Chapter 12 Somatosensory System Flashcards
Four MAIN senses
Touch, body position, pain and temperature
Hairless skin
glabrous
Two layers of skin
Epidermis (outer) and dermis (inner)
Mechanoreceptors
Most of the receptors in the somatic sensory system. These are sensitive to physical distortion such as bending or stretching. ‘Feel’ everything from force against blood vessels and the bladder, to contact against the skin. Unmyelinated axon branches contain mechanosensitive ion channels which can gate open and closed depending on stretching and other conditions of the surrounding membrane. All touch receptors have an axon and (except for free nerve endings) all have non-neural tissue.
Pacinian Corpuscle
Lies deep in the dermis and senses touch. Found in both hairy and glabrous skin, it is the largest of the mechanoreceptors. Can be seen with naked eye. Has connective tissue around the receptor which makes it look like an onion, when this tissue is squashed, ion channels open on the distored membrane beneath and depolarization occurs. If the receptor is depolarized enough it will make an action potential. The connective tissue’s slick surface causes slipping which makes sure that ion channels will close quickly once the corpuscle slips into a non-distored shape. The capsule makes the corpuscle sensitive to vibratios, but reduces it’s ability to ‘feel’ steady pressure
Ruffini’s Ending
Is found in both hairy and glabrous skin, skightly smaller than Pacinian corpuscles
Meissner’s Corpuscle
About the tenth of the size of Pacinian Corpuscles, are shallowly located in the epidermis of glabrous skin (eg. Fingertips).
Merkel’s Disks
Consist of a nerve terminal and a flattened, non neural, epithelial cell. The epithelial cell is like a skin cell, though it may be the mechanically sensitive part because it is joined with synapses.
Krause end Bulbs
Border of dry skin and mucous membranes, eg. Lips and genitals, look like knotted balls of string.
Rapidly Adapting Mechanoreceptors
Like Pacinian Corpuscles and Meissner’s corpuscles, these mechanoreceptors are quick to respond to stimulus, but stop firing shortly after stimulation (even if it is prolonged).
Slowly Adapting Mechanoreceptors
Like Merkel’s disks and Ruffini’s endings, these generate a more sustained response during a prolonged stimulus.
Follicles
Hairs grow out of follicles, which are richly innervated with free nerve endings that either wrap around it or run parallel to it. Several type of follicles have erectile muscles that can cause goosebumps. Hair follicle mechanoreceptors may quickly or slowly adapt when they are bent or stretched by moved hair.
Mechanoreceptors and Frequency
Touches that have vibration are only felt by certain mechanoreceptors. Pacinian corpuscles can only feel high frequencies, Meissner’s corpuscles can feel mid to low range frequency and low frequencies can stimulat Ruffini’s endings and Meissner’s corpuscles creating a ‘fluttering’ feeling
Two Point Discrimination
The distance it takes between stimuli, for them to feel like one point. This measure varies across the body. Fingertips have the highest resolution (making us able to read braille). This is because there is a higher density of mechanoreceptors in the fingertips, the brain has more tissue devoted to fingertip sensations, the receptors on the fingertips have smaller fields of reception and there may be specialized neural mechanisms to detect high-resolution discriminations
Axon sizes from Skin
Axons from skin carry the terminology (from fastest-largest to slowest-smallest) Aα (13-20 μm) > Aβ (6-12 μm) > Aδ (1-5 μm) > C . C axons are unmyelinated.
Axon sizes from muscle
Axons from muscles carry the following terminology (fastest-largest to slowest-smallest): Group I >II > III > IV. The largest is for proprioception of skeletal muscles, second largest is for mechanoreceptors of the skin, third smallest are for pain and temperature. The C fibers are only for temperature, pain and itch.
Cutaneous Mechanoreceptors
Touch receptors with the Aβ axons
Spinal Segments
30 notches along the spinal cord where central and dorsal roots enter. They are divided into four groups, each segment is named after the vertebra where the nerves originate. The four sections are; cervical (C 1-8), thoracic (T 1-12), lumbar (L 1-5) and sacral (S 1-5 coming off the sacral cord)
Cauda Equina
(eg, the horse’s tail) is a section of the spinal cord (lumbar and sacral) that has spinal nerves streaming down. These cauda equina flow through a sack of CSF called the dura, this fluid can be tapped in a lumbar puncture (AKA spinal tap).
Second Order Sensory Neurons
Neurons that recieve sensory info from primary afferent neurons, the mostly lie in the dorsal horns of the spinal cord’s grey matter. receive branches from the Aβ axon, these can initiate a reflex. Other branches of Aβ axons ascend straight to the brain, enabling perception.
Dorsal Column-Medial Lemniscal Pathway
The pathway serving touch to the brain, large sensory axons Aβ enter the ipsilateral (same side as sensation was felt) dorsal column. The dorsal columns carry information about tactile sensation (and limb position) towards the brain, they are composed of spinal grey matter, composed itself of primary sensory axons and second order axons from neurons in spinal grey matter.
Dorsal Column Nuclei
Where axons of the dorsal column terminate, lies at the junction between the spinal cord and medulla. Axons from dorsal column nuclei arch toward the ventral and medial medulla and decussate (cross into the opposite hemisphere from the side of the body that experienced stimulus).
Medial Lemniscus
Axons of the dorsal column nuclei ascent within this white matter tract, which rises through the medulla, pons and midbrain. Axons synapse at the ventral posterior (VP) nucleus of the thalamus. Then the thalamic neurons of the ventral posterior nucleus project to specific regions of the primary somatosensory cortex (SI).
