#14106 - Dorsal Coloumn Pathway - Neuroscience 1

Somatosensory pathways

Somatosenosry system three major functions

-Proprioception – sense of oneself, receptors in skeletal muscle, joint capsules, skin enable us to have conscious awareness of posture, movements of body

-Exteroception: Sense of direct interaction with external world, e.g sense of touch ( contact, pressure, stroking, motion, vibration) Sense of hot and cold, Sense of pain/ nociception

-Interocetion: Sense of function of major organ systems of the body and its internal state, primarily chemoreceptors

Ascending Somatosensory pathways

-Somatosensory information is conveyed to the thalamus and cerebral cortex by 2 asending pathways

a) Dorsal column medial lemniscal system: tactile and proprioceptive information- wide diameter Aa, AB fibres

-Tactile sensation can be divieded into nondiscriminative (Crude) touch and discriminative (fine touch)

-Crude touch is transmitted via anterolateral system and is sensed following gentle stroking of skin with fine cooton strand but this sensation doesn’t include detailed information about stimulus

-Tactile examination of an object is dependent on discriminative fine touch sense- enables to detect fine detail regarding location size, shape and texture of an object even when eyes are closed.

-Conscious Proprioception may be categorised into static proprioception (awareness of position of a body part such as a limb) and dynamic proprioception (awareness of movement of a body part and balance)

Discriminative fine touch and pressure sense from the body

Receptors

-Free nerve endings responding to touch, pressure and proprioception in skin, muscles and joint capsules

-Merke’s discs- respond to touch and pressure in skin

-Peritrichial endings stimulated by touch of hair follicles

-Meissner’s corpuscles activated by touch of skin

-Pancinian corpuscle- stimulated by touch, pressure, vibration and proprioception in deep layers of skin and in visceral structures

Receptive fields

-To stimulate a fibre, stimulus must lie within particular area – receptive field

-In case of fibres innervating hair follicles- one finds that each fibre innervate as many as hundred follicle and each follicle in turn receives branches from several fibres

-So receptive fields of individual fibres are large and show overlap- if stimulus lies within area of overalp then it will stimulate two fibres in different proportions depending on exact psotion- so by comparing frequency of firing of one with that of the other, CNS can determine position of stimulus much more accurately than if fields were discrete

-Overlap has further advantage that it makes the system less vulnerable to damage

-Receptors converge onto first order pseudounipolar neurons whose cell bodies are located in the dorsal root ganglia.

-Neurons are medium size Aβ and larger size Aa fibres

i) SPINAL CORD.

-Ascending branch of large sensory dorsal root ganglion axons (Aa, Ab) enters the ipsilateral dorsal coloumn of the spinal cord (white matter tract medial to the dorsal horn) via the medial division of dorsal roots of spinal nerves

-Upon entry into posterior funiculus of spinal cord, afferent fibres bifurcate into long ascending and short descending fibres. These fibres give rise to collateral branches that synapse with several distinct cell groups of dorsal horn interneurons and with ventral horn motor neurons. These fibres collectively form dorsal coloumn pathways –either the fasciculus gracilis/ fasciculus cuneatus

Below T6: Central processes that enter spinal cord below T6 (lower thoracic, lumbar and sacral levels) which bring info from lower limb and lower half of trunk- enter ipsilateral fasciculus gracilis and ascend to medulla and terminate in ipsilateral nucleus gracilis. Fasciculus gracilis present at all levels.

Level T6 and above

-Bring information from upper thoracic and cervical levels- enter ipsilateral fasciculus cuneatus, situated laterally in spinal cord, and ascend to medulla to synapse with second order neurons in the ipsilateral nucleus cuneatus.

-fasciculus cuneatus is present only at upper 6 thoracic and at all cervical spinal cord levels and is separated from fasciculus gracilis by septum

Information transferred through posterior coloumns: Discriminate touch (ability to localise the area of the body touched, aware points touched simultaneously, 2 point discrimination. Information from muscles and joints pertaining to movement and osition of different parts of the body. Vibratory sensations

ii) MEDULLA:

-At the junction between the medulla and the spinal cord : Neurons from each fascicle synapse on the second order neurons in the ipsilateral dorsal column nulcuei: gracile and cuneate nuclei-medulla, dorsal coloumn nuclei

