Flashcards in Motor Systems Deck (154):
What part of the nervous system innervates smooth muscle?
Autonomic nervous system
What are two types of striatal muscle?
- Contracts rhythmically, even in absence of any innervation
What does autonomic NS innervation of cardiac straital muscle do?
It functions to accelerate or slow down the heart rate.
Each muscle fibre (cell) is innervated by how many axon branches from the CNS?
A single axon branch for each muscle fibre
What is the somatic nervous system?
Muscles and the parts of the nervous system that controls them
What major muscle cause an arm flexion? What are these muscles categorically called?
brachialis, tendons insert into the humerus (upper arm) at one end and into the ulna (forearm) at the other.
The biceps brachii and coracobrachialis also cause flexion at this joint.
These muscles are called flexors of the elbow joint.
What are muscles that work together called?
Flexors and extensors are called _______ to each other
What are axial muscles? What do they do?
Muscles that are responsible for movement of the trunk.
What are proximal muscles? What are they critical for?
Muscles that move the shoulder, elbow, pelvis and knee
AKA girdle muscles
Critical for locomotion
What are distal muscles? What are they specialized for?
Muscles that move the hands, feet and digits
Specialized for manipulation of objects
What neurons directly command muscle contractions? Where are they found?
Somatic motor neurons, AKA lower motor neurons, are found in the ventral horn of the spinal cord. Final common pathway for the control of behaviour.
Where does the ventral horn of the spinal cord appear swollen?
C3-TI (arm) and LI-S3 (leg musculature), this is to accommodate the abundance of motor neurons that control the arms and legs.
How are lower motor neurons distributed in the ventral horn?
Distributed depending on their function. The cells innervating the axial (trunk) are medial to those innervating the distal muscles. Cells innervating flexors are dorsal to those innervating extensors. That is, they make a topographic map of the muscles in which they innervate (eg. the arm).
What are two categories of lower motor neurons of the spinal cord?
Alpha motor neurons
Gamma motor neurons
What are alpha motor neurons?
Directly trigger the generation of force by muscles. One alpha motor neurons and all the muscle fibres it innervates collectively make up the elementary component of motor control. AKA the motor unit. Muscle contraction results from the individual and actions of these motor units.
What is a collection of alpha motor neurons that innervates a single muscle called?
A motor neuron pool
What is the result of varying firing rate of motor neurons?
Because of the high reliability of neuromuscular transmission, the ACh released in response to one presynaptic action potential causes an excitatory postysynaptic potential in the muscle fibre (aka an endplate potential) large enough to trigger one postsynaptic action potential, this causes a twitch. Continue firings causes a sustained contraction. Twitch summations increases the tension in the muscle fibres, and smoothes the contraction. The rate of firing of motor units is therefore one important way the CNS grades muscle contraction.
What are two ways that the CNS grades muscle contractions?
- By varying firing rate of motor neurons
- By recruiting additional synergistic motor units (strength depends on how many muscle fibres are in that unit)
How can quantity to size of motor units affect fine motor control?
Muscles with large numbers of small motor units can be more finely controlled by the CNS. Large motor units tend to control things like the leg and arm muscles, with an innervation ratio of more than 1000 muscle fibres per single alpha motor neuron.
What accounts for greater muscle control under light loads, as apposed to heavy loads?
Motor units are recruited in the order of smallest first, largest last. This orderly recruitment explains why finger control is possible when muscles are under light loads than when they are under greater loads.
Small motor units have small alpha motor neurons and large motor units have large alpha motor neurons. How does this explain orderly recruitment of motor units from small to large?
As a consequency of their geometry, small neurons might be more easily excited by signals descending from the brain. This is called the size principle.
What do alpha motor neurons excite?
What are the three major inputs to alpha motor neurons?
- Dorsal root ganglion cellswith axons that innervate a muscle spindle. (feedback about muscle length)
- Upper motor neurons in the motor cortex and brain stem (Initiation and control of voluntary movement)
- Interneurons in the spinal cord (largest input, excitatory or inhibitory, is part of the circuitry that generates the spinal motor programs)
Why is dark meat dark (eg. dark muscles) and where is it found? What are these types of motor units called?
Dark muscles contain many mitochondria and enzymes for oxidative energy metabolism. Slow to contract but can sustain contraction for a long time.
Typically found in antigravity muscles of the legs and wings of birds.
Called slow motor fibres
What are fast motor fibres?
