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.
Polymodal Nociceptors
These are the majority of nociceptors, they respond to mechanical, thermal and chemical stimuli.
Nociceptor Distribution
Nociceptors are found in most body tissues, including skin, bone, muscle, most internal organs, blood vessels and the heart. They are absent in the brain, but exist in the meninges.
Hyperalgesia
Increased pain from tissue that has already been damaged. It can be from increased sensitivity for pain, reduced threshold for pain or even spontaneous pain. Primary hyperalgesia occurs within the area of the damaged tissue, surrounding tissue may become supersensitive as well in secondary hyperalgesia. A variety of substances from damaged cells can cause this, including bradykinin, prostaglandins and substance P. Bradykinins depolarize nociceptors and also stimulate long lasting intracellular changes that make heat activated ion channels more sensitive.
Prostaglandins
chemicals generated by lipid membrane breakdown, they do not elicit pain but increase gently the sensitivity of nociceptors to other stimuli. Aspirin and other non-steroidal anti-inflammatory drugs can help treat hyperglasia because they ihibit the enzymes required for the synthesis of prostaglandins.
Substance P
A peptide synthesized by the nociceptors themselves. Activation of one axon branch can lead to the others secreting this, it causes vasodilation and the release of histamine from mast cells. Nociceptor sensitization around the site of the injury by substance P is one cause of secondary hyperalgesia.
First and Second Pain
First pain is sharp and fast, cause by activation of Aδ axons. Second pain is dull and slow, caused by activation of C fibres. These axons have (like Aβ afferent mechanosensory axons) enter the dorsal root and have cell bodies in the dorsal root ganglia. They enter the spinal cord via the dorsal horn and the travel up and down the Zone of Lissauer (up and down cord) before synapsing at the substantia gelatinosa (a region inside the spinal cord).
Pain Neurotransmitter
The neurotransmitter for pain fibres is thought to be glutamate, but substance P may also play a part as it is kept in secretory granules and is required to experience moderate to intense pain
Viscera Nociceptor Axons
AKA autonomic nervous system nociceptor axons, enter the spinal cord by the same route as the cutaneous nociceptors (sensory information from skin). These two types of axons engage in cross talk, mixing of information. This causes referred pain where the visceral nociceptor activation is perceived as cutaneous sensation (eg. Angina, pain around upper chest and arm when heart isn’t receiving enough oxygen).
Spinothalamic Pain Pathway
The pathway that information about pain and temperature takes (as opposed to the dorsal column-medial lemniscal pathway). Axons of the second order neurons immediately decussate and ascend through the spinothalamic tract (running along the ventral surface of the spinal cord). Axons then project up the spinal cord, through the medulla, pons and midbrain without synapsing until they reach the thalamus. The axons of the two pathways lie against each other in the brain stem, but remain distinct. Information about touch ascends ipsilaterally, information about pain ascends contralaterally (opposite side of the midline).
Brown-Séquard Syndrome
Sensory and motor syndromes following damage to one side of the body due to the differing pathways. Eg. Having the sensation of light touch but not pain and vice versa, or sense function but no motor function.
Trigeminal Pain Pathway
Small diameter fibres from the trigeminal nerve carry pain and temperature information from the face and head and takes a path to the thalamus. First the small diameter fibres synapse on second order sensory neurons in the spinal trigeminal nucleus of the brain stem. The axons then cross and ascend to the thalamus in the trigeminal lemniscus. Closely related pathways send pain and temperature axons to all areas of the brain stem before the thalamus. Some of these pathways can lead to slow, burning agonizing pain and others are involved in arousal.
Spinothalamic and Trigeminal lemniscal path with thalamus and cortex
The spinothalamic tract and trigeminal lemniscal axons synapse over wider region of the thalamus than those at the medial lemniscal pathway. Some terminate in the VP nucleus (same as medial lemnsical axons, although touch and pain has segregated regions in VP). Other spinothalamic axons end in the small intralaminar nuceli of the thalamus. Axons from the thalamus from the spinothalamic tract and trigeminal lemniscal path project to a wider area of the cortex than the axons from the dorsal column-medial lemniscal pathway
Afferent Regulation
Large Aβ axons from mechanoreceptors can reduce pain when they are activated. This is explained by the gate theory of pain, this is that certain projection neurons in the dorsal horn (which project axons up the spinothalamic tract) are excited by both large diameter neurons and small unmyelinated ones. Interneurons inhibit the projection neurons are excited by large diameter axons and inhibited by small pain fibres, this means that normally pain fibres activate the projection neuron and send signals to the brain, but if large mechanoreceptor axons are firing concurrently they activate the interneuron and suppress nociceptive signals.
Descending Regulation
Regions of the brain can suppress pain due to different genetic and/or environmental circumstances. A region that does this often is the periaqueductal gray matter (PAG) in the midbrain. Stimulation of this part of the brain can lead to profound analgesia (decreases pain in damaged tissue). It does this by sending axons to the raphe nuclei (in the midline medulla), which the sends axons to the dorsal horn and depresses the activity of nociceptive neurons.
Exogenous Opiates
Opiates can achieve analgesia when taken systematically. They can also produce moood changes, drowsiness, mental clouding, nausea etc. They act by binding tightly to opiod receptors in the brain.
Endogenous Opiates
Also known as endorphins, these and their receptors are distributed widely in the CNS. Systems of endorphin containing neurons in the spinal cord and brain stem prevent the passage of nociceptive signals through the dorsal horn and into higher levels of the brain where the perception of pain is generated. This is done by blocking glutamate release along pain pathways and hyperpolarizing post synaptic membranes.
