NeuroBio Sensory systems

Somatic sensation

Enables body to feel, ache, sense temperature, and pressure.

3 Functions of skin

Protects body, prevents evaporation of bodily fluids, provides direct contact with world.

Layers of skin

Epidermis, Dermis

2 Types of skin

Hairy, Glabrous

Hairy skin

Skin with hair follicles (arms and face)

Glabrous skin

Skin without hair follicles (palms of hands, soles of feet)

4 types of mechanoreceptors

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

Pacinian corpuscles

Located in dermis, high density (high sensitivity), tracks movement, position and vibration. Has a large, dull receptor field that fires rapidly.

Ruffini’s endings

Located in dermis, hairy and glabrous skin, track movement, position and stretch. Has a dull, large receptor field that fires slowly.

Meissner’s corpuscles

On boarder of epidermis and dermis, Glabrous skin, raised parts of finer tips. Responds to light touch/a coarser appraisal of the surface. Has a small, sharp receptor field that fires rapidly.

Merkel’s disks

On Boarder of epidermis and dermis, responds to fine touch (like distinguishing small textures. Has a small, sharp receptor field that fires slowly.

How is touch stimulated?

Mechanosensitive ion channels convert mechanical force into the change of the ionic current when pressure is applied to part of the channel. This pressure could come from stretch, intracellular structures or extra cellular structures.

Skin mechanosensory receptors

pseudo unipolar neurons that respond to mechanical stimulus.

Primary afferent axons

Aα, Aβ, Aδ, and C axons

what does the Aδ axon do

They mediate pain and temperature. They are heavily milenated and deliver fast pain/sensation

What does the C axon do

Mediates pain and temperature, less milenated so delivers slow pain and itch.

What does the Aα axon do

It is heavily milenated so delivers fast, sharp pain. Helps detect where skin is in space, lives in skeletal muscle.

What does the Aβ axon do

mediates touch sensation, heavily milenated, delivers fast pain,

Spinal cord sections

Cervical, Thoracic, Lumbar, and sacral

Order of sensation pathway

First order is located in the dorsal root ganglia. Then the pathway travels to the dorsal column of the caudal medulla through the medial lemniscus (second order). Then to the thalamus where third order is located. The cell bodies of the neurons are in the thalamus and then stretch their axons to the somatosensory cortex.

First order neurons

Primary sensory neurons. They are pseudo unipolar meaning they have one axon that leaves the soma but branches into 2 axons. One of the axons goes to the spinal cord or brain stem (touch vs pain) and the other goes to the mechanoreceptors.

Dermatomes

Area of skin dictated by sensory nerve fibers from one spinal nerve root. Shingles activates along one dermatome.

Dorsal columns

The medial lemniscus pathway runs through it after the first and to the second order.

Trigeminal Touch Pathway

transports somatosensory information from face.

What is the path the trigeminal pathway takes?

Large mechanoreceptor axons rum from face to the principal sensory trigeminal nucleus where the first order synapse occurs. Then to the second order synapse in the thalamus. Finally its axon extends to the primary somatosensory cortex.

Where is the principal sensory trigeminal nucleus located

The caudal pons.

Sensory regions of the cerebral cortex

3a, 3b, 1, 2, 5, and 7. 5 and 7 are the posterior parietal cortex where as the somatosensory cortex are the other areas.

Posterior parietal cortex

Involved in somatic sensation, visual stimuli, movement planning, and attentiveness.

What happens if damage occurs to the posterior parietal cortex?

Neurological disorders like agnosia, the inability to recognize objects even though sensory skills are intact, Astereognosia, the inability to recognize common objects by touching them, and neglect syndrome, part of body or world is ignored.

Pain

Feeling of sore, aching, throbbing sensations

Nociception

sensory process, provides signals that trigger pain.

Nociceptors

Responsible for the transduction of pain with free, branching, unmyelinated nerve endings.

How do ion channels open?

strong mechanical stimulation, temperature extremes, oxygen deprivation, and chemicals.

What happens during an injury?

The damaged cells release prostaglandins, potassium ions, and Bradykinin which activates nociceptors. The signal travels to the spinal cord. The branches of the activated nociceptors release substance p to trigger the release of histamine and vasodilate blood vessels.

Hyperalgesia

The reduced threshold for main, an increased response to painful stimuli, or a spontaneous pain the follows localized injury.

Primary hyperalgesia

Occurs within the area of the damaged tissue

Secondary hyperalgesia

occurs in the surrounding tissue

Phases of transmitting a pain signal

First pain: fast and sharp (Aδ fibers) second pain: slow and dull (C-fibers), referred pain: Experience pain somewhere other than the original pain site

referred pain

Experience pain somewhere other than the original pain site. caused by cross talk between cutaneous nociceptors and visceral nociceptors (angina: the chest pain experienced during a heart-attack is referred pain)

If you feel a fast sharp pain, what nerve was effected?

Aδ

If you feel a slow and dull pain, what nerve was effected?

C-fibers

What is the Spinothalamic pathway

The pain pathway, ascends contralaterally. Small dorsal root axons → lateral spinothalamic tract → spinal cord → medulla → thalamus → cerebral cortex

Trigeminal Pain Pathway

pain and temperature info from face to head. Goes from the trigeminal nerve, to the spinal trigeminal nucleus in the tegmentum of the pons, to trigeminal lemniscus, to the thalamus, to the somatosensory cortex. Diameter of fibers are smaller than sensory.

Periaqueductal gray matter

located in the midbrain. Stimulating it can cause pain relief by releasing endorphins and opioids and by projecting to serotonin producing areas.

Organization of temperature pathway

identical to pain pathway, cold receptors are coupled to Aδ and C fibers, hot receptors are coupled to C fibers. Axons of second-order neurons cross.

Temperature TRP channels

Different TRP channels respond to different temperatures. Cold is sensed faster because TRPM8 and TRPA1 are myelinated whereas TRPV4 and down are unmyelinated.