14. Lower Motor Neurons and Neuromuscular Junction

What are the two types of neurons that comprise that two neuron pathways that is responsible for movement?

Upper Motor Neurons and Lower Motor Neurons

____ neurons originate in higher regions of the brain and influence ___ neurons usually through an interneuron

Upper motor neurons originate in higher regions of the brain (cortex and brainstem) and influence activity of LMNs

Efferent neuron of the PNS that connects the CNS with the muscle to be innervated

Lower motor neuron

What are the two main components of the LMNs

1) somatic motor neurons (traditional motor neuron that directly innervates the skeletal muscle)

2) visceral motor neurons (component of the ANS that regulates smooth muscles and glands)

T/F - the entire function of the CNS is manifested through the LMN

TRUE

T/F - the somatic efferent motor neuron of the LMNs are unmyelinated

FALSE; they are myelinated

NT of the somatic motor neurons

ACh

NT of the autonomic motor neurons

ACh (presynaptic) and then NE or ACh (postsynaptic)

What are the effectors/target organs of the somatic motor neurons (somatic efferents)

skeletal muscle

What are the effectors/target organs of the autonomic motor neurons (visceral efferents)?

Smooth muscle, cardiac muscle, glands

Where are the neuronal cell bodies of the somatic efferent system located?

Ventral gray columns throughout the entire spinal cord including cervical, thoracic, lumbar and sacral segments (remember: these are a type of LMN)

LMN in ventral grey matter leave through the ________ ______ where as LMN of cranial nerves leave through the _________

ventral root

skull

Where are the neuronal cell bodies of the visceral efferent system located?

Intermediolateral gray columns - T1L4 region for sympathetic, sacral region for parasympathetic

Where are the cell bodies of the cranial nerves with motor functions (all but 1, 2 and 8) located?

Specific brainstem areas

Single motor neuron and the muscle fibers it innervates

The motor unit

The strength of a muscle contraction depends on what?

The number of motor units activated in a muscle (THINK: pelvic limb has more motor units than the face because it is stronger than the face)

Synapse between the axons of a motor neuron and skeletal muscle fiber

Neuromuscular junction

What type of receptor does ACh bind to at the motor end plate?

Nicotinic

What triggers the release of ACh by exocytosis from the active zone of the axon terminal?

Ca (voltage gated Ca channel)

Membrane proteins that are involved in the fusion, docking and release of ACh at the active zone

SNAREs

How many AChs need to bind to a nicotinic receptor to open it at the motor end plate?

Two

Transient depolarization of the Motor end plate region due to ionic currents (Na and K) in the postsynaptic terminal

Endplate Potential

T/F - Endplate potentials are an example of an excitatory postsynaptic potential

TRUE

When the ACh binds to nicotinic receptors at the motor end plate and results in influx of Na, what kind of channels also open due to this local current flow?

Voltage gated Na channels

The ACh is cleaved into acetate and choline by what enzyme

acetylcholinesterase; then choline is reuptaked into the axon terminal to be re-synthesized

How does curare lead to paralysis?

Competes with ACh for postsynaptic nicotinic receptor sites and therefore there is no AP and no muscle movement

T/F - curare binding is irreversible

FALSE; curare binding is reversible

What is a non-depolarizing muscle relaxant?

Curare

How do organophosphates work?

It is an IRREVERSIBLE inhibitor of acetylcholinesterase and therefore, ACh is never broken down, so there is no choline and therefore less ACh released eventually and no muscle movement

What are the main clinical signs for nicotinic receptors (somatic lower motor neuron signs)?

Tremors progressing to paralysis, neuromuscular weakness, seizures

What are the main clinic signs for muscarinic receptors (visceral lower motor neuron signs)?

excessive salivation, increased defecation/diarrhea, miosis, vomiting, bronchospasm, brachycardia

What are some disorders of the neuromuscular junction?

-Myasthenia gravis

-Botulism

-Tick paralysis

*Signaled by a various degree of muscle weakness and wastage

Disease that antibodies form against the nicotinic Ach receptors which results in diminished neuromuscular conduction and is irreversible

Myasthenia gavis (associated with hypothyroidism, thymoma and other neoplasms)

Disease that interferes with docking proteins by blocking one or more of the synaptic vesicle proteins which results in diminished neuromuscular conduction and weakened muscles

Botulism

Disease that decreases calcium entry and consequently interfere with vesicular exocytosis and NT release

Tick paralysis