Lower Motor control Flashcards
Explain the following picture:
Explain the following diagram (motor structure):
- Pay attention to thickness of the arrows
- Basal Ganglia and Cerebellum are not considered upper motor n. since they don’t influence them directly
Describe the inner structure of the spinal cord? Which horn and columns are relevant for you now?
Explain the research in the picture:
A dye substance was injected to parallel muscles (names in the picture) in an animal model -> substance absorbed by neuromascular junction and transported back to its cell bodies -> slices of the spinal cord
Results:
- For each muscle there is a column of cells distributed in the ventral horn
- organization may NOT overlap - but there could be some order from medial to lateral aspect of the ventral horn
What is the somatotopy of the ventral horn? What white matter projections supply each part (+their general purpose)?
- Medial part = muscles in more proximal positions e.g.trunk
- descending projections connect via the anterior aspect of the spinal cord = posture
- Lateral = distal e.g. hands
- descending to the lateral column (lateral cortico-spinal tract) = skill, behavior
What is a motor unit? How does it work?
Leaving the spinal cord -> spinal nerve supplying a particular muscle divides into multiple branches -> innervates set of muscle fibers within the muscle
Motor unit = alpha motor neuron + muscle fibers
What types of motor units do we have?
Depending on which fibers are innervated (aka diff motor n.):
1. Slow motor unit
- small force, slow
- smallest size (small number of fibers)
2. Fast fatigue-resistant m.u.
- rapid, doesn’t fade as much (will at some point)
3. Fast fatigable
- the largest MU (greatest number of fibers innervated + biochemical specialization)
How do motor units act if we repeatedly stimulate them for a prolonged duration?
Slow
- each contraction takes some time -> builds up until hits constant
Fast fatigue-resistant
- quickly generates the force
Fast fatigable
- greatest amount of force
- very rapid -> BUT it decreases slightly over time
What is the principle that maps MU onto specific movements? How does it work?
To maintain posture - slow MU
- don’t need much force, has to be constant, no changes in velocity
-> more explosive - need for fast fatigable MU (but most be strategic
= Size principle = principle through which we recruit MU of different sizes
What is the myotatic reflex? What organ does the job?
= a system that monitors and maintains the length of a muscle
- Muscle spindle = collection of muscle fibers found within a capsule (= inrafusal muscle fibers)
What types of intrafusal fibers do we have? What is the morphological and function difference?
- Nuclear bag fibers = more boulbus collection of nucleui in the center
- n. fire for changes in muscle length (e.g. suddently stretched)
- Nuclear chain fibers = a single row od nuclei (picture)
- n. fire for stable states (e.g. maintans the stretched version)
What neurons are innervating the intrafusal fibers? How do they differ?
- Sensory axons
- Ia = center, close to nucleui of fibers, wraps around
- II = contractile portion, create flower spray ending (terminals look like petals)- Gamma motor neuron
How does the myotatic (stretch) reflex work (in a clinical setting)?
Clinically we may elicit it with a hammer
-> hammer hits a tendon underneath the knee bone -> stretches extensor muscle -> activation of Ia axons (dynamic stretch regostered by nuclear bag fibers) -> enter dorsal column region (some travels up)
-> EXCITES alpha motor neuron via
monosynaptic connection e.g. glutamite =>
contraction of the extensor
-> INHIBITES alpha motor neuron via interneuron => releases the flexor muscle
How would it work in real life scenerio e.g. someone pours us a beverage?
What is happening in this experiment of gain adjustments? What issue do you see? How can we solve it?
The extrafusal fibers are stimulated -> activates the motor n. -> contraction of the muscle
-> BUT due to contraction muscle spindle becomes loose -> no longer sends out info (becomes insensitive)
=> BUT adding gamma motor n. to the mix fixes the issue (sensitivity restored)
- can be part of the reflex loop or modulated by the descending pathways
Look at the anatomy of the Golgi Tendon Organ
Found in tendons (insensitive to muscle stretches since most of that energy gets absorbed by the muscle fibers)
- Ib afferent n. innervates its nerve endings throughout collagen fibrils
How does the GTO get activated? (up until spine circuitry)
Muscle contracts -> tags the GTO -> deformation of the strech sensitive channels in Ib -> opening for cations -> depolarization -> propagates into the spinal cord
What happens to the GOP info in the spinal cord?
Negative feedback system - helps protect the muscle
GOT sends afferent input -> dorsal root
1. Ib inhibitory interneuron -> makes connections to alpha motor n. of the corresponding muscle -> diminishing the force from the motor n.
e.g. release of flexor muscle
2. Disynaptic excitation of the “opposite muscle” alpha motor n.
e.g. activation of extensor muscle
How do Muscle Spindles and Golgi Tendon Organ work together (what happens during passive muscle stretching X active muscle contraction)
Passive muscle stretching
- force applied -> muscle stretches -> afferent n. is firing like crazy
- Nothings happening in GOT
Active muscle contraction
- activating motor n. -> muscle contraction -> muscle spindle loosens up -> firing lost
- GOT gets stretched -> activation
What are central pattern generators? What are its main elements/features (4)?
= a neuronal system capable of producing a rhythmic movement
- Network of alpha motor n. and excitatory or inhibitory interneurons
- some connections cross the midline, some ascend up
- Descending input e.g. brainstem
- initiates the rhythm or modulate it
- Sensory signals
- sometimes could be removed without affecting the circuit
- Can produce different movement patterns
- e.g. walk, run
Do we always need the descending input?
Experiment: a transection was made in the spinal cord of a cat -> put on a treadmill -> rhythmical activity in extensors and flexors didn’t change
- Pattern is intrinsic to the circuitry, no need for descending input (BUT for us seems to be more important
