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Descending Motor Systems
N/A
52
Medical
Graduate
03/09/2012

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Term
Alpha Motorneurons
Definition
Innvervate skeletal muscle
Term
Gamma Motorneurons
Definition
Innervate muscle spindles
Term
Beta Motorneurons
Definition

innervate both spindles and skeletal muscle

 

Term
Where are motoneurons located?
Definition
In the ventral horn of the spinal cord (gray matter), dendritic trees expanding out into white matter, zones 8 and 9
Term
How are motorneurons in the spinal cord organized?
Definition
Columnar organization if they innervate the same muscle; the pools can span multiple verterbrae
Term
Where do the motoneurons exit?
Definition
Axons leave the spinal cord at ventral roots, eventually forming a neuromuscular junction on the target muscle.
Term
What is the primary neurotransmitter at the neuromuscular junction?
Definition
Acetylcholine
Term
Synonym for endplate potential?
Definition
Action potential
Term
Describe Excitation-Contraction coupling
Definition
When a neuromuscular junction experiences an electrical stimulus, a muscle is contracted as a response.
Term
What are the purposes of the junctional folds?
Definition

Provide increased surface area to hold nicotinic receptors on the myocyte.

 

Can be diminished to provide less nicotinic receptors, less chance of an endplate potential

 

Term
Describe the contraction process from the Neuromuscular Junction onward
Definition

1. Ach released

2. Nicotinic receptors in NMJ bind Ach, causing Na+ influx, Action Potential

3. Action Potential Propagates to transverse tubule, where Ca+ channels are activated

4. The depolarization causes Ryanodine receptors in sarcoplasmic reticulum to release Ca+ into the cytoplasm of the myocyte

5. Ca+ binds troponin, uncovering myosin binding sites

6. Myosin hydrolyzes ATP to ADP, forms a crossbridge with actin, creates a power stroke

7. Calcium is released, actin relaxes

Term
Describe a Slow Twitch Muscle Fiber
Definition

Aerobic, mitochondria, fatigue resistant

(Type 1 muscle Fibers)

Attatch to Type S motorneurons

Term
Describe a Fast Twitch Muscle Fiber
Definition

Anaerobic, use glycogen, produce lactic acid

(Type 2a and 2b)

Innervated by Type FR and FF motor units

Term
What are the characteristics of a Type S motor unit?
Definition

slow twitch, small force, high resistance to fatigue, innvervate type 1 fibers

 

small cell bodies, easy to depolarize, steady firing rates, postural support (skeletal muscle)

Term
What are the characterstics of a Type FR motor unit?
Definition

fast twitch, moderate force, somewhat fatigue resistant, innervate 2a muscle units, which have both aerobic and anerobic mechanisms

 

Fast powerful movements, Large Cell bodies and arbors, Depolarize slowly, fast firing rates

Term
What are the characteristics of a Type FF motor unit?
Definition

fast twitch, highest force, fatigues easily, innervates type 2b muscle fibers, which have anerobic mechanisms mainly.

 

Fast powerful movements, Large Cell bodies and arbors, Depolarize slowly, fast firing rates

Term
What is Henneman's principle?
Definition
First small motor units are recruited, then larger ones as load increases, and the active units discharge faster the more units become active.
Term
Describe the function of muscle spindles
Definition

Encapsulated fiber bundles that lie parallel to muscle fibers, allow for proprioception of muscle length, as the ends of the spindles move with the stretch of the muscle.

 

Increases firing rate as it stretches.

 

Have both sensor and motor innervation (to adjust spindle length, gamma motoneurons primarily)

Term
What information does a Type 1a muscle spindle carry?
Definition
Rapid changes in muscle length (dynamic) or vibration
Term
What information does a type II muscle spindle afferent carry?
Definition
tonic firing rate about sustained stretch
Term
Describe the Stretch reflex.
Definition

A muscle spindle detects a change (type 1a)

 

Spindle transmits to spinal cord exciting motoneurons of stretched muscle, and also synergistic muscles, but inhibit antagonists 

 

(knock on knee test)

 

 

Term
Describe the function of golgi tendon organs
Definition

encapsulated receptors embedded in tendon-muscle junction, sensitive to forces from muscle contraction against a load

 

innervated by type 1b afferents

Term
Where are most spinal interneurons located?
Definition
intermediate zone of the spinal gray matter
Term
Describe the functions of the interneuron
Definition

Relay sensory input from the periphery

 

Relay and modulate descending brain signals

 

Form networks to control reflexes, patterned behavior, movement

Term
Describe Type 1a Interneurons
Definition
Receive input from descending motor systems (esp. corticospinal), modulate reflex sensitivity by changing its excitability, and can coordinate movements.
Term
What is pre-synaptic inhibition?
Definition
Type 1a interneurons can inhibit each other axon to axon, by degrading the response of action potentials along other interneurons, dampening the response
Term
Describe Renshaw Cells
Definition

Receive excitatory input from collateral motoneurons, and inhibit the same collateral motorneurons (recurrent inhibition)

 

Inhibit the inhibitory interneurons of antagonists, net excitation

 

Feedback loop that keeps muscles from becoming spastic

Term
Describe the Flexion reflex
Definition

Raise leg after stepping on glass

 

nociceptive neuron -> interneuron -> excite flexors on the side with the pain, but excites extensors on the side without the pain

 

Stabilizes the other leg as you raise your foot with a piece of glass.

