Motor Neurons/ Motor Units/ Neuromuscular synapses Flashcards
ACH (Acetylcholine NT)
Synthesis (precursor enzymes):
- Enzymes
- Cell body
- Acetyl CoA & Choline
- in terminal
Storage & Release:
- Small clear vesicles
Effect:
- Nicotinic
- EPSP
- Can also be muscarinic
Removal:
- AChE reputake by Choline
Dopamine (DA)
Synthesis (precursor & enzymes)
- Tyrosine hydroxylase & Tyrosine
Effect:
- EPSP on metabotropic
Removal:
- Reuptake- neuron or glia by NET/DET
- Diffusion, inactivation (COMT) Enzymes MAO , COMT
Norepinephrine (NE)
Synthesis (precursor enzymes):
- Tyrosine hydroxylase, tyrosine
Effect:
- EPSP on metabotropic
Removal:
- Reuptake- neuron or glia by NET/DET
- Diffusion, inactivation (COMT) Enzymes MAO , COMT
Glutamate
Synthesis (precursor enzymes):
- Glutamine in terminal
Storage & release:
- Small, clear vesicles
Effect:
- EPSP on metabotropic and ionotropic
Removal:
- Reuptake by neuron & glia, recycled, enzymes
GABA
Synthesis (precursor enzymes):
- Glucose to glutamate
Storage & release:
- Small, clear vesicles
Effect:
- IPSP on metabotropic and ionotropic
Removal:
- Reuptake by neuron & glia, recycled, enzymes
Neuropeptides
Synthesis (precursor enzymes):
- Prepropeptide made in cell body, processed in mature terminal
Storage and release:
- Large dense vesicles
Effect:
- Metabotropic
Removal:
- Peptidases
Lower Motor Neurons
(Commanded by the muscle to move in the CNS)
Somatic Motor Neurons (2 types):
- Alpha motor neurons
- Innervate skeletal muscles- main force generating muscles
- Referred to LMN–> final command
- Gamma motor neurons
- Innervate the stretch sensing organ within a muscle
Visceral motor neurons:
- Input from CNS (cell bodies in spinal cord & brain stem) to organs, cardiac muscle and smooth muscle)
- Autonomic nervous system telling the gut how to contract
Motor Unit (LMN)
- Single alpha motor neuron that has multiple terminals that synapse on individual muscle cells
- Called a motor unit
Muscle cells
- Muscle fibers (myofibers/myocytes)
- 1 muscle cell is ONLY innervated by 1 motorneuron
Motor Unit:
- 1 Motor neuron innervates multiple muscle cells = myofibers/myocytes
- A muscle cell contails mutiple myofibrils (actin/myosin)
- 1 motor neuron has multiple terminals which synapse on individual muscle cells
- Single axon branches innervate multiple muscle cells
- Fibers are scattered over a large area of muscle
Motor neuron pool refers to:
All the alpha motor neurons (motor units) that innervate a single muscle
Motor Unit to Skeletal Muscle
- 1 alpha motor neuron with multiple
- terminals which synapse on
- individual muscle cells = muscle fibers (myofibers/myocytes) which contain
- myofibrils
The cell bodies of Alpha motor neurons are located:
in the ventral horn of the spinal cord
Sensory Afferent
Motor Efferent
(SAME)
Motor Units & Integration
- Alpha motor neurons are integrators
- Inputs to alpha motor neurons
- Receive many synapses from multiple sources both excitatory and inhibitory
- Large neurons
- Lengthy, branched dendrites
- Synapses on muscle fibers (one neuron to many fibers)
ONLY generate AP in the muscle after a voltage threshold is exceeded!
CNS: Activation of Movement via Muscle Contraction
- Inputs from alpha motor neurons
- ONLY pathway for CNS to activate movement via muscle contraction
- Synapse with Smooth and Cardiac Muscle
- Mutiple cells/synapse (2 neuron chain to reach smooth muscle, ANS)
- Can have Gap Junction Coupling
- Norepinephrine (NE) major transmitter
- Ach, seretonin, and others too
- Synapse with skeletal muscle
- 1 alpha motor neuron/muscle cell
- Ach is the only NT
- Synapse with Smooth and Cardiac Muscle
Neuromuscular Junction: Basics
CHEMICAL SYNAPSE!
