L38 Somatosensory Pathway

• Sensation

  The detection of environmental changes (stimuli) by sensory receptors and the transmission of this information to the brain
• Components of sensation

  • Sensory receptors detect/carry the environmental changes (stimuli that get transduced) to the brain (conscious level)
  • They transmit information to the cerebral cortex (awareness)
  • Sensation is experienced at the site of the receptor

  
• Perception

  Awareness of the source of the stimulus where the sensory input is correlated with past or present information
• Sensory systems

  • Visual
  • Auditory (acoustic, vestibular)
  • Somatosensory (cutaneous, proprioceptive)
  • Gustatory
  • Olfactory (chemical/flavor)
• Distal senses

  Visual, auditory, olfactory
• Proximal senses

  Somatosensory (cutaneous, proprioceptive), gustatory
• Mechanoreceptors

  Receptors that detect mechanical deformation
• Other somatosensory transducers

  Free nerve endings that detect temperature, proprioception, and nociception (pain)

  
• Receptor

  • Specific and activated by only one type of energy
  • Converts (transduces) different forms of energy into impulses (action potentials) in sensory neurons
• Transduction

  The conversion of different forms of energy into impulses (action potentials) in sensory neurons
• Coding of stimulus intensity and duration

  Action potentials in sensory neurons encode the quality of the environmental stimulus
• Adequate stimulus

  A stimulus of quality and of sufficient intensity to excite a sensory receptor
• Adequate stimuli for somatosensation

  • Touch (light touch, pressure, vibration)
  • Thermal (infrared radiation, contact)
  • Pain and itch (chemical, thermal, mechanical)
  • Proprioception (mechanical; stretch or pressure)

  
• Bipolar neurons - receptors - sensors


  Physical "strain" at the nerve terminus forces Na+ ion channels to open, resulting in rapid depolarization and an action potential
• Receptive field (RF)

  The area in the periphery within which sensory stimulus can modulate the firing of the sensory neuron
• Receptive field properties

  • Size: smaller RF - higher spatial resolution
  • Density: higher density - higher spatial resolution
• Two-point discrimination test

  Measures the spatial resolution of the receptive field
• Cutaneous sensory receptors

  • Free nerve endings
  • Merkel disks (slowly adapting)
  • Meissner's corpuscles (rapidly adapting)
  • Pacinian corpuscles (very rapidly adapting)
  • Ruffini corpuscles (slowly adapting)

  
• Nociceptors

  Free nerve endings that respond to mechanical, thermal, or chemical stimuli
• Receptor classification by energy detected

  • Mechanoreceptors
  • Chemoreceptors
  • Proprioceptors
  • Photoreceptors
  • Osmoreceptors
  • Baroreceptors
  • Thermoreceptors
  • Nociceptors

  
• Receptor classification by location

  • Telereceptors (detect stimuli from a distance)
  • Exteroreceptors (detect stimuli from outside the body)
  • Interoreceptors (detect stimuli from within the body)

  
• Somatosensory receptor classification

  • Touch (rapidly adapting, slowly adapting)
  • Touch & pressure (slowly adapting)
  • Vibration (Meissner's, Pacinian)
  • Temperature (cold, warm)
  • Pain (fast 'pricking', slow 'burning')

  
• Nerve fiber classification

  • Aa (proprioception, somatic motor)
  • A (fine touch, pressure, vibration)
  • Ag (motor to muscle spindle)
  • A (pain, cold, touch)
  • C (crude touch, deep pressure, tickle, aching pain, thermal)

  
• Specific sensory pathways

  Convey impulses from one type of somatosensory receptor to specific primary receiving areas in the cerebral cortex
• Non-specific pathways

  Convey impulses from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus
• Major somatosensory pathways

  • Dorsal column-medial lemniscal pathway
  • Spinothalamic pathway
• Sensory information from the somatic segments enters the spinal cord through the dorsal roots of the spinal nerves
• Almost all sensory signals from the entry point into the spinal cord to the brain are carried in the dorsal column-medial lemniscal system or the anterolateral system
• Dorsal-column leminiscal pathway

  Transmits sensations that need rapid transmission and finer sensations

  
• Spinothalamic pathway

  Transmits sensations that do not need to be transmitted rapidly and cruder sensations
• Innervation Via Spinal Nerves
• Bell-Megendie Law

  dorsal = sensory afferents, ventral = motor efferents
• SENSORY PATHWAYS FOR TRANSMITTING SOMATIC SENSATIONS INTO CNS
• Sensory pathways

  1. Fibers carrying different types of somatic sensations enter the spinal cord through dorsal roots

  2. Inside the cord, a rearrangement takes place, fibers carrying similar type of sensation tend to form a bundle – SENSORY/ASCENDING TRACT

  
• Tract


  A bundle of nerve fibers carrying one or a group of sensory or motor impulses in the CNS
• Almost all sensory information from the somatic segments of our body enters the spinal cord through the dorsal roots of the spinal nerves
• Almost all sensory signals, from the entry point into the spinal cord to the brain, carried either in THE DORSAL COLUMN-MEDIAL LEMNISCAL SYSTEM OR IN THE ANTERO-LATERAL SYSTEM
• Dorsal column medical lemniscal system

  Transmits sensations that need rapid transmission and finer sensations, composed of large, myelinated fibers (Aa-g), limited to transmit discrete types of mechanoreceptive sensations

  
• Antero-lateral system


  Transmits sensations that do not need to be transmitted rapidly and cruder sensations, composed of smaller myelinated (A)/ unmyelinated fibers (C) that transmit signals at low velocities, ability to transmit a broad spectrum of sensations
• Dorsal column medical lemniscal system

  1. 1st order sensory neurons (dorsal route ganglion) occupy the dorsal white column of the spinal cord traverse on the same side & synapse with 2nd order neurons in the dorsal column nuclei (nucleus gracile, nucleus cunate) in lower medulla oblongata

  2. 2nd order neurons cross the opposite side in lower medulla oblongata and carry the signal to the upper medulla oblongata and ascend through the medial lemniscus

  3. Passes through the brain stem and terminates in the thalamus and 3rd order neurons arise and project to the cortex