Reading: Haines, Ch. 16
Pressure: Refers to a type of stimulus/sensation felt. This could be like compression through stress or strain.
Flutter: Refers to a type of stimulus/sensation felt. This is usually a rapid vibration or pulsation feeling.
Vibration: Refers to a type of stimulus/sensation felt. This is a rapid movement to and fro or an oscillation.
Pain: Sensory and emotional experience assoc with actual tissue damage (which is the stimulus) or potential damage or described in terms of such damage. You can have pain without actual damage – phantom pain or chronic pain – may be in the CNS but not in the periphery.
Thermal sensations: Refers to temperature sensations of hot or cold.
Proprioceptor: Refers to position sense of the body in relation to objects or of the parts of the body to itself. It is the conscious awareness of body position in space.
Primary afferent fiber: This is the neuron from the skin or periphery that is first set off from some stimulus that then travels to the spinal cord and further on up one of the tracts.
Free nerve endings: These are the endings of the dendrites that are closest to the periphery
Meissner's corpuscle: Found in glabrous skin or hairless skin – ie. Palms of hands, soles of feet, finger pads, lips. These are mechanoreceptors that are rapidly adapting (RA) fibers that send a tap and flutter sensation and are of the A b fiber type. They are attached to myelin fibers. They respond to a transient, phasic or vibratory stimuli. The receptors respond to each initial application or removal of a stimulus but fail to respond during maintained stimulation.
Merkel's receptor: Also comes from glabrous skin. These are slowly adapting (SA) fibers that undergo depolarization in response to a stimulus. They influence the associated primary afferent axon by vesicular release of a transmitter substance. Depolarization of these cells changes the rate at which the transmitter is released, leading to depolarization of the axon. Merkel cell stimulation gives a light pressure sensation and responds to indentations of the skin – displacement and velocity of a stimulus. Because they are SA fibers they will respond and be active as long as the stimulus is present. are of the A b fiber type.
Pacinian corpuscle: Comes from hairy skin. Is RA and responds to transient, phasic or vibratory stimuli. Is also of the A b fiber type. These are found within the dermis of the skin and in the fascia surrounding muscles and bone. They are called deep tactile mechanoreceptors. Pacinian cells also respond to stretch, joint movement and vibration and distention.
Ruffini's corpuscle: Comes from hairy skin. Is SA and responds to skin stretch, joint movement and displacement velocity. Are of the A b fiber type. Are also deep tactile mechanoreceptors and respond as the Pacinian corpuscles do.
Hair end organ: These receptors are both SA and RA. They can signal motion, its direction or orientation, and its velocity.
Rapidly adapting (RA) receptor: The receptors respond to each initial application or removal of a stimulus but fail to respond during maintained stimulation. This tells info about a change in stimuli and at the end of the stimuli is may also show some inhibition and has no spontaneous activity. Once the initial stimulus is responded to the RA fibers can’t tell any further info about it.
Slowly adapting (SA) receptor: These receptors respond to a stimulus and continue to maintain and adapt to the stimulus. The firing rate goes up and it may adapt a little but not much and then when the stimulus is stopped the firing stops. The SA fibers tell you the amt of pressure and keeps on firing until the pressure is gone.
Dorsal columns: As discussed last exam, these are the columns in the spinal cord that have laminae I – VI. They carry info ascending to the brainstem and on up to the cortex carrying discriminative touch.
Dorsal column nuclei (gracile and cuneate): Gracile carries info from the sacral, lumbar and lower thoracic. The cuneate carries info from the cervical and upper thoracic areas. The nuclei are located in the caudal medulla. Info comes up the dorsal column on the ipsilateral side and synapses here in the nuclei and then crosses over to the medial lemniscus – carrying discriminative touch.
Ventral posterior lateral nucleus: The area of the thalamus that the medial lemniscus and anterolateral system synapse in carrying somatosensory info from the body.
Ventral posterior medial nucleus: The area of the thalamus that is carrying somatosensory info from the face.
Primary sensory cortex: This is on the postcentral gyrus located in the parietal lobe.
homunculus: Translates into "little man." The mapped out picture drawn on the cortex to show the area representation of sensation for each section of the body. It is set up in a foot to tongue representation along the medial to lateral axis. Body regions, such as the hand and the lips, with a high density of receptors have a disproportionately large amount of cortical tissue dedicated to their central representation. In contrast, regions, such as the back, with low receptor density have small cortical representations. See p 228 in Haines for a picture.
Brodmann's area 1: the histological distinct section located in the postcentral gyrus that feels skin and has mostly RA fibers. Input is also projected here from 3B to give a higher level of processing of perception. Texture and edge detection occurs here.
Brodmann's area 2: located next to 1 in the postcentral gyrus. This area gets its info from deep tissue, joint and pressure position, and some skin touch. This area gives rise to the idea of what type of form you are touching.
Brodmann's area 3a: Receives input from deep tissue and muscle stretch areas and also has input and crossover to area 4.
Brodmann's area 3b: Receives info from cutaneous SA and RA fibers. This area projects to 1 and 2 to get a higher level perceptual processing.
Brodmann's area 5: located on the upswing of the postcentral sulcus – this region gets its info from somatosensory of area 2 but then projects the info back to the motor to be able to manipulate things and not drop them.
