Part 3 Sensory Function of the Nervous System. I Sensory pathways. Sensory systems allow us to detect, analyze and respond to our environment “ ascending pathways ” Carry information from sensory receptors to the brain Conscious: reach cerebral cortex
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Sensory Function of the Nervous System
D1. Sensory receptors
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissner’s corpuscle (touch)
D: Muscle spindle (stretch)
Primary somatosensory cortex (S1)
Small sensory fibres:
Pain, temperature, some touch
spinal cord injury
Primary somatosensory cortex (S1) in parietal lobe
Large sensory nerves:
Touch, vibration, two-point discrimination, proprioception
spinal cord injury
4. Somatosensory cortex
Located in the postcentral gyrus of the human cerebral cortex.
2)The lips, face and thumb are represented by large areas in the somatic cortex,
whereas the trunk and lower part of the body, relatively small area.
Spatial orientation of signals.
3)The head in the most lateral portion, and the lower body is presented medially
II . Pain the somatic cortex,
“ the somatic cortex, Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”
International Association for the Study of Pain
1. Nociceptors the somatic cortex,
free nerve endings in skin respond to noxious stimuli
Nociopectors the somatic cortex,
Hyperalgesia: the somatic cortex,
The skin, joints, or muscles that have already been damaged are unusually sensitive. A light touch to a damaged area may elicit excruciating pain;
Primary hyperalgesia occurs within the area of damaged tissue;
Secondary hyperalgesia occurs within the tissues surrounding a damaged area.
3. Fast and Slow Pain the somatic cortex,
spinothalamic the somatic cortex,
Impulses transmitted to spinal cord by
somato- the somatic cortex,
Impulses ascend to somatosensory cortex via:
4. the somatic cortex, Visceral pain
Notable features of visceral pain:
Often accompanied by strong autonomic and/or somatic reflexes
may be “referred”
Mostly caused by distension of hollow organs or ischemia (localized mechanical trauma may be painless)
Afferent innervation of the viscera. the somatic cortex,
Often anatomical separation nociceptive innervation (in sympathetic nerves) from non-nociceptive (predominantly in vagus).
Many visceral afferents are specialized nociceptors, as in other tissues small (Ad and C) fibers involved.
Large numbers of silent/sleeping nociceptors, awakened by inflammation.
Nociceptor sensitization well developed in all visceral nociceptors.
Convergence theory: the somatic cortex,
This type of referred pain occurs because both visceral and somatic afferents often converge on the same interneurons in the pain pathways.
Excitation of the somatic afferent fibers is the more usual source of afferent discharge,
so we “refer” the location of visceral receptor activation to the somatic source even though in the case of visceral pain.
The perception is incorrect.
The convergence of nociceptor input from the viscera and the skin.
Melzack & Wall (1965)
A gate, where pain impulses can be “gated”
The synaptic junctions between the peripheral nociceptor fiber and the dorsal horn cells in the spinal cord are the sites of considerable plasticity.
A “gate” can stop pain signals arriving at the spinal cord from being passed to the brain
descending nerve the somatic cortex,
fibers from brain
axons from touch receptors
axons from nociceptors
“THE PAIN GATE”
The gate = spinal cord interneurons that release opioids.
The gate can be activated by:
Stimulation of touch fibres for pain relief:
Release of natural opioids