Chapter 9 - The Somatosensory System Flashcards
The sensations mediated by the somatosensory system are…
touch, pressure, vibration, limb position, heat, cold, itch, pain
Somatosensory subsystems
Tactile (cutaneous: fine touch/vibration/pressure), proprioceptive (position in space), pain/T°/sensual
Where are the somatosensory receptors located?
Skin, muscles, joints
What is the general pathway for somatosensory information?
Periphery —> spinal cord & brainstem —> CNS
What is a pseudounipolar neuron?
Neuron that has 1 extension from its soma: an axon that splits into 2 branches (to PNS and CNS)
What is sensory transduction?
Conversion of stimulus energy into an electrical signal
How does sensory transduction occur in somatosensory afferents?
Somatosensory timulus opens cation channels in afferent nerve endings —> receptor/generator potential —> APs in afferent fiber if reaches threshold
What is the name of the family of mechanotransduction channels?
Piezo1 and Piezo2
What is a mechanoreceptor?
Afferent fiber terminal that detects & transmits touch sensory stimuli (more sensitive ie lower threshold)
What is a free nerve ending?
Afferent fiber that lack specialized receptor cells (less sensitive ie higher threshold: for pain)
What are the characteristics that differentiate afferent types?
- Axon diameter (determines conduction speed)
- Receptive field size determined by arborization size & density of afferent innervation (determines 2-point discrimination, spatial accuracy)
- Speed of adaptation ie temporal dynamics (info about spatial VS temporal attributes)
- Channel & filter properties (determines response to qualities of somatosensory stimulation)
What are Pacinian corpuscles?
Receptor cells for rapidly adapting afferents
What are Nociceptors?
Receptors specialized in pain stimuli
What does Glabrous mean?
Hairless
What does Haptics mean?
Active touching
What is Stereognosis?
Capacity to identify an object by manipulating it with the hand
What are the 4 classes of mechanoreceptive afferents that innervate glabrous skin of hand?
- Meissner corpuscule
- Merkel cell
- Ruffini corpuscule
- Pacinian corpuscule
What are the 3 classes of mechanoreceptive afferents innervating the hair follicles in hairy skin?
- Merkel cell afferents innervating touch domes
- Circumferential endings
- Longitudinal lanceolate endings (rapidly adapting, low-threshold)
What does Dermatome mean?
Geographically constrained zones that may present sensory loss in patients with nerve/spinal cord injury
What characterizes Merkel cell afferents?
- Slowly adapting fibers
- 25% hand mechanosensory afferents
- Info from epidermis
- Merkel cells & afferents express Piezo2 (respectively for static & dynamic aspects of stimuli)
- Highest spatial resolution, precise info about shape & texture
What characterizes Meissner afferents?
- Rapidly adapting fibers
- 40% of hand mechanosensory afferents
- In tips of dermal papillae, closest to skin surface
- Elongated set of flattened lamellar cells
- More sensitive to skin deformation, large receptive fields, less spatial resolution
- Info about textured objects moving across skin (grip control)
What characterizes Pacinian afferents?
- Rapidly adapting
- 10-15% hand mechanosensory afferents
- Deep in dermis/subcutaneous tissue
- Concentric layers of membranes surrounding afferent fiber –> filter
- Low response threshold, but large receptive fields
- Info about vibrations when making & breaking contact (tool use)
What characterizes Ruffini afferents?
- Slowly adapting
- 20% skin mechanosensory afferents
- Elongated, spindle-shaped, capsular specializations
- Deep in the skin & in ligaments/tendons
- Info about digit or limb movements causing skin stretches? (finger position & hand conformation)
What is a Proprioceptor?
Receptor providing information about the position of limbs & body parts in space
What are the 3 classes of proprioceptors?
- Muscle spindles
- Golgi tendon organs
- Joint receptors
What are the characteristics of muscle spindles?
- In skeletal muscles
- 4-8 intrafusal muscle fibers (parallel to extrafusal fibers of muscle) surrounded by capsule of connective tissue
- Works with mechanically-gated ion channels
- Innervated by primary (group Ia afferents: rapidly adapting) & secondary (group II afferents: slowly adapting) endings
- Piezo2 expressed
- Intrafusal fibers also controlled by separate set of motor neurons, which changes sensitivity of spindle afferents
- Density varies in different muscles
What are the characteristics of Golgi tendons?
- Formed by branches of group lb afferents arranged with extrafusal muscle fibers
- Info about tension in muscle
What are the characteristics of Joint receptors
- Contribute less to limb proprioception, but important for judging position of fingers
- Protective role
What is the central pathway for tactile information from body (aka PNS)?
It’s called the Dorsal Column-Medial Lemniscal System
- Cutaneous mechanosensory afferents enter spinal cord through dorsal roots, terminate in dorsal horn
- Ascending branches extend ipsilaterally through dorsal columns aka posterior funiculli
- Both these first-order neurons & projection neurons from dorsal horn laminae project to the dorsal column nuclei (brainstem)
- Second-order neurons in dorsal column nuclei project contralaterally to the VPL in the thalamus via medial lemniscus
- Third-order neurons in VPL project ipsilaterally to the SI & SII in cortex via internal capsule
What are First-order neurons?
Primary sensory neurons that detect a stimulus and transmit a signal to the spinal cord
What are Second-order neurons?
Projection neurons that transmit a received sensory signal to the gateway (thalamus)
What are Third-order neurons?
Neurons that carry a received sensory signal the rest of the way to the sensory region of the cerebral cortex
What is Postsynaptic dorsal column projection?
Indirect projection of somatosensory information to the dorsal column nuclei of the brainstem by second-order neurons from the laminae III, IV, V of the dorsal horn
What is Postsynaptic dorsal column projection?
Indirect projection of somatosensory information to the dorsal column nuclei of the brainstem by second-order neurons from the laminae III, IV, V of the dorsal horn
What is the Gracile tract?
Aka fasciculus gracilis, medial bundle of fibers that conveys ascending information from lower limbs in the dorsal columns of the spinal cord
What is the Cuneate tract?
Aka fasciculus cuneatus, lateral bundle of fibers that conveys descending info from upper limbs, trunk, neck
What is the Gracile nucleus?
Aka nucleus gracilis, subdivision of the dorsal column nuclei where fibers carrying ascending info from the lower limbs synapses
What is the Cuneate nucleus?
Aka nucleus cuneatus, subdivision of the dorsal column nuclei where fibers carrying descending info from the upper limbs, trunk, and neck synapses
What are Internal arcuate fibers?
Axons exiting the forsal column nuclei, towards the somatosensory portion of the thalamus (contralateral)
?What is the Medial lemniscus
Tract formed by the internal arcuate fibers that have crossed the midline
- Ventrally: axons carrying info from lower limbs
- Dorsally: axons carrying info from upper limbs
What does Decussation mean
When fibers cross the midline
What is the Ventral posterior lateral nucleus (VPL)
Portion of the thalamus responsible for somatosensory info
What is the Internal capsule
Axon bundle via which sensory info from the thalamus VPL is projected to SI (postcentral gyrus of the cerebral cortex)
What is the Primary somatosensory cortex
SI, aka postcentral gyrus, processing region in cerebal cortex
What is the Secondary somatosensory cortex
SII, smaller processing region in cerebral cortex (upper bank of lateral sulcus)
Central pathway for tactile information from face
