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.

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