Motor Neurons/ Motor Units/ Neuromuscular synapses Flashcards
(27 cards)
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
