Neurology: Lecture 1- LMN Flashcards
Describe the sequence of events in neuromuscular transmission including naming and the receptor
- AP arrives at terminal of motor neuron
- Voltage gated Ca2+ channels open and Ca2+ enters the cytoplasm
- Ca2+ entry triggers release of ACh by exocytosis
- ACh binds to nicotinic AChR on motor-end plate, opening ligand-gated channels
- Passage of Na+ into muscle cells generates EEP
- EEP initiates AP, which spreads via voltage-gated Na+ channels, ultimately stimulating contraction
- Neuromuscular transmission terminated AChEase, which degrades ACh to acetate and choline.
Briefly explain the role of motor unit recruitment and size in force generation in whole muscles.
Single motor neuron innervates >1 muscle fibre
Motor neurona and all muscle fibres it innervates =motor unit.
Greater number of motor units= more force generated.
With recruitment of progressively larger motor units in motor neuron pool, progressively larger forces generated
Briefly explain the roles of the action potential rate, muscle fibre length, and muscle fibre type in force generation in whole muscle.
- Action potential rate -slide 15- should know, more action potentials get summation of forces
- Mucscle fibre length -18
- Muscle fibre type -
Force production of individual muscle fibres depends on:
- Frequency of stimulation–> summation of contractions (build up of cytoplasmic Ca2+)
- Fibre diameter –> number of sarcommeres in parallel
- Resting Fibre Length–> overlap of thick/ thin filaments in sarcomeres
Describe the sequence of events that result in extensor contraction and flexor relaxation in response to tapping the patella tendon
Slide 24
- hammer tap stretches tendon, which in turn stretches sensory receptors in leg extensor muscles.
2a) Sensory neuron synapses with and excites motor neuron in the spinal cord
2b) Sensory neuron also excited spinal interneuron
2c) Interneuron synapse inhibits motor neuorn to flexor muscles
3A) Motr neuron conducts action potential to synapses on extensor muscle fibres, causing contraction.
3B) Flexor muscle relaxes because the activity of its motor neurone has been inhibited.
4) Leg extends
Describe the pathway and explain the sequence of events involved in the withdrawal and crossed-extensor reflex and explain the Importance of this event in normal locomotion.
Slide 26
-In response to activation of nociceptor, afferent neuron activates excitatory and inhibitory interneurons that stimulate flexors and relax extensors, causing ipsilateral leg to withdrawal
- In contralateral leg, extensor is stimulates and flexor is relaxed, extending leg to provide compensatory support.
- basic reflex for maintenance of posture and locomotion
Explain the role of y motor neurons in the regulation of muscle spindle activity
Slide 31
Describe the structure of the muscle spindles and explain their function
Slide 28, slide 30 and 31
Muscle Spindles:
-capsulated groups of skeletal muscle fibres (intrafusal fibres)
-arranged in parallel with striated (extrafusal) fibres
-attached to rest of muscle via connective tissue
-dont contribute to force od muscle contraction
-extrafusal fibres in main body of muscle innervated by a motor neurons
-Intrafusal fibres have separate sensory (Ia and II) and y motor neuron innervation.
Ia- sensitive to muscle length and rate of change of length
II- muscle length (little rate sensitivity)
-Myelinated y motor neuorns innervate contractile polar regions of intrafusal fibres
Function of Muscle Spindles:
- Provide info about muscle length and changes in length (velocity)
- reflex firing of y motor neurone causes intrafusal fibre polar regions to contract.
Explain how muscle spindles detect muscle length and changes in length.
Type Ia-
Receptor: Primary spindle endings
Sensitive to: Muscle length and Rate of change of length
Type II
Receptor: secondary spindle endings
Sensitive to: Muscle length (little rate sensitivity)
Describe the structure of Golgi tendon organs and explain their function.
How do they do their function?
Muscle spindles signal changes in muscle length, GTO provides info about changes in muscle tension.
-encapsulated afferents nerve endings located at junction of muscle and tendon, in series with extrafusal fibres
- innervation by single Type Ib sensory neuron
- through circuits in the spinal cord GTO inhibits a’ motor neuron that innervated the same muscle
Negative feedback regulation:
- GTO affects contact Inhibitory interneurons that decease activity of a motor neurons innervating same muscle
- inhibitory interneurons also recieve input from other sensory fibres, as sell as from descending pathways
- prevents muscles from generating excessive tension
- reciprocal excitation of a motor neurons to antagonist muscle dampens oscillations of moving limb
Dynamic and static phases of skeletal muscle activity. What are the 2 phases?
Lengthening or shortening of skeletal muscles associated with 2 phases Dynamic phase- muscle length is changing Static phase (steady state)- muscle has stabilised at new length after lengthening or shortening -separate components of muscle spindles signal each of these 2 phases
Relate the general neurological deficits that may be assosiated with LMN disease to what may be seen in patients with peripheral nerve disease or damage
Signs of LMT dysfunction:
- Weakness/ paralysis
- Hyporeflexia (reduced reflexes)
- Flaccidity/ hypotonia
- muscle fasiculation
- Neurogenic atrophy
Describe the types of INTRAfusal fibres slide 33
- 2-3 large nuclear bag fibres
- 5 small nuclear chain fibres
- Nuclear chain fibres have nuclei arranged in chain, near midpoint
- nuclear bag fibres have nuclei arrange in centre of fibre
- also have separate dynamic and static nuclear bag fibres
How are these intrafusal fibres innervated?
Primary type Ia sensory➡ inner ages go centres of all fibres
Secondary (type 2) sensory ➡ innervation to chain and static bag fibres
Dynamic y motor neuron ➡ innervation to contractile region of dynamic bag fibres
Static y motor neuron➡ innervation to contractile region of chain and static bag fibres
Sensory nerves respond via stretch-sensitive ion channels
Primary Type Ia afferents respons to muscle length, but more powerfully to changes in length
Secondary type 2 afferents much better at registering length alone (mainly via nuclear chain fibres)
Refer to pg 37
Read the slide on summary of function of muscle spindles and GTO
Side 41
What are the different types of muscle fibres in motor units?
SLOW:
- small
- slow oxidative red fibres
- contract slowly
- generate small forces
- resistance to fatigue
- good for activities that require sustained muscle contractions
FAST FATIGUABLE:
- large
- fast glycolytic pale fibres
- contract quickly
- generate large forces
- fatigue quickly
- good for brief exertions that require large forces
FAST-FATIGUE RESISTANT (FR)
- fast oxidative red fibres
- intermediate between S and FF
