Somatosensory

• Sensory receptors


  Respond to stimulation e.g., light energy, mechanical energy, chemical energy and thermal energy
• Sensory receptors

  • Made up of specialised cells known as receptor cells
  • In response to stimulation, the membrane potential of receptor cells changes generating a receptor potential
  • Receptor potentials are converted into action potentials in afferent axons
  • Information is transmitted from the receptor cell to the CNS
• Sensory receptors

  Provide info on the strength, type, duration and location of stimulus
• Stimulation

  1. Sensory receptor (receptor cell – stimulants energy is transduced into receptor potential)

  2. Action potential travels along afferent nerve

  3. CNS
• Primary afferent nerves

  • Axons that bring sensory information from the receptor to the spinal cord
  • Enter spinal cord via dorsal root
  • Axons vary in diameter
  • Can be myelinated
  • Diameter and myelination determine speed of action potential conducted
• Describe how sensory systems are hierarchically organised

  ·       Complexity of analysis increases

  ·       Neurons respond to stimuli of greater specificity and complexity

  ·       Receptors -> thalamus -> primary sensory cortex -> secondary sensory cortex -> association sensory cortex

  Types of receptors 

  ·       Majority are mechanoreceptors – respond to bending and stretching 

  ·       Pacinian and meissners corpuscles -> fast adapting, respond to sudden displacements of skin 

  ·       Merkels disks and ruffni endings -> slow adapting, respond to gradual change and stretching of skin 
• Primary afferent nerves

  • A-beta fibres carry info on touch – touch afferents
• Pain

  An unpleasant sensory and emotional experience associated with actual or potential tissue damage (IASP, 1994)
• Feeling pain

  • Acts as a danger signal and protects us from injury
• Congenital insensitivity to pain

  • Can't differentiate between stimuli, unusual autonomic response to pain, lower life expectancy
• Nociceptors

  Sensory receptors that respond to (potential) tissue damage, e.g. exposure to chemicals/extreme temps – free nerve endings in most tissues
• A delta fibres

  Info from unimodal receptors, Short duration pricking pain, bigger
• C fibres

  Info from polymodal receptors, Long lasting dull pain, smaller
• Difference in conduction speed can explain why pain can feel like it has two phases
• Feeling of pain conduction (from receptor to cortical level)

  1. Noxious info from periphery to spinal cord

  2. Ventral posterior thalamus

  3. Brain (the somatosensory cortex)
• Medial lemniscal pathway

  1. Sensory neurons enter spinal cord via dorsal root

  2. Ascend ipslaterally via dorsal column (white matter tract)

  3. Synapse in the dorsal column nuclei of the medulla

  4. Axons decussate/cross and ascend in the medial lemniscus (white mattertract) to the contralateral ventral posterior (VP) nucleus of the thalamus

  5. Majority of neurons project to the primary somatosensory cortex
• Nociception

  Peripheral and central processes linked to noxious stimuli – has potential to cause tissue damage
• Nociception can occur without pain
• Mechanoreceptors

  • Respond to different types of mechanical stimulation e.g. touch, vibration, and light pressure
• Pain conduction pathway

  1. Nociceptors detect noxious stimuli

  2. Primary afferent nerves (A-delta and c fibres) carry noxious signals enter the CNS via the dorsal horn of the spinal cord

  3. Afferents immediately synapse with the second order pain neuron in the spinal cord

  4. Second order pain neuron crosses-over and ascends contralaterally to the ventral posterior nuclei of the thalamus

  5. Axons from neurons in the thalamus (third order neuron) send the signal to the primary somatosensory cortex
• Touch signal conduction

  1. Primary afferent nerves (A-beta fibres) carrying signals about touch, vibration, light pressure enter the central nervous system (CNS) via the dorsal horn of the spinal cord

  2. Signal is carried along these afferent nerves ipsilaterally (i.e., on the same side as the touch/vibration / light pressures stimulus) along white matter tracts (called the dorsal column), until it reaches the medulla of the brain

  3. Afferent nerves synapse in the medulla (i.e., connect) with cell bodies in the medulla

  4. Signal transferred to the second order neuron

  5. Axons of neurons in the medulla cross-over and ascend to the ventral posterior (VP) nuclei of the thalamus and synapse with these neurons (third order neuron)

  6. Axons from neurons in the thalamus (third order neuron) sends signal to the primary somatosensory cortex
• Nuclei

  Cell bodies in the CNS
• Primary somatosensory cortex

  • Lies on the postcentral gyrus
  • Receives inputs from the ventral posterior nucleus of thethalamus
  • Neurons respond to somatosensory stimuli
  • Lesions impairs sensation
  • Electrical stimulation induces sensory experience – mapped by Penfield
• Pain info ascends contralaterally, touch info ascends ipslaterally
• Somatosensory information processing

  1. Inputs from both S1 and S2 project towards theassociation cortex in the posterior parietal lobe

  2. The association cortex is involved in the complexintegration of somatosensory information with othersensory information (e.g., vision)
• Damage to the association cortex can result in neurological disorders
• Strong emotion, stress and determination


  Can suppress feeling of pain
• Neurological disorders


  • Neglect syndrome: Part of the body or world is ignored
  • Asomatognosia: The man who fell out of bed
• Despite severe injury during action, service personnel reported little to no pain – found meaning behind the wounds mediated the pain response – Beecher
• Civilian cases (Larbig, 1991)
• Evidence suggests pain is influenced by central factors
• Attention

  • Perceived pain is less when individuals are distracted from source of pain (Bantick et al., 2002)
• Expectations

  • Appraisal of how noxious a particular stimuli is influences perception of pain (Atlas & Wager, 2012)
• Emotions


  • Negative emotions enhance pain, positive emotions decrease painful experience (Berna et al., 2016)
• Hypoalgesic effects of swearing

  • Swearing reduced participants pain perception and heart rate rise after putting hands in ice water – (Atkins et al, 2009)
• Gate control theory - Peripheral modulation


  1. Gate mechanism in the substantia gelatinosa in the dorsal horn of the spinal cord influences the transmission of pain

  2. Gate open - pain signals transmitted to the brain

  3. Opened by activity in the A delta and C pain fibres

  4. Gate closed - pain signals can't be transmitted to the brain

  5. Closed by activity in the larger touch fibres

  6. Primary afferent nerves carry touch signals (A-beta fibres) enter the dorsal horn of the spinal cord

  7. Activated pain fibres inhibit the inhibitory interneuron -> suppress activity (double negative) -> Pain signals transmitted to the second order neuron and sent to the cortex (= gate open)

  8. Activation of touch fibres will have an excitatory effect on the inhibitory interneuron -> exerts 'inhibitory' effect -> Pain signals blocked from reaching the brain (= gate closes)
• Descending pain control


  1. PAG located in tegmentum -> receives input from many other brain areas (including the cortex and amygdala)

  2. Electrical stimulation of the Periaqueductal gray PAG can block and supress pain

  3. Activated PAG neurons descend into midline regions of medulla (raphe nuclei) -> send pain supressing signals

  4. Medullary neurons project down into the dorsal horn of the spinal cord -> suppress the painful response

  5. Pathway sends signals from higher order brain areas (frontal areas) and emotional areas (amygdala) to the PAG, which has a pain supressing effect

  6. Activated PAG neurons descend to the medulla, particularly the raphe nuclei sending pain supressing signals

  7. Medullary neurons project down into the dorsal horn of the spinal cord -> suppresses painful response -> Pain signals blocked from ascending to the cortex