Download ANPS 019 Beneyto-Santonja 11-07

ANPS 019 Beneyto-Santonja 11/07/12
Somatosensation
What sensations does the brain detect?
 Special senses
o Olfaction (smell)
o Gustation (taste)
o Audition (hearing)
o Vestibular (balance)
o Vision (sight)
 Somatosensations
o Superficial cutaneous senses: light touch and pressure, vibration, hot, cold, pain,
itch, tickle
o Deep senses: position sense, pain, intense pressure
o Visceral senses: visceral pain, hunger, nausea, thirst
Somatosensation utilizes many receptors
 Somatosensation= senses detected through body
 Many different receptor types
o Nociceptors (pain)
 Free nerve endings sense tissue damage, chemical signals, temperature
extremes
o Thermoreceptors  free nerve endings sense temperature
o Chemoreceptors  respond to chemicals in intersititial fluid (H+, O2, CO2)
o Mechanoreceptors
 Superficial cutaneous – highly sensitive fine touch and pressure for
specific localization
 Deep cutaneous – less sensitive crude touch and pressure for less specific
localization
 Barorecptors detect internal pressure (blood vessels, bladder, GI)
 Proprioceptors/Joint receptors (monitor limb position)
Receptor characteristics
 Receptive field – part of the body from which the receptor can be stimulated (e.g., area of
skin for touch sensation)
o Small receptive fields can discriminate finer sensations
o When receptors synapse in spinal cord, many receptors converge; therefore,
spinal cord neuron receptive fields are much more complex
 Adapation – reduction in sensation with repeated stimulation; some receptors adapt
whereas other do not
o Slowly adapting receptors respond through the stimulus duration
o Rapidly adapting receptors only respond at the beginning of the stimulus
 Receptor Potentials differ from Action Potentials:
o Action Potentials are All-or-None
o Receptor Potentials are Graded and Summed
o Amplitude of receptor potential is due to intensity of the stimulus
o Once of sufficient intensity the receptor fires an action potential
o Action potential firing rate codes stimulus intensity
Superficial Cutaneous Mechanoreceptors
 Monitor small areas of skin (small receptive field) to detect very fine sensations
 Meissner’s corpuscle – rapidly adapting, low frequency vibration
 Merkel cell – slowly adapting, pressure
Deep Cutaneous Mechanoreceptors
 Monitor larger areas of skin (large receptive field) to detect crude sensations
 Pacinian corpuscle – rapidly adapting, vibration
 Ruffini ending – slowly adapting, pressure
Muscle Spindle; Muscle Receptor
 Monitors muscle length and rate of change
 Organized in parallel with extrafusal muscle fibers
Extrafusal vs. Intrafusal Muscle Fibers
 Sensory receptor is associated with intrafusal muscle
 Requires two types of motor neurons:
o Alpha motor neurons innervate extrafusal muscle (muscle bulk)
o Gamma motor neurons innervate intrafusal muscle
o Both motor neurons must fire simultaneously!
Golgi Tendon Organ (GTO); Joint Receptor
 Monitors muscle tension
 Organized in series with extrafusal muscle fibers
What happens after a receptor is activated?
 Stimulus  Detected by Receptor  Activates First-order neuron in PNS  Activates
Second-order neuron in CNS (Axon of second order neuron crosses to opposite side) 
Activated Third-order neuron in thalamus  Activates Neuron in primary sensory cortex
 provides Conscious perception of stimulus
Ultimately, somatosensory info is transmitted to the Postcentral Gyrus in the Parietal Lob
The 3 main somatosensory pathways can be understood by receptor types
 Mechanoreceptors (Joint & muscle receptors) 
o Spinocerebellar Cuneocerebellar= unconscious sensations
o Dorsal Column-Medial Lemniscal= conscious sensations
 Nociceptors (pain) & Thermoreceptors (temperature)
o Anterolateral (aka Spinothalamic)= pain & temperature
 Chemoreceptors (H+, O2, CO2)