- Location where alpha motor neuron terminals synapse on skeletal muscles
- Model for synaptic action
- Synaptic structure is similiar to neuron-neuron synapse
- Presynaptic Axon
- Presynaptic Membrane
- Synaptic Cleft
- Postsynaptic membrane
- Muscle Cell
Neuromuscular Junction Logistics:
Motor end-plate: muscles post-synaptic membrane which motor neuron innervates
- 1 motor neuron can innervate multiple muscle fibers
- Motor axon loses its myelin sheath and then it branches to synapse on multiple cells at the motor-end plate region
- Junctional folds in motor end-plate region and a cleft filled with mucopolysaccharide “glue”
- Varicosities (contact from alpha motor neuron) covered by Schwann cells- each acts as a synaptic bouton
- Ach released from presynaptic boutons
- Ligand gated ion channels
- Voltage gated Na+ channels
*
Neuromuscular Junction- Synaptic Cleft
- Within the synaptic cleft, basement membrane is present
- Basement membrane holds the neuron to the muscle fiber
- Collagen and ECM proteins
- Forms an anchor for acetylcholinesterase (recycles Ach)
- Junctional folds on postsynaptic cell = Ach receptors & voltage gated channels
Muscle Receptor at Neuromuscular Junction
- Ach released at NMJ
- Nicotinic, Ionotropic receptor
- Very fast transmission
- Muscle receptors located in the crests of the folds
*
Nicotinic Receptors
- Neuromuscular Junction
- Found in muscle
- Curare (antagonist)
- Ach = only muscles
Electrical Events at Neuromuscular Junctions
(Initiating an AP in Muscle)
Alpha motor neurons excite muscle by opening channels at the endplate - Na+ ligand-gated channels on muscle cell
- AP travels down motor neuron axon to terminal
- Voltage gated Ca2+ channels open
- Ca2+ enters the terminal causing Ach release via exocytosis
- Ach diffuses across the synaptic cleft and binds Ach receptors on the muscle cell
- ACH binding causes ligand-gated cation channels to open
- Na+ enters muscle fiber & K+ leaves the muscle fiber
Net influx of Na+ into muscle fiber= membrane potential less (-)
- Acetylcholinesterase breaks down Ach into choline and acetate- channels close
- Reuptake of Ach @ presynaptic membrane
Threshold reached = AP propagates along sarcolemma
- 70 mV depolarization
- EPP (way more dramatic than EPSP)
** Caused by ionic current through Ach channel
EPP vs. EPSP
Endplate Potential (EPP): Always Excitatory
- Multiple synapses on endplate
- Input from only 1 cell
- Dramatic
- Large synaptic potential
- Endplate potential = 70 mV depolarization
- ALWAYS EXCITATORY- Ach used
- Refers to a change in membrane charge that causes the initiation of an action potential –> that propagates down the muscle fiber
- Non-integrating after motor neuron integrates
Excitatory Post-Synaptic Potential (EPSP):
- NT causes change in postsynaptic cell membrane potential (Vm)
- Input from 1000’s of neurons
- Functional diversity: excitatory and inhibitory–> NT diversity
- Small post-synaptic potential
- Small amount of NT released
- Integrating = summation
- Depolarization
- Increased Vm
- Moves toward threshold
What happens at NMJ when motor neuron is damaged?
Happens in skeletal muscle, smooth muscle, glands
Alpha motor neuron was secreting trophic growth factors that kept it healthy and Ach receptors localized at the end plate
Axon destroyed= dismantling of NMJ
Damage results in the receptors spreading further away from the endplate region
Don’t have a mechanism to get rid of the Ach
Properties of EPP
- Synaptic potential
- increases rapidly, decreases slowly
- rapid rise b/c of flood of Ach into cleft
- Ach diffuses through cleft to receptors on endplate
- Not all Ach reaches endplate
- Removed by
- diffusion out of cleft
- inactivation via hydrolysis
EPP is NOT ALL OR NONE
- Stimulate motor axons
- Spreads passively
- Membrane potenntial falls with distance from motor end plate
