Midterm 1 Study Flashcards
Define a Motor Neuron Pool
- All motor neurons that innervate a specific muscle
- Motor Neurons that go down to muscle
Where do Cell bodies reside?
In ventral/anterior horn
How are Cell bodies grouped?
- in columns, by function
- for proximal muscles, found more medially
- for distal muscles found more laterally
What is a motor unit?
- motor neuron (cell body, axon, dendrites) and all muscle fibres it innervates
How many muscle fibres does a single motor neuron innervate?
- Many
- Varies depending on type of muscle
Muscle Unit
- All the muscle fibers innervated by 1 motor neuron
What are the 3 types of Skeletal Muscle?
- Slow Oxidative (SO)
- Fast oxidative glycolytic (FOG)
- Fast Glycolytic (FG)
What is the input resistance like for small MU? Why?
- Bigger, easier to elicit
- Due to high input resistance and slow conduction velocity
Twitch. What does it consist of?
- Physiological, quantal response of MU to stimulation
- key to starting movement
- change in force over time
- consists of rise in force, peak force and relaxation
Which motor neuron supplies fast twitch fibers?
- Large MN
Which motor neuron supplies slow twitch fibers?
Small MN
How are motor units grouped?
- Based on behaviour and function
Twitch characteristics of slow contracting, FR fibers?
- Slow to reach peak, slow to dissipitate
- long twitch
Twitch characteristics of fast contracting, FR fibers
-Quick to reach peak, quick to dissipitate
- Greater force at peak than slow contracting
What determines the physiological behaviour of the 3 types of MU (slow contracting FR, Fast contracting FR and Fast Contracting F)
- The neuron and muscle fibers they connect with
In what ways is muscle force controlled?
- frequency of motor units firing
- Number of motor units recruited
Why are motor units recruited in an orderly fashion?
- larger, more powerful MU recruited near end of contraction where increment will have effect on net force
- brain does not have to control hundreds of twitches, just adjust the level (think of a volume button)
- Con: unable to selectively activate MU out of order
Example when FG are activated
- Jumping
Receptors that transduce info from the environment?
- Exteroceptors
- Pick up info from the environment including rods and cones in the eyes
Role of efferent/sensory receptors?
- Carry info from periphery to spinal cord (centrally) to cell body in dorsal root ganglion
- Signal then goes to afferent fibers - relayed to SC if reflex and central projections move to superior structures
Subdivisions of I (largest) muscle fibers and characteristics?
- Ia = primary afferent fibers, greater number than Ib, connected to spindle *fastest conducting connected to spindle
- Ib = connected to golgi tendon, organ afferent, lower force production
Synaptic Potential
- inc AP = inc Ca2+ in synapse released from pre synaptic membrane = NT released from vesicles in synaptic cleft and bind to post synaptic membrane
(-40 to -70 mV)
Convergence of MN
- leads to greater spatial summation
- 3 excitatory neurons fire close to one another
- all individually are below threshold
- sum of all 3 = summate = reach threshold = AP generated
Primary Ia afferents?
- lots of myelin, conduct faster, greater current
- spiral bag1, Bag 2, and chain fibers
- spiral near center of spindle
Why is there a motor system for muscle fibers?
- gamma system tightens end of spindle (acts on polar ends where contractile proteins are), while alpha helps with force contraction of extrafusal muscle.
- keeps tension in middle of spindle to prevent sudden unloading
- maintains firing rate of Ia fibers in optimal range
- alpha and gamma can be co-activated and independently activated depends on moving fast vs slow/stationary/posture
Describe the Pathway of the Medial Lemniscus?
- First order neurons enter spinal cord in dorsal roots of spinal nerves
- info from hands/arms in cervical region, located higher up an later in pw
- info from legs/lower body in lumbosacral region = located earlier in pw
- fibers turn rostrally (toward head) and ascend in ispilateral funisculus (dorsal column) - All fibers make first synapse with nuclei in lower medulla (primarily)
- fibers from lumbosacral connect to neurons in gracile nucleus
- fibers from cervical region connect to neurons in cuneate nucleus
- this synapse = 2nd order neurons
- these neurons decussate at lower medulla and ascend in medial lemniscus, reach thalamus - 2nd order neurons synapse onto new set of neurons in thalamus = 3rd order neurons
- 3rd order neurons reach somatosensory cortex
- medial side = for legs/lower limbs
- lateral side = for arms/hands
- L cortex for R side of body, R cortex for L side of body
What is the sensory cortex needed for?
- object recognition, localization
Describe the Spinthalamic Pathway?
- 1st order neurons enter SC in dorsal roots of spinal nerves (lumbrosacral = legs/lower limbs, cervical = arms/hands)
- 2 fibers immediately synapse in dorsal grey horn
- (more importantly note that 1st order motor neurons do NOT ascend right away, they just immediately synapse) - 2nd order neurons decussate in ventral white commisure (contralateral side) and turn rostrally as they ascend.
