Neurophysiology (Part 3) Flashcards
Descending motor pathways are divided among what 2 tracts?
pyramidal and extrapyramidal tracts
What are the 2 types of pyramidal tracts?
- corticospinal tract
- corticobulbar tract
Where do the pyramidal tracts descend down to?
They pass through the medullary pyramids and descend directly onto lower motor neurons in the spinal cord
What are the 5 types of extrapyramidal tracts?
- rubrospinal tract
- potine reticulospinal tract
- medullary reticulospinal tract
- lateral vestibulospinal tract
- tectospinal tract
Where does the rubrospinal tract originate from and where does it project to?
It originates in the red nucleus and projects to the motoneurons in the lateral spinal cord
Stimulation of the red nucleus (rubrospinal tract) produces what?
activation of flexor muscles and inhibition of extensor muscles
Where does the pontine reticulospinal tract originate from and where does it project to?
It originates in nuclei of the pons and projects to the ventromedial spinal cord
Stimulation of the pontine reticulospinal tract produces what?
General activation of both flexor and extensor muscles, with its predominant effect on extensors
Where does the medullary reticulospinal tract originate from and where does it project to?
It originates in the medullary reticular formation and projects to motoneurons in the spinal cord
Stimulation of the medullary reticulospinal tract produces what?
General inhibition of both flexor and extensor muscles, with its predominant effect on extensors
Where does the lateral vestibulospinal tract originate from and where does it project to?
It originates in the lateral vestibular nucleus and projects to the ipsilateral motoneurons in the spinal cord
Stimulation of the lateral vestibulospinal tract produces what?
Activation of extensors and inhibition of flexors
Where does the tectospinal tract originate from and where does it project to?
It originates in the superior colliculus and projects to the cervical spine
What is the tectospinal tract involved in?
Control of neck muscles
If a brainstem lesion occurs above the pontine reticular formation and lateral vestibular nucleus, but below the midbrain what will occur and why?
It will cause a dramatic increase in extensor tone (called decerebrate rigidity) because the pontine reticular formation and lateral vestibular nucleus have powerful excitatory effects on extensor muscles
Will lesions above the midbrain cause decerebrate rigidity?
No
What does the cerebellum regulate and play a role in?
movement and posture and plays a role in certain kinds of motor learning
What does the cerebellum help to control?
The rate, range, force, and direction of movements (collectively called synergy)
The cerebellum is connected to the brainstem via what?
3 cerebellar peduncles that contain both afferent and efferent nerve fibers
What are the 3 main divisions of the cerebellum?
- vestibulocerebellum
- spinocerebellum
- pontocerebellum
What is the vestibulocerebellum dominated by and what does it control?
It is dominated by vestibular input and controls balance and eye movements
What is the spinocerebellum dominated by and what does it control?
It is dominated by spinal cord input and controls synergy of movement
What is the pontocerebellum dominated by and what does it control?
It is dominated by cerebral input and controls the planning and initiation of movements
How many layers are there to the cerebellar cortex and how are they described?
3, all of which are described in relaation to its output cells, the Purkinje fibers
List the 3 layers of the cerebellar cortex staring with the innermost layer
- granular layer
- Purkinje cell layer
- molecular layer
What does the granular layer contain?
the granule cells, Golgi II cells, and glomeruli
Mossy fibers from which 2 tracts synapse on dendrites of the granule and Golgi type II cells in the granular layer of the cerebellar cortex?
spinocerebellum and pontocerebellum tracts
What does the Purkinje cell layer contain?
Purkinje cells
Output from the Purkinje cell layer is always ______.
inhibitory
What does the molecular layer contain?
Outer stellate cells, basket cells, dendrites of Purkinje and Golgi II cells, and axons of granule cells
What 2 input systems provide excitatory input to the cerebellar cortex?
- the climbing fiber system
- the mossy fiber system
Excitatory projections from the cerebellar cortex modulates the output of what? How?
These excitatory projections activate secondary circuits, which modulate the output of the cerebellar nuclei via the Purkinje cells
Which of the 2 fiber systems constitutes the majority of the cerebellar input?
the mossy fiber system
Describe the synapses between the climbing fibers and the Purkinje cells
Each Purkinje cell receives input from only one climbing fiber, however a single AP from a climbing fiber can elicit multiple excitatory bursts (complex spikes) in the dendrites of the Purkinje cell
Describe the synapses between the mossy fibers and the Purkinje cells
Mossy fibers project to excitatory granule cells, that ascend to the molecular layer and give rise to parallel fibers that contact the dendrites of many Purkinje cells
In summary one climbing fiber activates __ Purkinje cell and one mossy fibers activates ___ Purkinje cells
one
many
In summary climbing fibers cause ____ spikes that produce multiple action potentials whereas mossy fibers cause _____ spikes that produce single action potentials.
complex
simple
What is the function of cerebellar intenreurons?
to modulate Purkinje cell output
With the exception of the granule cells, all of the cerebellar interneurons are ______.
inhibitory
The only output of the cerebellar cortex is via what?
axons of Purkinje cells
Purkinje cell output is always inhibitory, why?
Because the NT released at these synapses is GABA which is an inhibitory neurotransmitter
What do cerebellar lesions result in?
an abnormality of movement called ataxia
Cerebellar ataxia is a lack of coordination due to what?
errors in rate, range, force, and direction of movement
What are the 5 ways in which ataxia can be exhibited?
- Delayed onset
- Overshoot
- Dysdiadochokinesia
- Intention tremors
- Rebound phenomenon
Describe delayed onset ataxia
Movement or poor execution of the sequence of a movement is delayed causing the movement to appear uncoordinated
Describe overshoot ataxia
A limb may overshoot its target or stop before reaching its target
Describe dysdiadochokinesia ataxia
A person is unable to perform rapid, alternating movements
Describe intention tremors
These are tremors that occur perpendicular to the direction of a voluntary movement, increasing near the end of the movement
Intention tremors are seen in what type of pathology?
