Sematosensory neurons have …
Receptive field
- Area of the sense organ, (e.g. skin or joints) that when stimulated, will activate a single afferent neuron.
- The center of a receptive field is most sensitive to the stimulus.
The two points of a stimulating compass
- can be discriminated as separate points provided that each point stimulates a separate receptive field
- When both points of the compass activate the same receptive field, the brain can not discriminate the points as separate (b) because the profile of neural activity in the cerebral cortex from the three receptors (R1, R2, R3) contains only a single peak.
The smaller the receptive field….
the greater the sensory acuity.
Ascending somatosensory pathways
- Dorsal column–medial lemniscal system (DCML)
- -discriminative fine touch, proprioception - Anterolateral system (ALS)
- -crude touch, pressure
- -thermal and nociceptive
* *Examples of parallel processing. There is a redundancy of info in both tracts. A lesion in 1 tract does not cause complete loss of discrimination in any of these senses. However, a lesion in both tracts causes severe loss of sensation.
Topographical organization of spatial information in sensory pathways
- neurons are organized to provide a spatial representation of the body and face, i.e. the somatotopic map.
- Neurons with adjacent receptive fields are located nearby each other within a somatosensory structure
- Body and face (i.e., the receptive fields) are represented spatially within nuclei and cortex.
- Different features of an object are processed in parallel in different parts of the somatosensory cortex.
The proportions of the sensory homunculus or the amount of cortex devoted to a given body region is proportional…
to how richly innervated that region is.
Receptive fields in somatosensory cortex
- cortex are not fixed in size and can be altered with experience
- Example: amputation, and motor learning
Define Perception
–Integration of sensory information from many sensory sources (without integration with sensory memories)
Example: see, hear, smell & touch a dog (3 different sensation)
–With integration with sensory memories
Example: this is my dog (or cat)
–Perception NEEDS the cortex
Tests of cortical sensation/perception
- Graphesthesia: ability to identify characters that are written on the skin
- Stereognosis : ability to identify objects placed in the hand with eyes closed
- *Need to bring up stored memories from the cortex for cortical sensation/perception
Body Image/Schema Disorders
- awareness of body parts and their relationship to one another and the environment.
- Associated with parietal and temporal lobe lesions
- Most commonly occurs in patients with right-sided (nondominant) brain lesions with left hemiparesis, especially the right parietal lobe.
Body Image/Schema Disorders-Body Neglect (2)
- Somatognosia or asomatognosia
2. Hemi-asomatognosia
Somatognosia or asomatognosia
(neglect of body parts)
Lack of awareness of the body structure and the relationship of body parts to one another
Hemi-asomatognosia (L neglect)
- Some patients do not even comprehend that affected limbs belong to them
- Happens more with R stroke/head injury for L neglect- L side of the body is neglected/don’t know it is their own hand—loss of perception!
Body Image/Schema Disorders:Hemispatial Neglect
Most common in lesions of the right (nondominant) posterior parietal cortex (sometimes frontal lobe), causing patients to neglect the left side.
**missing part of the picture the pt is trying to draw
Tests of Hemispatial Neglect
-Find letter Ts among Ls. Eye movements during visual search in an individual with left-sided neglect. Were recorded. Red dots-visual fixations. Yellow lines- saccadic eye movements.
Apraxia
- inability to follow a motor command (usually a series of tasks) that is not due to a primary motor deficit or a language impairment. (cannot sequence a series of motor tasks)
- caused by a deficit in higher-order planning or conceptualization of the motor task PLUS an inability to carry out the motor plan.
Apraxia: Common Lesion
Most common in lesion of the left frontal and posterior parietal cortex
Ideomotor apraxia
- easiest to treat bc you can break it down to steps
- Can plan/conceptualize task, but not carry out the motor task
- Pts can make certain gestures/movements spontaneously, but have trouble in making these same gestures/movements if asked to do so.
Ideational apraxia
- hard to treat bc they can’t conceptualize plan
- Cannot come up with the plan or the concept of the task
- Inability to carry out an action, either automatically or on command
How to test for apraxia
asking the patient to do complex tasks, using commands such as “Comb you hair” or “Pretend to strike a match and blow it out” and so on. Patients with apraxia perform awkward movements that only minimally resemble those requested, despite having intact comprehension and an otherwise normal motor exam.
