M3 L1 Flashcards
What does motor control come from
1) the spinal cord
2) the brain
characteristics of somatic motor system
1) skeletal muscles (33 somites)
2) nervous system controls it
3) voluntary
Before Sherrington what was the belief about reflexes?
They believed that reflexes occurred as an isolated activity local to the area it occurred in (called reflex arc)
What did Sherrington do
He figured out how the nervous system controls muscles and found that reflexes are integrative activities
3 types of skeletal muscles
axial, proximal, distal
Skeletal muscle: axial
* what are
* what do
- muscles of the trunk
- maintain posture
Skeletal muscle: proximal
* what are
* what do
- muscles of the shoulders, elbows, thighs, knees
- locomotion
Skeletal muscle: distal
* what are
* what do
- muscles of hands, feet, digits
- manipulation of objects
What are skeletal muscles innervated by? What do the roots form?
- innervated by alpha motor neurons
- roots form mixed spinal nerve
What are the lower motor neurons
- alpha motor neurons
- gamma motor neurons
Alpha Vs Gamma motor neurons:
- what do alpha motor neurons do? What do they activate
- what do gamma motor neurons do? what do they activate
- alpha motor: control skeletal muscle contraction, making muscles move
- activate extrafusal (main muscle fibers) muscle fibers to produce movement.
- gamma motor: adjust muscle spindle sensitivity, keeping them responsive to stretch.
- activate intrafusal muscle fibers (inside muscle spindles) to maintain spindle sensitivity during movement.
Alpha Motor Neurons
* what innervate
* what NT is released and what do
- motor neurons that innervate muscle cells
- vary in size
- Acetylcholine (ACh) is the
neurotransmitter released by alpha motor neurons at the neuromuscular junction. It binds to nicotinic receptors on muscle fibers, triggering muscle contraction.
What are some inputs to alpha motor neurons
- spinal interneurons
- sensory input
- input from upper motor neurons from brain
What do alpha motor neurons innervate? Where are they located?
- extrafusal muscle fibers
- located in ventral horn and exit through ventral root
Which is the muscle vs muscle spindle in the pic?
What was the experiment they did with muscle spindles and the stretch reflex? What did they find
They measured the length of muscles and added a weight and watched how the muscle reacted.
They observed that the muscle stretches due to the weight but then it goes back to its optimal length after.
what are extrafusal muscles.
* ex?
* innervated by?
Regular muscle fibers responsible for force generation and movement.
- sarcomere, SR
- Innervated by alpha motor neurons.
what are intrafusal muscles.
* innervated by?
Specialized sensory fibers within muscle spindles that detect stretch and help with proprioception.
- Innervated by gamma motor neurons.
What is a muscle spindle
- encapsulated by what
- what wraps around the muscle fibers
A muscle spindle is a sensory receptor found within muscles that detects changes in muscle length and stretch.
- encapsulated by muscle fiber
- group 1a sensory axons wrap around the muscle fibers of the spindle in the middle
Muscle spindles consist of BLANK which are BLANK wrapped in sensory nerve endings.
They are surrounded by BLANK, which are the regular muscle fibers responsible for BLANK.
Muscle spindles consist of intrafusal fibers which are specialized muscle fibers wrapped in sensory nerve endings.
They are surrounded by extrafusal fibers, which are the regular muscle fibers responsible for movement.
What do muscle spindles do
Muscle spindles help regulate muscle tone and reflexes by sending signals to the spinal cord, triggering adjustments to maintain posture and coordination.
1a Axons
what are they?
- where is the 1a axon wrapped around
- why is the 1a axon the fastest?
- what causes the 1a muscle to fire? Where do they synapse
- where do they enter from
sensory nerve fibers that send information from muscle spindles to the spinal cord. They detect how much and how fast a muscle is stretching.
- the muscle belly of the muscle spindle
- bc its myelinated
- if muscle gets disturbed (such as by heavy weight) it stimulates 1a to fire. THEN they synapse onto Alpha motor neuron or interneurons
- inter via dorsal root
What are mechanosensitive channels
stretch gated channels that detect muscle length changes
what is the knee jerk reflex
when tapping on the tendon the muscle briefly stretches
Why can this never happen: stretch muscle (extrafusal fiber) and then the intrafusal fibers (muscle spindles) will become loose which would stop the 1a muscle from firing (reducing the spindle’s ability to detect further changes in length)
The gamma motor neurons sure the intramural fibers stay tight and sensitive, even when extrafusal fibers stretch.
Without gamma motor neurons, the muscle spindle would become less sensitive as the muscle shortens because the intrafusal fibers would be slack, and Ia axons would not detect small changes in length effectively AND the Ia afferents wouldn’t fire properly when needed, disrupting reflexes and muscle control.
Think of it like a rubber band inside a bigger band—gamma neurons adjust the inner band so it stays ready to detect changes, no matter what the outer band (extrafusal fibers) is doing.
