Sensory pathway into the CNS by Adejare A. A Sensory pathway into the CNS by Adejare A. A. Department of Physiology College of Medicine University of Lagos

Dorsal Column Medial Lemniscal Pathway For sensory information that must be transmitted rapidly (30-120m/s) And with high temporal and spatial fidelity Information in it is limited to discrete types of mechanoreceptive sensations The fibres are large and myelinated

The pathway carries: Torch sensations Vibratory sensations Position sensations Sensations that signal movement against the skin Pressure sensations

The pathway

Cross section of the spinal cord: showing the anatomy of the cord gray matter and of ascending sensory tracts in the white columns of the spinal cord.

Branches Lateral branch: Forms synapses and re-enter the dorsal column (15%) Forms the spinocervical tract that also rejoins the dorsal column in the medulla Others give rise to the spinocerebellar tract Many of these elicit local cord reflexes

Medial branch The 1st order neurones (fibers) pass uninterrupted up to the medulla where they synapse in the dorsal column nuclei (cuneate and gracile nuclei) 2nd order neurones decussate to the opposite side and continue to the thalamus through the medial lemnisci (bilateral brainstem pathways) Fibers from the sensory nuclei of the trigeminal nerve (CN 5)(sensory function from head and neck) joins the medial lemniscus in the brainstem

All fibers terminate in the ventrobasal complex (nuclei) of the thalamus. The complex consists of Ventral posterolateral nucleus receiving medial lemniscal fibers Ventral posteromedial nucleus receiving fibers fron trigeminal nuclei and Posterior thalamic nuclei receicing fibers from the anterolateral system

From the VBC of the thalamus, 3rd order nerve fibers project to the postcentral gyrus of the cerebral cortex (SSA 1) Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented on the right side of the thalamus while the right side is represented on the left side of the thalamus

DCMLP

Projection of the dorsal column–medial lemniscal system through the thalamus to the somatosensory cortex.

Somatic sensory cortex The cerebral cortex is divided into about 50 areas called (Brodmann areas) The large central fissure (central sulcus) separates the cerebral cortex into anterior and posterior halves. Sensory signals from all modalities of sensation terminate in the cerebral cortex posterior to the central fissure

somatosensory areas I and II.

Representation of the different areas of the body in somatosensory area I of the cortex

Functions of SSA 1 (Brodmann 3, 1, 2) To localize discretely the different sensations from the body parts Judge critical degrees of pressure against the body Judge the weight of objects Judge shapes of forms of object (inability: astereognosis) Judge texture of materials

Functions of somatic sensory association areas (Brodmann areas 5 & 7) To decipher the sensory information that enters the SSAreas Removal of these association areas causes amorphosynthesis

Anterolateral pathway The pathway transmits: pain, heat, cold, crude tactile, tickle, itch & sexual sensations. Pain: is a protective mechanism for the body. It shows that a tissue is being damaged and the pain stimulus has to be removed Types of pain Fast: sharp, pricking, acute, electric pain Stimuli: mechanical , thermal Not felt in the visceral

Causes of visceral pain Ischemia of visceral tissue Spasm of the smooth muscle in a hollow viscus Distension of a hollow viscuss Stretching of the ligaments Transmitted in C-fibres

Slow pain: aching, throbbing, nauseous, chronic Stimuli: mechanical, thermal and chemical (bradykinin, 5HT, Histamine, K+, ACh, Substance P)

Pain receptors Free nerve endings Location: skin, periosteum, arterial walls, joint surfaces, falx and tentorium of the cranial vault Pain impulses are transmitted to the CNS by: Aδ fibers: small, myelinated, 2-5µm dia, 12-30m/s Terminate on neurones in lamina I and V Secrete glutamate C-fibers: unmyelinated, 0.4-1.2µm dia., 0.5-2m/s Terminate on neurones in laminas I and II Secrete substance P Both fibers end in the dorsal horn

Transmission of both “fast-sharp” and “slow-chronic” pain signals into and through the spinal cord on their way to the brain.

Gating of pain The synaptic junctions between the peripheral nociceptor fibers and the dorsal horn cells in the spinal cord are the sites of considerable plasticity where pain impulses can be modified. The dorsal horn is thus referred to as a gate

The pathway The anterolateral fibers originate mainly in laminae I, IV, V and VI The fibers then cross in the anterior commissure of the cord to the opposite anterior and lateral white columns, where they turn upward toward the brain. As they ascend, the fibers form the following divisions Anterior division called anterior spinothalamic tract for fast pain Lateral division called lateral spinothalamic tract for fast pain Spinoreticular pathway to the reticular substance of the brainstem for slow pain Spinotectal tract to the tectum of the mesencephalon

Anterior and lateral divisions of the anterolateral sensory pathway.