- This contralateral ascending pathway is known as the spinothalamic tract (ventro-lateral white matter of SC)
- Reach thalamus and synapse with 3rd order neurons
- same as medial leminiscus pw: hand and arm info on lateral side of somatosensory cortex and info from leg in medial side
How are spindles spread and how do they lie?
- Throughout the muscle belly and lie in parallel to muscle fibers that have a large force generation
Describe the structure of GTOs
- As muscle fibers wind down, they lose actin/myosin associated with sarcomeres. Structural collagen starts coming together that intertwines/tangles - some pass through encapsulated structure - form into band of CT, know as tendon - GTOs pass through capsule
What do GTOs code for?
- Tension and Force
What do GTOs code for?
- Tension and Force
How many muscle fibers per GTO?
10-20
What relationship does GTO discharge rate have with F production?
- Linear relationship, except for at start where it is not exactly linear
Describe dendritic trees of MU? About how many synapses are found?
- extensive, go in all direction
- receive many inputs
- 3-D shape
- Between 10k and 50k
How many motor neurons innervate a specific muscle?
- One
Innervation Ratio (IR)
- number of muscle fibers per motor neuron
- varies for muscle: bigger muscle has larger IR due to need for larger force production
What causes IR ratio to change? How does it change
- With age = number of motor units dec, size inc, so does number per fiber = IR inc
- Due to disease = ie. ALS causes MN to die, so more muscle fibers are innervated by those few MN left
- causes greater IR and therefore, greater force production
- twitching of muscle produced
Innervation for SO fibres
- Slow MU
- Lots of Mitochondria
- Generate ATP = key for contractions
Innervation for FOG type muscle fibers
- Fatigue resistant (FR) type MU
- Contract with resistance to fatigue
Innervation for FG muscle Fiber?
- Fast Fatiguing (FF) type
Rheobase
Minimum current needed for action potential
Afterhyperpolarization, is it shorter in small or large MU?
- period after AP fires
- Shorter in larger MU = can fire again sooner
Twitch characteristics of fast contracting, fast to fatigue (FF) fibers?
- amount of time to reach peak and dissipate similar to FR
- Much greater force at peak generated
- larger sarcomeres
- twitch force is large due to large force generation
Heneman’s Size Principle?
- Motor units are recruited sequentially in order from smallest to largest motor unit
- first motor unit recruited is smallest and stays on as other ones become recruited
Recruitement threshold?
- The force at which a motor unit begins to discharge action potentials repetiviely during a voluntary contraction
- MU needed to be reached in order for MN to fire and generate a contraction
Sequential inactivation
- In relation to Heneman’s principle
- MU is deactivated in sequential order by size as well
- First MU to be activated is the first to be deactivated (so the smallest)
Sequential inactivation
- In relation to Heneman’s principle
- MU is deactivated in sequential order by size as well
- First MU to be activated is the first to be deactivated (so the smallest)
give an example when SO motor units are
activated
standing
Example of FOG activated
walking, running
What are the 3 classification of receptors?
- Interoceptors
- exteroceptors
- proprioceptors
Interoceptors?
- receptors that sense info from within the body
- sensory receptors
- include receptors for hunger, pain
proprioceptors?
- Transduce info about position of body segments in space
- relative position/configuration of body segments
Key points about afferent fibers
- Vary in size, larger produce larger conduction due to more channels, causing leaking AP
- Larger fiber diameter= inc speed and conduction potential carry signal for actual movement
- Numbered I, II, III, IV
- I is largest and most abundant
- small fibers = un-myelinated
Which muscle fiber connect to muscle spindle
Ia (primary) and II (secondary afferent)
- spindle has 2 axons: primary and secondary
Post Synaptic Potentials
- can be excitatory or inhibitory (EPSP or IPSP)
- interplay between EPSP and IPSP determine whether neuron will fire AP
Excitatory Post Synaptic Potential (EPSP)?
- Local and graded, occurs in membrane
- not all at once
- magnitude of EPSP proportional to amount of NT released
- NOT followed by refractory period = can summate
- NT binds to post synaptic receptors = open NA+ channels = release Na+ in cell = becomes less negative
- less negative = more positive post synaptic potential
Inhibitory Post Synaptic Potential (IPSP)?
- Becomes more negative, then goes back to normal
- Cl- released
- Most common inhibitory NT = GABA
communication of neurons divergence vs
convergence
divergence = single MN synapses to multiple
convergence = multiple MN synapse and converge to one
Spatial summation when inhibitory is present (IPSP)?
- 2 excitatory MN diminished by one inhibitory motor neuron summation of these 3 = below threshold = no AP
Spatial Summation?
- No refractory period, sequentially fire in a simultaneous order
Small motor neurons vs large
- small = inc input resistance, little current = low threshold
- large = low input resistance, large current due to large diameter = large threshold (EPSP may not fire due to large threshold)
What does a muscle spindle look like? where is it located? What are the characteristics?
- Circular cross-section with cross tapering (like a football) in most skeletal muscle
- runs parallel to large force producing skeletal muscle (extrafusal), allows it to change in length when muscle changes in length
- about 1cm in length, middle or equatorial region covered by capsule - made of connective tissue
- Polar ends contain myofilaments = contractile proteins
Where are cell nuclei located in muscle spindle? how are they grouped?