Explain the 5 steps of a stretch reflex
- Muscle induced to stretch
- Intrafusal fibers stretched
- Stretch detected by group 1a sensory axons
- 1a sensory axons fire → input sent to 𝛼MNs
- 𝛼MNs stimulate extrafusal muscle fibers to
contract
gamma motor neurons
* what do they innervate
* where located
- they innervate the intrafusal muscle fibers within the spindle
- located at the two ends of the spindle
main function of gamma motor neurons
prevents spindle from being slack
What is gain
the level of gamma motor neuron activation
Amount of force generated in response to given stretch of intrafusal fibers
● Gain = output (contraction of extrafusal)/input(stretch of intrafusal)
What neurons are activated during normal movements?
Alpha and gamma motor neurons are activated together to keep the muscle spindle sensitive even as the muscle shortens, allowing for continuous feedback for smooth, controlled movement.
What is hypotonia
floppiness of muscle - baby unable to keep head up
Extrafusal muscles:
* width
* length
* fiber amount
* innervated by how many alpha motor neurons
Extrafusal
* Fiber is 50-100 um wide
* Fiber is 2-6 cm long
* Muscle has 103 – 109 fibers
* A muscle fiber is innervated by only one alpha motor neuron
Intrafusal muscles:
* name?
* located where
* how many muscle fibers
* what wraps around
* what do
* what used for
* innervated by?
Intrafusal
* Known as muscle spindles
* Embedded in the extrafusal fibers
* A spindle consists of 8-12 muscle fibers
* Ia sensory axon wraps around them
* Continuously monitors length of muscle
* Used for feedback control of movement
* Innervated by gamma motor neuron
When are 1a sensory valuable (irl ex)
- walking on a rope
- walking and trying to maintain balance
What axon is located in the Golgi tendon organs? What do they do
the 1b axons
detect muscle tension/force. They inhibit alpha motor neurons via interneurons, helping prevent excessive force that could damage the muscle.
Where are 1a axons located? Where are 1b axons located?
1a: Ia axons are located in muscle spindles within skeletal muscles
1b: Ib axons are located in Golgi tendon organs (GTOs) at the junction between muscle and tendon
What happens to 1b when alpha motor neuron axons are active?
What do the 1b sensory neurons do?
they detect the increased tension in the tendon caused by muscle contraction.
1b sensory neurons may protect the muscle from being overloaded by synapsing on inhibitory interneurons in the spinal cord, which inhibit the alpha motor neurons of the contracting muscle. It also regulates muscle tension within an optimal range
What 2 things is optimal length controlled by?
- muscle spindles (prevent overstretching)
- Golgi tendon organ (prevent over contraction)
1a axon action:
1b axon action:
gamma motor neuron action:
Ia axons excite alpha motor neurons to contract muscles in response to stretch. (while inhibiting opposite girl)
Ib axons inhibit alpha motor neurons to prevent excessive force.
Gamma motor neurons adjust muscle spindles so Ia axons stay responsive.
give an example with lifting an object of how 1a and 1b work together
Ia axons in the muscle spindle sense stretch and activate motor neurons to contract the muscle.
If the muscle tension becomes too high, Ib axons in the Golgi tendon organ sense the force and activate inhibitory interneurons to reduce motor neuron activity, making the muscle relax.
Where do 1a axons originate and synapse?
Originate: From the muscle spindle itself.
Synapse: Directly in the spinal cord on alpha motor neurons.
Where do 1b axons originate and synapse?
Originate: From the Golgi tendon organ, not muscle spindles.
Synapse: In the spinal cord on alpha motor neurons and interneurons.
Where do gamma motor neurons originate and synapse?
Originate: In the spinal cord.
Synapse: On the intrafusal fibers of the muscle spindle.
Where do alpha motor neurons originate and synapse?
Originate: In the spinal cord.
Synapse: Directly on extrafusal muscle fibers (the regular muscle fibers that contract to cause movement).
where does most of the innervation to the alpha motor neuron come from?
interneurons of the spinal cord
What are spinal interneurons?
Spinal interneurons are neurons within the spinal cord that act as intermediaries between sensory neurons and motor neurons. They help process and modify signals before they reach the muscles or the brain.
where do spinal interneurons receive input from?
- primary sensory axons
- descending axons from the brain
- collaterals of lower motor neurons axons
Once a load is added to a muscle, the BLANK will be detected by BLANK sensory neurons. This causes a rapid increase in BLANK. Next BLANK will fire. Next BLANK will contract. Next BLANK will become slack. This causes BLANK to stop BLANK. This causes BLANK to fire. This leads to BLANK contract. This leads to the recovery of BLANK to detect BLANK.
