Motor 1 Flashcards
Skeletal muscles ___ at the ___ to move bones
Pull at the tendon
Types of muscle
- Striated: skeletal (we focus on this!), cardiac
- Smooth muscle (in your organs, innervated by autonomic nervous system)
Classifications of muscles by contraction
- Flexors
- Extensors
- Agonists/synergists
- Antagonists
Diagram of flexors and extensors
Flexors
Flexion at the joint (e.g. bicep)
Extensors
Extend at the joint (e.g. triceps)
Agonists/synergists
Muscles that contract together (e.g. biceps and brachialis)
Antagonists
- These are the muscles working against agonists (e.g. triceps oppose biceps)
- While agonists contract, antagonists relax to create smooth movement
Anatomical classifications of muscle
- Axial muscles: move trunk, maintain posture
- Proximal/girdle muscles: move shoulder, pelvis, elbow, knee; locomotion
- Distal muscles: move hands, feet, digits; fine motor skills, specialized in humans
Fast twitch (white) muscle
- High contraction force, rapid fatigue
- High level of stored energy
- Low mitochondrial density, use anaerobic metabolism
- Low capillary density and blood supply
- Used for sprinting
- Force increases rapidly, peaks at a higher level, and diminishes rapidly
Slow twitch (red) muscle
- Lower contraction force, slow fatigue
- Low level of stored energy
- High mitochondrial density, use aerobic metabolism
- High capillary density and blood supply
- Used for endurance exercise/continuous use of muscle (e.g. neck muscles)
- Longer time for force to be generated, peaks at lower level, generated force lasts longer
Composition of fast and slow twitch muscle
- Different muscles have different compositions of muscle fiber types (soleus - posture control and endurance, all slow; gastrocnemius - mixed function, both fast and slow)
- Different people have different compositions of muscle fiber types (genetics, training can change this slightly)
- Motor neuron innervation determines whether a muscle fiber is fast/slow
The spinal cord sends motor neuron axons to the brain through the ___
Ventral horns
Describe the enlargements of the ventral horn in certain parts of the spinal cord
- The cervical and lumbar segments have enlargements due to extra gray matter (cell bodies) needed for fine motor control
- Cervical enlargement: arms/hands/digits
- Lumbar enlargements: legs
Organization of the spinal cord
- Lateral ventral horn controls lateral muscles (e.g. hands)
- Medial ventral horn controls medial muscles (e.g. torso)
- Neurons innervating flexors are more dorsal to neurons innervating extensors
Neurons innervating flexors are more ___ to neurons innervating extensors
Dorsal
The lateral ventral horn controls ___ muscles, e.g. ___
Lateral muscles, e.g. hands
The medial ventral horn controls ___ muscles, e.g. ___
Medial muscles, e.g. torso
What is the basic unit of muscle?
Muscle fiber/muscle cell
Where do muscle fibers get input from?
From only one alpha motor neuron at the neuromuscular junction
What neurotransmitter does the neuromuscular junction use?
ACh
At the neuromuscular junction, the alpha motor neuron synapses on the muscle fiber at the ___
Motor end plate
Neuromuscular junction
- Gets input from only one alpha motor neuron (AKA lower motor neuron) at NMJ
- Very fast, very reliable – each AP always generates an AP in the muscle fiber
- Uses ACh as neurotransmitter
- Synapses on the muscle fiber at the motor end plate
Motor Unit
- 1 alpha motor neuron (αMN) and all the muscle fibers it innervates
- 1 muscle fiber gets input from 1 αMN, but 1 αMN can innervate many muscle fibers
- Smaller motor unit = less muscle fibers per αMN = finer control, more precise
- Each motor unit contains muscle fibers of the same type
- αMN determines fast/slow twitch type
One muscle fiber gets input from one aMN, but can one aMN innervate many muscle fibers?
Yes
What is the motor pool?
All the αMNs that innervate 1 muscle
What are the two ways to generate greater muscle force?
- Fire more APs
- Increase the number of active motor neurons (i.e. recruitment)
Firing more APs to generate greater muscle force
Increase firing rate of active motor neurons
a. Temporal summation of twitches
b. Fasciculation: spontaneous firing of individual
MNs → spontaneous motor unit contractions → uncoordinated muscle twitches
Increasing the number of active motor neurons (recruitment) to generate greater muscle force
- Size principle: small motor units recruited before larger ones
- Smaller motor units have smaller MNs, so easier to activate
A smaller motor unit has ___ control
Finer
The muscles that are capable of the finest controlled movement are ___
Constructed of small motor units
What factor determines whether a muscle fiber is fast- or slow-twitch?
The alpha motor neuron
Diagram showing alpha motor neuron control
Where does sensory input to alpha motor neurons come from?
- 1a and 1b fibers
- Input from same and nearby spinal segments:
a. From interneurons
b. Most amount of input - Brain: upper motor neurons from descending motor pathways
Where does sensory input to muscles come from?
A-alpha axons
What is proprioception?
