CNS 7 - Somatic Senses

4 somatic senses

touch, temperature, proprioception, and nociception

Proprioception

awareness of the position of body parts relative to each other

Nociception

detects tissue damage or the threat of it, and is perceived as pain or itch.

Somatosensory receptor cells are…

all neurons

Receptors for somatic sensation below the chin have their cell bodies in the…

dorsal root ganglia

Receptors for the head have their cell bodies in the

brain

Nerve endings

parts of these neurons that transduce touch, pressure etc

• in the tips of their fibers, in the skin and viscera.

Epidermis

outer layer of skin — has free nerve endings

Free nerve endings

detect mechanical stimuli, temperature, and chemicals

Merkel receptors (merkel disks)

mechanoreceptor nerve endings in contact with specialized epithelial cells called Merkel cells

• detect fine pressure, steady pressure. concentrated at fingertips

• very sensitive to deformation of the skin

• more tonic than phasic,

• i.e. they send a sustained message as long as the deformation persists.

• Signal contact

Encapsulated receptors (e.g. Meissner and Pacinian corpuscles)

mechanoreceptors sheathed in connective tissue

At the bottom of the epidermis are saucer shaped…

Merkel disks

Most mechanoreceptors are…

phasic

• Given a sustained, constant stimulus, the nerve ending’s membrane depolarizes but then returns to baseline in ~3 ms

• registers changes — not steady levels

At the top of the dermis are egg shaped

Meissner corpuscles (meiser, money)

Meissner corpuscles

mainly in the tongue and hairless skin — erogenous zones, palms and fingertips

• detect sideways shearing, as when you stroke a surface or lift something with your fingertips

• phasic - sense changes in shear (rather than sustained pressure)

Deep in the dermis are onion-shaped…

Pacinian corpuscles

Pacinian corpuscles

They can sense tiny displacements (10 μm) if the motion is quick

• nerve endings are sheathed in many layers.

• phasic - they respond strongly to vibration and other fast-changing stimuli

Receptors are not uniformly distributed over the body surface

Palms, fingertips, and lips are the foveas of the somatosensory system

• they have more densely packed receptors, and therefore higher acuity, than other areas.

Acuity test of 2-point discrimination

if your skin is touched at 2 places simultaneously, can you tell whether there are one or 2 contact points?

• On your lips and fingertips you can distinguish points 2–4 mm apart, but on your calves you need 40 mm

Thermoreceptors are…

free nerve endings

Cold receptors

respond maximally at ~30°C (which is well below body temperature)

• phasic-tonic

• why we get used to cold lake

Warm Receptors

respond at ~45°C

• phasic-tonic

• why we get used to hot bath

Pain receptor activation temperature

Above 45°C

paradoxical cold

a hot object, touched briefly, may feel cold

We have more cold receptors than warm, and few thermoreceptors in total

as few as 1000 fibers may carry temperature information up the spinal cord to the brain (precise localization isn’t crucial for temperature)

Nociceptors

free nerve endings that respond to noxious (harmful) stimuli

• Some respond to damaging mechanical stimuli, others to damaging heat or chemicals

• Some respond to chemicals released from damaged cells (K+, histamine, prostaglandins) or to serotonin released by platelets in response to injury

Somatosensory afferents fall into 2 groups

small and large

Somatosensory afferents

Carry signals from sensory receptors to the CNS

The small fibers

C and Aδ (A-delta) — come mainly from free nerve endings

C fibers

unmyelinated, and conduct spikes at speeds up to 2 m/s. — slower

Aδ’fibers

thicker than C’s, myelinated, and conduct at up to 30 m/s — faster

Large fibers — Aβ (A-beta),

come from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles

• myelinated, and conduct at 70 m/s.

Large and small fibers project differently

• Large fibers turn upward on reaching the spinal cord, and run ipsilaterally up to the medulla in tracts called the dorsal columns.

