Sensory Systems: Tactile and Proprioceptive Sensation

Modalities and Nerve Endings- Skin sense modalities

Fine, discriminative touch (2 point discrimination): Require small receptor near surface ( epidermis)

• Merkel (II), Pinkus (hairy; = Merkel), Meissner (II, hairless skin), Ruffini organs (deeper in dermis)

Vibration sense: Used to appreciate textures, tested with tuning fork

• Pacinian (“cut onion”, very fast), Meissner (fast)

Pressure: Slower adaptation

• Pacinian (very fast: good for fast changing but not ideal for slow changing pressure), Merkel (slow)

Specialized Sensory Endings (all have group II nerve fibers)

Hair shaft receptors

Merkel cell receptors

Meissner's corpuscles

Pacinian corpuscles

Ruffini endings

Proprioceptors (muscle and joint senses)

Muscle spindles: muscle stretch (length), primary (Ia) and secondary (II) endings

• primary have larger (faster) axons

Golgi tendon organs: muscle & tendon tension, group Ib

• almost as fast as primary spindles

Joint receptors: position sense, groups Ib, II

• several types

Physiological Properties

Modality-specific - each is tuned to type of mechanical stimulus

Receptive field: defined surface area of skin- can be mapped out on body surface as dermatomes (area of skin where the particular receptor receives stimulus/ differ by receptor and location of skin)

*photoreceptor cells: exact location in visual field

Smaller receptive field = greater sensitivity (fingertips)

Wider receptive field = lower sensitivity (back)

Spinal Cord Segments

Cervical

Thoracic

Lumbar (anterior)

Sacral (posterior)

Adaptation and Transduction

Adaptation determined by structure and electrical properties of ending

• changing stimulus- want fast adaptation

• slow adaptation = “true representation of stimulus”; keeps firing even after stimulation stops

Transduction thought to be by mechanical deformation of ion channel - differs with receptor type

Central Connections (body) (receptor → medulla)

Name of tracts: “medial lemniscus pathway” (medulla) or “dorsal column pathway” (spinal cord”

1o afferents have cell bodies in DRG

First synapse in spinal cord (posterior horn) for reflexes only (different from pain & temperature)

• if reflexes not needed, info will continue tract w/o synapses

1o afferents from body ascend in ipsilateral dorsal columns (gracile MEDIAL(receive sensory info from lower body) and cuneate LATERAL (upper body) fasciculus) to medulla (final destination of axon- first synapse if no reflex)

Central Connections (body) (medulla → VPL)

Synapses in medulla (dorsal column nuclei)

• nucleus gracilis (lower body) and nucleus cuneatus (upper body) different

2nd order fibers cross midline in medulla (sensory decussation) to form medial lemniscus

• lower body (gracile) more ventral in tract

Central Connections (body) (VPL → parietal lobe)

Medial lemniscus ascends to ventral posterior lateral thalamus (VPL)

• in pons, moves dorsally and laterally; lower body is most lateral (rearrange)

Third order fibers from VPL ascend to postcentral gyrus in somatotopic order

• same locations as pain & temperature: leg medial; arm dorsal; face ventral (homonculous)

Tactile and Proprioception Pathways for the Face

Primary afferents (CN V) synapse in principal sensory nucleus (of trigeminal) in pons - cell body in trigeminal ganglion

Local reflexes to facial motor nucleus (e.g., blink - V1 afferent)

Second order fibers cross midline (some ipsilateral), join trigeminothalamic tract

• more dorsal than pain & temperature fibers

Thalamic relay in ventral posterior medial (VPM) nucleus - same as pain & temp

Third order fibers ascend to inferior aspect of postcentral gyrus

Some primary afferents have cell bodies in mesencephalic nucleus (of trigeminal), participate in jaw reflexes

Muscle and Joint Sense Input to Cerebellum

All muscle and joint receptors, some skin receptors (proprioceptors)

• Primary afferent may travel in dorsal column

First synapses in spinal cord (Clarke's column- lower) or medulla (lateral cuneate nucleus- upper)

• Clarke’s: relays info from lower body (leg)

• Lateral cuneate: relays info from upper body (arm)

Destination in ipsilateral cerebellar vermis (most fibers do not cross midline- no decussation)

• Spinocerebellar tract from lower body (leg)

• Cuneocerebellar tract from upper body (arm)

Does not participate in perception: info used for muscular coordination