 

Divergence to enhance neural response

 

 

Term
Describe a 1b interneuron
Definition

Receives input from Golgi tendon organ, joint and skin afferents, and descending brain input (modulates sensitivity)

 

inhibits other motorneurons that innervate the same tendon. Causes relaxation when muscle is relaxed, and excitation when the muscle is moving.

 

Convergent input -> complex behavior

 

r

Term
Why can spinalized cats walk?
Definition
Central pattern generator neurons contain stereotyped movements, and allow each joint to flex at the appropriate time.
Term
What do descending motor systems do?
Definition

Plan and execute voluntary movement

 

Maintain Posture and Balance

 

Sensorimotor integration

Term
What does a lateral lesion create?
Definition
Fine movements are gone, but bulk movements are fine (walking)
Term
What does a medial lesion create?
Definition
Severe postural deficits (no walking, standing), but can do fine movements (grasp food)
Term
What are the lateral motor systems?
Definition
Corticospinal and Rubrospinal, control fine hand and finger movements, however, some overlapping function
Term
What are the medial motor systems?
Definition
Vestibulospinal and Reticulospinal control postural adjustments and proximal muscles, however some overlapping function
Term
What are some common features of motor systems?
Definition
Distributed activity, convergence, divergence, plasticity, somatotopy (rough)
Term
Describe the organization of the corticospinal tract.
Definition
From Primary motor cortex, decussate at the pyramids in the brainstem (90%), or remain uncrossed (10%) down to motorneurons (exiting through ventral horn)
Term
Differences and similarites between M1 and Pre-motor cortex
Definition

M1 -> Execution

Premotor-> planning

 

Both receive sensory inputs, but mostly premotor

 

Both can project to motor neurons, but mostly M1

Term
Name the specific Kinematics M1 controls.
Definition
Force, Movement Velocity, and Joint angles
Term
Are M1 Neurons directionally tuned?
Definition
Yes.
Term
What is the name of the neurons that provide a direct pathway between M1 and a hand muscle? Describe their location and connections. 
Definition

corticomotorneurons

 

Found mostly in primates, make monosynaptic connections with motoneurons, located in layer V of cortex, buried in the central sulcus

Term
What does an increased corticospinal representation provide in function?
Definition
Improved dexterity, it has evolved in parallel with the hand.
Term
Describe the tuning of M1 Neurons
Definition
Could be either broadly tuned, responding to different flexion/extension of digits, or sharply tuned to one digit. They respond to movement, not planning.
Term
How does M1 control the hand?
Definition
Through a distributed network that provide some encoding of specific postures of the hand. Different populations of neurons in M1 control specific hand movements, and some overlap.
Term
Describe the difference between Spike triggered averaging and stimulus triggered averaging.
Definition

the SpikeTA is the average stimulus preceding a spike. Provides an idea of a recptive field.


Stimulus triggered averaging is the average spike waveform.

Term
What role does the red nucleus have in motor control?
Definition

Control goal directed limb movements (reach to grasp). Can alter outputs to compensate for injury.

 

 

Term
Describe the pathway of the vestibulospinal tract.
Definition

The lateral tract arises from the lateral vestibular nucleus, descends ipselaterally.

 

The medial tract arises form the medial vestibular nucleus and descends bilaterally, in the medial longitude fasciulus.

Term
What does the Medial Vestibulospinal tract do? 
Definition
Mediates reflexive head movement (upper body neurons in trunk and neck)
Term
What does the Lateral Vestibulospinal tract do?
Definition
Keeps you standing, through antigravity postures (contracts throughout the entire body)
Term
Describe the Reticulospinal tract pathways.
Definition

Lateral 

From the medulla, descends bilaterally in the ventrolateral region of the spinal cord.

 

Medial

From the caudal pins and dorso-rostral medlla, descends ipsilaterally in the ventomedial region of the spinal cord.

Term
Describe the inputs and outputs of the reticulospinal tract
Definition
Primarily the motor cortex, supplementary cortex and premotor cortex. Also from cerebellum and ascending nociceptive information.
Term

What does the reticulospinal system do?

 

 

Definition

Control voluntary reaching movements, bilateral movements, stabilize posture.

 

reticulospinal neurons can target the same motor neurons as the reticulospinal neurons.

 

They touch many things.

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