Relay Nuclei
Refers to nuclei of the thalamus (such as the VP nucleus), and the assumption that they simpley relay information to the cortex for processing. In truth, in both the dorsal column and thalamic nuclei, specific transformation of information takes place. Indeed information is altered every time is passes through a set of synapses in the brain. Inhibitory interactions between adjacent sets of inputs in the dorsal column to edial lemniscal pathway enhance the response to tactile stimuli. The output of the cortex can influence the input of the cortex.
Trigeminal Nerves
Allow your face to feel via cranial nerve V. The V nerve enters the brain at the pons. The trigeminal nerve branches into two (for both sides of the head) before branching into three to innervate the mouth (and outer two thirds of the tongue), face and dura mater of the brain. Sensations from the ears, nasal areas and pharynx are supplied from other cranial nerves: the facial nerve (VII), the glossopharyngeal nerve (IX) and the vagus nerve (X). Axons from the ipsilateral trigeminal nucleus decussate and project into the medial part of the VP nucleus, from there information is relayed (via thalamic neurons) to the somatosensory cortex.
The Somatosensory Cortex
Located in Brodmann’s area 3b in the parietal lobe of the cerebral cortex, also called SI. Deals with somatosensory processing. It lies right on the postcentral gyrus (right caudal to the central sulcus). Surrounding areas on the postcentral gyrus 3a, 1 and 2 on the postcentral gyrus also deal with somatic sensory information, also areas 5 and 7 on the posterior parietal cortex.
Area 3B
Is the primary somatosensory cortex because it receives many inputs from the VP nucleus, has responsive neurons to somatosensory stimuli (but not to other types) and lesions impair somatic sensation/electrical stimulation invokes somatosensory feelings
Area 3A
Also receives many connections from the thalamus but is more concerned with proprioception than touch.
Areas 1 and 2
Receive dense inputs from area 3b. Area 1 mostly deals with texture information and area 2 mostly deals with shape and size.
SI Layers and Columns
Thalamic inputs to the SI terminate mainly in layer IV, neurons here then project to other layers. There are also vertical columns, each having a function (eg. Fingers being represented by 1 column of rapidly adapting neurons and another column of slowly adapting neurons.
Cortical Somatotopy and its Researcher
Stimulating certain parts of the cortex allowed mapping of it, Wilder Penfield (McGill 1930s) did this to neurosurgery patients to map out the SI. Mapping body’s surface sensations onto a structure in the brain is called somatotopy. A somatotopic map is often called a homunculus. Often the size of a certain part of the body on a homunculus is often a direct representation of how often it is used or how useful it is for survival (eg. Mouth is large because it tastes and manipulates potentially dangerous foods and forms speech). Area 3b and 1 make mirror images of different parts of the body (there are more than one somatotopic map).
Cortical Map Plasticity
When a part of the body is removed, major circuitry rearrangement happens where the cortical part that would have once processed information about this part, begins processing information from adjacent areas. Also, using a part of the body more expands that part’s representation on a somatotopic map, this type of map plasticity is common in the brain. V. S. Ramachandran (UC) found that phantom limb sensations usually follow stimulation of a part that is near on a somatotopic map to the missing part (eg. Touching face might cause phantom sensations in a missing thumb). This means that cortical processing that would have once been used to process the missing part is then redirected to process an adjacent part represented on the somatotopic map. These types of plasticity might not have an effect on processes involved in learning and memory (eg. Violinists enlarged left hand representation for fingering).
Posterior Parietal Cortex
The neurons of the posterior parietal cortex are complex, having large receptive fields (with a lot of overlap) and dealing with somatic, visual and movement planning. Informations from segregaded sources converge to form a meaningful representation of something. Seems to be essential in perception and interpretation of spatial relationships, accurate body image and the learning of tasks involving coordination of the body in space.
Agnosia
Can occur with damage to the posterior parietal cortex. It is the inability to recognize objects even when the sensory skills seem to be normal.
Nociceptors
Somatic sensations depend on nociceptors, the free branching unmyelinated nerve endings of Aδ axons and C fibres that signal endangered or damaged body tissue, for pain perception. Nociceptors are not mechanoreceptors. May fire wildly and continually and pain may come and go, the opposite may also happen. There are polymodal nociceptors (majority), mechanical nociceptors, thermal nociceptors and chemical nociceptors.
Nociception
The conscious experience of pain. Pain is the feeling, or the perception, of irritating, sore, stinging, aching etc. Sensations that arise from a part of the body. Nociception is the sensory process that proves the signal to trigger pain. Out of all the sensory systems, cognitive qualities of nociception can be controlled the most by the brain. Can arise from extremes in temperature, oxygen deprivation, exposure to certain chemicals and strong mechanical stimulation. Nociceptors contain ion channels that are activated by these stimuli.
Nociceptor Activation
A nociceptor of a certain type can only be activated by stimuli that is potentially harmful or harmful. Eg. A sharp mechanical pressure will cause stretching of the membrane and cause ion channel openings, but the destruction of cells will also release things that will activate the ion channels of nociceptors (eg. Proteases, ATP and potassium ions). Proteases can breakdown an abundant extracellular protein called kininogen ro form bradykinin. Bradykinin binds to specific receptor molecules that activate ionic conductances in some nociceptors. ATP causes nociceptors to depolarize by binding directly to ATP gated ion channels. Elevated K directly depolarizes neuronal membranes.
Itch
The smallest of C fibres are selectively responsive to histamine and cause the perception of itch.
