Chapter 12 - Touch/Nociceptors

Somatic sensation

Enables perception of touch, pain, and temperature. Receptors are broadly distributed across entire body (some are present in internal organs as well)

Glabrous skin

Skin that does not have hair, tends to have higher densities of sensitive, high-resolution mechanoreceptors

Receptive fields

Varies by shapes and size, related to anatomy and branching characteristics of underlying receptor cells.

Different receptor types provide different degrees of precision w/ regards to touch location.

Slowly adapting afferents

Exhibit more sustained responses to continuous pressure.

May be important for shape vs. size perception

Rapidly adapting afferents

Become more silent in response to continuous pressure.

May be important for rapid changes to ongoing stimulation

Stimulus frequency

Analogous to "texture", the function by which each receptor type exhibits thresholds.

Pacinian corpuscles

Deep pressure, fast vibrations.

Respond to light pressure at higher frequencies

Meissner's corpuscles

Changes in texture, slow vibrations.

Respond to lower frequency stimulation but requires more force

Merkel cells

Touch receptors in fingertips, have small endings w/ high spatial resolution.

High density of mechanoreceptors, brain dedicates more tissue to processing these signals.

Mechanisms by which mechanical force is converted to a neural code

Ion channels sensitive to stretching of lipid membrane.

Ion channels that open with force applied to extracellular structures

Ion channels that open w/ deformation and stress on cell's cytoskeleton

Piezo1 and Piezo2

Recently discovered mechanosensors

Action potentials

Caused by physical pressure on mechanoreceptors which opens ion channels in order to cause depolarization.

# is typically proportional to magnitude of the stretch/pressure

Two-point discrimination

The ability to discriminate detailed stimulus. Only a single point of contact is perceived if within the receptive field of a single sensory neuron, two are perceived if extended across the fields of two different sensory neurons.

Receptive fields map to areas of the body and their density varies greatly.

Which of the following has receptive fields that may cover an entire finger or half of the palm?

Pacinian corpuscles

Afferent axons

Axons that carry information to CNS - enter the spinal cord through dorsal roots

Aα fibers

Mediate proprioception

Aβ fibers

Mediate touch perception

C fibers

Mediate pain, temperature, and itch

Dorsal root ganglia

Cell bodies of sensory neurons gather here and primary afferents enter to spinal cord through them

Spinal nerves

Innervate segments of the body. Dermatomes are innervated by the dorsal root in single spinal segments

Dorsal column-medial lemniscal pathway

Mediates signals about touch and proprioception, Alpha/beta axons enter the ipsilateral side and terminate in dorsal column nuclei.

Trigeminal nerve pathway

Carries somatic signals that aren't carried by spinal cord (face, head).

Somatosensory information from face comes primarily through this pathway. Has similar sensory connections as dorsal root ganglia BUT crosses at trigeminal nucleus

Primary sensory afferent axons have widely varying diameters, and their size correlates with the type of receptor to which they are attached. Which of these axons are the smallest and slowest?

Temperature, pain and itch

A dermatome is an area innervated by:

Fibers of the cells from a single nucleus in the brain stem

Primary somatosensory cortex

Responsive to somatosensory stimuli

Lesions impair somatic sensation

Electrical stimulation evokes somatic sensation

Layered like visual cortex

S1 neurons

Neurons w/ similar inputs and responses are stacked vertically into columns

Contains neurons innervated by both rapid/slow adapting mechanoreceptors.

Somatotopy

Receptive fields of s1 neurons create a map of the body on the cortex.

There are more cortical spaces dedicated to certain areas

Somatotopic plasticity

Removing a digit in a reptile hand causes the cortical space to reorganize to fit the change.

Overstimulating causes the OPPOSITE effect

Posterior parietal cortical areas

Higher order somatic cortical areas. Involved in extracting perception of objects

Lateral inhibition

Provides contrast enhancement for somatic receptive fields. Mechanism by which contrast between neighboring points is enhanced

Allows brain to differentiate between the responses of two nearby sets of neurons.

No lateral inhibition

Difference between center and surround is small

Somatotopy

Cortical neurons are arranged in same topology as peripheral receptive fields on the skin.

Areas w/ denser receptive fields have bigger cortical representation, more neurons dedicated to processing!

Feature extraction

Cortical somatosensory neurons have more complex receptive fields than just location.

Orientation of pressures and direction of movement of touch across multiple receptive fields also cause a neuronal response.

What type of information does the dorsal-column-medial-lemniscal pathway carry?

Touch and vibration

Nociceptors

Expressed in branching unmyelinated nerve endings.

Activated by strong mechanical stimulation, temperature extremes, oxygen deprivation, chemicals, etc. Pain perception!

Transduction of painful stimuli

-Protease can breakdown kininogen to form bradykinin, which binds and activates nociceptors.

-ATP release activates ATP-gated ion channels located on nociceptors, causing an increase in extracellular K+ which then causes depolarization

TRPV1

Mediates heat nociception, activated at temperatures greater than 43C. Tissue damage may release chemicals that open this channels

Also activated by capsaicin, which may mimic chemicals released from damaged tissue. Capsaicin cannot be detected w/o TRPV1

Itch

Pain can modulate or suppress itch (imagine scratching a bug bite!)

Histamine binds to TRPV1 which causes itchiness.

Dual axon pathways

Nociception is transmitted via Aδ and C axons, nociception is much slower than somatic touch.

Two distinct pain perceptions - first pain (fast, sharp) and second pain (slow, dull)

Pain pathway

Nociceptive primary afferents have cell bodies in dorsal root ganglia.

Primary afferents branch on ipsilateral side at a short distance on spinal cord

Projections from neurons in substantial gelatinosa cross and ascend

Referred pain

Damage/issue w/ an internal organ results in specific sensations of pain across the body. Caused by visceral nociception (from organs) mixing w/ cutaneous nociception

Spinothalamic pathway

The route from the spinal cord to the brain that carries most of the information about skin temperature and pain.

1. Synapses on 2nd order neurons in substantia gelatinosa

2. Decussates

3. Ascends to thalamus

Trigeminal pathway

Carries pain and temperature information from the face and head.

1. Primary afferent innervates 2nd order neurons in spinal trigeminal nucleus

2. Decussates

3. Ascends to thalamus

Touch vs. pain

Touch ascends ipsilaterally (same side) and is mediated by the dorsal column-medial lemniscus pathway

Pain ascends contralaterally (opposite side) and is mediated by the spinothalamic/trigeminal pathway.

Nociceptive vs. mechanosensory pathways

Nociceptive information decussates at the level of spinal cord innervation, while mechanosensory information doesn't cross until reaching the dorsal column nucleus

Gate theory of pain

Mechanical stimulation could activate an interneuron that inhibits spinothalamic tract. Concurrent activation of somatic and nociceptive neuron leads to a reduced nociceptor response

Gate theory of pain

IN normally inhibits PN - decreasing pain.

During pain - (C) inhibits IN, increasing PN

Pain + touch - partially activates IN, inhibiting PN

Endogenous opioids

Endorphins, enkephalins, and dynorphins. Directly modulates nociceptive fibers by binding to receptors in the CNS.

Descending feedback

Electrical stimulation of periaqueductal gray matter (PAG) results in analgesia (being pain-free while conscious)

PAG receives input from parts of brain involved w/ emotional state, which descend to spinal cord.

TRP channels

Mediate temperature sensing, an array of different receptors which allow for perception of very small differences in temperature

Adaptation

Thermoreceptors exhibit this - are usually most sensitive to initial temperature changes which then eventually falls off.

Hot receptors

Coupled to C fibers

Cold receptors

Coupled to Aδ and C fibers