- in the equatorial region in intrafusal fibers grouped by how nuclei stay together.
- 2 kinds: Bag and chain
Differentiate between bag and chain intrafusal fibers?
- Bag = nuclei clumped in a group, longer than chain fibers (2 types, dynamic and static)
- Chain = nuclei organized in a row
How many intrafusal fibers per muscle spindle? how many are bag fibers? how many are chain?
- 2-20 intrafusal fibers
- 2-3 bag and 3 or more chain
Secondary II afferent
- Innervate bag 2 and chain, Never bag 1
- off center
- can be spiral or spider like
Why are muscle spindles unique?
- they have efferent/motor system
- fusimotor and gamma system
Gamma System?
- Only innervate intrafusal muscles, have own motor system
- has 2 components: dynamic and static
dynamic gamma motor neuron?
only connect to bag 1
- during stretch, Ia afferent inc info about velocity of stretch
Static Gamma Motor Neurons
- Connects to bag 2 and chain
- connect to static bag fibers, which allows to code for muscle length change; important for posture
How many Ia and II afferents are there per muscle spindle?
- varies, usually one Ia and one or more II endings
Mechanoreceptor mechanism of action
- muscle stretch = pulls on muscle = membrane stretches = causes ions to leak = increase AP = graded potential = fires AP
Slow adapting receptors?
- little adaptation to periphery
- provides static info, info about persistence
- more pressure added by probe = smaller gap b/w potentials = quicker firing
- codes for how deep pressure goes from outside source like probe
Rapid adapting receptors?
- Signal change – change in pressure (in vs out)
- Highest rate of AP firing is while adpating
Spindle Afferent response to muscle stretch?
- primary afferent = better at signalling change in velocity (how fast muscle is changing length) can also detect change in length but to lesser degree
- sensitive to taps and vibration
- AP fires at greater rate during release and during stretch
- Secondary Afferent = signals change in length, but can also detect change in velocity only to a smaller degree
- AP fires more rapidly when holding stretch
What are primary afferent fibers sensitive to
- taps and vibration
- quick change in muscle length
- allows for quick correction in movement
- respond to dynamic and static motor neurons
When do primary afferents fire most?
- During linear stretch and during a tap/vibration
Dynamic Index?
- Diff between peak (overshoot during stretch) and steady state
- peak (overshoot) varies with how fast stretch is = the faster = the greater the peak
Beta Fibers?
Innervate intrafusal and extrafusal fibers
- allows greater flexibility in controlling muscle spindle during different motor tasks
- allows ‘fine tuning’ of muscle spindle info in order to produce most appropriate output for specific task
Human microneurography?
- small electrade is placed in muscle nerve, records for AP near axon
Monosynaptic Reflex
- Afferent neurons carry sensory info to CNS, transduction occurs where sensory info is converted to motor info via one interneuron synapse - goes to efferent neuron and initiation action in effector organ
what is the fusimotor system dependent on?
- the task
Differentiate between Ia fibers and II fibers?
Ia = info about velocity
- influenced by dynamic and static
- sensitive to rapid change in muscle length such as tap, vibration, twitch
II = info about change in length
- receives info only from static and chain
Research on Muscle Spindle activity during repetitive movement?
- Recorded for muscle spindles during reptitive movement
- gave controlled stretch to ankle = imposed passive movement
- when asked to attend to movement:
- could inc/dec spindle modulation, length dependent
- inc length code, gamma static inc
- burst of movement seen
When asked to recognize letter being drawn out by foot - continuous firing seen, inc velocity code (gamma dynamic)
Research evidence you can regulate gamma static?
- participant told to vary position of movement
- asked which position was further
- as a result, inc firing in length code seen = inc gamm static activation
Research Evidence you can regulate Gamma dynamic?
- varying speed of movement conducted on participants, in between told to relax
- then told to judge which movement was faster/slower/intermediate
- showed inc in dynamic firing, code in Ia afferent= coded for velocity more frequently
What is the key point in regards to the importance of the gamma motor system?
- it is sensitve to the receptor desired
- for example, allows for selective tuning when choosing to attend to speed vs distance/position of movement
What is the purpose of length and velocity being coded in 2 different channels (Ia and II)?
- allows gamma system to be used for selective tuning
What do ascending pathways include?
- sensory neuronal signals from skin, joint , and muscle receptors
What are the 2 pathways sensory neuronal signals are segregated in (the 2 ascending pathways)?
- Medial lemniscus and spinothalamic
what are the main functions of the medial lemniscal ascending pathway?
- gives perceptio of body movement = proprioception
- involved in tactile recognition, change in position of stimulus
- relays info about discriminative aspects of sensation
*especially involves sensory info from skin receptors
What is the main functions of the spinothalamic pathway?
- relays conscious info about sensation - such as pain, temperature, and pleasure
- less discrimination in terms of tactile info
Where are GTOs located and how are they arranged?
- in junction between muscle and tendon = tendon junction
- lie in series