Temperature Reception
There are spots on the skin where there only receptors that can feel either cold or hot, areas in the middle tend to have no temperature reception. The sensitivity of a sensory neuron to a change in termperature depends on the type of ion channels the neuron expresses.
Trp Channels
Transient receptor potential ion channels that are usually sensitive to changes in temperature. TrpV1 (over 43* painful temps) was found with the molecule capsaicin. There are six distinct Trp channels in thermoreceptors that confer different temperature sensitivities. TrpM8 was found with menthol for minus 25 degrees nonpainful temperatures.
Thermoreceptors
Different thermoreceptors tend to only express one type of Trp channel, explaining how different regions of skin can have different sensitivities to temperature. There are some cold thermoreceptors though that also express TrpV1, so really hot things can feel like cold.
Thermoreceptors Adaptation
The differences between te response rates of warm and cold receptors are greatest during and shorly after temperature changes. After a bit of stimulus the feeling can often be transient, feeling less cold/hot then just before.
The Temperature Pathway
Same as the pain pathway except cold receptors are coupled to Aδ fibres and C, warm receptors are only coupled to C fibres.
Size and structure of Pacinian corpuscle
2mm by 2mm. Layers are separated by fluid layers, which cause the corpuscle to stop firing when fluid redistributes. The Pacinian is very sensitive to change and rapid stimuli
Ruffinini’s endings detect
Sensitive to dragging and other slow stimuli
Meissner’s corpuscles and merkel’s disks are found where
Commonly on glabrous skin, especially in fingertips
Nociception
The stimuli and physiology that encodes pain
Meissner Corpuscle receptive field
Small, can pinpoint stimuli based on which neuron is firing
Pacinian corpuscle receptive field
Huge receptive field with many overlaps
Slow mechanoreceptor adaption
Leads to continuous firing as long as stimulus is present
Pacinian Corpuscle ___ when stimulus is removed
Fires, even if it has already adapted
Pacinian Corpuscle most sensitive at _____
300 Hz
Meissner’s corpuscle most sensitive at ___
40 Hz, can’t even detect up to 300 Hz
The three frequency sensitive mechanoreceptors and the feeling they encode
Pacinian Corpuscle (vibrations)
Meissner’s Corpuscle (roughness)
Ruffini’s Ending (flutter)
Two point discrimination distance in fingers and back
Fingers: 1-2 mm apart
Back: 40 mm apart
Why does the lumbur puncture go in the lumbar region
Because there is more room for needle in lumbar gap and the spinal cord won’t be punctured..
Shingles and Dermatomes
Shingles reveal dermatomes because sensory nerve endings take up the virus and it stays dormant in the dorsal root ganglia. When virus is activated a rash along that dorsal root ganglia’s dermatome shows.
Why are the face and head not part of dermatomes
They are serviced by the 12 trigeminal nerves
How many trigeminal nerves are there?
Branches into 3 on each side of the head for a total of 6 branches.
5th Cranial Nerve
Trigeminal nerve
Bifurcation
When a nerve (or any part of the body) branches into two
2 Dorsal column nuclei
The paired nuclei in the brainstem that form the junction between the spinal cord and medulla. They contain secondary sensory neurons of the dorsal column-medial lemniscus pathway. Send axons to decussate and eventually synapse with third order thalamic neurons. The cuneate nuclei are for the upper body and the gracile nuclei is for the lower part of the body (trunk and legs)
3 Somatosensory Pathways
Dorsal Column-medial lemniscus pathway
Trigeminal Nerve Pathway
Spinothalamic Tract (pain pathway)
Dorsal Column-medial lemniscus pathway and receptive fields
Neurons along path will inherit the recptive fields of the sensory receptor
Spinothalamic Tract (simplified)
Immediate decussation and synapsing in the spinal cord. And then axons run up the spinal cord with no stops until thalamus.
Clinton Woolsey
Neurosurgeon, would tap parts of the brain and simultaneously record where touching stimulated, mapped out human homonculus.
Penfield
Awake neurological patients in the OR, stimulated cortex with electrode and asked what they felt
Example of discontinuities in homonculus map
Face next to hand
Merzenich
Figured out why the somatosensory system is spread over 4 Brodmann areas on the postcentral gyrus. There are four different maps of body surface. Also found out about compensation for missing limbs and whatnot
Where is the sensory representation of the face held?
The trigeminal nerve nuclei
Brodmann area 3b sensitive to
Light touch
Brodmann area 1 sensitive to
Light touch, rapidly adaptive
Brodmann area 2 sensitive to
Joint position and deep touch, directions of motion
Stereognosis
Picking up an object and identifying it by touch alone
When an animal is exploring by touch, which Brodmann area is most active?
Area 2
Digital Syndactyly
Abmormal fusion of two digits. Merzenich explored somatosensory plasticity by getting this surgically imposed on a howler monkey. Found that when both cortical areas are stimulated at same time they fuse, when the digits are cut again the cortical fusion remains. Led Merzenich to believe that rewiring must happen in brain. Neurons that fire together, wire together.
Hemi-neglect
Neglecting one side of the body (usually contralateral to damage)
William Syndrome
Leads to lack of coordination and spatial problems
Melzak and Wall
Neuroscientists. Found the gate control theory of pain.
Raphe Nuclei
Part of the frontal cortex to below down regulation of pain. It uses serotonin. Terminate in the dorsal horn of the spinal cord, where they regulate the release of enkephalins to inhibit pain.
