Somatosensory and Motor Processes Flashcards
dermatomes
two-point threshold sensitivity
basic anatomy and physiology:
Spatial and Temporal Properties of Receptors
- Cutaneous (skin)
- Merkel’s disk, free nerve endings, Meissner’s corpuscle, Pacinian corpuscle, Ruffini’s endings
- Detects touch, temperature, pain, body position
- Proprioceptors: Muscle & joint receptors (along with vestibular input)
mechanoreceptors
- Mechanoreceptors – sensitive to physical distortion
- Mechanical energy: vibrated, pressed, pricked, stroked
- Different receptors differ in preferred frequency
Spatial Properties of Meissner & Pacinian Receptors
- Electrode recordings from nerves emanating from single receptors indicate the spatial sensitivity of the receptor
- Meissner
- Narrow receptive fields
- Lower freq sensitivity - Requires greater pressure
- Pacinian
- Broad receptive fields
- Higher freq sensitivity - Requires less pressure
Temporal and Spatial Responding of Mechanoreceptor Types
Cortico-Spinal Pathways:
Primary afferent axons
- Bipolar cells: one ending is associated with receptor functions; the other ending with transmission of information (axon like functions), cell body located just outside of the cord in the dorsal root ganglion
- Axons enter the spinal cord through the dorsal root
- Axons vary in diameter and may be myelinated or unmyelinated: both factors influencing speed of transmission
Cortico-Spinal Pathways: Primary afferent axons
Bipolar cell
- One ending is associated with receptor functions
- The other ending with transmission of information (axon like functions)
- Cell body located just outside of the cord in the dorsal root ganglion
Primary afferent axons:
Skin-based receptors
- Axons from cutaneous receptors travel in superficial peripheral nerves and diameter-size is designated by Arabic and Greek letters:
- A-beta - largest & myelinated (FAST)
- C -smallest & unmyelinated (SLOW)
Primary afferent axons:
Muscle-based Receptors (proprioceptive)
- Axons from muscle or tendon receptors travel in deep peripheral nerves and diameter-size is designated by Roman numerals:
- Group I- largest & myelinated … (FAST)
- Group IV- smallest & unmyelinated (SLOW)
somatosensory pathways (2)
- Discriminative Touch / Vibration / Sharp Pain
- Fast, large, myelinated fibers
- Pain / Temperature / Diffuse Pressure
- Slow, small, unmyelinated/less myelinated fibers
Route to the Brain:
Medial Lemniscal vs. Spinothalamic Pathways
Medial Lemniscal Pathway:
DISCRIMINATIVE TOUCH SYSTEM
- Light touch & light pressure, vibration, fast (sharp) pain, proprioception
- lemniscus = “ribbon”
- Synapses:
- First synapse of the ascending ipsilateral pathway: Medulla (dorsal column nuclei), crosses & ascends as the medial lemniscus
- Next synapse: Thalamus – ventral posterior nucleus (VP)
- Thalamo-cortical projection to contralateral side of cortex: BA 3-1-2
Spinothalamic Pathway
(aka antereolateral pathway)
- First synapse: in ipsilateral spinal cord, crosses to ascend contralaterally
- Next synapse: Thalamus (Ventral Posterior & intralaminar nucleus)
- Thalamocortical projection to BA 3-1-2
Cranial (head/face) Sensation:
Trigeminal (= 3 + twin) Nerve Pathway
- 3 peripheral nerves
- Trigeminal nerves
- Pons (crosses over)
- Thalamus VP nucleus
- Sensory Cortex
Proprioceptive Input / Spino-cerebellar Tract (body position)
Does not cross and projects ipsilaterally to the cerebellum
somatosensory areas in the cortex
- Primary projection area: Postcentral gyrus (BA 3-1-2)
- Somatotropic organization: distorted “homunculus” from contralateral side of body
- Plasticity of the cortical map
- Secondary areas: Posterior parietal (5 & 7)
- Tertiary zones (BA 39, 40) and motor areas (BA 4 & 6)-input from secondary areas
somatotopic organization
Distorted “homunculus” from contralateral side of body
consequences of cortical damage
- Primary projection area: loss of tactile sensation, loss of proprioception
- Secondary areas: Astereognosia (failure to recognize objects by touch), Asomatognosia (failure of body schema), Disruption of body orientation in extrapersonal space, neglect syndrome
- Tertiary zones: Ideational (plan use of tools), ideomotor (gesture), or motor apraxia; Constructional apraxia (drawing, copying), dressing apraxia, agraphia (writing), acalculia (math), disruption of geographical knowledge, environmental agnosia
- Left parietal deficits: Gerstmann syndrome (dysgraphia, dyscalculia, finger agnosia, L/R confusion) autotopagnosia (inability to name and point to body parts)