Once a load is added to a muscle, the STRETCH will be detected by 1a sensory neurons. This causes a rapid increase in 1a SENSORY NEURON FIRING. Next ALPHA MOTOR NEURONS will fire. Next EXTRAFUSAL FIBERS will contract. Next INTRAFUSAL FIBERS will become slack. This causes 1a SENSORY NEURONS to stop FIRING. This causes GAMMA MOTOR NEURONS to fire. This leads to INTRAFUSAL FIBERS contracting. This leads to the recovery of 1a SENSORY NEURONS to detect STRETCH.
What type of synapses can spinal neurons form
- inhibitory synapses (reciprocal inhibition)
- excitatory synapses (flexor withdrawal reflex and cross extensor reflex)
Spinal Interneurons: Reciprocal
Inhibition
* what is it
* ex:
Reciprocal Inhibition is a spinal reflex where interneurons inhibit the antagonist muscle to allow smooth movement.
When you contract your biceps, reciprocal inhibition relaxes your triceps so the movement is smooth. (stimulate one inhibit the other)
where do spinal interneurons receive input from?
○ Primary sensory axons
○ Descending axons in brain
○ Collaterals of lower motor neuron axons
Spinal Interneurons: Excitatory Inputs Flexor Withdrawal Reflex
* what is it
* process of how it works
- a fast, protective reflex that pulls a limb away from pain using excitatory interneurons.
- How It Works:
- Pain receptor (nociceptor axons) activates it → Sends signal via sensory neuron to the spinal cord.
- Excitatory interneurons activate flexor muscles → Causes rapid withdrawal of the limb.
- At the same time, inhibitory interneurons relax the extensor muscles to allow movement.
basically stimulate flexor and inhibit extensor to move one leg (do opposite for other leg to keep it standing)
why is the flexor reflex slower than stretch reflex?
The flexor reflex is slower than the stretch reflex because it involves multiple synapses (polysynaptic reflex) in the spinal cord, while the stretch reflex is monosynaptic (only one synapse between the sensory and motor neuron).
Stretch Reflex (Faster) → Direct Ia sensory → Alpha motor neuron activation (e.g., knee-jerk reflex).
Flexor Reflex (Slower) → Pain sensory neuron → Multiple interneurons → Alpha motor neurons of flexors of affected limb (more processing = slight delay).
Spinal Interneurons: Excitatory Inputs Cross Extensor Reflex
* what is it
* process of how it works
a spinal reflex that helps maintain balance when withdrawing from pain.
How It Works:
Pain stimulus activates sensory neurons → Signal sent to the spinal cord.
Excitatory interneurons activate flexors on the injured side → Withdraws the limb.
At the same time, excitatory interneurons activate extensors on the opposite limb → Provides stability.
what are the 2 primitive reflexes?
1) root reflex -suckle breast tissue for food
2) babinski reflex (toes fan out in response to stimulation)
what is the building block for locomotion
the cross extensor reflex
list the 3 components of local circuits within the spinal cord that generate and coordinate rhythmic motor movements without continuous brain input.
Central Pattern Generator (CPG) → A network of spinal neurons that generate rhythmic movements.
Neurons with Pacemaker Properties → Special neurons within CPGs that can self-activate and maintain rhythm.
Timing & Coordination Control → Ensures the correct sequence of muscle activation for smooth movement.
In bipeds, the movement of a single limb is a cycle of two phases, name them
Phase 1: stance phase (standing - limb extended in contact w ground)
Phase 2: swing phase (moving foot forward - limb flexed to leave ground)
How does locomotion occur
Locomotion happens through alternating activation of flexor and extensor muscles, controlled by spinal circuits called Central Pattern Generators (CPGs).
How It Works:
* Flexor-Extensor Alternation
When you step, one leg flexes (lifts off the ground) while the other extends (pushes off for support).
This alternating pattern allows smooth movement.
- Reciprocal Inhibition
When flexors activate, inhibitory interneurons suppress extensors (and vice versa). This prevents both muscles from contracting at the same time. - Rhythmic Activation by CPGs
CPGs in the spinal cord generate rhythmic bursts of flexor-extensor activity.
These circuits are influenced by sensory feedback (e.g., ground contact) and descending brain signals (e.g., walking speed).
How did the experiments on cats show that locomotion is controlled by spinal circuits (CPGs), even when the brain is disconnected?
Decerebrate Prep → Brain removed, but brainstem intact → Cat still walks → CPGs generate rhythmic movement.
Spinal Prep → Spinal cord completely isolated → Cat still walks → Proves CPGs work without brain control.
Deafferented Prep → No sensory feedback, but adding neurotransmitters restores walking → CPGs don’t need sensory input to function.
What is locomotion dependent on
It is dependent on spinal cord neurons that exhibit a central pattern generator rhythm
When the excitatory interneuron activates what else activates and what does it do
it activates the inhibitory interneurons to ensure one goes at a time
