NeuroMotor Flashcards
What are direct and indirect connects
There are direct and indirect pathways onto the motor neuron that determine the out put activity.
Direct can be seen as alpha motor neurons while indirect can be noted as interneurons.
What are Betz cells
Upper motor neurons are present in the cerebral cortex where they synapse at the spinal cord.
Travel down the spinal cord via corticospinal tracts.
Key roles of skeletal muscle
- Movement
- Maintain posture
- Stabilizes joints
- Generates heat (core body temp determined by muscle mass)
- Lymphatic drainage
- Venous return
- Vision Reproduction
- Digestion
- Excretion
Why are there folds at the neuromuscular junction
Note the folds present at the neuromuscular junction, this allows the AcH receptors to be very close to the alpha motor neuron facilitating a fast synapse. However it allows for more voltage gated Na channels causing a faster transition from resting membrane potential and threshold.
What are active zones at the motor terminal
AcH is release upon calcium entry at these active zones
What type of receptor does AcH bind to at the motor endplate
Ligand gated ion channels
Diseases that will causes total failure of the neuromuscular synapse
Total failure of the neuromuscular synapse leads to muscle paralysis which can result in death due to respiratory failure
- Motor neuron disease - 80% die 3-5 yr post due to resp failure. Death of lower motor neurons
- Spinal muscular atrophy (SMA) - autosomal recessive disease caused by a defect in a SPECIFIC gene (SMN1). The SMN1 gene is vital in survival of alpha motor neurons
What is the result of partial failure of the neuromuscular junction and where does this occur.
Causes muscle weakness which may be due to inability to recruit enough larger motor units for strong muscle contractions. Causes defects in either the motor nerve terminal or in the region of muscle below the motor nerve ending known as the post synaptic region. Both have the same result of muscle weakness.
Process of AP generation at the neuromuscular junction
- AP arrives at motor nerve terminal triggering the opening of VGCC at the active zones and subsequent entry of Ca+
- Calcium causes the release of AcH from synaptic vesicles at the active zones
- AcH travels the synaptic cleft to bind to the ligand gated ion channels which causes large Na movement in and small K out.
- Results in local depolarisation which creates a current flow with the adjacent membrane of the muscle causing Na influx at this adjacent segment.
- Causes a rise in membrane potential to threshold triggering an AP
- ACh is destroy via the enzyme Acetylcholinesterase
2 examples of disease causing partial failure at the neuromuscular junction
- Defects in the motor nerve terminal (Lambert Eaton Syndrome) -auto immune disease effecting the pre synaptic voltage gated calcium channels resulting in poor neurotransmitter release → muscle weakness
- Defects in the muscle’s post synaptic region (Myasthenia Gravis) - Causes a decrease in the post synaptic AcH receptors causing the frequency of muscle APs to drop → weakness
What are muscular dystrophies
Muscular dystrophies → Duchenne’s Muscular dystrophies cause muscle weakness due to a mutation in the dystrophin gene (none).
This means that during eccentric contractions the membrane can be damaged causing calcium to flow out and proteases to flow in.
Two ways in which the neuromuscular system grades the force of contraction
The neuromuscular system grades the force of contraction in two main ways, rate modulation and motor unit recruitment.
What is rate modulation
frequency in which muscle APs can be generated
Single AP generates short twitch so to generate larger contraction multiple APs need to arrive from MU in close succession. This will result in twitch summation. This will eventually reach a point known tetanus.
This is only relevant for SINGLE muscle fiber tension. Whole muscle tension needs this as well as motor unit recruitment.
Different MUs have different firing properties.
What is the safety factor at the neuromuscular junction
The neuromuscular synapses has a safety factor, there are more AcH receptors than will ever be used meaning that regardless of the amount of neurotransmitters that is release an excitatory response can always cause an AP.
What is a motor unit
1 motor neuron and all of the muscle fibres it innervates
