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Dorsal column pathwayDorsal column pathway
DR SYED TOUSIF AHMED
General Sensory Receptors
Sensory pathways
Spinocerebellar Pathway
Posterior Column Pathways
Anteriolatheral Pathways
Posterior Tract
Anterior Tract
Fasciculus Cuneatus
Fasciculus Gracilis
LatheralTract
Anterior Tract
Sensory PathwaysSensory Pathways
Cutaneous sensory Cutaneous sensory receptorsreceptors
Smooth skin(glaborous)
Hairy skin
Subcutis
Dermis
Epidermis
Freenerve
endingsMerkel disks detect steady pressure & are slowly adapting
Free nerve endingsaround hair root can be either rapid or slowlyadapting - depends on hair type
Meissner’s corpusclesdetect flutter & arerapidly adapting
Pacinian corpuscles detect vibration & are very rapidly adapting
Ruffini corpuscles detect steady pressure at higherthreshold & are slowly adapting
Free nerve endings in the skinare modality specific and candetect either pain or touch or pressure or temperature
The receptor location and its associated structure can alter thestimulus and influencethe response
Tactile SensationsTactile Sensations arise by activation encapsulated
mechanoreceptors– touch– pressure– vibration
arise by activation of free nerve endings– itch– tickle
TouchTouch Crude touch
– ability to perceive that something contacted skin– exact location, shape, size, or texture cannot be
detected Fine touch
– provides specific information about location, shape, size, and texture of stimuli
ProprioreceptorsProprioreceptors Intrinsic knowledge of limb position is known as
kinaesthesia. Information is provided by sensory input from muscle
spindles (Ia & II) and Golgi tendon organs. These are mechanoreceptors and provide the CNS with
information on muscle length, position and tension. Muscle spindle group Ia afferent fibres are rapidly
adapting (dynamic) and are sensitive to rapid changes in muscle length.
Muscle spindle group II afferent fibres are slowly adapting (static) and single the fixed length of the muscle.
Joint Kinesthetic ReceptorsJoint Kinesthetic Receptors located within and around articular capsules of synovial
joints perception of body movements several types
– free nerve endings and type II cutaneous mechanoreceptors in capsules of joint and respond to pressure
– small lamellated corpuscles respond to acceleration and deceleration of joints
during movement– articular ligaments contain receptors similar to tendon
organs adjust the contraction of adjacent muscles when
excessive strain is placed on joint
Two-Point DiscriminationTwo-Point Discrimination
Processing at Receptor Processing at Receptor LevelLevel
Receptor must have specificity for stimulus activity
Stimulus must be applied to receptive field Transduction must occur Generator potential in 1st order neuron must reach
threshold How is information about stimulus encoded? Tonic receptors (slow adapting) Phasic receptors (fast adapting) Adaptation
Rapidly adapting cutaneous mechanoreceptors signal the onset and offset (phasic) of a stimulus and give rise to sensations such as vibration, touch, and movement
Slowly adapting mechanoreceptors continuously signal (tonic) the intensity of the stimulus and give rise to the sensation of pressure.
Cutaneous receptors with small receptive fields are involved in fine spatial discrimination, whereas receptors with larger receptive fields are less spatially precise.
Overlap of receptive fields allows lateral inhibition to occur in the ascending pathways and increase sensory acuity.
Cutaneous Cutaneous mechanoreceptorsmechanoreceptors
Somatosensory system Somatosensory system receptor classificationreceptor classification
First-Order Neuron First-Order Neuron Sensory neuron delivers information to CNSCell body is located in dorsal root ganglionSynapses with interneurons in CNS
Second-Order NeuronSecond-Order Neuron
Usually interneurons receiving information from first order neurons
Axons cross to opposite sides of body– Decussation
May be located in the spinal cord or brain stem
Third-Order NeuronThird-Order Neuron
Located in thalamusCarries information to cerebrumSynapses with neurons of the primary
sensory cortex
Ascending Pathways to Ascending Pathways to BrainBrain What type of information do the neurons carry?
3 chains of successive neurons to brain– 1st order neurons
Where is cell body? Conduct impulses from cutaneous receptors and proprioceptors Synapse w/ 2nd order
– 2nd order neurons Cell bodies in dorsal horn or in medullary nuclei Transmit impulses to thalamus or cerebellum; synapse
– 3rd order neurons In thalamus Where do they conduct information to? No 3rd order neurons in cerebellum
Posterior Column PathwayPosterior Column Pathway Carries sensations of highly
localized (fine) touch, pressure, vibration, proprioception
Spinal Tracts Involved:
– Left/right fasciculus gracilis – Left/right fasciculus cuneatus
Peripheral nerves systemPeripheral nerves system
Sensory Pathways and Sensory Pathways and Ascending Tracts in the Spinal Ascending Tracts in the Spinal
CordCord
dorsal cloumnpathway
Dorsal column pathwayDorsal column pathway
Dorsal-column leminiscal Dorsal-column leminiscal pathwaypathway
Principally conveys tactile discrimination, vibratory and position senses (A,large fibres).
1st order sensory neurones run on the same side & synapse with 2nd order neurones in the dorsal column nuclei .
2nd order neurones integrate the input and their axons cross to the opposite side. These ascend through the medial leminiscus
Further integration in the thalamus & 3rd order neurones project to the cortex.
The area of sensory cortex devoted to a body region is relative to the number of sensory receptors.
The Posterior Column Pathway The Posterior Column Pathway and the Spinothalamic Tractsand the Spinothalamic Tracts
Dorsal column pathwayDorsal column pathway
Large sensory nerves:
Touch, vibration, two-point discrimination, proprioception
Primary somatosensory cortex (S1) in parietal lobe
Thalamus
Medulla
Mediallemniscus
Spinal cord
Dorsal column
Dorsal columnnuclei
Dorsal Dorsal column column damagedamage
dorsal column pathway
Leftspinal cord injury
Loss of sense of:•touch•proprioception•vibrationin left leg
Dorsal column damageDorsal column damage
Sensory ataxia
Patient staggers; cannot perceive position or movement of legs
Visual clues help movement
Central Pathways
3.3 Spinocerebellar pathway3.3 Spinocerebellar pathway Carries unconscious
proprioception signals Receptors in muscles &
joints 1st neuron: enters spinal
cord through dorsal root 2nd neuron: ascends to
cerebellum No 3rd neuron to cortex,
hence unconscious
Coding in the sensory Coding in the sensory system could system could theoretically follow:theoretically follow:
1. The labeled-line principle in which each receptor responds to a limited range of stimuli and sends a direct line to the brain.
2. Across-fiber pattern in which each receptor responds to a wider range of stimuli and contributes to the perception of each of them.
THANK YOUTHANK YOU
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