Flashcards in Somatosensory I Deck (24)
What kinds of info do somatosensory receptors convey? (4)
What is meant by the intensity coding?
Increased stimulus results in increased amplitude of receptor potentials, which produces increased frequency of action potentials.
Thus, intensity of stimulus may be represented by the frequency of APs.
Describe sensory adaptation and what it communicates to the CNS.
Decrease in neural activity with sustained stimulus.
Tells CNS when the stimulus changes.
What impact do slow vs fast adapting receptors have on the message sent to the brain?
Fast adapting receptors have rapid onset and offset, they communicate that something occurred.
Slow adapting receptors gradually decrease activity and communicate the status of the stimulus, or what is occurring.
What are the different somatic modalities?
Tactile: touch, pain, temperature
Proprioception: joint position, muscle sense, and movement
How are sensory modalities distinguished by peripheral nerves?
Individual sensory neurons respond maximally to one modality.
Modalities can also be distinguished by grossly by conduction velocity.
What are cutaneous receptive fields and how do they contribute to tactile localization?
A cutaneous RF is the area of skin innervatd by the branches of a single sensory neuron. RFs of mechanoreceptors vary in size, so you have different degrees of acuity in different areas of the skin.
It's easier to localize touch in areas of skin that have smaller RFs.
Describe the compound action potential and what it tells you. How is an unmyelinated axon designated in terms of the compound AP?
Recordings of the summed APs measured over time at a single point on a nerve. The APs are coming from different neurons in the same nerve. The farther down the nerve you measure, the more the peaks separate due to different conduction speeds of the axons. This allows you to judge how fast different axons are.
An unmyelinated axon would be more likely a C fiber, which are the slowest.
Which receptors for discriminative touch are slow-adapting and which are fast-adapting?
Slow: Merkel cell, Ruffini corpuscle
Fast: Meissner corpuscle, Pacinian corpuscle, Hair follicle
How are the tactile modalities (other than discriminative touch) sensed?
Free nerve endings, slow adapting, C and A-delta axons.
Temperature: within dermis
Pain: up to stratum granulosum
What are each of the discriminative touch receptors sensitive to? (5)
Merkel: form, texture (fingers scanning a surface)
Ruffini: skin stretch (hand shape and position)
Meissner: skin movement and slip (grip control)
Hair follicle: motion/direction of stimulus
What modalities are transmitted along the dorsal column lemniscal system?
Describe the dorsal column lemniscal system.
First-order neurons are the A-beta sensory neurons from the periphery. They travel as the dorsal columns to the gracile and cuneate nuclei in the mudulla.
Second order neurons begin at the dorsal column nuclei and decussate at the level of the medulla, then travel up to synapse in the ventral posterolateral nucleus of the thalamus in the internal capsule.
Third order neurons convey the information from the thalamus to the primary somatosensory cortex.
What modalities are transmitted along the anterolateral system?
Describe the path of the anterolateral system.
First-order (C and A-delta sensory) neurons synapse onto second-order dorsal horn neurons.
Second-order neurons are located in the dorsal horn. They decussate at the level of the spinal cord to the anterolateral system (in the white matter).
Then the path splits. The neospinothalamic tract projects to the lateral thalamus and then on to the cortex. The paleospinothalamic tract projects to the reticular formation and the medial thalamus, then the cortex and limbic system.
Describe the neospinothalamic and paleospinothalamic tracts.
The neospinothalamic tract does localization of sensation and projects from the anterolateral to the lateral thalamus, where third order lateral thalamic neurons project to the somatosensory cortex.
The paleospinothalamic tract conveys qualitative aspect of pain/temperature to the reticular formation and medial thalamus. Third order neurons project from the medial thalamus to the insula and cingulate gyrus.
What is the reticular formation?
A set of nuclei in the brainstem that govern very fundamental processes for higher organisms. One of the most ancient parts of the brain.
Compare the medial and lateral projections of the anterolateral system third order neurons in terms of location and perception.
Medial projections convey affective qualities of pain and temperature to the insula and cingulate gyrus.
Lateral projections are arranged to provide stimulus localization at the primary somatosensory cortex (postcentral gyrus).
Account for the peculiar symptoms in syringomelia and Brown-Sequard syndromes.
Syringomelia: bilateral loss of pain and temp sensation but not discriminative touch or proprioception from the cervicothoracic area due to a cyst in the cervical grey matter interrupting the anterolateral decussation.
Brown-Sequard: Hemisection of spinal cord cuts dorsal and anterolateral columns on one side, leading to loss of ipsilateral discriminative touch and motor control, and loss of contralateral pain and temperature.
What are dermatomes? How do body and face dermatomes differ?
Area of skin innervated by a single spinal segment.
Body: innervated by spinal nerves
Face: innervated by trigeminal nerve
Describe the anatomy of the trigeminal nerve.
Projects from the pons, each branch passes through different foramen:
Ophthalmic--superior orbital fissure
Cutaneous branches exit through supraorbital, infraorbital, mental, etc.
Describe the divisions of the trigeminal nerve and what areas they innervate.
Each division innervates a cavity + a dermatome
Ophthalmic: orbital cavity + forehead and nose
Maxillary: nasal cavity + cheeks
Mandibular: oral cavity + chin, side of face, ear
What are the trigeminal nuclei, and what modalities do they mediate?
Principal Sensory: discriminative touch from face
Spinal Trigeminal: pain and temp from face
Mesencephalic: proprioception/reflexes from masticators
Motor: Motor to masticators