Sense of body position and movement so your body knows what you’re doing and where you are in space
Where is proprioception information received from?
Golgi tendon organs and muscle spindles
Overview of muscle spindles
- Muscle spindles run in parallel alongside muscle fibers and are made up of capsules containing muscle fibers inside them (intrafusal muscle fibers)
- Wrapped around these muscle fibers are Ia afferent axons
- The normal muscle fibers outside of these
spindles are extrafusal muscle fibers
Characteristics of Golgi tendon organs
- Force/tension receptors
- In series with muscle fibers
- At the junction between muscle and tendon
- Innervated by 1b axons
- Depolarized by deformation of the membrane
- More force → more depolarization
Characteristics of muscle spindles
- Stretch receptors
- In parallel with muscle fibers
- Within the muscle
- Innervated by 1a axons
- Sense changes in length of muscle
- Mechanism is more complicated than more force = more depolarisation
___ are in series with muscle fibers
Golgi tendon organs
___ are in parallel with muscle fibers
Muscle spindles
___ are innervated by 1a axons
Muscle spindles
___ are innervated by 1b axons
Golgi tendon organs
Which of the following are stretch receptors: muscle spindles or Golgi tendon organs?
Muscle spindles
Which of the following are force/tension receptors: muscle spindles or Golgi tendon organs?
Golgi tendon organs
Where are Golgi tendon organs found?
At the junction between muscle and tendon
Where are muscle spindles found?
Within the muscle
What is the purpose of the Golgi Tendon Organ (GTO)?
Allows for regulation of how much force to use for different tasks
Mechanism of action of GTO
- αMN activates → causes muscle to contract
- Muscle contraction pulls GTO and squeezes collagen fibrils
- Ib axons interwoven with collagen deform and activate
- Mechanosensitive ion channels on Ib open → depol
- Ib axon fires more to encode increased tension in muscle
Negative feedback in GTO
- GTO innervates inhibitory interneurons → reduces aMN activity → reduces muscle tension if it’s overloaded
- Force feedback reflex
Mechanism of muscle spindles
- When muscle stretches, they also pull the spindles and intrafusal muscle fibers
- This stretch opens mechanosensitive ion channels in 1a fibers → depolarization
- 1a fibers fire more
Diagram of muscle spindle stretch
Gamma Motor Neurons and Muscle Spindles
- αMN activation → muscle contracts/shortens → spindles aren’t stretched and spindles/intrafusal fibers are slack and baggy → 1a fibers stop firing
- Gamma motor neurons (GMNs) innervate intrafusal fibers and make them contract/re-adjust length until 1a fibers come online again
○ Adjusting the baseline – respond to changes in length, not absolute length
Diagram of Gamma Motor Neurons and Muscle Spindles
Extrafusal fibers
- Most skeletal muscle in the body
- Generates force
- Innervated by alpha motor neurons → contraction
Intrafusal fibers
- Fibers within muscle spindles
- Senses changes in muscle length
- Innervated by gamma motor neurons → spindle length normalization
Which type of fiber generates force?
Extrafusal
Which type of fiber senses changes in muscle length?
Intrafusal
Which type of fiber is innervated by alpha motor neurons?
Extrafusal
Which type of fiber is innervated by gamma motor neurons?
Intrafusal
Damage to the ___ would result in difficulty in adjusting the proper amount of force needed for movements
Golgi tendon organs
What is a reflex?
An involuntary coordinated muscle pattern caused by peripheral stimuli
The stretch reflex
● The simplest reflex
○ 1 synapse, no interneurons, no brain
involvement
● AKA myotatic reflex/knee-jerk reflex
● Function: maintain muscle at a constant length
Mechanism of the stretch reflex
- Tap patellar tendon
- Briefly stretches quadriceps muscles
- Spindle stretch receptors → 1a axon activation
- 1a axon synapses onto αMN innervating quads
- αMN activates → quads contract
Reciprocal inhibition
- Through reciprocal inhibition, as agonists contract, antagonists relax
- Activate agonists, inhibit antagonists
- Helps with load adjustment, maintaining muscle length
Reciprocal inhibition in the stretch reflex
- Spindles stretch → 1a activates
- 1a excites agonist αMNs
■ 1 synapse, direct - 1a excites inhibitory interneuron, which inhibits antagonist αMNs
■ 2 synapses, indirect
Overview of flexor-crossed extensor reflex
- Usually involved in withdrawal from painful stimuli (eg: a thumbtack)
- When we step on a tack, we want to flex the affected leg and extend the other so we
don’t fall over (let’s say you stepped on a tack with your right foot)
Mechanism of flexor-crossed extensor reflex
- Pain stim → nociceptive Aδ axons carry signals to spinal cord
- Aδ axons excite excitatory interneurons that synapse on other interneurons
- Excitatory interneurons onto R leg flexors and L leg extensors
- Inhibitory interneurons onto R leg extensors and L leg flexors
■ So R leg flexes smoothly and L leg extends smoothly
Activation of pain receptors in the left foot would lead to excitation of ___ in the ___
Extensors in the right leg
If action potentials in gamma motor neurons were blocked, action potentials in
a) Golgi tendon organ axons would increase
b) Golgi tendon organ axons would decrease
c) 1A afferents would increase
d) 1A afferents would decrease
d) 1a afferents would decrease
If action potentials in gamma motor neurons were blocked, the muscle spindle would lose this sensitivity. Consequently, Ia afferents would become less responsive to muscle stretch because the spindles would be less taut, leading to a decrease in action potentials in Ia afferent neurons.
