Sensory systems

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Psy 111 Basic concepts in Biopsychology Lecture
7 Somatoensory Systems
Website http//mentor.lscf.ucsb.edu/course/summer
/psyc111/
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The Somatosensory system
Somatosensory system gathers info from all parts
of the skin and from the internal organs
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Somatosensation

• Somatsensory system is 3 separate and interacting
  systems
• Exteroreceptive external stimuli
• Proprioceptive body position
• Interoceptive body conditions (e.g.,
  temperature and blood pressure)
• Exteroreceptive 3 divisions
• Touch (mechanical stimuli)
• Temperature (thermal stimuli)
• Pain (nociceptive stimuli)
• Specialized receptors respond to the various
  stimuli

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Somatosensory system(s)

• Somatosensory information is transduced by a
  variety of receptors with diverse properties, all
  are sensitive to mechanical (pressure) force.
• Two distinct pathways mediate (1)
  touch/proprioception
• (2) pain/temperature.

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Mechanosensitive Ion Channels

• Membrane proteins open in response to mechanical
  stimuli which cause confirmational changes in the
  proteins to open pore generating an ionic current
  that leads to depolarization.

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Cutaneous (touch) Receptors

• Free nerve ending
• temperature and pain
• Pacinian corpuscles
• touch adapt rapidly, large and deep onionlike
• touch sudden displacements of the skin
• Merkels disks touch gradual skin indentation
• Ruffini endings touch gradual skin stretch

• We respond to change when there is no change,
  no sensation
• Stereognosis identifying objects by touch

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Adaptation of touch receptors

• Response profiles are diverse and complex for
  individual somatosensory transducers.
• Typically, changes in pressure produces high
  firing with varying adaptation speeds.

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Regional Sensitivity Differences
point-to-point discrimination measures
sensitivity
Different regions of skin have different
sensitivities due to variation in receptor type
and amounts.
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Chemical Mediators of Pain
In addition to pressure responsiveness, pain
sensation involves chemical detection of
noxious molecules associated with tissue
damage. Chemical and mechanical transduction is
performed by free nerve endings.
Pain is multi-modal (chemical and mechanical)
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Thermoreceptors
Thermoreceptors are sensitive to changes in
temperature rather than absolute temperature
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Afferent fiber diversity
Touch fibers are fast (large and myelinated) Pain
fibers are slow (smaller and/or lack myelin)
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Fast and Slow Pain Afferents
Pain fibers have different response profiles.
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Spinal Afferents

• Dermatome the area of the body innervated by
  the left and right dorsal roots of a given
  segment of spinal cord

• Nerve afferents enter spinal cord between each
  vertebrae.
• Each nerve innervates a specific body region.

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Organization of the spinal cord
All somatosensory info enters the spinal cord
through the dorsal horn
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Touch Somatosensory Afferents
Touch info travels up the spinal cord in the
dorsal columns
To interneurons

• Primary sensory neuron bifurcates, axons go
• Through dorsal column up to brain (ipsilateral,
  cross over later).
• To s.c. interneurons (important for reflexsive
  motor actions)

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Pain Somatosensory Afferents

• Pain info travels up the spinal cord in the
  lateral column spinothalamic tract.
• Afferents bifurcates to interneurons and
  ascending pathways.
• Spinothalamic tract has ipsilateral and
  contralateral ascending fibers.

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Ascending Touch Pathway (for body)

• Ascending axons of
• primary neuron goes
• Through dorsal column up to medulla relay,
  dessucates in medial lemniscus and projects to
  Ventro-Posterior Lateral (VPL) thalamus, then to
  primary somatosensory cortex.
• Dorsal column-medial lemniscus pathway

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Ascending Pathway (for facial touch)
Somatosensory receptors in the face form the
trigeminal nerve (cranial), synapse in trigeminal
nucleus of the pons, then to VPL thalamus, and
to primary cortex.
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Pain and Temperature Paths
Pain and Temperature Info is relayed and
decussates (or not) at spinal levels and travels
to VPL thalamus, then primary cortex.
Spinalothalamic tract
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Ascending Somatosensory Pathways

• Dorsal-column medial-lemniscus system
• Touch and proprioception
• 1st synapse (axon decussating to other
  hemisphere) in the dorsal column nuclei of the
  medulla
• (Antero)lateral system
• Pain and temperature
• Synapse upon entering the spinal cord (axon
  decussating to other hemisphere)
• 3 tracts spinothalamic, spinoreticular,
  spinorectal

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Ascending Regulation of Pain Gate theory
Touch and pain paths interact at the spinal level
to gate pain info.
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Descending modulation of Pain

• Electrical stimulation of the periaqueductal gray
  (PAG) has analgesic effects
• PAG and other brain areas have opiate receptors

Brain activity can help to decrease pain info
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Cortical mapping of somatosensation

• Postcentral gyrus is site of primary
  somatosensory cortex.
• Body parts are dis-proportionally mapped in the
  primary cortex.
• Area of cortex is proportional to sensitivity of
  the body region.

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Somatosensory cortex

• Primary somatosensory cortex (SI)
• Located in postcentral gyrus
• somatotopic
• input largely contralateral
• Secondary somatosensory cortex (SII)
• mainly input from SI
• somatotopic, input from both sides of the body
• Much of the output from SI and SII goes to
  association cortex in posterior parietal lobe

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Secondary Association Somatosensory Cortex

• Secondary somatosensory cortex -bilateral info
  (both sides of the body)
• Damage complex effects
• Asterognosia inability to recognize objects by
  touch
• Asomatognosia the failure to recognize parts of
  ones own body

• Parietal lobe association cortex
• Apraxiadifficulty making specific movements when
  requested to do so
• Contralateral neglect inability to respond to
  stimuli on one side of the body opposite the
  lesion

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Plasticity in Somatosensory Maps
Loss of input or sensory experience can alter the
somatosensory mapping in the cortex.
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Acupuncture
Stimulation of define points of the somatosensory
system(s) via mechanical, thermal, or electrical
energy produces changes brain function.
Conditions that purportedly respond well to
Acupuncture Acute and chronic pain Headaches
Fibromyalgia Smoking/Drug Addiction Insomnia/Depre
ssion/Stress/Anxiety Weight Loss/Appetitie
Control Celiac Disease (Gluten-intolerence)