Touch/Pain

Somatosensory system function

transduces mechanical stimulation of the skin, injury to the skin, and/or changes in temperature into neural impulses

sensory transduction

conversion of energy from a stimulus into a change in membrane potential in a receptor cell

Aspects of somatosensation?

mechanosensation (touch, pressure, vibration) and nociception (pain & temp)

Meissner’s corpuscle

skin receptor type that detects light touch, sits in the dermis but comes from the hypodermis, has receptive field with small & sharp borders

Merkel’s disc

skin receptor cell type that detects fine touch (touch in a specific spot), in the dermis but kinda goes into the epidermis, has receptive field with small & sharp borders

Free nerve ending

detects pain and temperature, are in the dermis and can go into the epidermis

hair follicle receptor

wraps around the hair follicle in the dermis, detects touch

Pacinian corpuscle

detects vibration or pressure, located in the hypodermis, has receptive field with large & vague borders

Ruffini’s ending

detect stretch, also located in the hypodermis, has receptive field with large & vague borders

Receptive field

the space in which a stimulus will alter a neuron’s firing rate

Effect of touch on a cortical neuron’s receptive field?

touching outside of it has no effect, touching in the center of the receptive field excites it, but touching in the surround inhibits it

What receptor would be best at detecting braille?

Meissner’s corpuscle

What receptors in the skin are fast-adapting?

Pacinian corpuscle and Meissner’s corpuscle

What receptors in the skin are slow-adapting?

Merkel’s discs and ruffini’s ending

Where are the cell bodies for all somatosensory cells found?

in the dorsal root ganglia (part of the somatic nervous system)

location of all parts of the pacinian corpuscle?

cell body found in the dorsal root ganglia, is unipolar and has one axon heading towards corpuscle structure in the skin (specifically in hypodermis), other heads into the dorsal spinal cord

How does the pacinian corpuscle detect vibration?

mechanical displacement of the membrane layers results in Na+ channels being opened and the corpuscle being depolarized, which generates an action potential once reached

What kind of receptors have A beta axon type?

Pacinian corpuscles, Ruffini’s endings, Merkel’s discs, and Meissner’s corpuscles

Sensory pathway from the face?

receptors — sensory cranial nerves → brainstem → thalamus → primary sensory cortical areas (but can also go back from thalamus from there)

Sensory pathway from the body?

receptors — sensory peripheral nerves → spinal cord → brainstem → thalamus → primary sensory corical areas (but can also go back from thalamus from there)

Dorsal column system pathway

skin (mechanoreceptors) → unipolar neurons (in dorsal root ganglion) → join dorsal columns → ascend to the medulla → decussate joining medial lemniscus (brainstem) → synapse within the ventral posterior and ventral lateral nuclei of thalamus → thalamic cells relay info to the post-central gyrus (somatosensory cortex in parietal lobe)

Number cervical vertebrae and type of info they receive

8 vertebrae, info from back of head and arms

Number thoracic vertebrae and type of info they receive

12 vertebrae, infro from trunk of the body

Number lumbar vertebrae and type of info they receive

5 vertebrae, info from lower back and the front of your legs

Number sacral vertebrae and type of info they receive

5 vertebrae, infrom from the very back of your legs

homunculus map

a spatial map of the body represented in the cortex

Primary somatosensory cortex

Located just behind the central sulcus, each receives info from the other side of the body, neurons mapped according to body part

Plasticity

ability of the brain to change and adapt, such as if you lose a finger (surrounding fingers would take over representation of the lost finger)

Why might some body parts have more representation than others in the primary somatosensory cortex?

Are used more and receive higher stimulation

When is the percept of pain generated?

when mechanical pressure on skin is enough to cause tissue damage - bleeding, swelling, high temps, burns, chemical damage

What are nociceptors

pain receptors, such as free nerve endings, whose job is to detect painful stimuli and transmit them to the nervous system

What do damaged cells secrete?

serotonin, prostaglandins, and leukotrienes

What acts on free nerve ending receptors?

serotonin, prostaglandins, and leukotrienes

How do aspirin, ibuprofen, and acetopminophin work to make you feel less pain?

inhibit the synthesis of prostaglandins (so decrease activation of free nerve endings!)

Pain pathway (detailed)

1. damaged cells release substances that excite free nerve endings

2. action potential generated in periphery can excite blood vessels and other cells to produce inflammation

3. info enters through the dorsal root ganglion and synapses on neurons in the dorsal horn

4. pain fibers release glutamate and substance P in the spinal cord, dorsal horn cells send info across the midline and up to the thalamus

5. histamines are released in response to itch by mast cells

C fibers

unmyelinated fibers that can detect both hot and cold, CMR1 detects cold and TRPV1 detects heat; release glutamate and substance P when stimulated

A delta fibers

myelinated fibers that are good at sensing heat at very hot temperatures

What kind of receptors do C fibers act on?

AMPA, NMDA, NK1, and ion channels

What kind of receptors do A delta fibers act on?

AMPA and NMDA

How do A delta and C fibers get info to the brain?

come in through the dorsal root ganglion and synapse onto the dorsal horn, axons within the spinothalamic tract ascend to synapse on thalamic neurons within the ventral posterior nucleus of the thalamus, whose neurons can then project up into the somatosensory cortex

Neuropathic pain

may be due to inappropriate signaling of pain by neurons, dorsal horn neurons can become hyperexcitable and cause chronic pain

Example of neuropathic pain?

phantom limb pain

What can be released to decrease pain?

serotonin

dermatomes

topographical representations of sensory information

Pain pathway (simplified)

spinothalamic tract → ventral posterior nucleus (in thalamus) → S1