Sensory2_0708

Sensory system 2

Contents

Dorsal column Anterolateral pathway Sensations Role of thalamus Role of somatosensory cortex

Spinal cord

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

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

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

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

Dorsal column.

Fibres from the arm and leg cross at the level of caudal medulla

Dorsal column medial lemniscus

1. Fasciculus gracilis 2. Fasciculus cuneatus

3. Dorsal horn 4. Lateral corticospinal tract

5. Ventral horn 6. Ventral corticospinal tract

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

Anterolateral system

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

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)

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)

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.

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.

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

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

Function of sensory cortex (3)

Stereognosis The ability to recognize objects by touch

without the aid of vision

Primary somatosensory cortex

Post-central gyrus Brodmanns area 3, 1 and 2

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)

Somatosensory cortex

Somatosensory area II-localization is poor

Occipital lobe-visual cortex Temporal lobe-auditory cortex

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

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.

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

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)

Effects associated with pain sensation (3)

Emotional reactions Impulse from pain receptors reach the

limbic system Anxiety, crying, depression, anguish

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

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

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)

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)

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

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

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

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

Paleospinothalamic tract

Poor capability to localize pain Multisynaptic and diffuse connectivity

of the tract Function1. Unpleasant character of pain

sensation2. Arousal

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

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

PAIN TRANSMISSION IS NOT INHIBITED

PAIN TRANSMISSION IS INHIBITED

Summary

2 pathways Dorsal column Anterolateral spinothalamic tractPain Fast Slow