Mechanosensation Domina Petric, MD

Functions of somatosensory system shape and texture of objects that interact with our skin prevention of possible injury monitoring of the forces that are acting upon the body proprioception (self awareness)

Basic organisation of somatosensory system labeled lines (labeled line system) mechanosensation; pain and temperature two pairs of pathways (pair of pathways for the back of the head and body; pair of pathways for the face)

Peripheral mechanosensory elements Dorsal root ganglion cells are pseudomonopolar cells. Axon bifurcates and grows in two directions: one direction grows out to the body and other grows out to the spinal cord. Peripheral process of pseudomonopolar cell is connected with specialised receptor ending. The mechanosensory central process of the pseudomonopolar axon enters the dorsal root and makes a sharp upward bend and enters the dorsal column of white matter. The pain and temperature central process of pseudomonopolar axon enters the dorsal horn and makes a synaptic connection.

Pseudomonopolar (pseudounipolar) neuron Central process Peripheral process Peripheral process Central process https://images.fineartamerica.com

Peripheral mechanosensory elements Sensory function Receptor type Afferent axon type Axon diameter (μm) Conduction velocity (m/s) Proprioception Muscle spindle Ia, II (myelinated) 13-20 80-120 Touch Merkel, Meissner, Pacinian, Ruffini cells Aβ (myelinated) 6-12 35-75 Pain, temperature Free nerve endings Aδ (myelinated) 1-5 5-30 Pain, temperature, itch Free nerve endings (unmyelinated) C (unmyelinated) 0,2-1,5 0,5-2

Mechanosensation Meissner corpuscle and Merkel-cell neurite complex are responsible for light touch sensation. Ruffini corpuscle is sensitive to stretch. Pacinian corpuscles (encapsulated with many Schwann cell lamellae) are sensitive to vibration

Afferent systems and their properties Small receptor field Large receptor field Merkel Meissner Pacinian Ruffini Location Tip of epidermal sweat ridges Dermal papillae (close to skin surface) Dermis and deeper tissues Dermis Axon diameter 7-11 μm 6-12 μm Conduction velocity 40-65 m/s 35-70 m/s Sensory function Form and texture perception Motion detection, grip control Perception of distant events through transmitted vibrations, tool use Tangential force, hand shape, motion direction Effective stimuli Edges, points, corners, curvature Skin motion Vibration Skin stretch Receptive field area 9 mm2 22 m2 Entire finger or hand 60 mm2 Innervation densitiy 100/cm 2 150/cm 2 20/cm2 10/cm2 Spatial acuity 0,5 mm 3 mm 10+ mm 7+ mm

Muscle spindle, proprioception It is formed by specialized muscle fibres and certain types of receptor endings that innervate those specialized muscle fibres. Muscle fibres within muscle spindle are called intrafusal muscle fibres. Intrafusal muscle fibres have collection of nuclei that are bundled near the centre of spindle shape. Those muscle fibres have their contractile elements extending away from the central region out to pole of the muscle spindle. Group Ia afferent axons innervate the central region of muscle spindle and group II afferent axons end in a flower spray ending on the contractile elements.

Muscle spindle Wikipedia.org

Golgi tendon organ Group Ib afferent axons innervate the junction of the muscle fiber and the tendon. Golgi tendon organ is sensitive on the muscle force, while the muscle spindle is sensitive to muscle stretching.

Golgi tendon organ Wikipedia.org

Ventral posterior complex of the thalamus It is divided into two main nuclei: ventral posterior lateral nucleus (somatosensory pair of pathways from posterior part of the head and body) ventral posterior medial nucleus (somatosensory pair of pathways from face)

Primary somatosensory cortex in postcentral gyrus https://faculty.washington.edu /chudler/gif/twoptctx.gif

Primary somatosensory cortex (S1) somatotopy Areas 1, 2, 3a and 3b (Broadmann´s areas). Representation of contralateral somatosensation of foot is in the area of paracentral lobule. Expansive representation for contralateral hand is in the S-shape area of the central sulcus. The face is conveyed by inputs from the ventral posterior medial nucleus of the thalamus which terminates in the inferior segment of the postcentral gyrus, below the S-shape bend of the central sulcus.

Thalamic Ventral Posterior Medial Face Nucleus Thalamic Ventral Lateral Nucleus Paracentral lobule S-shape bend of the central sulcus Thalamic Ventral Posterior Medial Nucleus Face https://www.studyblue.com/notes/note/n/lec-10/deck/7007528 (Neuroscience 5th edition. Sinauer Associates, Inc. 2012.)

Cortical magnification How much more does cortical circuits magnify the representation of the body that is related to the density of peripheral receptors and the density of neurons at all the antecendent stations in the somatic sensory pathway.

Primary somatosensory cortex (S1) somatotopy Area 3a is concerned with proprioceptors (muscle spindle, golgi tendon organ, joint receptors) and proprioception. Area 3b is concerned with cutaneous receptors. Area 1 is going to respond to complex stimuli activating multiple skin surfaces. Area 2 is concerned with the shapes of objects that we encounter.