Brodmann's area 7: Receives input from visual system for specific eye-hand coordination and reaching movements.
dorsal column-lemniscal system: As discussed above and in exam 1 section.
anterolateral system: as above.
describe the functional roles of slowly and rapidly adapting receptors: Slowly adapting fibers when stimulated will continue to fire during the entire time the stim is applied and maintained. The firing rate goes up at the beginning of the stim and may adapt slightly but then continues to fire until the stim is removed. SA receptors tell the CNS the amt of pressure is present – it doesn’t disappear from the consciousness. Rapidly adapting fibers tell you info about a change in stim. They fire initially and then adapt quickly. There is no continued sensation being felt other than the original stimulus. Once the stim is removed the RA fibers may even show some inhibition but there is no spontaneous activity.
state the submodalities of proprioception, describe the receptors involved in mediating these sensations: Proprioceptors are also called deep receptors that are also located in the dorsal column/lemniscal system. There are 3 types:
All of these reflexes travel to the cerebellum.
compare and contrast the two major ascending somatic systems: The dorsal column/medial lemniscal system carries discriminative touch, flutter vibration and proprioceptive info. The fibers here are the cutaneous receptors on glabrous and hairy skin and all the proprioceptors mentioned above. The ALS carries pain, temp, and crude touch. These fibers don’t have specialized nerve endings in the periphery. The endings are all free nerve endings and are all served by the A d (delta) fibers or unmyelinated ones called C fibers.
describe the columnar organization of primary sensory cortex: The gray matter in the cortex is arranged in a columnar fashion as is the visual and motor systems. The columns receive specific thalamic input and output. Within the columns are neurons organized to process certain kinds of info. There are chunks devoted to certain areas of the body. See the handout – the digits project to specific areas of the columns and each one is subdivided to RA and SA fibers.
describe the different functional areas of somatic sensory cortex: See the descriptions of Brodmann’s areas for the functional differences.
outline the projections to and target areas from somatic sensory cortex: Projections to the sensory cortex come from the gracile and cuneate fasciculi that comprise the dorsal columns and synapse in the corresponding nuclei, decussate and go up through the medial lemniscus to synapse in the VPL of the thalamus. Axons then continue through to the cortex and end in their respective corresponding areas of the different regions of the sensory cortex along the postcentral gyrus. There are also projections from the sensory cortex to the motor cortex and to the visual cortex to coordinate all the info to know where the object is or where your body is in relation to other things and itself.
be able to identify the approximate location of the representation of the body surface on primary somatic sensory cortex: see class handout. Know that area 4 is on the precentral gyrus and that 3a is in the central sulcus, 3b moves up the side of it into the postcentral gyrus. Area 1 is on the top of the postcentral gyrus, 2 on the down slope into the postcentral sulcus and 5 is on the upslope of the postcentral sulcus.
explain the significance of receptive field size and surround inhibition for the tactile senses:
Receptive field is the size of area that a particular neuron serves – in other words the patch of skin that each neuron is responsible for. If the fibers are dense in a particular area then the receptive size will be small since the neurons compete for space. The 2-point discrimination will be low as in the fingertips or the tip of the tongue. If the field is less dense then the area will be large and the sensitivity to 2-pt discrimination will be less such as on the back. Surround inhibition or lateral inhibition relates to the convergence of the sensory neurons synapsing in the secondary neurons of the dc/ml system in the medulla. There is a smearing view as all the neurons converge together. There are then inhibitory neurons from surrounding regions that clear up the edges of stimulation coming from the periphery to make the image more like reality. Look at the handout description. With no lateral inhibition you see 2 peaks and no real distinct valley on the 2-pt discrimination. When the lat inhib is added you see a deepening of the valley which allows for better discrimination. This process of surround inhibition occurs in the dorsal column and again in the thalamus. They still manage to label line code up through the system but they enhance it and centrally spruce up the images.
describe at least two higher order functions that are performed by cortical areas 1 and 2: Area 2 is responsible for the info coming from the VPL. This area along with 3a receives proprioceptive inputs arising from muscle spindle afferents (mainly 3a), Golgi tendon organs, and joint afferents (mainly area 2). These two area are capable of processing kinesthetic info related to muscle length and tension, as well as static and transient joint position. Areas 3b and 1 are mainly targeted by the core region of the VPL. They receive cutaneous afferents from receptors such as Meissner’s corpuscles (RA) and Merkel cells (SA). Damage to area 3b has a more profound effect than damage to either area 1 or 2 alone, producing deficits in both texture and size/shape discrimination. This difference suggests that there is hierarchical processing of tactile info in SI cortex, with area 3b performing the initial processing and distributing the info to areas 1 and 2.
describe the roles of Brodmann's areas 5 and 7: Area 5 is located on the upswing of the postcentral sulcus – this region gets its info from somatosensory of area 2 but then projects the info back to the motor to be able to manipulate things and not drop them. Area 7 receives input from visual system for specific eye-hand coordination and reaching movements. Together they give you the ability to integrate at a higher level what the body is doing to things outside the body.
Last Updated 04/10/00 12:27:16 PM
Return To The MNA2001 Homepage