Week 2 Sensory Receptors, Sensory Pathways, Lesions Flashcards
Define stimulus transduction.
The process by which a sensory receptor converts the sensory stimulus into an electrical signal that is carried by sensory axons
What is a graded potential (aka generator potential or receptor potential)?
- changes in ion fluxes across the membrane in varying degrees (not all or none)
- an increase in magnitude of a receptor potential (stimulus energy) causes an increase in frequency of action potentials
Define adaptation or desensitization.
-decrease in frequency of action potentials in a sensory neuron despite maintenance of the stimulus at constant strength
What is the difference between rapidly adapting (phasic) receptors and slowly adapting (tonic) receptors?
- rapidly adapting: response quickly to onset of stimulus then decreases over time or may stop responding. important for indicating change in stimulus. E.g. putting on a sock, then forgetting it is on your feet
- slowly adapting: responses that maintain response to stimulus over time, e.g. receptors in joints/muscles that convey positional info, receptors that measure oxygen tension in blood
What are the 4 attributes coded for by a sensory stimulus?
- modality (what it is)
- touch, temperature, pain receptors - location (where it is)
- related to size of receptive fields of neurons - intensity
- conveyed via freq of action potentials - duration
How do receptive fields of neurons convey the degree of acuity of a particular sense?
- overlapping receptive fields give more precise location (e.g. if recorded by 3 neurons, stimulus is at the intersection of those receptive fields)
- size of receptive field: small on fingertips and large on back (less able to distinguish between two points close together) [2 point discrimination]
- lateral inhibition: ability of excited neuron to reduce activity of it neighbors and sharpen spatial profile of excitation
Where is neuron 1 and its axons for the DC/ML pathway?
Neuron 1 is in DRG
Its axons for C1-T5 are in Fasciculus cuneatus in spinal cord
Its axons for T6-S5 are in Fasciculus gracilis in the spinal cord
Where is neuron 2 and its axons for the DC/ML pathway? Where is neuron 3?
Neuron 2 is the Nucleus cuneatus and Nucleus gracilis in the lower medulla. Its axons take the ML pathway after decussation. Neuron 3 is VPL in the thalamus.
Where is neuron 1 and its axons for the STT pathway?
Neuron 1 is DRG
Its axons doesn’t have a name
Where is neuron 2 and its axons for the STT pathway? Where is neuron 3?
Neuron 2 is nucleus proprius in the dorsal horn where the axons of neuron 1 enter the spinal cord.
Axons of neuron decussate 1-2 segments above nucleus proprius through white commissure and follow STT tract to Neuron 3-VPL in thalamus
Where is neuron 1 and its axons for the TL (trigeminal lemniscus) pathway?
Neuron 1: trigeminal ganglion
Its axons aren’t named
Where is neuron 2 and its axons for the TL pathway? Where is neuron 3?
Neuron 2: Chief/main sensory nucleus of V in mid pons
The axons decussate in the mid pons and follow the TL pathway
Neuron 3: VPM in thalamus
Where is neuron 1 and its axons for the TTT (Trigeminal thalamic tract) pathway?
Neuron 1: trigeminal ganglion
The axons follow spinal tract of V in mid pons-descending to lower medulla
Where is neuron 2 and its axons for the TTT pathway? Where is neuron 3?
Neuron 2: Spinal nucleus of V
Axons follow TTT pathway, decussating in lower medulla and ascending to Neuron 3
Neuron 3: VPM in thalamus
What is the sensory pathway for taste?
Neuron 1: sensory ganglion of VII, IX, and X Neuron 1 axons: solitary tract Neuron 2: nucleus solitarius Neuron 2 axons central tegmental tract Neuron 3: insula, SS1 for tongue
List the four sensory pathways for the head/neck and body and what sensory information they carry.
Body
1. DC/ML: fine touch, proprioception, vibration, and 2 point discrimination
2. STT: temperature and pain
Head/neck
1. TL: fine touch, proprioception, vibration, and 2 point discrimination
2. TTT: temperature and pain
What type of injury does a decreased deep tendon reflex indicate? What type of injury does a increased deep tendon reflex indicate?
decreased reflex: Peripheral injury
increased reflex: CNS injury
Identify the connections that mediate the corneal blink reflex.
- afferent limb: opthalmic nerve (V) innervates cornea and conveys sensation of touch
- sensory nuclei are parts of spinal trigeminal nucleus and chief sensory nucleus that projects laterally to the facial motor nuclei (VII)
- efferent limb: cranial nerve VII to orbicularis oculi muscles that contract to close eye
What are the features of isolated peripheral nerve lesion (mononeuropathy)?
- Sensory: numbness and loss of light touch, pinprick, and vibration sense in a targeted dermatome or area of an individual limb or trunk dermatome
- motor: weakness or loss of limb or digit movements, possible atrophy of muscle. decreased deep tendon reflex
- common causes: contusion, crush, or laceration of a ventral rams of a peripheral nerve
What are the features of distal symmetrical polyneuropathy? [degeneration of distal axons]
- Sensory: bilateral sensory loss-Glove and/or stocking distribution in all modalities. vibration may be earliest sensor modality effected
- motor: muscle wekaness, areflexia, flaccid paralysis
- Common causes: metabolic diseases (diabetes), deficiency syndromes, acute inflammatory disease causing demyelination, chemical toxins (cancer drugs)
What are the features of radiculopathy?
- lesion at spinal nerve at intervertebral foramen
- numbness and pain along course of nerve or dermatome
- causes: compression of nerve root/area of spinal nerve, disc herniation
What are the features of transverse cord lesion?
- Sensory: bilateral loss of all sensory modalities at and below the lesion
- motor: upper motor neuron syndrome bilateral
- common causes: trauma, tumors, multiple sclerosis
What are the features of a hemicord lesion: brown-sequard syndrome?
- Sensory: loss of vibration and proprioception ipsilateral to lesion (DC/ML pathway). Loss of pain and temperature contralateral but slightly below lesion (STT pathway)
- motor: upper motor neuron syndrome ipsilateral to lesion
- common causes: penetrating injury, lateral compression from tumor
What are the features of a central cord syndrome (springomyelia-small lesion)?
- Sensory: segmental loss of pain and temperature bilaterally at the level of lesion. Lose crossing fibers in 1-2 segments (STT)
- motor: only occur with larger lesion that effects the anterior horns of spinal cord
- common causes: central cavitation can be congenital or acquired due to trauma, tumor, etc.
