Sensory Pathways At the end of this lecture you should understand: OBJECTIVES: At the end of this lecture you should understand: Name of the Ascending tracts responsible for carrying different type of sensations. Formation, course & functions of each. Lesions associated with these Tracts

Transmission of Somatic Sensations All sensory information enter the spinal cord through dorsal route (Bell Magendie Law) from where they are carried through any of the following Ascending Pathways: Dorsal column-medial Laminiscal System. Fasciculus Gracilus (Medial) Fasciculus Cuneatus (Lateral) Antero-lateral system (Spino-thalamic). 1. Anterior Spinothalamic Tract 2. Lateral Spinothalamic tract Spinocerebellar tracts 1. Dorsal 2. ventral 4. Spino tectal, Spino olivary, Spino Vestibular

Dorsal column-medial Laminiscal System.

Cross Section of the Spinal cord showing Ascending tracts Rexed Laminae

Dorsal Column-Medial Laminiscal system Ist Order N. From Receptor Enter the spinal cord & divide into Lateral & Medial Branches. - The medial Branch ascends ipsilaterally to the Medulla to relay in the Gracile or Cuneate Nucleus. - The lateral branch divide & relay in the Ventral horn to serve as: 1. Local spinal Reflex action 2. for formation of Spino-Cerebellar Tract 3. May accompany dorsal column Tract

Dorsal Column-Medial Laminiscal system 2nd order nuclei Fibers from these nuclei will pass to the contralaterl side of the brain stem as internal arcuate fibers Here fibers of 5th Cranial N also join (Carrying same sensations from the face as Dorsal column from the rest of the body) and relay in the VPL nucleus of the thalamus.

3rd Order N. From VPL (Ventro Postero-Lateral ) nucleus of the thalamus the fibers Pass through Internal Capsule to the primary sensory cortex.

Dorsal Column-Medial Laminiscal system Large myelinated nerve fibers----30-110m/sec. High degree of spatial orientation of nerve fibers with respect to their origin. Spatial orientation is maintained in the thalamus. Sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted mainly in the DCML system.

Broadman’s Areas

Somatosensory Cortex I & II

Representation of different areas of the Body in the Somatosensory Cortex-I Humunculous

Sensory Pathways Dorsal Column-Medial Laminiscal System. Functions: (Types of sensations carried); Fine Touch requiring high degree of localization & gradations of Intensity. Two point discrimination. Phasic Sensations (Vibration). Sense of Position (Proprioception). Fine Pressure. Sensations that signals movement against the skin

Characteristics of Signal Transmission in Dorsal Column

Two Point Discrimination

Lesions of DCML system Tabes Dorsalis Cause; Bacterial (Treponema Pallidum)- Neurosyphilius --- Demylination Sign & symptoms: -weakness, diminished Reflexes -Hypoesthesia (Diminished Tactile Sensations) - Paresthesia (shooting & burning pain) -Tabetic gait -Tabetic bladder -Romberg’s Sign is +ve -Argyll Robertson pupil.

Anterolateral System

Sensory Pathway Ist. Order N. from receptors to the Laminae I IV, V &VI on the same side

Antero-Lateral Path 2nd Order N. From Laminae I,IV,V & VI Cross to opposite side obliquely, Ascend up & relay in Anterior & Lat White columns of the spinal cord 1. Reticular formation of Brain stem. 2. VPL N & Intralaminar Nuclei of the Thalamus 3rd Order N. From Thalamus – Pass through Internal capsule (Post.1/3rd. Of Post Limb) to the Primary Sensory Cortex

Antero-Lateral Path Small myelinated fibers------ few meters/sec to 40 m/sec Sensations that does not need to be transmitted rapidly or with great spatial fidelity. Ability to transmit broad spectrum of sensory modalities

Antero-Lateral Pathway Types of Sensations Carried; Crude touch & Pressure Pain Thermal sensations Heat & Cold Tickle & Itch Sexual Sensations

Differences between DCML/AL system The DCML system carries signals upward toward the brain mainly in the dorsal columns of the cord. After synapse they continue upward through the brain by way of medial lemniscus. Large myelinated nerve fibers having conduction velocities of 30 to 110 m/sec. High degree of spatial orientation of nerve fibers with respect to their origin.

Anterolateral system ascend through anterior and white columns of the cord. Smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second upto 40 m per sec. Much less spatial orientation Ability to transmit a broad spectrum of sensory modalities.

Spinocerebellar Pathway

Spinocerebellar Pathway Includes : -Ventral spinocerebellar T. (Crossed fibers) -Dorsal spinocerebellar T. (Uncrossed fibers) Dorsal Carries sensation to the cerebellum concerning position of muscles, tendons and joints. Ventral Carries unconscious muscle sense.

Ventral Spinocerebellar Tract

Dorsal Spinocerebeller Tract

Spinotectal Tract Origin: Posterior horn of the opposite side. Termination: Tectal lamina of mesencephalon Function: Passes pain, thermal tactile information to superior colliculus for spinovisual reflexes. ( brings about the movement of eye and head towards the source of information.)

Spino-olivary Tract Situated in anterolateral part of white column Origin is not specific Termination: Olivary nucleus of medulla oblongata. Function: Proprioception

Spinovestibular Tract Situated in lateral white column of spinal cord Origin: All the segments of spinal cord Termination: Lateral vestibular nucleus Function: Proprioception