Chapter 12: The Somatic Sensory System

Meissner's corpuscles

-1/10 the size of Pacinian corpuscles are are located in the ridges of glabrous skin -small receptive fields -rapidly adapting

the mechanical sensitivities of the receptors allow us to detect different sensations: based on the data in the slides, what can you tel me about what these receptors are responding to?

-Meissner's corpuscle: responds to low stimulus frequency (ex. feel rough texture on speaker) -Pacinian corpuscle: responds to high stimulus frequency (ex. feel vibration of music if you close your eyes)

what structural properties affect the response of the receptor with onion like tissue attached to end of receptor?

-applying pressure initially initiates a receptor potential (small depolarizations EPSP/IPSP) -continue to touch decreases receptor potential in fast adapting receptor -stimulus off causes another axon receptor potential

dorsal column-medial lemniscal pathway

-ascending branch of large sensory axons enters ipsilateral dorsal column of the spinal cord, the white matter tract medial to the dorsal horn -dorsal column carries primary sensory axons and second-order axons from neurons in the spinal gray matter -dorsal column axons terminate in the dorsal column nuclei -dorsal column nuclei decussate at the medulla and ascend within the medial lemniscus white matter tract -medial lemniscus neurons synapse onto ventral posterior (VP) nucleus of the thalamus -ventral posterior (VP) nucleus neurons project to specific regions of the primary somatosensory cortex (S1)

primary afferent axons

-axons bringing information from the somatic sensory receptors to the spinal cord or brain stem -enter spinal cord through the dorsal roots -cell bodies lie in the dorsal root ganglia -size depends on receptor

how can force be applies to channel of a membrane?

-by the membrane itself when it is stretched or bent -connections between the channels and extracellular proteins or intracellular cytoskeleton components -trigger release of 2nd messengers

agnosia

-caused by damage to posterior parietal cortex -inability to recognize objects

astereognosia

-caused by damage to posterior parietal cortex -inability to recognize objects by touch

cauda equina

-collection of spinal nerves below the end of the spinal cord -courses down the spinal column within a sack of dura filled with CSF

Merkel's disks

-each consist of a nerve terminal and a flattened, non-neural epithelial cell (the Merkel cell) -located in the epidermis -small receptive fields -slowly adapting

follicles

-embedded in the skin and grow hair -richly innervated by free nerve endings-the termination of single axons-that either wrap around the follicle or run parallel to it

posterior parietal cortex

-essential for perception and interpretation of spatial relationships, accurate body image, and learning tasks involving coordination of the body in space -similar to the area IT in the visual system, its neurons have large receptive fields with stimulus preferences that are a challenge to characterize because of their complexity -also deals with visual, movement planning, and a person's state of attentiveness

pain

-feeling, or the perception, of irritating, sore, stinging, aching, throbbing, miserable, or unbearable sensations arising from a part of the body -our body puts together from our nociceptors

Ruffini's endings

-found in both hairy and glabrous skin -slightly smaller than Pacinian corpuscles -large receptive fields -slowly adapting

how does hair play a role in sensing?

-help distinguish stimuli (whiskers especially) -bending of hair deforms follicle and surrounding skin tissue which in turn alters the nearby nerve endings which then increase or decrease in APs

Pacinian corpuscles

-largest and lies deep in the dermis -large receptive fields -rapidly adapting

similarities across sensory systems in the neocortex with S1?

-layered structure -similar inputs and responses are stacked vertically into columns that extend across the cortical layers

Krause end bulbs

-lie in the border regions of dry skin and mucous membrane -nerve terminals look like knotted balls of string

several notes on somatotopic maps

-map is not always continuous but can be broken up -not scaled like the human body: the relative size of cortex devoted to each body part is correlated with the density of sensory input received from that part, how often it is used, and its importance -map varies across species to meet different needs -not limited to a single map

mechanoreceptor: how do you generate a potential?

-mechanical force stimuli opens ion channels to allow Na+ to enter -extracellular proteins tethered to ion channels that can be pulled -intracellular proteins can help channels open from the inside of the cell -multiple receptor potentials add up RIGHT NEXT TO AXON (no need to travel to axon hillock) -AP rapidly propagated to the spinal cord -second-order sensory neurons decussate at medulla and synapse to the thalamus then synapse to cortex

mechanoreceptors

-most of the sensory receptors in the somatic sensory system -sensitive to physical distortion such as bending or stretching -unmyelinated axon branches that are sensitive to stretching, bending, pressure, or vibration

ways to map somatosensory cortex (cortical somatotopy)

-moving stimulus across S1 causes sensation in different parts of the body -record activity of a single neuron and determine the site of its somatosensory receptive field on the body (somatotopy)

stimuli somatic sensation is sensitive to?

