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NERVOUS SYSTEM Ⅲ
The sensory function of brain
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Senses
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Five basic types of sensory receptors
Mechanoreceptors Thermoreceptors Nociceptors(pain receptors): physical or
chemical damage Electromagnetic receptors:light on the
retina of the eye chemoreceptors
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Sensory receptor
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissner’s corpuscle (touch)
D: Muscle spindle (stretch)
A
B C
D
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sensation
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Somatic senses Mechanoreceptive somatic senses
Stimulated by mechanical compression or stretching of some tissue of the body
Tactile sense Touch , pressure, vibration , tickle senses
position sense Static position and rate of movement senses
Thermoreceptive senses Detect heat and cold
Pain senses
sensation
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• Sensory pathway:– 1st : enters spinal cord from periphery– 2nd : crosses over (decussates), ascends in spin
al cord to thalamus– 3rd : projects to somatosensory cortex
sensation
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Sensory pathway Spinothalamic pathway(anterolateral syste
m)
Dorsal column pathway (dorsal column–medial lemniscal system)
Spinocerebellar pathway
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Spinothalamic pathway Function
Carries pain, temperature, crude touch and pressure signals (superficial sensations)
Three-order neuron 1st order neuron enters spinal co
rd through dorsal root 2nd order neuron crosses over in
spinal cord; ascends to thalamus
3rd order neuron projects from thalamus to somatosensory cortex
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spinothalamicpathway
Pain , temperature, crude touch and pressure signals
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Spinothalamic Pathway
Small sensory fibres:
Pain, temperature, some touch
Primary somatosensory cortex (S1)
Thalamus
Medulla
Spinal cord
Spinothalamic tract
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spinothalamic pathwayspinal cord injury
Loss of sense of (superficial sensations):•crude Touch•Pain•Warmth/coldin right leg
Spinothalamic damage
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Dorsal column pathway• Function
– Carries fine touch, vibration and conscious proprioception signals
(deep sensations)
• Three-order neuron– 1st order neuron enters spinal co
rd through dorsal root; ascends to medulla (brain stem)
– 2nd order neuron crosses over in medulla; ascends to thalamus
– 3rd order neuron projects to somatosensory cortex
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dorsal cloumnpathway
fine touch, vibration and conscious proprioception signals
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Dorsal column pathwayDorsal column pathway
Large sensory nerves:
Fine Touch, vibration, two-point discrimination, proprioception
Primary somatosensory cortex (S1) in parietal lobe
Thalamus
Medulla
Mediallemniscus
Spinal cord
Dorsal column
Dorsal columnnuclei
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Dorsal column pathway• Function
– Sensory nuclei of the trigenimal nerve which subserve the same sensory functions for the head
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Dorsal column damage
dorsal column pathway
spinal cord injury
Loss of sense of (deep sensations):•Fine touch•proprioception•vibrationin left leg
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Central Pathways
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Differences between the two system
Velocities
The dorsal column–medial lemniscal system: large, myelinated nerve fibers 30 to 110 m/sec,
The anterolateral system :smaller myelinated fibers a few meters per second up to 40 m/sec.
