Sensory2_0708
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Transcript of Sensory2_0708
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Sensory system 2
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Contents
Dorsal column Anterolateral pathway Sensations Role of thalamus Role of somatosensory cortex
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Spinal cord
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Gray matter of the spinal cord
Divided into 10 layers (I-X) Lamina I-Marginal nucleus L II + III Substantia Gelatinosa L IV + V Nucleus Proprius
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Rexed lamina in the gray matter of spinal cord
Mechanoreceptors - AL III, IV, V and VI
Nociceptors - AL I and V
Nociceptors C
L I and II
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Dorsal Column (Lemniscal system)
Fine touch requiring a high degree of localization of stimulus or transmission of fine gradations of intensity
Vibratory sensation Movement against the skin Position sense Pressure (fine pressure) Consists of large myelinated fibers 30-110m/sec Deals with information that may require quick
action, information from skin mechanoreceptors and proprioceptors
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Figure 1. The neural pathway from the periphery to the post-central gyrus
Figure 2. The cross section of the spinal cord that demonstrate the location of gracile and cuneate fasciculi
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Dorsal column.
Fibres from the arm and leg cross at the level of caudal medulla
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Dorsal column medial lemniscus
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1. Fasciculus gracilis 2. Fasciculus cuneatus
3. Dorsal horn 4. Lateral corticospinal tract
5. Ventral horn 6. Ventral corticospinal tract
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Anterolateral system
Axons ascend in the anterolateral quadrant Crude touch Pressure sensation (crude pressure) Tickle and itch Pain and temperature-lateral spinothalamic
tract Few meters to 40m/sec (conduction velocity) Information less urgent, such as heat, cold,
and some kinds of pain
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Anterolateral system
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Spinal cord
Differences in spatial orientation of nerve fibres in the pathways
Anterolateral pathway -from medial to lateral- Cervical, thoracic, lumbar and sacral (most lateral)
The other way round in the dorsal column- Sacral (most medial), lumbar, thoracic and cervical
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Role of thalamus
All sensory tracts synapse in the thalamus (except the olfactory pathway)
Crudely aware of the sensation But not able to perceive all the fine details of the
stimulus Interpretation of the quality and localisation of the
stimulus occurs at the level of the cortex (Pain and temperature sensations are appreciated at the level of
thalamus)
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Somatosensory cortex
Part of cerebral cortex to which sensory fibresproject
Receive direct afferent fibres from the thalamus Area I- primary sensory area (postcentral gyrus) Area II-wall of the Sylvian fissure Area I- stimulated by impulses ariving from the
contralateral side of the body (face is bilaterally represented in both hemispheres)
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Organization Legs are represented on top and the head at the
bottom of the gyrus Area of representation of specific part of the body is
related to the density of the receptors in the part Sensory homunculus describe the distorted human
figure drawn to reflect the relative sensory space our body parts represent on the cerebral cortex.
Somatotopic organization-afferent fibres enter and make connections in an orderly way and preserve a 'map' of the sensory body surface in tracts, relay nuclei and cortex.
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The endings of the axons of the somatic pathways are grouped according to the peripheral locations of receptors
The parts of the body that are densely innervated lips, thumb are represented by large areas.
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Function of sensory cortex (1)Spatial recognition Localisation of the site of stimulus and two point
discrimination (Ability to perceive two touch stimuli applied simultaneously as two separate points when the eyes are closed)
Finger tips-2mm Back-30mm Acuity increases with 1. increase number of touch receptors per unit area 2. increase width of the area represented in the sensory
cortex
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Function of sensory cortex (2)
Recognition of relative intensities of different stimuli
Increase number of afferent fibresstimulated
Increase frequency of action potentials Indication of strength
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Function of sensory cortex (3)
Stereognosis The ability to recognize objects by touch
without the aid of vision
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Primary somatosensory cortex
Post-central gyrus Brodmanns area 3, 1 and 2
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Primary somatosensory cortex
The size of the areas can be modified with changing sensory experience- plasticity
Amputation of the fore limb expansion of hindlimb region in the area of forelimb stump (rats)
Amputation of the foot resulted in expansion of the area that represented the lower leg in the thalamus (monkeys)
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Somatosensory cortex
Somatosensory area II-localization is poor
Occipital lobe-visual cortex Temporal lobe-auditory cortex
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Pain
Pain is unpleasant sensory and emotional experience associated with actual or potential tissue damage, or describe in terms of such damage(IASP)
Role: protective to prevent tissue damage
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Properties of pain sensation All pain receptors are free nerve endings Pain receptors are widely distributed Pain can be produced by various types of stimuli e.g.
chemical, mechanical and thermal. Localisation is less exact than the other modalities Pain receptors adapt very little Pain threshold is high Certain visceral areas are insensitive to pain e.g. brain
tissue, liver parenchyma and lung alveoli. But the brain blood vessels, meninges, liver capsule, bile ducts, bronchi and parietal pleura are sensitive to pain.
