Sensory/somatosensory system Flashcards
List the five types of stimuli and the receptor types/modalities that are associated with them.
Mechanoreceptors (e.g., hair cells in the vestibular system and organ of corti), sound (i.e., hair cells in the organ of Corti)
Photoreceptors respond to light
Thermoreceptors respond to temperature
chemoreceptors respond to various chemicals in the environment (In the latter case, this can be taste, smell)
Nociceptors respond to noxious (painful) stimuli, some of which can be high threshold mechanical or chemical stimuli.
What are the common receptors of the somatosensory system and what do they detect?
Free nerve endings represent the majority of sensory receptors. These are usually pain and temperature detecting fibres which are often activated through the TRP family of receptors (transient receptor potential). These mostly respond to high-intensity mechanical stimuli and common mediators commonly associated with pain (e.g., inflammatory mediators, capsaicin, thermal stimuli, tissue metabolites such as potassium and acid, etc.). There are encapsulated nerve endings (such as Meisner’s corpuscles in the dermal papillae and Pacinian corpuscles in the deeper tissues. These are generally rapidly adapting receptor for touch or, in the case of Pacinian corpuscles, vibration. There are small free nerve endings that synapse with Merkle cells in the epithelium. These respond to touch stimuli.
What is the course and function of the dorsal column-medial lemniscus (DCML) system.
The dorsal column-medial lemniscal pathway (DCML) carries the sensory modalities of fine touch (tactile sensation), vibration and proprioception.
Its name arises from the two major structures that comprise the DCML. In the spinal cord, information travels via the dorsal (posterior) columns. In the brainstem, it is transmitted through the medial lemniscus.
What is the course and function of the anterolateral system (ALS)?
The anterolateral system consists of two separate tracts:
Anterior spinothalamic tract – carries the sensory modalities of crude touch and pressure.
Lateral spinothalamic tract – carries the sensory modalities of pain and temperature.
Much like the DCML pathway, both tracts of the anterolateral system have three groups of neurones.
What is the course and function of the trigeminal mechanosensory system.
The trigeminal lemniscus is a part of the brain that conveys tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities, and the eye, as well as proprioceptive information from the facial and masticatory muscles.
trigeminal ganglion -> the pons of the brain stem through the middle cerebellar peduncle -> chief (principal) sensory nucleus of the trigeminal nerve in the lateral pons -> the ventral posteromedial nucleus (thalamus) -> the head/face area of the postcentral gyrus
Define the concept of somatotopy and homunculus.
Somatotopy refers to the presence of a topography of the body in neural tissues. This is true with the primary somatosensory cortex in the postcentral gyrus, where the head is represented immediately dorsal to the lateral fissure, the hand represented about halfway between the lateral fissure and the vertex and the foot area wrapping over the dorsal aspect of the brain onto its medial aspect. This has been termed a homunculus (little man).
How is pain modulated by descending pain pathways and mechanosensory fibers in the spinal cord?
Mechanoreceptive sensory axons can activate small, inhibitory interneurons in the dorsal horn of the spinal cord and inhibiting activation of pain transmitting neurons. Descending spinal cord pathways, mostly using norepinephrine and serotonin, can activate local enkephalineergic interneurons in the dorsal horn, also powerfully inhibiting pain transmission.
What is the somatosenory association cortex, where is it located and what does it do?
The somatosensory association cortex is located immediately posterior to the postcentral gyrus. This receives input from the primary somatosensory cortex and processes it to determine what is being felt. Damage to this area can produce an inability recognize objects by touch even if they can still be felt normally (assuming that the sensory pathways and the the postcentral gyrus are intact)
Discuss the neurologic tests used to evaluate sensation in patients.
In order to detect abnormalities of conscious somatic sensation one needs a cooperative patient. You should at least use pain and/or temperature and vibration sense to check the ALS and the DCML, respectively (the tests for the ALS also check the health of lightly myelinated and unmyelinated axons and those for the DCML check for large, heavily myelinated axons. You can also check well localized touch, the ability to identify objects by touch and joint position sensitivity. Unconscious proprioception can really only be tested by tests of coordination (these sensations are used by the cerebellum to coordinate).
Merkel cell-neurite complex:
Nerve ending surrounded by a capsule of Merkel cells. High innervation density (finger), 25% of mechanoreceptors in the hand.Small receptive field, SA.Discriminative touch with fine (highest, 0.5 mm) spatial resolution (shape and texture).
Meissner corpuscle:
Several nerve endings in a capsule (connective tissue + lamellar cells).Dense innervation of the skin, 40% of mechanoreceptors in the hand.Small receptive field (but larger than Merkel), RADiscriminative touch with less spatial resolution than Merkel. Skin deformation, motion, grip control.
Ruffini corpuscles:
Nerve ending and capsule (collagen)Low innervation density (finger), 20% of mechanoreceptors in the hand.Large receptive field, SARespond to stretches of the skin and movement/finger position.
Pacinian corpuscles:
Layered onion-like appearance (layers of membrane), single afferent.Low innervation density (finger), 10-15% of mechanoreceptors in the hand.Very large receptive field, RADetect high-frequency vibratory and deep pressure stimuli (highest sensitivity for vibrations/motion), use of tools.
Proprioception
Perception/awareness of motion and sense of static position
Proprioceptors:
Muscle spindle
Golgi tendon organ
Joint receptor
Muscle spindle:
In skeletal muscles-Inform about changes in muscle length
4 –8 intrafusal fibers + capsule of connective tissue
Primary (Ia) and secondary (II) nerve endings
Golgi tendon organ
In tendon
Informs about changes in muscle tension
Primary (Ib) nerve endings
Central pathways for touch
Dorsal column –medial lemniscal pathway
Gracile tract/nucleus: lower body
Cuneate tract/nucleus: upper body
VPL in thalamus
Primary somatosensory cortex (SI)
Proprioceptive afferents for the lower part of the body synapse on neurons in the dorsal and ventral horn of the spinal cord and on neurons in Clarke’s nucleus. Neurons in Clarke’s nucleus send their axons via the dorsal spinocerebellar tract to the cerebellum, with a collateral to the dorsal column nuclei. Proprioceptive afferents for the upper body also have synapses in the dorsal and ventral horns, but then ascend via the dorsal column to the dorsal column nuclei; the external cuneate nucleus, in turn, relays signals to the cerebellum. Proprioceptive target neurons in the dorsal column nuclei send their ax- ons across the midline and ascend through the medial lemniscus to the ventral posterior nucleus