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Differences between the two system
spatial orientation : in spinal cord
the dorsal column–medial lemniscal system :high degree of with respect to their origin
the anterolateral system : much less spatial orientation
lower parts of the body---center of the cord
higher parts---lateral layers
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Spinocerebellar pathway
Function : receives inputs from golgi tend
on organs and muscle stretch receptors. proprioception signals
Receptors : muscles & joints Three-order neuron
1st order neuron: enters spinal cord through dorsal root
2nd order neuron: ascends to cerebellum
3rd order neuron to cortex
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Spinocerebellar pathway
Division
Peripheral Process of First Order the Neuron
Region of Innervation
dorsal (posterior) spinocerebellar tract
from muscle spindle (primarily) and golgi tendon organs
Ipsilateral Caudal Aspect of the body and legs
ventral (anterior) spinocerebellar tract
from golgi tendon organs
Ipsilateral Caudal Aspect of the body and legs
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Dorsal column damage
• Sensory ataxia (loss of coordination)
• Patient staggers; cannot perceive position or movement of legs
• partially compensated by visual surveillance
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Spinocerebellar tract damage• Cerebellar ataxia
– a failure of the fine coordination of muscle movements
• Clumsy movements• Incoordination of the limbs (intention
tremor)• Wide-based, reeling gait (ataxia)• Alcoholic intoxication produces simila
r effects
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anterolateral view
"inner chamber” in Greek
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Compositions of thalamus
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Lateral-lower part of bodyMedial –upper part
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thalamus
2 Associated nuclei
Receive their driving inputs fromOther cortical areas
ventral-lateral nucleus
cerebullum ,globus pallidus—motor Pulvinar nucleusMedial ,lateral geniculate--sensors
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thalamus
3 nonspecific projection nucleus
“nonspecific nuclei”connect to association areas of cortex and/or limbic structure.
Anterior nuclei Medial nuclei Intralaminar and reticular nuclei
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Projection system of thalamus
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Projection system of thalamus
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Projection system of thalamus
Function Specific projection system of thalamus
Specific senses: visual or auditory cortex (except olfaction)
Non-specific projection system of thalamus Maintain and alter the excitatory situation of cortex
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Somatosensory cortex
Located in the post-central gyrus of the human cerebral cortex Ⅰ
Sagittal section coronal section From side
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Somatotopic map of the somato-sensory cortex
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Somatosensory cortex Cross projection
Each side of the cortex receives sensory information exclusively from the opposite side of the body (the exception: the sensor information from face is bilateral projection).
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Somatosensory cortex
The finer the sense, the larger the somato-sensory cortex area
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.
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Somatosensory cortex
Inversely projection
“Up is down and down is up ” The head in the most lateral portion, and the lower
body is presented medially.
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Visual cortex Primary visual cortex Secondary visual area
s
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bitamporalnasal side
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Pain
: Easily localized, occur first
: occur second
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nociceptornociceptor
Aδ nerve C nerve
spinothalamicpathway
to reticularformation
Impulses transmitted to spinal cord by– Myelinated Aδ nerves: fast pain (80 m/s)– Unmyelinated C nerves: slow pain (0.4 m/s)
Pain
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reticular formation
spinothalamicpathway
thalamus
somato-sensorycortex
Impulses ascend to somatosensory cortex via:– Spinothalamic pathway (fast pain)– Reticular formation (slow pain)---cingulate
gyrus
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Viseral sensation: Pain ?
without proprioceptor few thermoceptor
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Visceral pain Poorly localized; may be “referred” Slow pain Mostly caused by distension of hollow organ
s or ischemia (localized mechanical trauma may be painless)
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Visceral pain
Agina:
due to ischemia of the heart muscle, generally due to obstruction or spasm of the coronary arteries.
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Refered pain
is pain perceived at a location other than the site of the painful stimulus
Site of pain may be distant from organ
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Visceral pain
myocardial infarction :
(heart attack), where pain is often felt in the neck, shoulders, and back rather than in the chest, the site of the injury.
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Mechanism of refered pain
Convergence theory The primary afferent axons of skin and viscera c
onverged on the same interneurons in the pain pathways.
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Mechanism of refered pain
Convergence theory This theory explains why referred pain is
believed to be segmented in much the same way as the spinal cord.
Fail to explain why there is a delay between the onset of referred pain after local pain stimulation.
Threshold for the local pain stimulation and the referred pain stimulation are different, but according to this model they should both be the same.
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Mechanism of referred pain Facilitated theory
The primary afferent axons of skin and viscera closed to each other
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Mechanism of referred pain Facilitated theory
to explain why there is a delay between the onset of referred pain after local pain stimulation.
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Summary Spinothalamic pathway Dorsal column pathway Specific projection system Non-specific projection system Sensory area of cerebral cortex Referred pain