731 Intro to Somatosensation Processing

mechanoreceptor

a sensory receptor that responds to mechanical stimuli such as touch, pressure, vibration, or stretch

meissner corpuscle

fast-adapting cutaneous mechanoreceptor in glabrous (hairless) skin; most sensitive to light tough and low frequency vibration; critical for discriminative touch

Merkel disc

slow-adapting cutaneous mechanoreceptor in the epidermins; responds to sustained pressure and fine spatial detail; important for two-point discrimination

Pacinian corpuscle

fast-adapting deep mechanoreceptor sensitive to high frequency vibration; activated by a vibrating tuning fork over a bony prominence

Ruffini ending

slow-adapting receptor in the deep dermis and joint capsules; respond to skin stretch and joint displacement; contributes to proprioception

muscle spindle

intrafusal proprioceptor within the muscle belly arranged in parallel with extrafusal fibers; detects muscle length and rate of stretch via Ia and II afferents; sensitivity regulated by gamma motor neurons

golgi tendon organ (GTO)

proprioceptor at the musculotendinous junction arranged in series with the muscle; detects tension via Ib afferents; mediates autogenic inhibition to protect the musculotendinous joint

1a afferent

large myelinated sensory fiber from the primary ending of the muscle spindle; carries velocity-senstive stretch information; the fastest-conducting sensory fiber in the body

dorsal column medial leminiscal pathway

ascending sensory pathway for fine touch, vibration, proprioception, and two point discrimination; ascends ipsilaterally in the dorsal columns and crosses the midline in the medulla

decussation

the crossing of nerve fibers from one side of the nervous system to the other; occurs at different levels for the DCML (medulla) vs spinothalamic (spinal cord) pathways

fasciculus cuneatus

lateral portion of the dorsal columns; carries somatosensory information from the upper extremities and trunk above T6

fasiculus gracilis

medial portion of the dorsal columns; carries somatosensory information from the lower extremities and trunk below T6

steregnosis

inability to identify objects by touch alone; results from lesions in the somatosensory cortex, particularly area 2

two-point discrimination

clinical measure of sensory resolution reflecting receptor density and cortical representation; fingertips 2-3mm threshold & lower back 40-70mm threshold

cortical plasticity

the ability of the cortex to recognize its functional maps in response to experience, injury, or rehabilitation; underlies sensory retraining after nerve injury and phantom limb phenomena

primary sensory areas

regions in the cerebral cortes dedicated to processing sensory inputs from various modalities, including touch (somatosensory), vision (visual), and hearing (auditory)

thalamus

a deep brain structure that acts as a relay station for sensory signals to the cerebral cortex, also plays a crucial role in controlling attention and regulating consciousness

brainstem

the part of the brain connecting the cerebrum with the spinal cord and containing important centers for autonomic functions (HR, RR); it also helps relay sensory info from the head and neck

somatosensory cortex

located in the post central gyrus of the parietal lobe, it is the main sensory receptive area for the sense of touch

sensory homunculus

a representation of how sensory inputs from different body parts are mapped to specific areas within the somatosensory cortex

primary motor cortex (M1)

a region in the frontal lobe of the brain responsible for controlling voluntary movements. neurons in this area send signals to activate muscles throughout the body

premotor cortex

situated anterior to the primary motor cortex, the area is involved in planning and coordinating complex movements, integrating information from the sensory systems and other parts of the brain

supplementary motor area (SMA)

located in the frontal lobe, this structure plays a crucial role in the preparation and sequencing of complex movements, particularly those that are internally generated

motor homunculus

a visual representation of the anatomical divisions of the primary motor cortex, where different body parts are represented in specific areas based on the region’s level of motor control

primary somatosensory cortex

located in the post central gyrus of the parietal lobes, this structure is responsible for processing tactile information from the skin and muscles

secondary somatosensory areas

these areas are involved in complex functions such as object recognition, spatial awareness, and attention

association areas

located throughout the cerebral cortex, these areas integrate different types of sensory information, allowing for complex perception, interpretation, and interaction with the environment

what can a region in a sensory homunculus with dense receptor packing resolve with two point discrimination

can resolve separate touch stimuli only 2-3mm apart

what can a region in a sensory homunculus with less dense receptor packing resolve with two point discrimination

typically seen in the back, can resolve separation of 40-70mm before two points can be distinguished

secondary somatosensory cortex (S2)

receives input from both ipsilateral and contralateral body and plays a role in tactile object recognition, learning new tactile tasks, and integration of somatosensory information with memory

posterior parietal cortex

integrates somatosensory input with visual and vestibular information, contributing to body schema, spatial awareness, and planing of reaching and grasping movements

basal ganglia

embedded deep within the cerebral hemispheres, plays a significant role in movement regulation and facilitate the initiation of movements and help in the automation of routine behaviors through their connections with the motor cortex and thalamus

cerebellum

at the back of the brain, is responsible foe fine-tuning motor commands, ensuring precision and coordination

alpha motor neurons

neurons that drive contraction

joint receptors

found in joint capsules and contribute to joint position sense, particularly at end range positions

AB fibers

these are the longest fibers that carry vibration and proprioception

infancy development of somatosensory system

early development includes rapid refinement of tactile and proprioceptive processing

childhood development of somatosensory system

sensory discrimination and integration become more sophisticated

adolescence development of somatosensory system

rapid physical growth and hormonal changes can temporarily influence body awareness and sensory processing

adulthood development of somatosensory system

maintaining somatosensory function becomes increasingly important over time

what are the motor and descending (efferent) pathways

• pyrimidal tract

• extrapyrimidal tract

what can be found in the pyrimidal tracts

• lateral corticospinal tract

• anterior corticospinal tract

what can be found in the extrapyrimidal tract

• rubrospinal tract

• reticulospinal tract

• olivospinal tract

• vestibulospinal tract

what are the sensory and ascending (afferent) pathways

• dorsal column medial leminiscus system

• spinocerebral tracts

• anterolateral system

what is found in the dorsal column medial leminiscus system

• gracile fasciculus (sensory info from LE)

• cutane fasciculus (sensory info from UE)

what tracts are found in the spinocerebellar tracts

• posterior spinocerebellar tract

• anterior spinocerebellar tract

what tracts are funf in the anterolateral (spinothalamic) system

• lateral spinothalamic tract

• anterior spinothalamic tract

bullet point list of DCML

• fine touch

• vibration

• proprioception

• two point discrimination

• ascends ipsilaterally

• decussates in the medulla

bullet point list of anterolateral (spinothalamic) system

• pain

• temperature

• crude touch

• synapses early in dorsal horn

• crosses within 1-2 spinal segments

• ascends contralaterally in anterolateral tract