of the effective stimulus (Sherrington’s classification). o Exteroreceptors receptors in the skin. o Telereceptor: remote from the body (eye, ear, olfactory). o Interoreceptor: receptors in visceral organs. o Proprioceptors: receptors concerned with body position. Classification of afferent nerves  Lloyd’s classification – based on size. Fibr e Ia Ib II III IV  Fibr e Aα Aβ Aγ Aδ B Origin Muscle spindle. Golgi tendon organ. Cutaneous tactile receptors. Cutaneous cold receptors and for sharp/fast pain. Cutaneous warmth receptors and for slow/burning pain. Muscle/deep pain. Diameter 12 – 20μm. 12 – 20μm. 5 – 12μm. Conduction velocity 70 – 120m/s. 70 – 120m/s. 40 – 70m/s. 2 – 5μm. 12 – 30m/s 0.5 – 1.0μm. 0.5 – 2.0m/s Erlanger and Gasser’s classification – based on conduction velocity. Origin Muscle afferents. Somatic motor. Tactile afferents. Muscle afferents. Motor fibres to muscle spindle. Pain and thermal afferents. Autonomic pre-ganglionic efferents. Diameter 12 – 20μm. Conduction velocity 70 – 120m/s. 8 – 12μm. 40 – 70m/s. 4 – 8μm. 15 – 40m/s. 2 – 5μm. 1 – 3μm. 12 – 30m/s. 3 – 14m/s. C Pain and thermal afferents. Autonomic and thermal afferents. 0.2 – 1.0μm. 0.2 – 1m/s. Thermal sensation  Cold fibres. o Group III/myelinated A fibres. o Respond to a decrease in temperature (i.e. become more active with a drop in temperature).     Warmth fibres. o Group IV/unmyelinated C fibres. o Respond to an increase in temperature (i.e. become more active with a rise in temperature). There are more cold receptors than warmth receptors. Integration of information from cold and warmth fibres underpin our capacity to detect changes in skin/surface temperature. Thermal sensation is innocuous (not harmful) at ~10 – 42oC. Anything below or above is no longer thermal, but rather painful. Pain sensation  An unpleasant sensory and emotional experience associated with actual or potential tissue damage. o Subjective. o Learnt experience.  Pain can be caused by two mechanisms: o Nociceptive pain arises due to the activation of nociceptors during tissue damage and the accompanying inflammation. o Neurogenic pain is caused by direct injury to nerves in the PNS or CNS.  Qualities of pain: o Pricking pain (first or fast pain): sharp, well localised and quickly subsides. It is associated with myelinated Aδ fibres. o Burning pain (second or slow pain): slower onset, less well localised and more persistent. It is associated with unmyelinated C fibres.      Mechanism of activation of pain fibres: o After tissue damage, proteolytic enzymes are released to break down gamma globulins. o Gamma globulins produce polypeptides and kinins (e.g. bradykinin) that activate nociceptors. o Nerve terminals of the nociceptors then release substance P, which contribute to inflammation. o Substance P also causes mast cells to release histamine, which in turn further sensitises nociceptor endings. There are free nerve endings which allow integration of multiple inputs (e.g. detect both burning pain and sharp pain), having a wide dynamic range. o These nociceptor channels are constitutive. o In a free nerve ending, these channels would be mixed. Spinal projections travel in lamina specific to their function: o Touch (low threshold)  lamina III – V. o Wide dynamic range  lamina V. o Nociceptive specific  lamina I. In patients who experience chronic pain, there is convergence of pain with low threshold stimulus. o Hyperalgesia: exaggerated painful response to a normally painful stimulus. o Allodynia: painful response to a normally non-painful stimulus. Localised and referred pain results from the spinal convergence of afferent input from deep and somatic sources (created during embryologic phases). Kinaesthesia  Sense of movement.  Dependent on: o Muscle length. o Joint angle. o Skin stretch.  Muscles are arranged in parallel fashion and respond to stretch/tension. o Primary/annulo spiral (group I) and secondary/flower-spray (group II). o Muscles need to be engaged for kinaesthetic sensation. o Vibrate Achilles tendon  activate muscle spindles  mimics stretching of posterior muscles  interpreted as forward lean  compensates by leaning backwards.  Joints are important because they protect against overextension and overflexion.  Skin stretching (as well as joint angulation) give contextual cues as to where the body is moving. Proprioception  Sense of body scale (relative size).  Large and small diameter afferent fibres contribute to our normal sense of scale and proportion.  Increasing the information at a certain area makes the body part feel bigger (e.g. painful cooling of thumb makes it feel bigger). Tactile sensation  Associated with large diameter myelinated afferent fibres.  Have specialised endings that confer sensitivity to different stimuli.  Can be subdivided based on their response properties.