-pressure of objects against the skin -the position of joints and muscles -distension of the bladder -temperature of the limbs and of the brain itself -origin of itch -pain

areas 1 and 2

-receive dense inputs from area 3b -3b to 1 is texture -3b to 2 is size and shape

trigeminal nerve pathway

-sensory axons of the large trigeminal nerve carry tactile information from skin to second-order neurons in ipsilateral trigeminal nucleus -trigeminal nucleus neurons decussate and project into the medial part of the VP nucleus of the thalamus -thalamus to S1

C fibers

-smallest axons that are unmyelinated -mediate pain, temperature sensation, and itch -slowest axons

contrast enhancement

-the intensification of the perception of edges -uses inhibition of nearby neurons to do so

memorize table in slides and complete filling it out

...

4 main types of somatic sensory receptors

1. Merkel's disks 2. Meissner's corpuscle 3. Pacinian corpuscles 4. Ruffini's ending

mechanoreceptors of the skin

1. Merkel's disks 2. Meissner's corpuscle 3. Pacinian corpuscles 4. Ruffini's ending 5. Krause end bulbs

based on the data in the slides, which of these (Merkel's disks, Meissner's corpuscle, Pacinian corpuscles, Ruffini's ending) are slow adapting ad which are fast adapting?

1. Merkel's disks: slow 2. Meissner's corpuscle: fast 3. Pacinian corpuscles: fast 4. Ruffini's ending: slow

why is area 3b the primary somatic sensory cortex?

1. it receives dense inputs from the VP nucleus of the thalamus 2. its neurons are very responsive to somatosensory stimuli 3. lesions here impair somatic sensations 4. when electrically stimulated, it evokes somatic sensory experiences

2 ways in which the somatic sensory system is different from the other sensory systems

1. its receptors are distributed throughout the body rather than being concentrated at small, specialized locations 2. it responds to many different kinds of stimuli, we can think of it like a group of at least four senses instead of a single one

why are fingertips so much better than the elbow for resolution?

1. there is a much higher density of mechanoreceptors in fingertip skin 2. fingertips are enriched in receptor types that have small receptive fields 3. there is more brain tissue devoted to sensory info from fingertips 4. there may be special neural mechanisms devoted to high-resoluation discriminations

4 main senses of somatic sensory system

1. touch 2. temperature 3. pain 4. body position

The ______ _____ ______ are divided into four groups and each is named after the vertebra adjacent to where the nerves originate cerviacal (C1-C8), thoracic (T1-T12), Lumbar (L1-L5), and Sacral (S1-S5)

30 spinal segments

other cortical areas that also process somatic sensory info flank S1. list them.

3a, 3b, 1, and 2 on the post central gyrus and areas 5 and 7 on the adjacent parietal cortex

two point discrimination test

A simple measure of spatial resolution

thalamic inputs to S1 terminate in what layer?

IV

receptive fields of Meissner's corpuscles vs Pacinian corpuscles?

Meissner's corpuscles have much smaller receptive fields and therefore are much more sensitive and thus takes up more room in the primary somatosensory cortex

PAIN

PAIN

TOUCH

TOUCH

neglect syndrome

The result of certain right parietal lobe lesions that leave a patient completely inattentive to stimuli to her left, including the left side of her own body.

two point discrimination

ability to discriminate the detailed features of a stimulus

shingles

all the neurons of a single dorsal root ganglion become infected with varicella zoster virus which reactivates after sometime of having chickenpox as a kid which is when the virus is introduced initially

why do we have so many somatic sensory receptors?

allow us to differentiate between different stimuli that we touch

what structural properties affect the response of the receptor without onion like tissue attached to end of receptor?

cell now responds like a slow adapting receptor

as with all other sensory systems, the most complex level of somatosensory processing occurs in the ___ ___

cerebral cortex

mechanosensitive ion channels

converts mechanical force into a change of ionic current in membranes of mechanoreceptor axons by causing alterations in their gating

how to lose sensation in a dermatome?

cut all 3 adjacent dorsal roots

name of pathway serving touch

dorsal column-medial lemniscal pathway

somatic sensory receptors: how do we measure their receptive fields?

