Principles of Neuroscience Lecture 12 The Lower Motor Neuron Flashcards
Describe the structure and location of the basal ganglia
Sub cortical grey matter in the frontal lobe
Which two regions in the brain are important for initiation of movement?
The basal ganglia and the cerebellum
What is the role of the cerebellum in movement?
Modulation of movement.
There cerebellum is almost completely devoted to motor movement
Where are motor neurons located?
In the spinal cord:
Ventral horn
… Are the final common pathway
Lower motor neurons
Which neurotransmitter do motor neurons use?
Acetyl choline
They are thus cholinergic
What is the relative speed of transmission of action potentials in the motor neurons?
How does this happen?
Transmission is very fast.
The axons are heavily myelinated and very thick
Motor neurons can only … skeletal muscle
Excite
What is the arrangement of motor neurons in the spinal cord?
Longditudinal motor columns
Describe the mapping of motor neurons in the spinal cord
Somatotopic arrangement
What are local circuit neurons?
Interneurons
What types of neurons make up the majority in the spinal cord?
Interneurons
What are commissural axons?
Axons that cross over to the other side of the spinal cord?
… make up the minority of neurons in the spinal cord
Motor neurons
Differentiate between motor units and motor neuron pools
Motor units: the muscle fibres that are innervated by one neuron
Motor pools: the neurons that innervate a single muscle
What is the name of the region at the neuro-muscular junction?
The motor end plates
How do the functions of small and large motor units differ?
Large: big movements that require much force
Axons are long and thick, complex dendrites and axonal branching
Small: fine movements that produce small amounts of force
Increased input resistance
Are large or small motor units more excitable?
Large show decreased excitability
Describe differential motor neuron recruitment
Smallest motor units are recruited first
Largest motor units are recruited last
…% of motor unit recruitment results in …% of maximum muscle force produced (small motor units)
25% recruitment (small motor units) gives 5% of maximum muscle force produced
Describe motor unit recruitment for activities like jumping that require 100% of a muscle’s force
All the motor units in the muscle are recruited. The large motor units are recruited last and contribute a large proportion of the force
Order of recruitment of motor units impinges on … ?
This order enable us to …
Size of the motor unit dictates when it will be activated
This order allows us to small movements first
Of what is the stretch reflex is an example?
A closed circuit pathway
What are the two types of muscle fibre?
How do their structures differ?
Intrafusal and extrafusal
Intrafusal is encased in a capsule of connective tissue
Intrafusal and extramural muscle fibres operate in …?
Parallel
What is the function of muscle spindle?
It is the muscle sensory structure
What is the structure of muscle spindle?
Intrafusal muscle fibres: chain and bag
Ia afferents
Gamma motor neurons
What are the two types of intrafusal muscle fibres?
How do they differ in function?
Nuclear Chain: responds to constant stretch: Slowly Adapting
Nuclear Bag: responds to change in length: Fast Adapting
Describe the Innervation of muscle spindle.
Muscle spindle is innervated by gamma motor neurons
What is the function of alpha-afferent in muscle?
Innervates the muscle fibres (extrafusal), causing the muscle to contract
What is the function of gamma-motor neurons in muscle spindle?
The gamma motor neurons makes the muscle spindle contract so that the spindle is tense
What are the names for the two sensory structures in muscle tissue?
Where are they both located?
The Muscle Spindle: within the muscle fibres
Golgi tendon bodies: mesh of connective tissue between the muscle and the tendon
Describe the steps involved in the stretch reflex
- Muscle becomes stretch (eg. By hammer hitting knee)
- The receptor cells become depolarised (mechanical opening of the channels)
- Nerve transmission to the spinal cord via the Ia afferents
- Mono-synaptic: Ia afferents connect to the alpha motor neurons
- Alpha motor neurons innervate the same muscle (homonymous)
- Muscle contracts to counter the stretch of the muscle
What is the function of the Golgi Tendon Organ?
This is a sensory structure that detects the amount of force produced by the muscle
What is the structure of the Golgi Tendon Organ?
The afferent nerve endings are embedded in connective tissue.
The Golgi tendon organ is found at the junction between the muscle and the tendon
G.T.O.s run in … with extrafusal muscle fibres
Series
What is the name of the afferents going from the muscle spindle to the spinal cord in the stretch reflex?
Ia afferents (sensory neurons)
What is the crossed extension reflex?
This is when we experience pain on one foot.
