Week 1 & 2 Flashcards
How is force controlled?
- Altering the # of active MUs (de/recruitment)
- Changing frequency of activation (rate coding - MU discharge rate)
______________ is the component of the motor neuron where the excitatory post-synaptic potentials summate to generate an action potential
axon hillock
What is saltatory conduction?
propagation of action potential down the axon
What is myelination?
Insulates the axon and prevents movement of ions (Na+ and K+) across the membrane
What are the benefits to saltatory conduction and myelination?
- increase conduction velocity without a change in axon diameter
- Reduces metabolic cost as only small segments of the axon require the Na+-K+ pump to restore the resting membrane potential
What is the ratio of motor neuron action potential: muscle fiber action potential
1:1 relationship
How does Curare, which prevent the binding of Ach to Ach receptors, prevent muscle contraction?
Muscle fibers can not generate an action potential
How long does curare last?
8-30 hours
How does botox, which prevents the release of Ach, prevent muscle contraction?
Inability for excitation along the sarcolemma
What should we know about botox?
- last 6-8 weeks
- causes botulism
Used to treat: - Overactive muscles (cerebral palsy)
- Temporary cosmetic improvements (e.g. wrinkle lines)
What is rheobase?
direct measure of the current that has to be put into a neuron to get it to fire (rheobase is low in small MNs)
Describe the concept of input resistance
small MNs (Type 1) have high input resistance; leads to greater response (easier to excite)
Describe the concept of conduction velocity
small in MNs are slow (think slow twitch)
What is after-hyperpolarization?
a phase of a neuron’s action potential that hyperpolarizes the cell membrane. It’s a key mechanism that regulates the frequency and timing of neuronal firing; shorter in larger MNs
What is the equation of Ohm’s Law? How can they be compared to parts of a hose?
current x resistance = voltage
- input resistance “diameter of hose”
- rheobase (current) “flow of water”
- voltage “pressure of water”
What is the size principle?
motor units should be recruited from smallest to largest
What are the functional consequences of the size principle?
- simplifies the task of modulating forve not necessary for higher centers to select specific combinations of MUs
- ensures a smooth increase in force production
- minimizes fatigue as slow twitch, fatigue resistance muscle fibers activated first
What is a negative aspect of the size principle?
cannot selectively choose which MUs to recruit
What is the recruitment threshold?
amount of force needed to turn on motor unit; can change within a motor unit
- order across motor units remain consistent
What does the force-frequency curve look like?
sigmoidal relationship; firing rates match contractile speed
How is force controlled?
- increase the number of motor units (recruitments)
- increase rate of firing of individual motor units (rate coding)
How am I able to maintain a steady contraction if m MUs are only firing/discharging at 8 Hz?
- each motor unit produces partially fused tetanus; because units fire asynchronously with each other, the net force is smooth
- inter-dispersed or asynchronously (each individual MU produces unfused force contraction, but together can produce smooth force)
What is electromyography?
recording of the electrical activity from muscle
What are the two types of EMGs to know?
surface and indwelling EMGs
What does indwelling EMG allow us to do?
somewhat invasive. but can observe single motor unit
This technique is somewhat invasive, but can be used to observe single motor unit action potential…
indwelling EMG
Describe communication between neurons
- communication not always a 1:1 event, can involve many neurons simultaneously
- divergence and convergence
What is divergence?
a single neuron synapses on multiple neurons
What is convergence?
multiple neurons “converge” on fewer neurons
What are the peripheral structures?
intrafusal = muscle spindles
extrafusal =skeletal muscle
What is the role of afferent/sensory inputs?
- wide variety of receptors in the periphery with specific roles
- each afferent projects centrally to the spinal cord (reflexes) and to superior structures
- cell body in the dorsal root ganglion
- afferent fibers
How are afferents labelled? What are the categories?
labeled based on cross-sectional diameter
- I (largest), II, III, and IV (smallest)
- subdivision of “I” into Ia and Ib
Conduction velocity depends on….
Diameter
- large diameter = faster conduction
What are group Ia afferent fibers and what do they sense?
- muscle spindles
- length and velocity (dynamic)
What are group II afferent fibers and what do they sense?
- muscle spindles
- static length
What are group Ib afferent fibers and what do they sense?
- golgi tendon organs
- tension
What are group III afferent fibers and what do they sense?
- free nerve endings
- chemical/mechanical
What are group IV afferent fibers and what do they sense?
- free nerve endings
- chemical
Describe muscle spindle receptors
- spindle/fusiform-shaped receptors (i.e. football) found in most skeletal muscles
- lie in PARALLEL with large force producing skeletal (extrafusal) muscle
- different muscles have different number of spindles (density)
- hand muscles have a higher density of spindles, while more proximal muscles have fewer muscle spindles
What are the two types of receptors?
- Bag (based on shape):
- Bag 1: static
- Bag 2: dynamic - Chain: static
What are the two types of afferents? Describe what they innervate
- Type Ia (primary) - length & velocity
- innervate bag 1, bag 2, chain - Type II (secondary) - length
- innervates bag 2 & chain
How do we record single unit action potentials?
mirconeurography
Stretch a muscle, get Ia and II responses to stretch that…
code for length and rate of change of length (velocity of stretch)
What is the monosynaptic stretch reflex?
- A muscle spindle detects how much and how quickly a muscle is being stretched.
- Sensory impulses travel from the intrafusal muscle fiber to the spinal cord.
- The impulses synapse with an alpha motor neuron in the spinal cord.
- The alpha motor neuron sends an action potential to the muscle’s neuromuscular junction.
- The muscle contracts.
Primaries (Ia) are very sensitive to taps and vibration…
and can stop firing on release (unloading)
What is the fusimotor or gamma system?
- only receptor that have its own efferent (motor) system
- consists of dynamic and static gamma MNs
- in contrast to alpha MNs that innervate skeletal muscle (skeletomotor vs fusimotor)
Dynamic gamme sends info to…
Bag 1
Static gamma sends info to…
Bag 2 + chain
What is the importance of the gamma system?
- turning on the gamma system prevents the spindle from becoming unloaded during shortening contractions, and keeps it sensitive to stretch
Gamma dynamic makes the spindle more….
velocity sensitive
Gamma static makes the spindle more…
length sensitive
What happens with alpha-gamma co-activation during voluntary contraction?
- without, muscle spindles would become unloaded and stop firing when the muscle shortens; making them ineffective as a feedback system during movement
- coactivate both = the muscle spindle maintains its sensitivity during a shortening contraction
