Sensory receptors and identifying sensory lesions Flashcards
1
Q
Stimulus transduction
A
- The process of a sensory receptor converting a sensory stimulus into an electrical signal
- A sensory receptor is specialized to respond to a type of stimulus, by having a lower threshold for APs to that stimulus (stimulus specificity)
- But other stimuli can still activate it if the stimulus is strong enough
- An increase in a magnitude of a receptor potential (stronger stimulus), causes an increase in frequency of action potentials
2
Q
Rapidly adapting (phasic) receptors
A
- Respond very rapidly at the onset of a stimulus (high frequency), then the frequency decreases over time and the axons my stop responding completely
- In part from rapidly decaying generator potential and part due to accommodation in the nerve
- Are important for indicating when there is a change in the stimulus intensity and designating onset/offset
- Ex: pressure receptors
3
Q
Slowly adapting (tonic) receptors
A
- These maintain their response to a stimulus overtime (tonic info) due to slowly decaying generator potential
- Useful for monitoring important aspects that should always be watched (O2 tension, proprioception, cold/hot and pain)
- Ex: proprioceptors, pain and temp receptors, O2 tension receptors
4
Q
Sensory coding
A
- Converting the sensory stimulus into a recognizable sensation
- Consists of 4 attributes: modality (what it is), location (where it is), intensity, and duration
5
Q
Sensory modality and duration
A
- Sensory modality is based on the particular type of receptor conveying the signal
- This distinguishes a touch signal from a pain signal
- Since each peripheral nerve contains many modalities for that area, a peripheral nerve lesion will result in the loss of all modalities to that dermatome
- However in a SC lesion (where the various modalities run in 2 separate pathways and somatotopic locations), it is possible to lose only some of the sensory modalities to a particular lesion (either touch/vibration/prop or pain/temp)
- For stimulus duration the most important feature is adaptation (whether the neuron is slow or rapid adapting)
6
Q
Stimulus intensity
A
- Conveyed by the frequency of APs sent to the CNS
- The stronger the stimulus, the higher the frequency
- As a stimulus increases in intensity it tends to spread over a larger area, by activating sensory units in adjacent areas
- This is called recruiting
7
Q
Stimulus location and acuity 1
A
- The degree of acuity (resolution) of a sense is related to the size of the receptive fields of those neurons
- The larger the receptive field, the lower the acuity (and vice versa)
- This is because small receptive fields means more neurons to cover a given area, thus the CNS gets more information about sensory modalities
- Small receptive fields allow for better 2 pt discrimination
8
Q
Stimulus location and acuity 2
A
- On top of this, most receptive fields overlap w/ those from another neuron to give a more precise location
- More neurons means more overlapping and more specificity
- Thus the brain increases resolution and location by decreasing the size of receptor fields and increasing the overlapping of receptor fields
- Location is largely dependent on somatotopic organization
9
Q
Lateral inhibition
A
- Ability of an excited neuron to reduce the activity of its neighbors and thereby sharpen the spatial profile of excitation
- Receptors at the edge of a receptive field are inhibited relative to the receptors in the center of the receptive field
- Lateral inhibition serves to enhance the boundaries of a particular stimulus, enabling accurate and precise localization of a stimulus
- Esp important for touch and vision
10
Q
Lesions of peripheral nerves
A
- Most peripheral nerves are SS, SM, sym/post, therefore lesions to peripheral nerves usually involved both motor and sensory deficits
- These are targeted to a dermatome where the nerve would’ve innervated, and the corresponding muscles
- Can be unilateral or bilateral deficits based on the cause of damage
11
Q
Lesions of the SC
A
- DCML and STT are separated in the SC, so the sensory modalities can be lost separately or together (based on site of lesion)
- Damage to one or both results in loss of sensory info at and below the level of damage
- Damage to DCML results in loss of sensory info to ipsilateral side (crosses at lower medulla)
- Damage to STT results in loss of sensory info to contralateral side, and 1-2 segments below lesion (once it crosses after entering)
- Does not affect sensory to face
- If pt has functioning touch but no pain/temp, damage must be to SC
12
Q
Lesions in the brainstem
A
- Sensory loss from face is characteristic of lesion in pons or midbrain (CN V)
- Brainstem contains long ascending tracts and cranial nerve nuclei, so lesions typically involved sensory and motor loss to body and face
- STT and descending tract of V lie close to each other in medulla, so damage there may cause loss of both (loss of only pain/temp to body and face, contralateral to lesion for body but ipsilateral for face since V has not decussated)
- DCML and STT have already decussated (DMCL in medullar-SC junction), so damage to brainstem results in contralateral sensory loss to body
13
Q
Lesions in the thalamus
A
- Loss of sensation of all modalities to the contralateral side of body and face
- If body has contralateral sensory loss, but face has ipsilateral sensory loss, then it must be before thalamus
- Lesions involving both thalamus and internal capsule (carries motor pathways) leads to loss of sensory and motor on contralateral side of body and face
14
Q
Lesions of the cerebral cortex
A
- Vascular lesions of the primary sensory cortex will result in the contralateral loss of sensory info to body and/or face depending on site of vascular injury (ACA: lower limbs, MCA: upper limbs and face)
- Lesions of parietal lobe will result in contralateral deficits in higher order sensory processing (asterognosis and agraphesthesia)
15
Q
Distal symmetrical polyneuropathy
A
- Bilateral sensory loss distributed to most distal areas (hands and feet), vibration may be earliest to be affected
- Accompanied by weakness, flaccidity (LMN symptoms)
- Common causes: metabolic diseases (diabetes, deficiencies in malnutrition), acute inflammation causing demyelination (guillan-barre), toxins/drugs