• Small fibers synapse directly or via interneurons on motoneurons (for reflex responses) or on dorsal-horn neurons whose axons cross the midline and run in the spinothalamic tracts, in the lateral part of the cord, between the dorsal and ventral horns

This anatomy reflects the fibers’ different functions

Large fibers provide feedback to…

the brain, especially to motor cortex, as it manipulates objects. Their information has to travel a long way (up to the brain) quickly — complex

Small fibers evoke simple responses…

to specific stimuli: withdrawing from pain, brushing away a bug, thermoregulatory and sexual responses. Many of these tasks can be handled in the spinal cord, without immediate input from the brain.

Signals travel via

thalamus to cortex

Signals from the spinal cord travel via the…

ventroposterolateral (VPL) nucleus of the thalamus.

• pass to the primary somatosensory cortex, S1

Signals from the head travel via the…

ventroposteromedial nucleus (VPM)

• pass to the primary somatosensory cortex, S1

Primary somatosensory cortex is somatotopic

Neighboring areas of skin project to neighboring cells in cortex, so S1 is a map of the contralateral body surface

• map is distorted, as areas of high sensitivity and acuity (such as hands and lips) get a lot of cortical space, just as the foveas do in the visual system

Somatotopic

cortex contains map of the body’s surface

Primary somatosensory cortex (S1) location

Parietal Lobe

lateral inhibition

enhances spatial differences, i.e. edges

There is lateral inhibition among somatosensory fibers

If you step into a very hot bath, most discomfort is felt ay the line formed by the water surface around your leg, because that is the temperature edge — bc of lateral inhibition

• somatosensory version of the Chevreul illusion

TRP

transient receptor potential (TRP) channels

Many nociceptors have ion channels of the TRP type

• eg: TRPV1 channels—vanilloid receptors: respond to damaging heat and to chemicals, including the capsaicin in chili peppers

• TRPM8 channels respond to cold and to menthol

Nociceptive signals

report damage or danger, and evoke pain or itch

congenital analgesia

cannot feel pain

• people with this usually die before they are 20, because of injury and infection

2 types of pain

fast and slow

• e.g. when you stub your toe, you feel an immediate sharp pain, followed ~1 s later by a duller sensation

Fast pain is carried by…

Aδ fibers

slow pain is carried by…

C fibers

reason for the 2 types is likely that pain evokes 2 distinct responses:

quick withdrawal and prolonged immobilization

quick withdrawal

to get away from the painful thing

prolonged immobilization

(to promote healing)

Nociceptive signals evoke responses from the…

CNS

Nociceptive signals trigger withdrawal

e.g. pulling your hand back from a hot stove.

Spinal reflex

doesn’t need immediate input from the brain

Nociceptive signals also reach the limbic system and hypothalamus

causing emotional distress, nausea, vomiting, and sweating

Descending pathways through the thalamus can…

can block nociceptive cells in the spinal cord,

• in emergencies where survival depends on ignoring pain

Referred Pain

Pain in an internal organ is often felt on the body surface

Nociceptors from different locations converge on a single…

ascending tract

• when that tract sends signals to the brain, the brain doesn’t know where the stimulus came from

As pain is more common in skin than in internal organs…

the brain assumes the problem is on the body surface

Pains from different organs are referred to different regions on the body surface

Pain can be gated by…

Aβ activity

• In the dorsal horn, C fibers contact secondary neurons.

• 'Those secondaries are inhibited by Aβ fibers via interneurons.

• Aβ’s can block or dampen pain signals, e.g. if you rub a sore shoulder, it feels better

Analgesics

Drugs that relieve pain

Acetylsalicylic acid (aspirin)

inhibits prostaglandins and inflammation and slows transmission of pain signals.

Opioids

(such as morphine and codeine) decrease transmitter release from primary sensory neurons and postsynaptically inhibit secondary sensory neurons.

Body’s natural painkillers

Endorphins, enkephalins and dynorphins