Exam 5: Upper motor neurons Flashcards
1
Q
what muscle are engaged in the stretch reflex?
A
quadriceps/hamstring and biceps/triceps
2
Q
what is the purpose of the stretch reflex?
A
negative feedback loop to keep the muscle length at a desired value
3
Q
what is the path from afferent to efferent in the stretch reflex? (2)
A
- Direct monosynaptic excitatory path from afferent to efferent of the same muscle
- Disynaptic inhibitory path from afferent to efferent motor neuron of the antagonistic muscle
4
Q
where are the lesions affecting the stretch reflex and what do they demonstrate? (2)
A
- Knee jerk reflex also a diagnostic for L2-L4 spinal nerve lesions
- Elbow jerk reflex is a diagnostic for C5C6 lesions
5
Q
how does the elbow jerk reflex work with addition of weight?
A
- when there is a passive stretch in the elbow there is descending facilitation and inhibition (from cerebellum on gamma motor neuron) that goes to the alpha motor neuron
- the alpha Moto neuron works on the upper arm muscles to hold a cup up with the hand
- when water is added to the cup there is a disturbance and the spindle receptor increase discharge to act on the alpha neuron and interneuron to contract the bicep (flexor) and relax the tricep (extensor) to adjust weight bearing
- (passive stretch of biceps leading to active tension in the biceps (flexor) and inhibition of the triceps (extensor))
6
Q
what is the general bicep/tricep stretch reflex pathway with a disturbance
A
- a disturbance of weight is on the hand
- the stretch muscle and spindle activate Ia and II sensory spindle neurons
- these activate inhibitory interneurons (inhibit alpha motor on flexor muscles), ascending circuits, and activate alpha motor neurons in flexor (flexor) muscle to contract
- the flexor (bicep) alpha motor neuron also activates ascending circuits which activate the cerebellum
- the cerebellum activates descending circuits and the gamma motor neuron
- the gamma motor neuron shortens the muscle spindle
7
Q
what is the purpose of the gamma motor neuron in the bicep/tricep reflex?
A
provides muscle tension and recalibration of muscle spindle to keep particular muscle tension and control
- the stretch reflex circuit is a negative feedback for stretch
8
Q
which afferents are used in the stretch reflex?
A
1a
9
Q
how many synapses are used in the general circuit for stretch reflex?
A
- 1 synapse for the flexor (bicep) muscle
- 2 synapses for the extensor (tricep) muscle
10
Q
contraction reflex
A
contraction or muscle tension limiter mechanism
11
Q
what is the purpose of the contraction reflex?
A
the GTO won’t let you pick up stuff that is too heavy ⇒ limits muscle tension
12
Q
what are the properties of the contraction reflex? (2)
A
- the GTO senses active contraction of the muscle (insensitive to stretch)
- Blocks the alpha motor neuron to that muscle ⇒ disynaptic inhibitory path from afferent to efferent of the same muscle
13
Q
what afferents are used in the contraction reflex?
A
1b Golgi tendon organ
14
Q
how many synapses are there in the contraction reflex?
A
2 synapses for each muscle path
- goes through a local inhibitory neuron to the bicep and a local excitatory neuron connected to the tricep
15
Q
what is the pathway for the contraction reflex?
A
- contraction of the pipe (flexor)
- pulling on the GTO and activation of 1b sensory neurons
1. activate excitatory interneuron in spinal cord => activate alpha motor neuron => contract extensor muscle
2. activate inhibitory interneuron in spinal cord => inhibit alpha motor neuron => relax flexor muscle
16
Q
is the direct connection excitatory or inhibitory for the contraction reflex?
A
excitatory;
- vs. indirect excitation vs. inhibition (contracted muscle)
17
Q
what are the properties of the Golgi tendon organ? (2)
A
- Senses/regulates muscle contraction
- Relatively insensitive to stretch
18
Q
what type of feedback does the GTO give?