White meat. Rely mainly on anaerobic metabolism. These fibres contract rapidly and powerfully, but they fatigue rapidly as well. Typical in escape reflexes, eg. jumping muscles in frogs and arm muscles in humans.
Each motor unit only contains muscle fibres of a certain type, which two types of motor unit are these?
Fast motor units (fast fatiguing white fibres)
Slow motor units (slowly fatiguing red fibres)
What are characteristics of alpha motor neurons of fast motor units? Slow?
- Fast motor unit alpha motor neurons are generally bigger with larger diameter, faster conducting axons. They have high frequency firing bursts of action potentials.
Alpha motor neurons of slow motor units are generally smaller with smaller diameter, slower conducting axons. Relatively steady, low frequency activity.
What probably controls whether a muscle fibre is slow or fast? How do we know?
The synaptic connection from the alpha motor neuron. We know this because of the experiment by John Eccles, where neurons that normally innervate fast twitch motor fibres were switched to slow. The slow then became fast. This is a biochemical switch in phenotype. This change may also be induced by changing the firing rate of the motor neuron.
What is ALS? (disease)
Amyotrophic Lateral Sclerosis is a disease that causes muscle weakness, that continues to worsen until death. It is the degeneration of the large alpha motor neurons, the large neurons of the motor cortex are also affected but other neurons in the brain are spared (selective neuron loss)
What is excitation-contraction coupling?
The excitation (EPSP) of a muscle fibre from an alpha motor neuron causes an action potential that triggers the release of calcium from an organelle inside the muscle fibre, which leads to contraction of the fibre. Relaxation occurs when calcium levels are lowered by reuptake into the organelle.
How are muscle fibres formed?
By the fusion of myoblasts which are derived from mesoderm, leaving each cell with more than one nucleus, multiple fusions form a fibre
What is the sarcolemma?
The excitable cell membrane that encolses muscle fibres
What are myofibrils? What surrounds each one?
Cylindrical structures within muscle fibres, which contract in response to an action potential sweeping down the carcolemma. They are surrounded by sarcoplasmic reticulum.
What is sarcoplasmic reticulum? What
An extensive extracellular sac that stores Ca. Action potentials sweepeing along the sarcolemma gain access to the sarcoplasmic reticulum deep inside the fiber by way of a network of tunnels called T tubules.
How does a muscle fibre respond to a action potential?
The action potential sweeps down the sarcolemma. When it arrives at a T tubule membrane, a conformational change in the voltage-sensitive tetrad of calcium channels to open, realeasing calcium intro the tetrad channels and the calcium release channels (in the sarcoplasmic reticulum membrane). The resulting increase of Ca within the cytosol causes the myofibril to contract.
In a nutshell, calcium flows from outside the muscle fibre through the T tubule (in the membrane of the sarcolemma) and into the cytosol of the muscle fibre. Then through the calcium release channel and into the Sarcoplasmic reticulum lumen.
Myofibril vertically divided into two types of filaments? What divides these filaments and what are they?
The Z line divides thick and thin filaments
Thick filament = myosin
Thin filament = actin
What is a sarcomere?
A segment of myofibril composed two Z lines and the myofibril inbetween with three bands of thin filaments and two of thick filament. The functional unit of a muscle fibre.
How does a sarcomere contract?
When thin filaments slide along thick filaments, bringing adjacent Z lines towards one another. This happens by a conformational change that causes the myosin to pivot, causing the thick filament to move with respect to the thin filament.
ATP causes the myosin head to disengage and uncock, so that it can repeat the movement, effectively walking along the actin filament.
Why can't myosin interact with actin when the muscle is at rest?
The protein troponin is covering myosin attachement sites on the actin. Calcium initiates muscle contraction by binding to troponin and exposing the sites where myosin binds to actin
What is the protein that convers myosin binding sites on actin?
What conditions enable contraction to continue? And to stop?
Contraction continues for as long as there is Ca and ATP. It relaxes when the Ca is sequestered by the sarcoplasmic reticulum. The reuptake of calcium requires on the action of calcium pumps and therefore requires ATP
What are the 5 steps of excitation of muscle fibres?
1. An action potential occurs in an alpha motor neuron axon
2/ ACh is released by the axon terminal of the alpha motor neuron at the neuromuscular junction
3. Nicotinic receptor channels in the sarcolemma open and the postsynaptic sarcolemma depolarizes (EPSP)
4. Voltage gated sodium channels open, and an action potential is generated in the muscle fibre, which sweeps down the sarcolemma and into the T tubules
5. Depolarization of the T tubules causes Ca release from the sarcoplasmic reticulum
What are the 6 steps of contraction of muscle fibres (after excitation)?
1. Ca binds to troponin
2. Myosin binding sites on actin are exposed
3. Myosin heads bind to actin
4. Myosin heads pivot
5. Myosin heads disengage at the expense of ATP
6. The cycle continues as long as Ca and ATP are present
What are the 3 steps of relaxation in a muscle fibre after excitation and contraction?
1. As EPSPs end, the sarcolemma and T tubules return to their resting potentials
2. Ca is sequestered by the sarcoplasmic reticulum by and ATP driven pump
3. Myosin binding sites on actin are covered by troponin
Why does rigor mortis occur?
Starving muscles cells of ATP prevents the detachment of th emyosin head and leaves the myosin attachment sites on the actin filaments exposed for binding. The end result is the formation of permanent attachments between the thick and thin filaments.
What causes Duchenne Muscular Dystrophy?
An absence of the mRNA region coding for the protein dystrophin, leading to paralysis. Only affects boys, passed down on X chromosome, paralysis starts in legs.
What is the condition called where released ACh into neuromuscular junctions is far less effective than normal, leading to muscle weakness? What causes this condition?
- This is an autoimmune disease where the body's own antibodies attach nicotinic ACh receptors, interfering with the normal actions of ACh at the neuromuscular junction. This also causes secondary changes in the structure of the neuromuscular junction that also make transmission much less efficient.
What is a good treatment for myasthenia gravis?
Admistering drugs that inhibit the enzyme acetylcholinesterase (AChE). This prevents breakdown of ACh in the synaptic cleft. This doesn't last very long however, because the receptor is quickly desensitized to the extra transmitter.
Another treatment is suppression of the immune system by drugs or removal of the thymus gland.
What is a muscle spindle?
Also called a stretch receptor, consissts of several types of specialized skeletal muscle fibers contained in a fibrous capsule. The middle third of the capsule is swollen, giving the structure the shape for which it is named. In this middle equatorial region, group Ia sensory axons wrap around the muscle fibers of the spindle.
The muscle spindle is an example of a proprioceptor
What type of sensory axons wrap around the muscle fibres of muscle spindles?
group Ia sensory axons
These are the largest and fastest type of axons in the body.
What is proprioception?
Body sense, tells us how our body is positioned and moving in space
Group Ia sensory axons from from muscle spindles go where and innervate what two types of neurons?
Ia axons enter the spinal cord via the dorsal roots, branch repeatedly and form excitatory synapses upon both interneurons and alpha motor neurons of the ventral horns.
A single Ia axon synapses on virtually every alpha motor neuron in the pool innervating the same muscle that contains the spindle.
What is the reflex called where a muscle that is pulls on pulls back (contracts).
A myotatic reflex
- Happens when sensory feedback from the muscle spindles Ia axons increases (when muscle is stretched). and decreased when it is shortened.
When do Ia axons from muscle spindles fire most?
When the muscle is stretched. This causes them to also excite alpha motor neurons, leading to contraction of the muscle. This is called a myotatic reflex.
What happens to muscles if the dorsal roots (sensory information going from muscle) of the muscle are cut?
Eliminated myotatic reflex and caused a loss of muscle tone
What is the name for the pathway involving Ia axon and the alpha motor neurons on which it synapses?
The monosynaptic myotatic reflex arc
- Monosynaptic because only synapse separates the primary sensory input from the motor neuron output. This pathway serves as an antigravity feedback loop.
How does stretching of muscle spindles cause action potentials at the end of Ia axons?
By the stretching of mechanosensitive ion channels (leading to the opening and causing depolarization).
How does the monosynaptic myotatic reflex arc serve as an antigravity feedback loop?
When a muscle is lengthened the muscle spindle induces Ia axons to fire, which excite alpha motor neurons, causing the muscle to contract. This allows the muscle to re-shorten back to its former length, even when it has a weight bearing it down.
What are muscle fibres inside muscle spindles called? Outside?
Inside: intrafusal fibres
outside: extrafusal fibres (form the bulk of the muscle)
What type of motor neurons innvervates extrafusal muscle fibres?
Alpha motor neurons
What type of motor neurons innvervates intrafusal motor fibres?
gamma motor neurons
What do gamma motor neurons do? Eg. what is their significance?
If intrafusal muscle fibres in the muscle spindle were innervated by alpha motor neurons, than the moment that they stop firing, the sensory output from the muscle spindle Ia axons would go dead. The gamma motor neuron (which innervates above and below the noncontractile equatorial region of the intrafusal muscle fibres) activates and prompts the muscle fibres to contract so that the Ia axons stay active.
Alpha motor neuron activation alone (no gamma motor neuron activity) would cause the Ia axons of the muscle spindle to do what?
To go dead, off the air.
What changes the 'set point' of the myotic feedback loop?
Changing the activity of the gama motor neuron by changing the activity of the gamma loop.
What are all 5 components of the gamma loop in order?
1. Gamma motor neuron
2. Intrafusal muscle fibre
3. Ia afferent axon
4. alpha motor neuron
5. extrafusal muscle fibre
What are two sources of control for alpha and gamma motor neurons?
- Both stimulated by descending commands from the brain
- The gamma loop provides additional control of alpha motor neurons and muscle contraction by regulating the set point of the myotatic feedback loop
What proprioceptive 'organ' monitors muscle tension?
Golgi tendon organs
What type of neuron leaves Golgi tendon organs? Where does it go? What type of reflex do they contribute to?
Ib sensory axons, which are slightly smaller than Ia axons.
These enter the spinal cord through the dorsal horn and branch repeatedly before synapsing on interneurons in the ventral horn. Some of these interneurons form inhibitory connections with the alpha motor neurons innervating the same muscle. This is the basis for the reverse myotatic reflex.
What reflex protects the muscle from being overloaded and is important for fine motor control?
The reverse myotatic reflex, modulated by Golgi tendon organs and their Ib axons innervating on inhibitory interneurons (which stop alpha motor neurons from causing further contractions when it may harm the muscle).
This reflex is good for tasks that need a steady but not powerful grip
The actions of Ib from Golgi tendon organs on alpha motor neurons are entirely polysynaptic, meaning?
They are all mediated by intervening spinal interneurons. Indeed most of the input to the alpha motor neurons comes from interneurons of the spinal cord. Spinal interneurons receive synaptic input from primary sensory axons, descending axons from the brain and collaterals of lower motor neuron axons.
What is process involving contraction of flexors and simultaneous relaxation of antagonist muscles?
How is reciprocal inhibition used to overcome myotatic reflex?
When flexion of a joint occurs, antagonist extensor muscles are lengthened, this would notmally activate the myotatic reflex arc, but the descending pathways that activate the alpha motor neurons controlling the flexors also activate interneurons, which inhibit the alpha motor neurons that supply the antagonist muscles.
What is the flexor reflex?
An example of a reflex mediated in part by excitatory interneurons. This is used to withdraw a limb from aversive stimuli. This reflex is slower than the myotatic reflex (more interneurons intervene).
1. The pain axons entering the spinal cord branch profusely and activate interneurons in several different spinal segments.
2. These cells eventually excite the alpha motor neurons that control all the flexor muscles of the affected limb
3. The crossed-extensor reflex occurs, which makes sure that extensor muscles are activated and flexors are inhibited on the opposite side of the body
What is the reflex used to compensate for the extra load imposed by limb withdrawal on the antigravity extensor muscles of the opposite leg? What happens?
- Opposite extensor muscle activated
- Opposite flexor muscle inhibited
Another example of reciprocal inhibition
What are the circuits that give rise to rhythmic motor activity (such as walking) called?
central pattern generators
How can intrinsic pacemaker activity in spinal interneurons (pacemaker neurons) occur with glutamate gated ion channels and calcium activated potassium channels? (8 steps)
1. The membrane depolarizes when glutamate binds to AMPA receptors and allows sodium in
2. Na and Ca flow into the cell through NMDA receptors
3. Ca activates potassium channels
4. Potassium flows out of the cell
5. The membrane hyperpolarizes
6. Ca stops flowing into the cell
7. The potassium channels close
8. The membrane depolarizes and the cycle repeats
What are two possible ways in which neurons create rhythmic motor actions such as walking?
- Pacemaker neurons
- Interconnected pattern-generating circuits (eg. two excitatory neurons (one for extensor motor neuron and one for flexor motor neuron) connected by two self-inhibiting inhibitory interneurons.
What is the function and structures of the highest level of motor control?
Strategy (determine goal)
- Association ares of neocortex, basal ganglia
What is the function and structures of the middle level of motor control?
Tactics (make plan to reach goal)
- Motor cortex, cerebellum
What is the function and structures of the low level of motor control?
Execution (activating motor neurons and adjusting posture)
- Brain stem, spinal cord
What are the two major groups of pathways descending from the brain through the spinal cord that communicate with motor neurons of the spinal cord?
- Lateral column
- Ventromedial column
What are the lateral pathways involved in?
Voluntary movement of the dital musculature.
The lateral pathways are under what type of control?
Direct cortical control
What are the ventromedial pathways involved in?
The control of posture and locomotion
The ventromedial pathways are under what type of control?
Under brain stem control
What are the two pathways in the lateral column?
- Corticospinal tract (most important)
- Rubrospinal tract
What are the 4 pathways in the ventromedial column?
- Tectospinal tract
- Vestibulospinal tract
- Pontine reticulospinal tract
- Medullary reticulospinal tract
Where do most axons from corticospinal tract originate?
The motor cortex (areas 4 and 6).
The others come from the somatosensory part of the parietal lobe.
List the path of the corticospinal tract in order
1. Motor cortex
2. Base of cerebral peduncle
4. Collect in medullary pyramid
5. Runs down ventral surface of medulla
6. Decussates at the pyramidal decussation
7. Lateral column of spinal cord
8. Corticospinal axons terminate in the ventral horn and intermediate grey matter, where motor neurons and interneurons that control distal muscles are.
List the path of the rubrospinal tract in order
1. Originates in red nucleus of the midbrain
2. Decussation in the pons
3. Join corticospinal tract in the lateral column of the spinal cord
What is the major source of input to the red nucleus of the midbrain?
The same region of frontal cortex that contributes to the corticospinal tract. During human evolution, the corticospinal tract took over mostly from the rubrospinal tract.
What is fractionated movement?
Moving different muscles independently of one another.
What is the result of lesioning the lateral pathways?
- Unable to make fractionated movements
- Slower and less accurate voluntary movements
- Posture and things like that are unharmed
Lesion in the corticospinal tract had the same results of above, but gradually recovered. Lesioning the rubrospinal tract prevented this recovery, suggesting that the corticorubrospinal tract takes over if the coricospinal tract is damaged.
Where do the ventromedial pathways originate and where do they terminate?
Originate in the brain stem and terminate among the spinal interneurons controlling the proximal and axial muscles.
What two groups of muscles does the ventromedial column service?
Proximal and axial
- Using sensory information about balance, body position, and the visual environment to reflexively maintain balance and body posture
What group of muscles does the lateral column service?
The distal muscles
What two ventromedial pathways control the posture of the head and neck?
The vestibulospinal tract and the tectospinal tract
What does the vestibulospinal tract do? Where does it originate?
The vestibulospinal tract originates in the vestibular nucleus of the medulla (related to vestibular labyrinth of the inner ear). It controls neck and back muscles by projecting bilaterally down the spinal cord to guide head movements (keeping visual field stable). It also projects ipsilaterally to lumbar spinal cord to maintain an epright and balanced posture facilitating extensor motor neurons of the legs.
Where does the tectospinal tract originate and what does it do?
It originates in the superior colliculus of the midbrain, which receives direct input from the retina. Using other sensory information from the afferent audio/somatosensory axons and the visual cortex, the superior colliculus directs our eyes and head to move so that a stimulus is imaged on the fovea. Basically allows us to respond to a stimuli by moving our attention to it.
Where do the pontine (medial) and medullary (lateral) reticulospinal tracts originate from? What do they do?
Originate from the reticular formation of the brain stem, which runs the length of the brain stem at its core, just under cerebral aqueduct and fourth ventricle.
- The pontine reticulospinal tract engances the antigravity reflexes of the spinal cord. Helps maintain s standing posture by resisting gravity.
- The medullary reticulospinal tract has the opposite effect of the pontine one. It liberates the antigravity muscles from reflex control.
- Activity from both reticulospinal tracts is controlled by descending signals from the cortex, a fine balance is required.
True or false? The control of voluntary movement engages almost all of the neorcortex.
True, including goal making, memory holding of plans, proprioception, tactics, Implementation of the plan etc.
Where is the motor cortex located? (don't just say a Brodmann's area, describe where it is)
Region of the frontal lobe, area 4 is anterior to the central sulcus on the precentral gyrus and area 6 is anterior to area 4.
How did Wilder Penfield demonstrate which areas contitute motor cortex in humans?
Penfield discovered that stimulating area 4 would cause a muscle twitch on the contralateral side of the body. Probing showed that there is a somatotopic organization in the human precentral gyrus much like that seen in the somatosensory areas of the postcentral gyrus.
Area 4 is often referred to as?
The primary motor cortex (M1)
The motor area in primates is located in what gyrus?
What is area 6 primarily responsible for in humans?
Higher motor functions, such as learned skills.
What are the two areas of area 6? What are they responsible for?
- Premotor area (PMA)
- Supplementary motor area (SMA)
These appear to do similar things but to different muscle groups
- SMA sends axons that innervate distal motor units directly
- PMA connects primarily with reticulospinal neurons that innervate proximal motor units
Which one of the premotor area or supplementary motor area innervate distal motor units directly?
The supplementary motor area (SMA)
The prefrontal and parietal cortex both send axons that converge on cortical area 6. Area 6 lies at the junction of what conversion of signal?
From what actions are desired to signals that specify how the actions will be carried out.
When subjects are asked to mentally rehears a movement without actually moving, which cortical areas remain active and which don't (of the two motor cortex areas)?
Area 4 remains active when thinking about a movement, but area 6 is not active until the movement is actually executed
Which cortical area increases its discharge rates about a second before the execution of a hand or wrist movement?
Cortical area 6 (SMA and PMA)
What is the selective inability to perform complex motor acts called?
Being 'ready' to execute a motor action depends on activity in what to lobes?
Parietal and frontal
-As well as brain centres that control alertness and attention levels
In the monkey and 'ready, set, go' experiment, when did premotor area (PMA) neurons of area 6 begin and stop firing?
Began firing at the instruction stimulus (set), continued to fire for as long as the trigger stimulus was present (go) and only stopped firing once the movement had been executed.
- This experiment also showed that PMA neurons are selectively active before movements are initiated, a type of coding takes place.
Where does the major subcortical input ro area 6 arise from? Where is the input to this from?
A nucleus of the dorsal thalamus, called the ventral lateral (VL) nucleus. Input to this part of VL (called VLo) arises from the basal ganglia.
- The basal ganglia in turn, are targets of the cerebral cortex, particularly the frontal, prefrontal and parietal cortex. Thus we have a loop where information cycles from the cortex through the basal ganglia and thalamus (VLo) and then back to the cortex, particularyly the supplementary motor area (SMA)
List the targets of the motor loop from cortex to cortex
What is a function of this loop?
1. Prefrontal cortex
2. Basal ganglia
3. Ventral lateral (VLo) nucleus in thalamus
4. SMA of area 6 of motor cortex.
5. Can go back to prefrontal cortex or to other parts of the cortex/spinal cord.
A function of this loop appears to be the selection and initiation of willed movements
What are the four components of the basal ganglia?
- Striatum = Caudate nucleus + putamen
2. Globus pallidus
3. Subthalamic nucleus
4. Substantia nigra (reciprocally connected, but in midbrain)
What is the target of cortical input to the basal ganglia?
The striatum (caudate nucleus + putamen)
What part of the basal ganglia is the source of output tot he thalamus?
What is the direct path of of the basal ganglia? (list the 5 structures in order)
2. Striatum (putamen and caudate nucleus)
3. Globus pallidus
5. Neocortex (SMA, supplemented motor area)
What is the direct motor loop through the basal ganglia? What are the areas of the brain involved (6) and are their neurons excitatory (+) or inhibitory (-)?
1. Frontal cortex (+)
2. Putamen (-)
3. Globus pallidus (-)
4. VLo (+)
5. SMA (+)
This loop shows that cortical activation of the putamen causes excitation of neurons in the SMA by the VL. Because it can be the frontal cortex that is initially stimulating the putamen, this is a positive feedback loop. This loop may function as a basal ganglia filtering funnel, where a threshold amount of activation of the SMA must be reached.
In a nutshell, the basal ganglia acts to inhibit the thalamus (VLo), it prevents inappropriate behaviours from being expressed.
Which part of the direct motor loop is continuously firing and therefore must be inhibited to ellicit excitation in the SMA?
The globus pallidus, it is inhibitory itself, inhibiting the VL nucleus. So when it is inhibited by putamen neurons, the inhibition on the VL stops and the VL is allowed to excite the SMA.
What is hyperkinesia? How can the basal ganglia cause it?
Hyperkinesia is an excess of movement. It might be a result of decreased inhibition on the thalamus by the basal ganglia.
What is hypokinesia? How might the basal ganglia cause it?
Hypokinesia is a lack of movement. Increased basal ganglia outputs (inhibiting the VLo of the thalamus) may cause this.
A symptom of Parkinson's disease.
What is bradykinesia?
Slowness of movement
What is the symptom of difficulty in initiating willed movement called?
Another term for increased muscle tone?
What are four symptoms of Parkinson's disease?
- Rigidity (increased muscle tone)
- Tremors of the hands and jaw at rest
- Deficiencies of cognition may develop as the disease progresses
What is the physiological basis of Parkinson's disease?
Degeneration of the substantia nigra inputs to the striatum (including Putamen), these are dopaminergic, they normally directly activate cells in the putamen (facilitating the direct motor loop). In essence the depletion of dopamine closes the funne that feeds activity to SMA via the basal ganglia and VLo.
What is the name for abnormal movement (such as in huntington's disease)?
What are the 6 symptoms of Huntington's disease?
- Chorea (most prominant, spontaneous movement)
- Impaired cognitive abilities
- Disorder of personality
What are pathological symptoms of Huntington's disease?
Loss of neurons in caudate nucleus, putamen and globus pallidus. Additional loss in cerebral cortex. Damage to basal ganglia is responsible for motor symptoms of Huntingtons, damage to cortical and other areas is responsible for dementia and impaired cognitive abilities.
Where does the signal for activating lower motor neurons originate in, in the motor cortex? (cortical area and layer)
Cortical area 4 (M1)
Layer V (pyramidal neurons called Betz cells)
What are Betz cells?
Pyramidal neurons in layer V of M1, receive inputs from other cortical areas and the thalamus.
Where does the thalamic input to M1 mainly originate from?
A part of the ventral lateral (VL) nucleus of the thalamus called VLc, which relays information from the cerebellum. Besides projecting directly to the spina cord, layer V pyramidal cells also send axon collaterals to many subcortical site involved in sensorimotor processing, especially the brain stem.
What two properties of movement do layer V pyramidal cells in M1 encode?
Direction and force.
Do layer V M1 pyramidal cells activate a single motor neuron pool, or many?
Many, from groups of different muscles as well.
Layer V M1 pyramidal cells are tuned for what?
Movement, firing before and after a voluntary movement. (Georgopolous discovered this with monkeys, joysticks and light stimulus, as well as recording from these neurons).
How do M1 pyramidal neurons encode movement?
Many neurons are tuned to make a population vector. They fire in different frequencies depending on what the vector of the force is and if they are tuned to that vector. That is that if a movement is 180 degrees, a neuron tuned for this direction will fire very strongle, and one tuned for 100 degrees will fire less strongly (but still fire). Each cell represents a single 'vote' for a particular direction of movement, but direction of movement is determined by a summation and averaging of cotes registered by each cell in the population.
In what two ways is the motor map malleable?
It can recruit more neurons to a task requiring more fine motor control (eg. fiddline). It can rearrange to accomadate for lost limbs etc.
What is the condition of uncoordinated and inaccurate movements called?
What is a lesion that will invoke ataxia (inaccurate and uncoordinated movements)?
What is dysnergia?
inability for continuous multijoint movement
What is dysmetric finger movement? What is it a symptom of?
Inability to aim fingers. A symptom of cerebellum lesions or damage
What are 3 symptoms of cerebellar damage?
2. Dysnergia (inability for continuous multijoint movement)
3. Dysmetric finger movement (inability to aim fingers accurately)
Why does alcohol intoxication make you clumsy?
Depression of cerebellar circuits.
What are the stalks that the cerebellum sits on called? Where do they rise from?
peduncles, rise from the pons.
What are the shallow ridges on the cerebellum called?
What are the fisures in the cerebellum (that appear to section it off in the saggital plane) called?
What two cerebellar cortex components work to increase surface area?
folia and lobules.
What cerebellar structure separates the two lateral cerebellar hemispheres? Where does it provide input to?
The vermis. It sends output to the brain stem structures that contribute to the ventromedial descending pathways (control axial muscles).
What do the lateral cerebellar hemispheres send input to?
The VLc of the thalamus, which then projects to area 4 (M1).