Termination The fast fibers terminate in: Reticular nuclei of the brainstem (most fibers): where they are relayed to the intralaminar nuclei of the thalamus the VBC, majorly in the ventral postero lateral and medial thalamic nuclei where they project to the somatosensory cortex for crude tactile signals and intralaminar nuclei of the thalamus

The slow pain fibers terminate in: The reticular nuclei of medulla, pons and mesencephalon and intralaminar nuclei of thalamus The tectal area of the mesencephalon deep to the superior and inferior colliculi The periaqueductal gray region surrounding the aqueduct of Sylvius

Xtics of transmission in the anterolateral pw: Velocity of transmission is reduced (8-40m/s) compared to DCMLPW The degree of spatial localization is poor The gradations of intensities are far less accurate The ability to transmit rapidly repetitive signals is poor

Role of the thalamus in somatic sensation Helps to relay tactile information to the cortex Play a major role along with brainstem in discrimination of pain and temperature Receives regulatory corticofugal fibers (majorly inhibitory) from the cortex Role of the cortex Interpretation of the quality of pain

Applied physiology Thalamic syndrome: Cause: blockage of posterior cerebral that supplies posteroventral portion of the thalamus by thrombosis. The nucleus does degenerate Effects Loss of pain sensation from the opposite side of the body Ataxia: inability to control body mvmts precisely The threshold for sensory perception becomes higher after some weeks Perception of many affective sensations of extreme unpleasantness or pleasantness Enhanced sensitivity to pain

Hyperalgesia: hypersensitivity to pain Causes: Primary hyperalgesia: excessive sensitivity of the nociceptors eg release of histamine ff heat burn Secondary hyperalgesia: facilitation of sensory transmission due to lesion in the spinal cord or thalamus

Brown-Sequard Syndrome Occurs when one half of the spinal cord is transected on a single side All motor functions are blocked on the side of the transection in all segments below the level of the transection The sensations of pain, heat and cold are lost on the opposite side (contralateral side) of the body in 2-6 segments below the level of transection Kinesthetic, position, vibration sensations, light touch, discrete localization, two-point discrimination are lost on the side (ipsilateral side) of the transection in all segments below

Analgesia: pain suppression system in the brain and spinal cord This refers to the capability of the brain to suppress the input of pain signals to the NS by activating a pain control system called an analgesia system

Cellular pathways of pain transmission and modulation Cellular pathways of pain transmission and modulation. Painful stimulation releases substance P from afferent fibers in the spinal cord (a). Substance P release is blocked (b) by a descending analgesic system using axo-axonic synapses on the afferent neuron.

The system consists of The periaqueductal gray and periventricular areas of the mesencephalon and upper pons surrounding the aqueduct of Sylvius and adjacent to portions of the 3rd & 4th ventricles. These neurones project to The raphe magnus nucleus located in the lower pons & upper medulla and the nucleus reticularis paragigantocellularis in the medulla. These also project to A pain inhibitory complex located in the dorsal horns of the SC Periventricular nuclei in the hypothalamus Medial forebrain bundle Most of the fibers involved in the transmission secrete enkephalins and serotonin

Analgesia system of the brain and spinal cord, showing (1) inhibition of incoming pain signals at the cord level and (2) presence of enkephalin-secreting neurons that suppress pain signals in both the cord and the brain stem.

Brains opiate system Morphine-like agents, mainly opiates cause analgesia when injected into any part of the analgesia system. This effect is mediated through activation of opiate receptors present in them. The opiate substances include β-endorphin, met-enkephalin, leu-enkephalin, dynorphin

Thermal sensations Gradations of cold & heat Freezing cold: -50°C-13°C, sensed by nociceptors Cold: Cool: sensed by cold receptors Indifferent: Warm: sensed by warm receptors Hot: Burning hot: 45°C >, sensed by nociceptors

The cold & warm receptors Have a diameter of 1mm There are 3-10 times as many cold receptors as warm receptors Total no of receptors: lips (15-25 cold spots) > finger (3-5coldspots/cm2) > trunk(<1 cold spot) Warm receptors use C-fibers while cold receptors use Aδ fibers Thermal signals are transmitted in the ALPW The signals travel in the Tract of Lissauer and then terminate in laminae I, II & III

The fibers also cross to the opposite side then ascend in the ALPW The fibers terminate The reticular areas of the brainstem The VBC of the thalamus Some to the SSA (Area 1)