A
Functions in a negative feedback loop to maintain steady level of force
19
Q
Pain withdrawal reflex
A
recruiting the gamma first pain afferents which plays a role in protecting the system and withdrawing the limb from the painful stimulus
- flexed crossed extension reflex
20
Q
T/F you want to activate the stretch reflex with a painful stimulus?
A
False
- you don’t want to activate stretch receptor
- relax the quad but hamstring needs to be activated
21
Q
how many synapses does the pain withdrawal reflex use?
A
3 spinal synapses on ipsi side and 4 synapses for opposite leg
22
Q
what is the pain withdrawal reflex pathway?
A
- painful stimulus on the foot activates the ADelta neuron
- this synapses on 4 different interneurons and each have another interneuron that synapses again (2 interneurons)
1. on the ipsilateral side there is one more interneuron that gets synapsed on and then each muscle is synapsed on - the extensor muscle is inhibited but the flexor is excited to contract
2. on the other side another interneuron is required to synapse on the muscles - the flexor muscle is inhibited on this side
- the extensor muscle is excited on this side
23
Q
are upper motor neurons required for the pain withdrawal reflex?
A
No => happens automatically
- Lift leg that stepping on the tack
- Shift weight to the other leg
24
Q
what makes it possible to shift weight without getting off balance?
A
Proprioceptive information being continuously sent to the upper motor neurons
25
T/F alpha motor neurons are always excitatory?
True
- alpha motor neurons are always excitatory leading to muscle contraction
26
how does descending motor control work from the upper motor neurons in the cerebral cortex?
- upper motor cerebral cortex neurons become the corticobulbar tract and the corticospinal tract
- corticospinal neurons synapse on lower motor neuron in the lateral ventral horn (lateral white matter)
27
what type of movements come from the motor cortex?
voluntary movements
28
what type of movements are lateral ventral horn neurons responsible for?
distal limb muscle for skilled movements through the hands and legs
29
how does descending motor control work from the upper motor neurons in the brainstem?
- upper motor neurons in the brainstem follow the anterior medial white matter tract
- this tract diverges into LMNs in the medial ventral horn on both sides of the body
30
what type of movement comes from the brainstem?
body position/posture and eye/head orientation
31
where does the corticospinal tract cross?
the medulla
32
T/F there is a 4th layer in the motor cortex?
No
33
Betz cells
large cells in the 5th layer of M1
- direct monosynaptic connection to alpha motor neurons (only place this happens)
34
properties of pyramidal multipolar neurons (betz cells)? (3)
- Largest neurons in the cortex
- Excitatory (glutamatergic)
- Betz go to limbs only
35
upper motor neurons purpose; locations?
planning and precise control of complex voluntary movement ⇒ not micromanaging movement control
- Premotor cortex
- motor cortex
36
what is the corticospinal tract pathway?
- starts in the cortex and goes through the internal capsule
- descends into the midbrain near the cerebral peduncle
- goes through the middle pons next to the pontine fiber bundles
- goes through the pyramid of the middle medulla
- crosses at the pyramidal decussation in the caudal medulla for the lateral corticospinal tract (the ventral tracts stays on the same side and the medial of the other side
- the ventral tract goes to more inferior segments and the lateral tract synapses in the ventral horn and goes to skeletal muscle
37
Corticospinal tract
control of body/limb muscles via descending axons from M1
- 90% of axons (solid line) cross to the contralateral side in pyramidal decussation and form the lateral corticospinal tract
- 10% of axons remain ipsilateral and form the ventral corticospinal tract that is important for control of axial and proximal limb muscles
38
T/F cortical neurons that control the forearm/hand may directly synapse on alpha motor neurons?
True
- Most synapse on local circuit neurons
39
T/f there is a precise homunculus in the M1?