All of the following directly synapse onto alpha motor neurons in the spinal cord EXCEPT
a) Neurons in primary motor cortex
b) axons from muscle spindles
c) axons from Golgi tendon organs
d) interneurons from the same segment of spinal cord
c) Axons from Golgi Tendon Organs
The axons from Golgi tendon organs (which are primarily 1b afferents) do not directly synapse onto alpha motor neurons. Instead, they primarily synapse onto interneurons within the spinal cord. These interneurons can then inhibit alpha motor neurons to prevent excessive tension on muscles, functioning as a protective mechanism against damage.
You are holding an empty plastic bucket in your hand with your arm bent 90 degrees at the
elbow. Your roommate then pours 4 gallons of blue cheese dressing into the bucket and your
motor system responds by generating more force in flexor muscles to counter the added weight
and keep your arm at 90 degrees.
In response to the increased load,
a) your motor system recruits more alpha motor neurons innervating flexor muscles
b) your motor system decreases the firing frequency of alpha motor neurons innervating
flexor muscles
c) the stretch reflex, which prevents contraction of flexor muscles in this case, must be
inhibited
a) your motor system recruits more alpha motor neurons innervating flexor muscles.
c) is also incorrect. The stretch reflex helps maintain muscle tone and stability. While increased load might stretch the muscles and activate the stretch reflex, it does not prevent contraction of the flexor muscles. In fact, the stretch reflex can facilitate the contraction of the extensor muscles to help maintain balance.
Voluntary flexion of the arm involves
a) increase in activity of alpha motor neurons innervating the biceps muscle
b) decrease in activity of alpha motor neurons innervating the triceps muscle
c) increase in activity of gamma motor neurons innervating the biceps muscle
d) More than one of the above
e) All of the above
e) All of the above
A tap to the left patellar tendon would initiate the stretch reflex and lead to monosynaptic
a) activation of alpha motor neurons innervating flexor muscles in the thigh
b) inhibition of alpha motor neurons innervating flexor muscles in the thigh
c) activation of alpha motor neurons innervating extensor muscles in the thigh
d) inhibition of alpha motor neurons innervating flexor muscles in the thigh
c) activation of alpha motor neurons innervating extensor muscles in the thigh.
Stimulating gamma motor neuron axons innervating your biceps muscle would cause
a) A decrease in action potentials in alpha motor neurons innervating the biceps muscle
b) Stretching of the biceps muscle
c) An increase in action potentials in 1a afferents originating from the biceps muscle
d) None of the above
c) An increase in action potentials in 1a afferents originating from the bicep muscle
When gamma motor neurons are stimulated, they cause the intrafusal fibers to contract. This does not directly cause the muscle (in this case, the biceps) to stretch; rather, it alters the sensitivity of the spindle to changes in muscle length.
Two muscles have the same number of muscle fibers and they are all fast-twitch. Muscle A has
100 neurons in its motor neuron pool while muscle B has 500 neurons in its motor neuron pool.
Which statement is true
a) Muscle A generates more force
b) Muscle B generates more force
c) Muscle A is capable of finer control of level of force
d) Muscle B is capable of finer control of level of force
d) Muscle B is capable of finer control of level of force
Which region of the spinal cord controls muscles in the arms?
Cervical
Where in the spinal cord are the cell bodies of alpha motor neurons found?
Ventral horn
The following statements about the flexor and cross-extensor reflexes are all correct
EXCEPT
a) the reflexes show local sign
b) the reflexes are graded in strength
c) the reflexes use circuitry confined to one spinal segment
d) the reflexes use circuitry well-suited to walking and swimming
c) the reflexes use circuitry confined to one spinal segment
Where in the spinal cord are the cell bodies of alpha motor neurons found
a) dorsal root ganglion
b) ventral root ganglion
c) dorsal horn
d) ventral horn
d) ventral horn
In the control of movement, the basal ganglia are thought to participate most in
a) Execution
b) Strategy
c) Tactics
b) Strategy
Execution (a): The execution of movement is primarily carried out by the motor cortex
The amount of force you can apply to flex your arm against a force acting to extend your arm
is determined by all of the following EXCEPT
a) the inhibition of antagonistic muscles acting to extend the arm
b) the number of motor units recruited in the flexing muscle
c) the frequency of action potentials in motor neurons innervating the flexing muscle
d) the speed of action potentials in alpha motor neurons
d) the speed of action potentials in alpha motor neurons
