The Somatosensory System

• Purpose of the Somatosensory System:

  • allows us to perceive a range of different sensory stimuli...(receptors have large diameter, fast conducting axons)

  *proprioception = awareness of where our body and limbs are

  *nociception = pain 

  *touch

  *pressure

  *temperature

  • Somatosensory receptor(Stimulus) - To spinal cord via peripheral afferent nerves (1st Order, PNS) - via ascending pathway through the thalamus (2nd Order, CNS) - to primary sensory cortex (3rd Order) - Processing of signal in primary sensory cortex to provide a conscious perception (Sensation)
• Nonencapsulated Touch Receptors:

  • sensory nerve endings that are free (not wrapped in connective tissue)

  *pain, temperature, itch

  *proprioception = densely packed in ligaments and tendons

  • Merkel's cells/disks = b/w epidermis and dermis

  *small receptive fields and slow adapting

  *respond to light pressure - densely packed in fingertips

  • Hair follicle receptors (root hair plexus)= bending of hairs on the skin

   *rapidly adapting

  *respond to light pressure
• Encapsulated Touch Receptors:

  • nerve terminals surrounded in a capsule of connective tissue or glial cells and differ in sizes -  pressure, vibration , stretch
  • Meissner's corpuscles = located beneath epidermis

  *small receptive fields + rapidly adapting

  *discriminative touch - densely packed into fingertips, lips, nipples, foot soles

  • Pacinian corpuscles = scattered deep within dermis

  *large receptive fields + rapidly adapting

  *on/off pressure = sensitive to vibrations

  • Ruffini endings = located in dermis

  *large receptive fields and slow adapting

  *deep + continuous pressure
• Touch Receptors:

  • specialised dendritic endings of neurones located in the skin, with their cell bodies in the dorsal root ganglia of afferent spinal nerves
  • vary in density for different parts of the body - skin of the fingertips is packed with receptors which makes them incredibly sensitive to touch

  Somatosensory homunculus/Somatotopy

  • mapping of body in primary somatosensory cortex - amount of cortex dedicated to body part is related to that region's sensitivity (corresponds to no. of receptors in that part of the body)

  *biggest representation = most sensitive = fingertips , lips , face

  
• Ascending Pathway to Brain: (1)

  • Dorsal column-medial lemniscal pathways provide direct transmission for each class of receptor 

  *Dorsal columnar = organised according to receptor type and body location

  *Exception = trigeminal system for the perception of touch to the face

  • 2 paired tracts of white matter in the dorsal column of the spinal cord - fasciculus gracilis(lower part of body) and fasciculus cuneatus (upper part of body)
  • Signals pass ipsilaterally to the dorsal column nuclei - nucleus gracilis and nucleus cuneatus medulla oblongata

  *This is in medulla oblongata of the brainstem
• Ascending Pathway to Brain: (2)

  • Medial lemniscus tract - formed by 1st order neurons making their synapses with 2nd order neurons and decussating (switch sides)
  • Ventroposterior nucleus of the thalamus - where medial lemniscus tract projects into and connects w/ trigeminal system
  • Primary somatosensory cortex - where 2nd and 3rd order neurones synapse
  • Spinocerebellar pathway consists of ventral and dorsal - send electrical signals from the muscles and tendons to convey information relating to stretch
  • Cerebellum synapse - unconscious proprioception + coordinating skeletal muscle
• Primary Somatosensory Cortex:

  • located in the postcentral gyrus of the parietal lobe, posterior to the central sulcus and motor cortex
  • consists of 4 different regions - each corresponds to different type of sensory input:
  • 3a = proprioception.
  • 3b = slow adapting fibers 
  • 1 = input directly from area 3b, but also from rapid adapting fibers
  • 2 = input directly from area 1, but also receives information about pressure and joint position (proprioception).
• Pain Processing (Nocipception):

  • Pain = unpleasant sensory or emotional experience associated with real or potential tissue damage (highly subjective and varies b/w individuals)
  • Nociception = physiological response to real tissue damage.

  ! possible to have pain w/out nociception and nociception w/out pain

  • Nociceptors are free nerve endings (unencapsulated) that signal tissue damage that can be either mechaninal (cuts and bumps), thermal (burns) or chemical (irritants)
  • Injured tissues release chemicals (prostoglandins, histamine, ATP, K+, substance P) that activate nociceptors
• Hyperalgesia:

  • can make damaged tissue more sensitive to pain

  Allodynia:

  • receptors become sensitive to a stimulus which is not usually painful

  *touching sunburnt skin
• 2 Types of Nociceptors:

  • High-threshold mechanoreceptors - activated by intense mechanical stimulation , responsible for sharp, localised, stabbing pain perceived at the time of injury (1st pain)

  *Aδ fibers = medium diameter , myelinated axons

  *Fast conducting fibers

  • Polymodal nociceptors = intense mechanical stimulation, temperatures in excess of 42°C , irritant chemicals, responsible for long-lasting dull/burning pain which is more difficult to localise (2nd pain)

  *C fibers = small diameter , unmyelinated axons

  *Slow conducting fibres

  
• Pain Pathway:

  • Nociceptors and thermoreceptors project into the spinal cord via the dorsal root
  • Nociceptors synapse with 2nd order neurones in the dorsal horn and project up spinothalamic tract
  • DECUSSATE IN SPINE - axons of the 2ND order neurones cross over to the opposite side at the level of the spinal cord
  • Spinothalamic tract terminates in the ventroposterior and intralaminar nuclei in the thalamus
  • Project to primary somatosensory cortex and anterior cingulate cortex

  *damage to these areas can alter perception of pain - lead to chronic pain syndrome or make people not feel pain at all
• Why does Referred Pain Occur?

  • Visceral receptors (nociceptors connected to organs) project to the spinal cord and synapse with the same interneurons as somatic (i.e. skin/muscle) pain fibres
  • Send a signal up the spinothalamic tract via 2nd order neurons
  • Visceral pain from overstretching of tissue, ischemia, muscle spasms, chemical irritants can produce the sensation of pain on the skin surface instead

  *a person experiencing a heart attack may experience this as pain in their left arm = same spinal cord segments innervate both the heart and the arm
• Gate Theory of Pain:

  • PAIN AND RUB IT BETTER
  • Input from nociceptors (C fibers) suppresses the activity in the interneuron = GATE OPEN = PAIN
  • Input from touch receptors (Aβ fibers) leads to additional stimulation of the inhibitory interneuron
  • Inhibits pain neurone from sending any more signals
  • All you feel is the rub of the knee, as interneuron does not stop the touch neurones from running its course
• Control of pain:

  • Damage and inflammation at the site of injury > Block the production of inflammatory mediators using non-steroidal anti-inflammatory drugs (paracetamol, ibuprofen)
  • Aδ and C fiber input > Block the ascending pain signalling pathway ( local anaesthetics)
  • Sympathetic innervation of skin and blood vessels > Block activation of nociceptors by sympathectomy (Cutting of sympathetic nerve)
  • Ascending nociceptive inputs (GATE THEORY) > Activate inhibitory interneurons that function as "gates" to decrease ascending signals
• • Reduce the transmission of nociceptive input via the activation of inhibitory interneuorns 
  • Descending enkephalinergic brainstem neurons release endorphins - act on opiod receptors to decrease nerve transmission
  • Exogenous administration of opiod analgesics (e.g. morphine, heroin) mimic the role of endogenous opiods 
  • Surgical interventions - lesioning of appropriate nerve root or the use of deep brain stimulation.