Secondary somatosensory cortex Secondary somatosensory cortex is in the inferior and posterior aspect of the parietal lobe and recieves inputs from primary somatosensory cortex. Informations are then passed to amygdala and hippocampus (creation of memories). All areas project to secondary somatosensory cortex, but area 2 also projects to parietal areas 5 and 7 (orientation relative to the environment).

Secondary somatosensory cortex (S2) Amygdala, hippocampus Secondary somatosensory cortex (S2) Parietal areas 5 and 7: orientation Area 3a: proprioception Area 3b: cutaneous receptors Area 1: complex stimuli Area 2: shapes of objects

Mechanosensory pathways II.

Two pairs of somatosensory pathways Post-cranial body: posterior part of the head and body below the head Dorsal column medial lemniscal system: mechanical stimuli Anterolateral system: pain and temperature Face and anterior portion of the head Pathway through the principal sensory trigeminal complex nucleus: mechanical stimuli Pathways through the spinal trigeminal complex nucleus: pain and temperature

Dorsal column medial lemniscus system, mechanosensation from the post-cranial body First order neuron is dorsal root ganglion neuron in the spinal cord. Central process enters the dorsal column in the white matter and runs the longitudinal length of the spinal cord. Dorsal column nuclei in the dorsal part of tegmentum of medulla oblongata: synapses for the second order neuron. Axons of the second order neuron cross the midline. Third order neuron is in the ventral posterior lateral nucleus of the thalamus.

First order neuron: dorsal root ganglion neuron Third order neurons: ventral posterior lateral nucleus of the thalamus After crossing the midline, fibres continue as medial lemniscus. Second order neuron: dorsal column nuclei in the tegmentum of medulla Crossing the midline Internal arcuate fibers are crossing the midline. Dorsal column of the spinal cord Fibers from cuneate tract (upper extremity) project to the middle of the postcentral gyrus. Postcentral gyrus Second order neurons are in cuneate nucleus and gracile nucleus. First order neuron: dorsal root ganglion neuron Fibers from gracile tract (lower extremity) project to the paracentral lobule. www.bioon.com Dorsal column medial lemniscus system: mechanosensation from the post-cranial body.

Trigeminal lemniscus pathway or V-lemniscus, mechanosensation from the face First order neuron is the ganglion cell in the trigeminal ganglion. Axons enter the brainstem through the trigeminal nerve. Second order neuron is in the principal sensory trigeminal complex nucleus. Second order axon crosses the midline and ascends the remaining divisions of the brain stem, the upper pons and the midbrain and enters the ventral posterior complex of the thalamic nuclei. This pathway is trigeminal lemniscus and it is near the medial edge of the medial lemniscus. Third order neuron is in the ventral posterior medial nucleus. Third order neuron projects to the inferior one third of the postcentral gyrus.

Inferior one third of postcentral gyrus Third order neuron: ventral posterior medial thalamic nucleus. Second order neuron: principal sensory trigeminal complex nucleus Axons continue ascending as trigeminal lemniscus. Axons cross the midline. First order neuron: trigeminal ganglion Inferior one third of postcentral gyrus Trigeminal lemniscus pathway: mechanosensation from face and anterior portion of the head.

Cerebellum It helps to facilitate agile movements of the body and agile movements of thoughts. The cerebellum gets inputs from the cortex about what we are trying to do. The cerebellum gets feedback from our sensory systems about what we are actually doing. If there is a problem, the cerebellum will generate an error signal that is sent back into our motor system for an adjustment (correction can be made).

Spinocerebellar pathways: dorsal spinocerebellar tract Receptor is for example proprioceptor in skin of the foot. The first order neuron is in the dorsal root ganglion of the spinal cord. First order axon enters the dorsal column and ascends to the thoracic spinal cord (from lower extremity). Clarke´s nucleus is in the intermediate gray matter of the thoracic spinal cord: synapse occurs with first order axon. Clarke´s nucleus extends throughout the thoracic spinal cord and then to the upper lumbar levels of the spinal cord.

Spinocerebellar pathways: dorsal spinocerebellar tract Clarke´s nucleus gives rise to a second order axon that ascends in the dorsal lateral white matter of the spinal cord. This is dorsal spinocerebellar tract. The dorsal spinocerebellar tract ascends to the cerebellum via the inferior cerebellar peduncle. This is the pathway for lower extremity.

Spinocerebellar pathway for upper extremity From the upper extremity, first order axons ascend in the dorsal column. From the dorsal column fibres interact with external cuneate nucleus in medulla oblongata From external cuneate nucleus second order axons project to the cerebellum.

Conscience awareness Proprioceptive informations from spinocerebellar tracts enter the medial lemniscus via colateralls and enter the conscience awareness.

Literature https://www.coursera.org/learn/medical-neuroscience/lecture: Leonard E. White, PhD, Duke University https://faculty.washington.edu https://www.studyblue.com Neuroscience 5th edition. Sinauer Associates, Inc. 2012. https://images.fineartamerica.com Wikipedia.org www.bioon.com