The leg experiencing the pain flexes to withdraw
The opposite leg extends to support the rest of the body
In this way, the leg is removed from the danger
What are the general names for antagonistic muscles?
Extensor and flexor
In the thigh, describe the position of the extensor and flexor muscles
Extensor: top
Flexor: underneath
What is the role of the commissural axons in the spinal cord during a crossed extension reflex?
These activate the extensor and inhibit the flexor in the opposite leg
What is the role of inhibitory motor neurons in the Stretch Reflex?
The inhibitory motor neurons inhibit the antagonistic muscle. In this way, the stretched muscle can contract more easily to correct the stretch.
What is the role of inhibitory interneurons in the Crossed Extension Reflex?
The inhibitory neurons make sure that the motor neurons do not fire. This means that the muscle (whether it be the extensor or flexor, depending on the leg) does not contract
What are the roles of the brain and the spinal cord in Pattern Generation?
Motor pattern generation is initiated and carrier out by the spinal cord
The brain is not required
What are two examples of pattern generation in the spinal cord?
- Babies making walking motions with their legs when they feel a surface on the soles of their feet.
- Cats walking on a treadmill after a transection of the spinal cord (brain not attached to the spinal cord)
Describe walking pattern generation in newborn babies
When a baby is held upright with the soles of its feet touching a surface, walking movements will be generated.
This is a result of sensory receptors activating a motor pattern generator in the spinal cord, which activates motor neurons in the leg muscles.
The brain has no input, as babies are yet to form connections between the brain and the spinal cord.
Describe the role of flexors and extensors during walking.
What are the two phases during walking?
Two phases of walking:
- Swing: flexion
- Stance: extension
Cats will walk on a treadmill even after … ?
The brain is removed
What are the three other modes of locomotion that cats use?
(Walk)
Pacing
Trot
Gallop
Relate structure to function of motor neurons
Motor neurons are very thick –> fast transmission of nerve impulses due to the thickness of the myelination
How many muscle fibres can a neuron innervate?
Could be just one, could be many.
One muscle fibre is innervated by only one neuron
What does the Ia inhibitory interneuron do?
Inhibits the alpha motor neuron that innervates the muscle antagonistic to the homonymous one that was stretched.
Inhibition: no firing of the motor neuron
No contraction of this antagonist muscle
What is the name of the inhibitory interneuron in the stretch reflex?
Ia inhibitory interneuron
How many synapses are there in the stretch reflex? Between which neurons are these synapses?
One -> it is a mono-synaptic process.
The synapse is between the Ia afferent sensory neuron from the muscle spindle and the alpha motor neuron innervating the muscle
Which afferent sends information from the muscle spindle to the spinal cord?
The Ia afferent sensory neuron
Describe the reciprocal activity of the stretch reflex
This reflex has a reciprocal Innervation.
The antagonist muscle is inhibited so the muscle that was stretched can contract to a greater extent.
- Muscle spindle detects stretch in muscle
- Message sent via Ia sensory afferent
- Spinal cord. Ia afferent synapses with Ia inhibitory interneuron
- Inhibitory neuron makes sure that the alpha motor neuron going to the antagonist muscle doesn’t fire
- Antagonist muscle is inhibited from contracting
Give an example of when the stretch reflex is activated
Knee jerk reflex
Holding an object eg. A glass
What is the stretch reflex and associated reciprocal activity? Don’t outline the steps
Stretch reflex: contraction of a muscle following the stretch of that muscle
Reciprocal process: inhibition of contraction of the antagonist muscle for the stretched muscle can contract
Describe the order of motor neuron recruitment for various movement tasks.
Why is it in this order?
The order of motor unit recruitment in a muscle is:
Smallest to Largest.
This order means that for simpler tasks that require less muscle force, the smaller units are activated.
This means we can have fine motor control
What are the different types of motor units possible?
Small or large (continuous scale)
How is the opposite leg activated/inhibited in the crossed extension reflex?
For stimulation and inhibition of the opposite leg, the Ib afferent splits into commissural axon that cross to the correct side.
How are muscles inhibited in the crossed extension reflex?
For inhibition, the Ib afferent synapses with a Ib inhibitory interneuron, which inhibits firing in the appropriate muscle
Which muscles are activated and which are inhibited in the crossed extension reflex?
Stimulated leg: flexor activated, extensor inhibited
Opposite leg: flexor inhibited, extensor activated.
What is the role of interneurons?
To communicate between the motor pools
What does a big motor unit mean?
This is a single neuron innervating many muscle fibres