Parkinson’s disease
Describe the rebound phenomenon
A person is unable to stop a movement
The basal ganglia are the deep nuclei of the telencephalon, list the 4 nuclei
- Striatum which consists of the caudate nucleus and the putamen
- Globus pallidus
- Amygdala
What are the associated nuclei of the basal ganglia?
- Ventral anterior and ventral lateral nuclei of the thalamus
- Subthalamic nulceus of the diencephalon
- Substantia nigra of the midbrain
What is the major function of the basal ganglia?
to influence the motor cortex via pathways through the thalamus
What are the 2 other functions of the basal ganglia?
To aid in planning and execution of smooth movements, and to also contribute to affective and cognitive functions
What are the 2 pathways into and out of the basal ganglia?
- indirect
- direct
The overall output of the indirect pathway is ______, whereas the overall output of the direct pathway is _____.
inhibitory
excitatory
What are 2 diseases of the basal ganglia?
Parkinson’s disease and Huntington’s disease
Describe Parkinson’s disease
Occurs when cells of the pars compacta of the substantia niagra degenerate, which reduces inhibition via the indirect pathway and reducing excitation via the direct pathway
What are the characteristics of Parkinson’s disease?
- resting tremor
- slowness and delay of movement
- shuffling gait
How is Parkinson’s disease treated?
By replacing dopamine by treatment with L-dopa or administration of dopamine agonists
Describe Huntington’s disease
It is a hereditary disorder caused by destruction of striatal and cortical cholinergic neurons and inhibitory GABAergic neruons
What are the symptom of Huntington’s disease?
choreic (writhing) movements and dementia
Where are voluntary movements directed by?
the motor cortex via descending pathways
What is the function of the motor plan
The motivation and ideas necessary to produce voluntary activity are organized in the cerebral cortex and then transmitted to the supplementary motor and premotor cortices to develop the plan. This plan will identify the specific muscles that need to contract, how much they need to contract, and in what sequence
After the motor plan is developed, where is it transmitted?
to upper motoneurons in the primary motor cortex, which send it through descending pathways to lower motoneurons in the spinal cord
What are the planning and execution stages of the motor plan influenced by?
motor control systems in the cerebellum and basal ganglia
What are the 3 areas of the motor cortex?
- primary motor cortex
- supplementary motor cortex
- premotor cortex
The supplementary motor cortex and premotor cortex are responsible for what?
generating a plan of movement
When is the supplementary motor complex active?
during “mental rehearsal” of a movement
Once the plan of movement is generated is transferred to the primary motor cortex, for what?
execution
Describe what happens as programmed patterns of motoneurons are activated from the primary motor cortex
This activation excites upper motoneurons which transmits the plan to the brain stem and spinal cord, where lower motoneurons are activated and produce coordinated contraction of the appropriate muscles
The primary motor cortex is topographically organized and is described as what?
the motor homunculus
The topographic organization of the primary motor cortex is dramatically illustrated by what type of seizures?
jacksonian seizures
The brain is composed of 80% fluid, most of which is what?
cerebrospinal fluid (CSF)
CSF is formed at a rate of ___ml/day by what cells?
500
by the epithelial cells of the choroid plexus
Describe the path the CSF takes after it is produced by the choroid plexus
It flows into the ventricles and the subarachnoid spaces, which surround the brain and spinal cord
The rate at which CSF is transferred to venous blood is equivalent to what?
the rate of CSF formation
How is CSF sampled?
Using a lumbar puncture in the lumbar cistern
What is the barrier between the cerebral capillary blood and CSF called?
the choroid plexus
What are the 3 layers of the choroid plexus?
- capillary endothelial cells and basement membrane
- neuroglial membrane
- epithelial cells of the choroid plexus
What is the barrier between the cerebral capillary blood and the interstitial fluid of the brain called?
blood-brain barrier
What are the 2 ways in which the blood-brain barrier differs from the analogous barrier in other tissues?
- the junctions between endothelial cells in the brain are so “tight” that few substances can cross between the cells
- only a few substances can pass through the endothelial cells (lipid-soluble substances can; water-soluble substances cannot)
What is CSF formed by?
the epithelial cells of the choroid plexus
What molecules are excluded from CSF?
Protein and cholesterol because of their large molecular size
Which molecules move freely and equilibrate between the CSF and the brain interstitial fluid?
Lipid-soluble substances such as oxygen and carbon dioxide
What substances are present at approximately the same concentration in the CSF and in the brain interstitial fluid?
- sodium
- chloride
- bicarbonate
- osmolarity
What substances are present at higher concentrations in the CSF compared to in the brain interstitial fluid?
- magnesium
- creatinine
What substances are present at higher concentrations in the brain interstitial fluid compared to in the CSF?
- potassium
- calcium
- glucose
- amino acids
- pH
What are the functions of the CSF?
- provide a constant, controlled environment for the brain cells
- to protect the brain from endogenous or exogenous toxins
- prevent escape of local transmitters into the general circulation
Do ionized or nonionized drugs readily penetrate the brain readily and why?
nonionized drugs do because they are lipid-soluble
What things may increase the permeability of the blood-brain barrier and allow substance normally excluded to enter the brain?
inflammation, irradiation, and tumors
What part of the brainstem do the rubrospinal and tectospinal tracts originate?
midbrain
What part of the brainstem does the pontine reticulospinal tract originate?
Pons
What part of the brainstem do the medullary reticulospinal lateral vestibulospinal tracts originate?
medulla