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Effects associated with pain sensation (1)
Motor reaction Withdrawal reflex- a reflex which remove
part or all the body from the painful stimulus
Reflex contraction of somatic muscles in response to deep somatic and visceral pain e.g. guarding over inflamed appendix
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Effects associated with pain sensation (2)
Autonomic reactions Mild pain in superficial tissues stimulates
posterior hypothalamic nuclei- sympathetic discharges (tachycardia, rise in blood pressure, pupil dilatation)
Severe pain in superficial and deep tissues or organs stimulates anterior hypothalamic nuclei-parasympathetic discharges (bradycardia, decrease in blood pressure)
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Effects associated with pain sensation (3)
Emotional reactions Impulse from pain receptors reach the
limbic system Anxiety, crying, depression, anguish
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Receptors 2 major types of pain1. Fast pain (A)2. Slow pain (unmyelinated C-fibers) Nociceptors Nerve endings of thin myelinated A and
unmyelinated C-fibers Distinguished from other receptors higher
threshold Mechanical, thermal (45C) and chemical Non-adapting nature
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Pain stimuli Thermal - 45C in average person (Begin to perceive pain) Chemical bradykinin, histamine, serotonin,
potassium ions,acetylcholine, proteolytic enzymes (Pain producing substances)
Mechanical pressure Tissue ischemia-1. Accummulation of lactic acid2. Cell damage-bradykinin stimulate the nerve
endings Muscle spasm-direct effect or ischemic effect
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Pain Fast-1. 0.1 sec after a pain stimulus 2. Sharp, pricking, electric3. Mediated by A fibers (velocity 6-30 m/sec)4. Usually well localised5. Superficial Slow-1. Burning, aching, throbbing2. Intensity increases with time3. Tissue destruction4. Can occur both in skin and in deeper tissues5. Transmitted by C-fibers (0.5-2 m/sec)
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Comparison of central pathways for pain transmission
TemperatureSimple touch
Other functions
Affective-arousal components of pain
Discriminative pain (quality intensity, location)
Role
Cingulate gyrusParietal lobe (SI cortex)Cortical location
Intra-laminar nucleiOther midline nuclei
Ventral posterolateral (VPL)Thalamic nucleus
HypothalamusLimbic system
Autonomic centres
NoneSub-cortical targets
YesNoSynapse in reticular formation
BilateralContralateralBody representation
NoYesSomatotopic organisation
Lamina I, IV,V, (and VII, VIII) Lamina I & IV, VOrigin
Lateral-STTSpinoreticular tract (SRT)
Lateral-STTTract
Indirect (slow)Direct (fast)
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Neospinothalamic tract
Fast pain Terminate in lamina I (lamina
marginalis) of the dorsal horn Cross immediately to the opposite side
through the anterior commissure Pass upward to the brain stem in
anterolateral column
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Capability to localize pain
Fast pain- more accurate than slow pain However, if the pain receptors are
stimulated without stimulation of tactile receptors-the localization may be poor
Tactile receptors-dorsal column-localization is more accurate
Glutamate-neurotransmitter Period of action-few millisecond
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Paleospinothalamic tract Peripheral fibers terminate in lamina II and III
(substantia gelatinosa) Pass through one or more neurons within the dorsal
horn before entering lamina V-VIII Last neuron give long axons and join the fibers from
fast pain pathway Cross to the opposite side upward in anterolateral
pathway Most of the fibres terminate diffusely in the reticular
formation of the brain stem Intralaminar nuclei, hypothalamus Connection with the Reticular Activating System- a
strong arousal effect The neurotransmitter-substance P
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Paleospinothalamic tractProjection into brain stem and thalamus
1) Reticular nuclei medulla, pons and mesencephalon
2) Tectal area of mesencephalon 3) Periaqueductal gray region Multiple short fibers into intralaminar
nucleus of the thalamus Hypothalamus Cingulate cortex and insula
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Paleospinothalamic tract
Poor capability to localize pain Multisynaptic and diffuse connectivity
of the tract Function1. Unpleasant character of pain
sensation2. Arousal
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Pain suppression(Analgesia system)
The ability of the brain to suppress pain inputs to the nervous system
Consists of1. Periaqueductal gray and periventricular
areas of mesencephalon and upper ponssurrounding the aqueduct of Sylvius and portions of third and fourth ventricle
2. Raphe magnus nucleus and nucleus reticularis paragigantocellularis
3. Dorsolateral columns in the spinal cord Transmitters-enkephalin & serotonin
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The brain opiate system
Morphine- an opioid that has analgesic action Opioid receptors- periphery and central endings of c
and A fibers, spinal cord and other areas in the brain e.g. thalamus
Large protein molecules-proopiomelanacortin, proenkephalin and prodynorphin
-endorphin, met-enkephalin, leu-enkephalin and dynorphine
Endogenous opioids are released during stress
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PAIN TRANSMISSION IS NOT INHIBITED
PAIN TRANSMISSION IS INHIBITED
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Summary
2 pathways Dorsal column Anterolateral spinothalamic tractPain Fast Slow