SLP528 Neurology
Guide to neurology for clinicians and students in speech-language pathology
Speech Production
LANGUAGE is a cognitive act while SPEECH is a motor act.
We will now take a look at the motor systems that are involved with producing speech.
We now know that the cortex is superior to the spinal cord, both anatomically and functionally.
The lower in the central nervous system, the more involuntary the act or reflex is (i.e. acts that are not well thought out and don’t need to be further processed).
The higher in the central nervous system, the more voluntary the act is or more cognition is required (i.e. acts that require more thought).
How Motor Activity is Carried Out
All motor activity involves the following:
PLANNING or INITIATING
*Includes the Pre-motor area and Supplementary motor area in the cortex
PROGRAMMING: The direct or pyramidal pathway
COORDINATING and EXECUTING: The primary motor area and pyramidal tracts and extrapyramidal tracts
*Clients with apraxia often experience issues with planning and programming. Click here to hear what apraxia of speech sounds like.
*Clients with dysarthria (muscle weakness) often experience difficulties with coordination and execution. Click here to hear what dysarthria sounds like.
PLANNING
PRE-MOTOR AREA
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Responds to external stimuli
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Voluntary
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Examples:
*You hear music that makes you want to dance
*You feel cold and put your jacket on
SUPPLEMENTARY MOTOR AREA
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Responds to internal stimuli
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Automatic
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Examples:
*You’re tired so you yawn
*Your body feels like giving up during a strenuous workout so you stop moving
Pyramidal Tracts and Extrapyramidal Tracts
These are the neural pathways that put motor action into place and provide feedback to modify those actions.
PYRAMIDAL a.k.a. DIRECT
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This pathway gets its name because they pass through the pyramids in the medulla (part of the brainstem).
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Begin in the primary motor cortex and travel through the spinal cord
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Responsible for putting a motor action in place
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For example: This tract will “tell” you to retract (i.e. spread) your lips!
EXTRAPYRAMIDAL a.k.a. INDIRECT
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This pathway gets its name because it does NOT pass through the pyramids in the medulla.
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Responsible for modifying the action that was projected by the pyramidal tracts
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Will make sure you don’t overshoot (too much force) or undershoot (not enough force)
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For example: This tract will tell you how MUCH lip retraction is necessary! Are you trying to say /i/ (as in seed) or /ɪ/ (as in bid)?
The Basal Ganglia
The basal ganglia is a subcortical structure in the middle of the brain.
It consists of several nuclei, including: the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus.
These nuclei form connections in order for different parts of the brain to communicate with each other.
The Basal Ganglia Circuit
The basal ganglia circuit is also part of the extrapyramidal system. It modifies output from the pyramidal pathway by inhibiting some things and exciting some things. All our actions have to do with increasing or decreasing something. Whether the pathway is excitatory or inhibitory depends on what type of dopamine is released from the substantia nigra.
EXCITATORY PATHWAY
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Main goal is to increase an action!
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Dopamine is released from the substantia nigra, triggering the excitatory pathway.
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The striatum (caudate/putamen) inhibits the globus pallidus internal (GPI).
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The GPI tries to inhibit the thalamus, but since it has already been inhibited, it cannot do its job of inhibiting the thalamus.
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Therefore, the thalamus is able to carry out its job of exciting the motor cortex.
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The cortex will send messages to the body (e.g. Keep doing what you’re doing but do it faster/better!)
INHIBITORY PATHWAY
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Main goal is to decrease an action!
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Dopamine is released from the substantia nigra to the striatum, triggering the inhibitory pathway.
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The striatum does its job of inhibiting the globus pallidus external (GPE).
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The GPE tries to inhibit the subthalamic nucleus, but since it is inhibited it cannot do its job.
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The subthalamic nucleus can now function properly and excite the GPI.
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Because the GPI is excited, it can do its job of inhibiting the thalamus.
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The thalamus, because it is now inhibited, cannot excite the cortex.
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Therefore, motor activity is now inhibited. The cortex will tell the body to slow down or apply less force.
Overshooting and Undershooting
While the basal ganglia circuit seems confusing at first, remember to think about the pathway as increasing or decreasing an action. We can think about it in terms of overshooting and undershooting.
Example of overshooting:
*Aiming to throw a piece of paper in the garbage but throwing it too far so it goes past the garbage can (Too much force!)
Example of undershooting:
*Aiming to throw a piece of paper in the garbage but throwing it too softly so it lands in front of the garbage can (Not enough force!)
If you’re OVERSHOOTING you’ll use the inhibitory pathway to do less.
If you’re UNDERSHOOTING you’ll use the excitatory pathway to do more.
The Cerebellum
The cerebellum is located at the back of the brain. Like the cortex, it is also divided into 2 hemispheres. It is connected to the brainstem by little structures called peduncles. (Think of these as “tree huggers” hugging the brainstem.)
3 lobes of the cerebellum:
1. Anterior
2. Posterior
3. Flocculondular
The cerebellum is responsible for equilibrium (balance), coordination, and proprioception.
Proprioception is the awareness of where parts of your body are without actually looking at them. For example, every day we are able to walk without actually having to look down at our feet. We know where they are because we can sense them.
Cerebellum Fine Tuning
The cerebellar pathway is also considered part of the extrapyramidal system. It helps modify motor activity.
2 ways the cerebellum helps adjust:
Feed forward: Uses sensory information from the environment in order to plan ahead and make decisions.
For example: you’ll ride a bike differently if you’re going up a steep hill versus pedaling on a flat road
Feedback: Detects errors in order to make further adjustments.
For example: while riding up the steep hill you may notice you’re not applying enough force so you’ll pedal harder
DYSKINESIA
What happens when there are issues in the motor movement circuits?
One possible result is dyskinesia, or a movement disorder.
To see an example of a child with dyskinesia: