Movement - 1 Flashcards
What are all movements based on?
All neuronal networks and behaviors arise from feedback circuits.
How many types of movement are there? What are some examples?
- Voluntary → hitting a tennis ball
- Involuntary → reflexes
- Rhythmic → walking, breathing
- Learned → playing the piano
- Habitual → scratching
The neuronal system for movement control can be divided into lower- and higher- level control. What is meant by this?
- Lower level → the spinal cord circuit that consists of a local circuit of neurons and a lower motor neuron pool (that innervate muscles).
- Higher level → upper motor neurons in the brain, located in the primary motor cortex and in the brainstem.
Are there other brain regions that are involved in motor control besides the lower- and upper level of motor control?
Yes.
There are some motor systems like the basal ganglia and cerebellum that innervate the upper level of motor control. And also the cortical brain regions are connected to this pathway.
What is ballistic movement?
Ballistic movement are muscle contractions that exhibit maximum velocities and accelerations over a very short period of time. They exhibit high firing rates, high force production, and very brief contraction times.
What is ballistic control of movement?
This control mechanism consists of a controller (our neurons) and a plant (our muscles). A desired result causes the controller to send motor commands to the plant, which results in an actual result in the real world (e.g. movement of legs).
What is the meaning of noise in the control for movement?
Noise means that some things can influence movement → e.g. change of weight, wind blowing, etc.
How can ballistic control be improved (so that noise doesn’t affect the movement of control as much)?
Ballistic control with parametric adjustment. This means that the body senses the noise around it and feed-forwards this information to the controller, so that the controller adjusts its parameter so that it isn’t influenced by noise anymore.
What’s even better than ballistic control with parametric adjustment?
Ballistic control with parametric adjustment and feedback.
This means that when the body has parametrically adjusted itself to the noise, a certain desired movement is initiated. Only, when the result of this desired movement is still not optimal (e.g. you’ve thrown a ball in a certain direction and the direction of the throw is still not correct), a feedback mechanism goes back to a comparator that sends a message to the controller to adjust its parameters so that the next time you perform the desired movement (e.g. throwing the ball in a certain direction), the movement is optimal.
So to summarize: ballistic control with parametric adjustment and feedback, means that learning (trial and error) is an important component in this.
Why is ballistic control with parametric adjustment and feedback not always optimal?
Because if the noise changes (e.g. when throwing the ball and the direction of the wind blowing changes), you cannot learn from your previous mistakes, since the conditions in the environment have changed.
What is the best option for movement control with trial and error?
Feedback guidance.
Here, the feedback circuit goes directly to the comparator, as well as the desired result. With this, the command actually becomes the error that tells the controller (neurons) to innervate the plant (muscles). It’s like you have a target and based on where the target is, you change the error of where you’re going to and finally, the target is reached/captured.
What is the simplest feedback circuit for movement?
Stretch reflex
What happens during a stretch reflex?
There’s a sensor in the muscle and this sensor sends input of the stretch of the muscle into the dorsal root ganglia of the spinal cord. The dorsal root ganglia are connected to alfa-motor neurons that control the tone of the muscle by increasing in firing-rate.
What is the motor unit?
The motor unit is made up of alfa-motor neurons (lower motor neuron) and all of the skeletal muscle fibers that are innervated by the alfa-motor neurons. The alfa-motor neurons are controlled by upper motor neurons.
There are 3 different types of motor units. By what are these three types distinguished and what three types exist?
By their response to a single AP in the nerve fiber. An action potential causes certain responses in the three different motor units:
- Fast fatigable motor units → exert a high amount of force, but their durability in exerting a maximum amount of force is low.
- Fast fatigue-resistant motor units → exert a medium amount of force, and their durability in exerting a medium amount of force is average.
- Slow motor units → exert a minimum amount of force, but their durability in exerting a low amount of force is very high/long.
What two forms of neuronal encoding are depicted in the picture?
- Rate coding → increased firing rates when we need more force.
- Spatial coding → more units are recruited when we need more force.
Describe the characteristics of alpha-motor neurons.
- Alpha-motor neurons are located in the ventral root/horn of the spinal cord (gray matter), whereas sensory neurons are located in the dorsal root of the spinal cord.
- In the more lateral part of the ventral horn, there are motor units for distal musculature.
- In the more medial part of the ventral horn, there are motor units for proximal musculature.
Why is the spinal cord thicker around the cervical and lumbar spinal cord (with mostly more gray matter)?
In these areas, motor units reside that are connected to hands and legs.
How is the stretch of the muscles sensed?
- The muscle spindle is important for this, it contains intrafusal muscle fibers that innervate the sensory apparatus.
- The intrafusal muscle fibers are innervated by gamma-motor neurons, innervation of these fibers causes contraction of the sensory apparatus itself. The sensory apparatus consists of sensory afferent axons that are wrapped around the intrafusal muscle fibers. These sensory afferent axons are stretch sensitive, so when the muscle spindle stretches/contracts, these axons will fire action potentials that send the signal to the dorsal root of the spinal cord.
- There are group 1a afferent axons that sense contractions in the middle of the muscle spindle and there are group II afferent axons that sense contractions in the bottom of the muscle spindle.
- Extrafusal muscle fibers are located above the muscle spindle and cause contractions when they’re innervated by alfa-motor neurons.
The picture depicts the nomenclature for different motor and afferent nerves. What can be concluded based on this picture?
That the bigger the diameter, the bigger the conduction velocity.
What happens to the muscle when the extrafusal muscle fibers are stimulated by an alfa-motor neuron?
The muscle contracts, but since only the extrafusal muscle fibers are innervated (and not the intrafusal muscle fibers), the contraction of the muscle is a bit crooked. This will cause the sensory afferent axons of the muscle spindle to stop firing action potentials.
What happens to the muscle when the extrafusal muscle fibers are stimulated by alfa-motor neurons and the intrafusal muscle fibers are stimulated by gamma-motor neurons?
The muscle contracts outside and around the muscle spindle (extra- and intrafusal muscle fibers), keeping the muscle spindle tight and in place, which causes the sensory afferent axons to fire action potentials that send it to the dorsal root.
Is it possible to control movement solely by gamma fibers?
Yes. When the gamma-motor neurons are activated, the muscle spindle becomes tighter. This activates the 1a fibers that send the signal to the dorsal root of the spinal cord. The signal is then send back to the alfa-motor neurons, that cause the contraction of the muscle. So you could initiate contraction solely by the acitvation of gamma fibers.
Note: this doesn’t really happen normally, only in some kinds of movements.
Depicted in the picture is the control of movement by feedback guidance. Now that you know what fibers, muscles and neurons are needed for the stretch reflex → think of how the different components of the stretch reflex fit into the feedback guidance and fill this in.
- Desired result → the desired lenght/stretch of the muscle, the information for this is the input of the gamma-motor neurons.
- Actual result → the actual lenght/stretch of the muscle
- The comparator → the muscle spindle
- The error → the 1a and II sensory afferent axons that are connected to the dorsal root of the spinal cord.
- The controller → the alfa-motor neurons that cause the contraction of the plant (→ the muscle)
- The plant → the muscle
So what happens when you are holding an empty cup and the cup is filled up with water?
The load of the cup changes. This causes a length change in the intrafusal muscle fibers of the muscle spindle, which results in the activation of Ia afferent axons. The signal of the Ia afferent axons is sent to the spinal cord where the cell bodies of alfa-motor neurons are located. These alfa-motor neurons are activated and innervate the extrafusal muscle fibers (some fibers are also inhibited, while the antagonist’s muscle is innervated). This all results in increased resistance of the arm and the position of the hand is corrected.
How does alpha/gamma-coactivation result in a force and position command?
- The input for the desired length is relayed into the upper motor neurons, which will result in a force command for the alpha-motor neurons that will innervate the muscles.
- Besides this, the input for the desired length is also relayed to the gamma-motor neurons that innervate the intrafusal muscle fibers in the muscle spindle. This input can be seen as a positional command, where the comparator (i.e. muscle spindle) will send input into the Ia and II afferent axons that relay this signal to the dorsal root of the spinal cord. Here, the signal is spread out to the upper motor and alpha-motor neurons so that these neurons can adjust their firing rate to the desired (positional) lenght.
Note: the command of the gamma-motor neurons can be seen as an assisting command or as a booster.
Besides the muscle spindle, there’s another proprioceptor that senses the muscle force. What other proprioceptor is there? Describe what happens when this proprioceptor senses force on a muscle.
The golgi tendon organ.
This organ resides in the tendons of muscles and senses the force/contraction of the muscle. The golgi tendon organ is composed of multiple Ib afferent axons. These axons terminate on some local interneurons in the spinal cord, where they will control the flexor and extensor muscles in a manner that protects them.
So when muscle is contracted, the shape of the muscle and thus golgi tendon organ changes and the afferent axons will activate. This is important for the protection of muscles.
The spinal cord contains many local circuits. Describe these local circuits.
The spinal contains two types of circuits:
- Long-distance local circuits, like excitatory neurons and interneurons that control alpha-motor neurons. There are also commissural axons, these long axons cross the midline of the spinal cord, which is important for the coordination at the two different sides.
- Short-distance local circuit neurons (see picture).
Together, they coordinate many muscles in different limbs.
What happens when you step on a nail with your right foot so that the withdrawal reflex is initiated?
The cutaneous receptor on the right leg is stimulated by the nail. This receptor sends its information through afferent fibers to the dorsal root of the spinal cord. The afferent fibers synapses on a number of local circuits: it synapses with an inhibitory circuit so that the alpha-motor neurons of the extensor are inhibited and the extensor inactivates and is able to relax (1) and it synapses onto excitatory alpha-motor neurons that are then activated to activate and contract the flexor of the leg (2).
Furthermore, commissural axons are also activated by the cutaneous afferent fibers. These commissural axons synapse on an inhibitory circuit that inactivate the flexor and synapse on an excitatory circuit that activate the extensor muscle. This causes the opposite left leg to extend to support the weight.
Our eyes move all the time and are fundamental for how we perceive the world.
A stabilized image on the retina will fade away from perception. What’s the only way prevent this?
When you attach a tiny little mirror on the eye by gluing it on a contact lense. This way the mirror moves whenever your eye moves. When a light is projected onto the mirror, the light is reflected back onto a screen that sends the image to another set of mirros. Only through this set up, a stabilized picture will stay on the retina.
Describe what muscles reside in the eye.
There are three pairs of extraocular muscles in each eye:
- Lateral and medial rectus muscle → move the eye in the horizontal plane
- Superior and inferior rectus muscle → move the eye in the vertical plane
- Inferior and superior oblique muscle → rotate the eye
What kind of control do saccades have (ballistic, ballistic control with parametric adjustment, feedback guided)?
They have ballistic control.
Describe the ballistic control of a saccade that follows an object from left to right.
When an object moves from left to right, the brain takes some time to analyze where the object has moved to. When the brain has analyzed this, a saccade occurs. But when the saccade is made, the object has already moved more to the right. So your eye will actually go to the previous position of the object, where at that point the objects’ location needs to be analyzed again.
So in order to follow a moving object, there need to be many ballistic controls that follow each other.
Saccades are not entirely based on ballistic control. Why?
There’s an internal model integrated in the ballistic control. This internal model is the model of the plant → model of where the eyes currently are. This model is updated by an efference copy that originates from the motor neurons (controller). The model of the plant then sends the signal to the comparator to control the eye movement with a prediction error → comparing where we send the eye and where we want the eyes to go.
This makes ballistic control more accurate and fast, where you don’t actually have to wait for the actual result to alter the eye movement.
Extraocular muscles are innervated by the lower motor neurons located in three positions of the midbrain and the pons. Three out of 12 cranial nerves are used for eye movement.
What three nerves are this? Also describe where in the brain these nerves come from and what eye muscle they innervate.
- Oculomotor nerve → comes from region in midbrain close to the midline called the ocolomotor nucleus → innervates inferior and superior recti, medial rectus and inferior oblique muscle.
- Trochlear nerve → leaves from part in caudal midbrain called the trochlear nucleus → innervates superior oblique
- Abducens nerve → comes from abducance nucleus in the pons → innervates medial and lateral rectus.
Cranial nerve mnemonic
Ok
What happens when the eyes are moving from medial to lateral side? And what happens when the eyes will go back from the lateral side to the medial site?
When the eyes move from the medial to the lateral side, the abducens neurons fire. When the eyes stay put in a certain position, the abducens neurons will tonically fire to maintain the contraction in the lateral rectus muscle.
When the eyes move more medial again, the abducens neurons will stop firing for a moment.
When a saccade needs to occur in the horizontal plane, the lateral rectus of right eye and the medial rectus of the left eye needs to be activated. This conjunct eye movement is coordinated by the PPRF. How?
The PPRF contains neurons that collateralize in two different streams:
- Into the right abducens nucleus → right abducens nerve → innervates lateral rectus muscle
- Into medial longitudinal fasciculus → left oculomotor nucleus → oculomotor nerve → innervates medial rectus muscle
What upper motor brain region contains neurons that control the PPRF?
The superior colliculus and the frontal eye field (cortical region)
Study picture.
You can see that the superior colliculus not only combines the information from the visual (superficial layer) and motor center (deep layer). But that these neurons from these two layers also synaps into each other, thereby controlling gaze center and the saccade.