electrophysiology: place stimulus on hand and measure action potentials by sticking in an electrode

somatic sensation

enables our body to feel, to ache, to sense hot or chill, and to know what its parts are doing

skin two layers

epidermis and dermis

what is the somatic sensory system really an umbrella term for?

every sense that is not seeing, hearing, tasting, smelling, and vestibular

what nerves supplies the skin around the ears, nasal areas, and pharynx?

facial (VII), glossopharyngeal (IX), and vagus (X)

what body part has the highest resolution?

fingertips

nociceptors

free, branching, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged

ganglion

groups of cell bodies

two major types of skin

hairy and glabrous (hairless)

how does the Pacinian corpuscle communicate info about rapid vibrations to the central nervous system in a timely way?

has largest and fasted axons that originate in the skin

somatotopic map is sometimes called a ___

homunculus

gray and white matter orientation of spinal cord

inner gray and outer white matter tracts (columns)

nociception

is the sensory process that provides signals that trigger pain

how do touch receptors help us grasp a bottle of water?

mechanoreceptors and somatosensory receptors

how is somatic sensation of the face supplied?

mostly by large trigeminal nerve (CN V) which enters the brain at the pons

lumbar puncture

needle in inserted into the cauda equina CSF sack in order to test medical diagnostics

second-order sensory neurons

neurons that receive sensory input from primary afferents and lie within the dorsal horns

does a receptor potential equal an AP?

no

Hereditary Sensory and Autonomic Neuropathy

no pain receptor feedback and answers why we need pain to survive

is sensory info transferred unchanged through the nuclei in the brain stem and thalamus on its way to the cortex, with the actual processing taking place only in the cortex?

no, in both dorsal column and thalamic nuclei, considerable transfer of information takes place

are the mechanoreceptors myelinated?

no?

in addition to mechanoreceptors for touch, what other receptors does the somatic sensory system rely heavily on?

nociceptors (take ea different pathway)

compare and contrast: rapidly-adapting and slowly-adapting receptors

objective 2

describe how a sensory even is encoded in APs in sensory fibers?

objective 3

where do neurons in layer IV project to?

other layers

Proprioception

our sense of body position

how is the net perception determined when holding a water bottle?

overall pattern of firing

location of S1

parietal lobe on the post central gyrus

answer to grasping water bottle example?

pattern: Merkel's disks stretching of skin: Ruffini ridges: Meissners pressure: Pacinian

phantom limb

perceived sensation, following amputation of a limb, that the limb still exists (often perceived when face is stimulated)

how do mechanoreceptors vary in their responses?

preferred stimulus frequencies, pressures, and receptive field sizes

essential protective function of skin

prevents the evaporation of body fluids into the dry environment

what happens to the character of neuronal receptive fields as info passes through the cortex?

receptive fields change and enlarge

what happens to the cortex when a digit use is increased?

representation of the stimulated digits has expanded in comparison with the adjacent, unstimulated ones

area 3a

sense of body position

largest sensory organ

skin

where does the sensation of touch begin?

skin

dermatome

the area of skin innervated by the right and left dorsal roots of a single spinal segment

what happens to the dermatome when a dorsal root is cut?

the corresponding dermatome on that side of the body does not lose all sensation because the adjacent dorsal roots innervate overlapping areas

what happens to the cortex when a digit is removed?

the cortex originally devoted to the amputated digit now responded to stimulation of the adjacent digits

how is the selectivity of a mechanoreceptor axon determined?

the structure of its special ending (ex. Pacinian has onion shape that helps senses vibration and not steady pressure unless onion is taken off)

Why do we need fast-adapting AND slow-adapting receptors?

to combine info and detect different kinds of changes

what nerve supplies the face, mouth areas, outer 2/3 of the tongue, and the dura mater covering the brain?

trigeminal nerve (CN V)

how is each half of the spinal cord divided?

ventral horn, intermediate zone, and dorsal horn

slowly adapting receptors

when stimulus is present, APs fire when stimulus is absent, APs still fire but slower

rapidly adapting receptors

when stimulus is present, APs fire when stimulus is absent, no APs

cortical barrels

whiskers are stamped in the mouse brian, meaning each barrel is associated with single whisker and that each one forms part of a functional cortical column

are the pathways of touch and vibration to the brain different than pain and temperature?

yes

can you have pain without nociception and the other way around?

yes