Somatosensory Pathways:

Ascending Pathways

From periphery to cerebral cortex-

• 1st order - Primary neuron for sensation
• cell bodies in dorsal root ganglion: collect sensory information from periphery
• Communicates with 2nd order
• Tracts have either 2 or 3 neuron organization so sometimes there is a 3rd order neuron.

light, touch, pain, pressure, temperature, and joint and muscle position sense (also called proprioception)

However, these modalities are lumped into three different pathways in the spinal cord and 
have different targets in the brain.

includes touch, pressure, vibration, and texture (i.e., anterior spinothalamic tract)

-The discriminative touch system crosses high - in the medulla.

Pain and temperature (i.e., lateral spinothalamic tract)

-The pain system crosses low - in the spinal cord.

-includes receptors for what happens below the body surface: muscle 
stretch, joint position, tendon tension, etc. (i.e., spinocerebellar tract)

-This modality primarily targets the cerebellum, which needs second-by-second feedback on what the muscles are doing.

-The proprioceptive system is going to the cerebellum, which works ipsilaterally- therefore this system doesn't cross.

Sensory axons can be classified according to diameter and therefore- conduction velocity.

1. The largest and fastest axons are called A-alpha, and include some of the proprioceptive neurons, such as the stretch receptor.
2. The second largest group is called A-beta, which includes all of the discriminative touch receptors, AKA: mechanoreceptors because there is physical displacement of nerve endings
   Instrumental in haptics
3. / 4. Pain and temperature include the third and fourth groups
   A-delta: intermediate in size but myelinated (fast pain)
   C fibers: smallest and also unmyelinated so conduction velocity is quite slow (slow pain)

Rapidly Adapting (RA)-response to unchanged stimulus is NOT sustained ( but do respond to changes in stimulation)

Slowly Adapting (SA) -response to unchanged stimulus IS sustained
Punctate - small receptive fields

Diffuse - large receptive fields

1. Meissner Corpuscles (RA-punctate) responds to active touch during object exploration (haptics) and light touch. They are concentrated in fingertips, palms, lips, soles, face (and other erogenous zones!)- Why you stop feeling your clothes.
2. Pacinian Corpuscles (RA-diffuse) sensitive to deep pressure over a large receptive field- Why you stop feeling a snug watch
3. Merkel Disks (SA-punctate) sensitive to constant sources of stimulation over a small area- you are holding a pebble
4. Ruffini Endings (SA-diffuse) constant stimulation over a larger area - also detects skin stretch- Only in glabrous dermis. Helps modulate grip?