-Terminates in the ipsilateral dorsal column nuclei- 90% axons of the dorsal root ganglion cells, remaining 10% are axons of spinal neurons. Post synaptic dorsal column axons course along midline adjacent to medial septum provide a specialised rapid pathway for visceral pain

-Primary afferent fibres terminate on neurons throughout rostrocaudal extend of each nucleus

-Rostral 1/3^rd of the dorsal column nuclei-dominated by neurons that process proprioceptive information (75% receive sensory information from muscle afferents), tactile inputs predominate in middle 1/3^rd where 90% of neurons process cutaneous info, neurons in caudal third are evenly divided between cutaneous and proprioceptive

-Dorsal-ventral gradient in dorsal columns and dorsal coloumn nuclei-proprioceptive neurons are more common ventrally, cutaneous neurons more common dorsally

-Lateral and rostral to the cuneate nucleus proper is the external cuneate nucleus-receives proprioceptive afferent from arm and hand. External cuneate nucleus projects to both the cerebellum and thalamus

Parallel signalling-focused and refined with surround inhibition in dorsal column nuclei, necessary for fine tactile discrimination

-Incoming fibres that relay onto ascending second order neurons- gracile and cuneate nuclei, they also exite local interneurons as well

-Interneurons in turn send inhibitory connections to neighbouring second order cells- so each incoming neuron stimulates own second order cell but inhibits one that surrounds it

-Prevents blurring

Damage to these tracts results in symptoms that appear ipsilateral to the affected dorsal columns in the dermatomes at and below the level of the spinal cord lesion. The symptoms include loss of tactile sense (vibration, deep touch, and two-point discrimination) and kinesthetic sense (position and movement). The patient with a lesion affecting the cervical cord cannot identify an object placed in his or her hand ipsilateral to the lesion. If the lesion is located at the level of the lumbar cord, then the loss of these forms of sensation will be restricted to an ipsilateral lower limb. The patient cannot perceive sensations, such as touch or pressure, and his or her movements are poorly coordinated and clumsy because of the loss of conscious proprioception of his or her position in space.

iii) The axons of the second order neurons- internal arcuate fibers cross the midline of the medulla towards the ventral and medial side and decussate

The axons then ascend white matter tract called medial lemniscus (- due to crossing/sensory decussation- left side of brain receives somatosensory input from the mechoreceptors n right side of the body)

-Medial leminiscus joined medially by homologous projection from trigeminal nuclei

-Somatic sensation of the face- large trigeminal nerves- enter brain at pons- carry tactile information from skin mechanoreceptors-synapse onto second order neurons in ipsilateral trigeminal nucleus- from here axons decussate and project into the medial part of the VP nucleus of the thalmus

-Medial leminiscus rises through medulla, pons, midbrain axons synapse upon neurons of the ventral posterior nucleus of the thalamus

iii) THALAMUS:

-Egg shaped cluster of nuclei in the centre of the brain- conveys sensory input to the primary sensory areas of the cerebral cortex/ neocortex? , subcortical brain- acts as a gate keeper- preventing/enhancing passage of specific information depending on behavioural state of animal

-4 nuclei in the thalamus: anterior, medial, ventrolateral, posterior

Ventral posterior Nucleus/ nucleus ventralis caudalis: Fibres of medial lemniscus-covney tactile and proprioceptive signals, terminate in ventral posterior nucleus of thalamus where they synapse with 3^rd order axons

-Medial zone of nucleus receives trigeminal nerve fibres from the head and the face, Lateral zone receives fibres from the dorsal column nuclei (cutaneous, proprioceptive)

-Inputs are arranged somatotopically- forelimb medial and trunk and legs lateral

-Tradittionaly thought of as a single nucleus-fibres carrying cutaneous signals terminate in large central and caudal region, proprioceptive information terminate in dorsal and rostral region. Jon Kaas- separate nuclei-ventral posterior nucleus proper-cutaneous information by medial leminscal and trigeminal axons, ventral posterior superior nucleus-proprioceptive info

iv) Cortex

-Fibres arising from thalamus ascend in posterior limb of internal capsule and corona radiate to terminate in primary somatosensory cortex of post central gyrus- brodmann’s areas (3a, 3b, 1 and 2 of parietal cortex)

-Nuclei send outputs to different subregions of cerebral cortex-ventral posterior nucleus-cutaneous info to area 3b (Brodmann’s area) and 1 of primary somatosensory cortex, ventral posterior superior nucleus-conveys proprioceptive info to...