66 - Sensory Systems Flashcards
Define proprioceptor
• Proprioceptors provide information about joint angles, muscle length and muscle tension. This information can be integrated by the brain (with vestibular input) to give you a perception of the position of your body in space and the relative position of your limbs.
Define adequate stimulus
Adequate Stimulus Under normal circumstances a specific receptor is affected by only one stimulus modality – this is the adequate stimulus.
Define nociceptor
pain receptors
Define sensory modality
Sensory modalities include five classical senses: vision, hearing, taste, smell and touch; and also pain, temperature, itch, proprioception and vestibular sense
Define receptive field
In the somatosensory system the receptive field of a neuron is the region of tissue (e.g. skin) within which a stimulus can evoke a change in the firing rate of the neuron.
Define graphesthesia
the sense through which figures or numbers drawn on the skin can be recognized
Define agraphesthesia
Loss of graphesthesia is called agraphesthesia
Define stereognosis
ability to recognize objects through touch alone
Define asterognosis
loss of stereognosis is called astereognosis
Be able to compare and contrast a receptor potential and an action potential. What is the relationship of the receptor potential to the action potential?
The receptor potential is a change in the membrane potential produced by a transducer mechanism. It is a local graded potential.
- Most receptor potentials are depolarizations
- If depolarization reaches threshold, generate action potentials
- Rate at which action potentials are generated increases as receptor potential rises higher above threshold.
- If the receptor potential is a hyperpolarization it does not generate an action potential.
How does the brain code for “what” the stimulus is?
The labeled line principle – tells you the sensory modality since there is a chain of connected neurons from the sensory receptor which responds to the stimulus to the brain area that perceives the stimulus
(think sensory modalities)
How do we know “where” the stimulus is coming from?
In the somatosensory system the receptive field of a neuron is the region of tissue (e.g. skin) within which a stimulus can evoke a change in the firing rate of the neuron.
How do we know “duration” of the stimulus?
Maintain continuous signal during stimulus
Have on-signal and off-signal
How does the nervous system code for stimulus strength (intensity)?
Signal increasing stimulus strength by increasing frequency of nerve impulses - frequency code.
Signal increasing stimulus strength by increasing the number of fibers activated - population code.
Describe adaptation of a receptor
Adaptation - When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potentials in the receptor’s sensory nerve decreases over time.
- adaptation is a general property of sensory receptors
- different receptors can have different mechanisms of adaptation
Phasic receptors
Rapidly adapting receptors
AKA rate receptors
Describe phasic receptors
- disadvantage - cannot give continuous signal in response to continuous stimulus
- useful for signaling beginning and end of stimulus
- can signal change in stimulus intensity
- number of action potentials transmitted related to rate at which change is taking place
- extract dynamic information e.g. velocity and acceleration
- knowing rate of change enables the nervous system to make prediction
Example of phasic receptors
Pacinian corpuscles
Meissner’s corpuscles
Tonic receptors
Slowly adapting receptors
Describe tonic receptors
- useful for giving continuous information about stimulus strength and signaling stimulus duration
- not as useful for signaling stimulus duration at low stimulus intensities
- when stimulus intensity low, firing rate low, difficult to decide when stimulus began or ended
Example of tonic receptors
Merkel cell ending or Merkel’s disks
Which category of receptors is often categorized with slowly adapting receptors?
Non-adapting receptors (never completely adapt)
Nociceptors
There are two different classification systems of nerve fibers. What are they?
- Roman number classification system
- Letter classification scheme
Describe the Roman system
Types I, II and III are myelinated, type IV is not.
If we exclude the A gamma fiber to the muscle spindles from the I to IV scheme, we can assign an “average” conduction velocity to each fiber type and the average conduction velocity
Type I
100 meters per second – A alpha
Type II
50 meters per second – A beta
Type III
20 meters per second – A gamma
Type IV
1 meter per second - C
Describe the letter classification scheme
A and B fibers are myelinated. C fibers are unmyelinated
A alpha function
Axon of alpha motor neuron; muscle spindle primary ending (Ia);
Golgi tendon organ afferent (Ib)
A beta function
Muscle spindle secondary ending (II)
And the axons of cutaneous mechanoreceptors
A gamma function
Axon of gamma motor neuron to muscle spindle fibers
A delta function
Fast pain, some temperature
receptors (Group III fibers)
B function
Sympathetic preganglionic axons
C function
Slow pain, some temperature (Group IV)
- and -
Sympathetic , postganglionic axons
Be able to describe how you could measure spatial resolution in the somatosensory system. What is “two-point discrimination”? Which areas of the body show the best discrimination? The worst?
Two point discrimination
- test of tactile acuity
- good discrimination on lips and fingertips
- poor on back and calf of leg
- high density of receptors in areas of good discrimination.
- more cortical tissue devoted to analyzing signals from areas of good discrimination.
Be able to list the receptors involved in proprioception.
Receptors involved in proprioception include:
a. joint receptors b. muscle spindles and Golgi tendon organs c. skin tactile receptors
Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
a. What kinds of information are carried by each system?
Dorsal column
- This system is responsible for fine tactile discrimination (discriminatory touch is only found in dorsal column)
- Vibratory sense (rapid, repeated touch/pressure)
- Proprioception
- There is also a visceral pain pathway in the dorsal column
Anterolateral column
- Responsible for conveying nociceptive and thermal sensations
- Some tactile information
Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
b. Where does information in each system cross the midline?
Dorsal column
- Brain stem
Anterolateral system
- Spinal cord
Be able to answer the following questions about the dorsal column and anterolateral spinal cord pathways.
c. What kinds of deficits are associated with damage to each system?
Dorsal column lesions
- Deficits in tactile discrimination
- Vibratory sense
- Proprioception (Rhomberg test is a test for proprioception
Anterolateral column lesions
- Lack of nociciptive and thermal sensations
- Some tactile information is carried, but a lesion does not destroy ability to sense touch (note that light touch is carried by both anterolateral and dorsal columns)
Be able to give the location of the somatosensory cortex and the general organization of this area. (Example: Which part of the body is represented medially?)
- Each of the four cortical areas in SI contains a separate and complete representation of the contralateral side of the body.
- Foot area always in medial cortex.
- Plasticity and reorganization
Humonculus on slide 22
Be able to predict functional losses associated with damage to different areas of the somatosensory system.
Functions (Do not have to know what each subarea does, just given as examples of how information analyzed in cortex)
• A cortical area could respond primarily to cutaneous stimuli (3b and 1), primarily to proprioceptors (3a) or it could respond to both tactile and proprioceptive stimuli (2). Largest portion of input from thalamus goes to Area 3b and lesions of this area in non-human primates cause profound deficits in all forms of tactile sensation mediated by cutaneous mechanoreceptors.
• Graphesthesia and sterognosis
• Somatosensory area II (SII) - neurons in this area show attention modulated responses, (That is, the responses of the neurons in this area to a stimulus depend on whether the subject is paying attention to the stimulus)
Or the answer could be to just look at the homunculus and know general locations