False
- the primary motor cortex is not a mirror image of the primary somatosensory cortex ⇒ no precise homunculus in M1
- Somatotopy of M1 is coarse compared to S1
- Somatotopy is the wrong way to think about the function of M1 ⇒ skin ≠ muscle
40
what regions of the body do the corticobulbar tract vs the corticospinal tract control?
face; upper extremities, trunk, lower extremities
41
what type of movements does the motor cortex have? how does this impact muscle movement?
goal directed movements
- not maps of individual muscles ⇒ why there is no homunculus for muscle by muscle
- You don’t activate muscle in isolation
42
what test did the U of M do on monkeys with direction tuning of M1 upper motor neurons?
The motor cortex can only solve the problem of the direction of movement by a population code of neurons => there is a tuning curve showing that individual neurons cannot encode all directions but a particular directions
- ex: If a monkey moves a joystick in particular directions around the clock, firing is higher around the time of movement for 180 degrees but on the opposite side at 0 degrees the neurons reduce signaling
43
how is the M1 direction tuning curve made?
different neurons in different regions of the body are tested for directionally selective responses => have inhibited firing in the opposite directions
- Individual neuron responses are fairly broad but the mean population neural activity matches the monkeys behavior ⇒ a local region of neurons in the motor cortex fire in a general direction which match behavior on average
- creates a bell curve look for each neuron
44
T/F all motor neurons are directionally selective?
False
- not all motor neurons are direction selective ⇒ different categories
45
how does stimulating different regions of neurons work to cause different movement effects?
a couple of local neurons get stimulated which leads to a region of movement
- these are not single muscle contractions but goal directed movements
- More of a microscopic scale ⇒ could be layers 2,3, or 5
46
what are different types of UMNs movement for monkeys? (7)
- hand to mouth
- defensive
- central space/manipulation
- reach
- other outward arm movement
- climbing/leaping
- no movement
47
Brain machine interface
use motor cortical neuron activity to drive artificial robot limbs and exoskeletons
- Companies do this if there is muscle paralysis or cut in a spinal cord ⇒ some level of skilled movement
- You can bypass the system in some cases
48
Premotor cortex
reciprocal connections with the motor cortex
- Motor planning (thinking) comes from lateral and medial areas
49
lateral premotor cortex
guided by sensory info and social communication including speech
- Has mirror motor neurons and a given neuron fires upon self motor action and observation of similar motor movements made by other individuals
50
medial premotor cortex
self initiated unrelated to sensory stimulation
51
what do mirror neurons respond to?
voluntary movement and to seeing someone else do the same movement
52
what happens when monkeys perform a movement vs see their handler do the same thing with their hand? pliers?
- If a monkey does the task and then it watches its handler do the task it will fire action potentials in a similar way ⇒ its less than when it just sees the handler do it rather than doing it by itself
- The neuron will stay silent if the person picks up something with a pliers rather than fingers
53
what are symptoms of UMN syndromes? (5)
- Muscle weakness
- Mild or no muscle atrophy
- Initial period of spinal shock followed by hyperactive reflexes
- Initial period of spinal shock followed by spasticity
- Loss of voluntary movement ⇒ gross motor movement may be relatively normal
54
what does spasticity result in? (3)
- Increased tone
- Clonus ⇒ oscillatory contract/reflex
- Decerebrate rigidity
55
what are symptoms of LMN syndromes? (6)
- Weakness or paralysis, decreased muscle tone (hypotonia) due to decreased gamma motor neuron regulation
- Severe atrophy ⇒ after fasciculations
- Hypoactive superficial and deep reflexes ⇒ stimulation of skin vs tap knee
- Initial sign and symptoms persist
- Regional distribution of symptoms corresponds to site of damage
- Impairment of reflexive and gross and or fine voluntary movements
56
babinski sign
if someone flicks the bottom of a baby's foot then their toes will fan up
- because upper and lower motor neurons are not connected in newborn babies
57
is babinski sign an upper or lower motor syndrome?
upper
- If there is a lesion in the upper motor neurons for adults then they will respond by fanning the toes like a baby does
58
end card
:)
