Lecture 2: Touch and Proprioception

finished 9/3

What types of somatosensation are cutaneous mechanoreceptors associated with?

fine touch, vibration, pressure

What types of receptors is proprioception associated with?

specialized receptors associated with musles, tendons, and joints

What is proprioception?

our ability to sense the position of our own limbs and other body parts in space

What types of somatosensation is associated with free nerve endings?

pain, temperature, coarse touch

somatosensory afferents convey information from ___ to ___

skin surface, central circuits

What are the properties of Ia, Ib, and II afferents?

Largest and fastest, supply sensory receptors to muscles for proprioception.

What are the characteristics of A-beta afferents?

smaller, convey touch

What are the characteristics of A-delta and C afferents?

Small and slow, convey pain and temperature.

sensitive areas of the body have a (higher/lower) two-point discrimination threshold

lower

Describe temporal dynamics of sensory afferents

Slowly adapting afferents generate a sustained discharge during the stimulus. this provides spatial info (size and shape of stim.).

Rapidly adapting afferents fire rapidly when a stimulus is first presented, then slow with continual stimulation. this conveys stimulus movement.

Determined by receptor type.

free nerve endings, meissner corpuscles, and merkel cells are all in the (deeper/shallower) part of the skin

shallower

ruffini corpuscles and pacinian corpuscles are in the (deeper/shallower) part of the skin

deeper

What do meissner corpuscles detect?

motion detection and grip

What do merkel cells detect?

shape and texture

what do ruffini corpuscles detect?

force, shape, internally generated motion

what do pacinian corpuscles detect?

vibrations

25% of the mechanoreceptors in the hand

Merkel cells

40% of the mechanoreceptors in the hand

Meissner corpuscles

10-15% of the mechanoreceptors in the hand

Pacinian corpuscle

20% of the mechanoreceptors in the hand

Ruffini corpuscle

What are proprioceptors?

receptors that provide information about the position of limbs in space

What are the three types of proprioceptors?

muscle spindles, golgi tendon organs, and joint receptors

What do muscle spindles do? how?

signal changes in muscle length. they are surrounded by sensory afferents and when a muscle is stretched, the tension activates the nerve endings. This triggers an action potential.

What are the two types of endings/afferents in the muscles? Describe both

Primary endings (group Ia afferents) are large myelinated sensory axons. rapidly adapt responses to changes in muscle length.

Secondary endings (group II afferents) produce sustained responses to constant muscle lengths.

What do golgi tendon organs do?

signal changes in muscle tension

What do joint receptors do?

Relay finger position for range of motion protection

Dorsal column-medial lemniscal pathway

tactile from body

Trigeminothalamic pathway

tactile from face

Spinocerebellar tract

proprioception

Where is decussation in the dorsal column-medial lemniscal pathway?

At the (caudal) medulla

How does info from the upper body differ from info from the lower body?

Upper body goes up the cuneate tract, lower body goes up the gracille tract. After decussation, both pathways are the same (up the medial lemniscus)

In the dorsal column-medial lemniscal pathway, the information synapses onto which nucleus in the thalamus?

ventral posterior lateral nucleus (VPL)

Where is decussation in the trigeminothalamic pathway?

At the brainstem (pretty much right away)

Which nucleus of the thalamus is involved in the trigeminothalamic pathway?

ventral posterior medial nucleus (VPM)

Describe the pathway for tactile information from the face to the central nervous system

cutaneous receptors, cranial nerve v (trigeminal ganglia), trigeminal brainstem complex, VPM, contralateral cortex

What are first/second/third order neurons?

First order neurons bring sensory information to the spinal cord. Second and third order neurons are the neurons involved in bringing the information to the brain.

What is the difference between cutaneous and proprio-mechanoreceptors?

cutaneous receptors provide information from external stimuli, proprioreceptors provides information from within the body itself.

What are the different characteristics of somatosensory (cutaneous) receptors?

Detect touch. merkel, meissner, pacinian, ruffini

What are the central pathways to process tactile information and proprioception?

dorsal column-medial lemniscal pathway, trigeminothalamic pathway, spinocerebellar tract

What is the role of the thalamus in processing of tactile information and proprioception?

The thalamus helps route information to the correct location in the brain for processing. The VPM is used for facial info, and the VPL is used for body info.

How do we process tactile information in the brain?

Through various pathways to the somatosensory cortex, which is organized into sections for each body part.

What is sensory cortex plasticity?

The ability of the sensory cortex to physically change and adapt.