L18: Sensory pathways and pain processing

What is the receptive field?

Area where stimulation changes the activity of a particular sensory neuron

Localization difference in small/large receptive fields?

Small= precise localization

Large= less precise localization

What is nerve plexus?

Network of interweaving anterior rami in the PNS

Which spinal nerves don’t form plexuses?

T2-T12 (intercostal and subcostal nerves)

What activates a nociceptor?

Noxious stimulus

Where does AP travel after activating nociceptor?

Primary afferent (first-order) neuron

Where is first-order neuron located?

Dorsal root ganglion

Where does central axon enter?

Spinal cord through the dorsal root

Where do neurotransmitters release/what does this activate?

Dorsal horn, and it activates second-order neurons

What are the primary afferents?

• A-delta fibers

• C fibers

Characteristics of A-delta fibers?

Thinly myelinated— faster, sharp, and well-localized pain

Characteristics of C fibers?

Unmyelinated—slower, dull or burning, poorly localized pain

What is the anterolateral system made of?

Anterior and lateral spinothalamic tract

Which tract transmits pain and temperature?

Lateral spinothalamic tract

Which tract transmits feelings associated with crude touch and pressure?

Anterior spinothalamic tract

Where are first-order neurons?

Cell bodies in DRG

What do FON enter?

Through dorsal roots

Where do FON synapse?

Dorsal horn

Where are second-order neurons?

Cell bodies in dorsal horn

Where do SON cross?

Through anterior white commisure

Where do SON ascend?

Contralaterally to thalamus

Where are third-order neurons?

Cell bodies in VPL nucleus of thalamus

Where do TON project?

Project to S1 in postcentral gyrus

What is the descending inhibitory control pathway?

Periaqueductal gray (midbrain) —> rostral ventromedial medulla—> dorsal horn—> activation of inhibitory interneurons that release enkephalins

What is ascending nociceptive signaling?

Primary afferent releases glutamate (w/ or w/o substance P)—> activation of SON

What is the presynaptic enkephalin effect?

Enkephalins bind to opioid receptors on presynaptic nerve terminal. Decrease in Ca2+ entry, which decreases neurotransmitter release

What is the postsynaptic enkephalin effect?

Enkephalins can act on postsynaptic neuron, which causes an increase in K+. This leads to hyperpolarization, which decreases neuronal excitability.

What does simultaneous presynaptic and postsynaptic do?

Lower nociceptive transmission toward brain

What is the transduction step for pain?

Noxious stimulus—> receptor potential—> AP in peripheral nociceptors

What is transmission step for pain?

Primary afferent—> dorsal horn—> SON crosses—> contralateral spinothalamic tract—> VPL thalamus—> S1

What does somatosensory cortex perceive in relation to pain?

Location and intensity

What does cortical and limbic networks perceive in relation to pain?

Unpleasantness and emotional response

What is the pathway for modulation of pain?

PAG—> RVM—> dorsal horn—> inhibitory interneurons release enkephalins—> nociceptive transmission decreases

What is proprioception?

Sense of body position and movement

What does muscle spindle detect?

Muscle length and rate of stretch

What is function of muscle spindle?

Body position sense and stretch reflexes

Where is golgi tendon organ?

Tendons near muscle-tendon junction in series with muscle fibers

What does golgi tendon organ detect?

Muscle tension

What is golgi tendon organ function?

Force feedback and regulation of muscle activity

Where is the joint receptor?

Joint capsules and ligaments

What do joint receptors detect?

Joint deformation and movement

What are examples of joint receptors?

Ruffini endings and Pacinian receptors

What is function of joint receptor?

Additional position and movement info, especially near joint movement limits

What does posterior column pathway carry?

Fine touch, vibration, and conscious proprioception

What neuron enters spinal cord and which side does it travel?

FON ascend ipsilaterally in spinal cord

Where does decussation occur?

First synapse in medulla where SON cross

Where is fasiculus gracilis located?

Dorsal column spinal cord; medial—lower trunk and lower limb

Where is fasciculus cuneatus?

Lateral portion of dorsal columns; lateral—upper trunk and upper limb (T6 and up)

Where does FON synapse in dorsal medial lemniscus pathway?

Synapse in gracile or cuneate nucleus of medulla

Where are SON cell bodies?

In gracile or cuneate nucles

Where do SON axons cross?

Medulla via internal arcuate fibers

Where do SON fibers ascend?

Contralaterally as medial lemniscus

Where do SON synapse?

VPL nucleus of thalamus

Where are cell bodies in TON?

VPL nucleus of thalamus

Where do TON axons pass through?

Posterior limb of internal capsule (info highway)

Where does TON project?

S1 in postcentral gyrus

What is the function of unconscious proprioception?

Provides info for automatic coordination of movement and posture

What is posterior spinocerebellar pathway?

Lower body to cerebellum

What is cuneocerebellar pathway?

Upper body to cerebellum

What direction are unconscious proprioception pathways and where do they enter through?

Ipsilateral and enter through inferior cerebellar peduncle

What is pathway for VPL (ventral posterolateral nucleus):body?

Touch/vibration/proprioception/pain/temp—> DCML/spinothalamic—> S1

What is pathway for MGB (medial geniculate body):hearing?

IC—> MGB—> primary auditory cortex (temp lobe)

What is pathways for VPM (ventral posteromedial nucleus):face?

Somatosensation—> trigeminal pathways—> S1

What is pathway for LGB (lateral geniculate body):vision?

Retina—> optic tract—> LGB—> primary visual cortex (occipital lobe)

What does S1 process?

Touch location and stimulus features

What does somatosensory association cortex integrate?

Sensory info to help recognize objects by touch

Where is somatosensory association cortex?

Posterior parietal cortex

What is stereognosis?

Recognizing familiar object w/o looking

When can cortical maps change?

With experience or injury