Neuromuscular Junction Quiz

Lower Motor Neurons and the Neuromuscular Junction

• Lower motor neurons are located in the gray matter of the spinal cord ventral horn.
• They are also called alpha motor neurons.
• Lower motor neuron cell bodies are grouped into clusters called motor neuron pools.
• Each pool innervates a specific muscle; axons in a pool project to the same muscle.
• Axial muscles are located more medially in the spinal cord.
• More distal muscles (dexterity muscles) are located more laterally.
• Motor neurons' dendrites and cell bodies are in the spinal cord.
• Their axons leave the spinal cord via the ventral root.
• Axons travel in bundles through segmental nerves and peripheral nerves.
• Terminal endings of motor neurons branch to many muscle fibers in a muscle, forming a motor unit.
• The fibers are scattered throughout the muscle, not clustered.

Motor System Organization

• Muscle activation occurs by lower motor neuron (LMN) activation.
• Lower MNs are activated by sensory neurons (reflexes) and upper MNs.
• Local circuits of interneurons also play a role, such as in stepping.
• Descending pathways, such as from the motor cortex, influence voluntary movements.
• Basal ganglia and cerebellum coordinate movement initiation and execution.

Motor Unit Types

• Motor units vary in size.
• Small motor units innervate small "red" muscle fibers and are slow (fatigue-resistant) motor units. These are good for sustained contractions.
• Large motor units innervate large, pale muscle fibers, often with few mitochondria.
• Such units are fast, fatigable units.
• Medium units have properties between small and large units (fast, fatigue-resistant).

Precision of Control

• Motor unit size correlates with level of precision of muscle control.
• Fine control uses small motor units with limited force.
• Gross control uses large motor units for large force.

Neuromuscular Junction (NMJ)

• The NMJ is a synapse between a motor neuron and a muscle fiber.
• Electrical signals are passed through the release of a chemical messenger.
• Acetylcholine (ACh) is the key neurotransmitter released at the NMJ.
• ACh is always excitatory for muscles.
• ACh diffuses across the synaptic cleft (a very tiny gap).
• The muscle cell membrane exhibits junctional folds that increase the surface area for ACh receptors.
• Receptors are called nicotinic ACh receptors.
• The enzyme acetylcholinesterase (AChE) breaks down ACh.
• Muscarinic ACh receptors are found in the brain and smooth muscle.

Activation of Muscle Fibers by ACh

• ACh binds to receptors, triggering the opening of chemically gated cation channels.
• A graded depolarization results in an end-plate potential (EPP).
• A larger EPP will lead to greater depolarization in the muscle fibers leading to an action potential.
• EPPs must reach a certain threshold to trigger the muscle fiber action potential, for a safety factor.

Definition of a Motor Unit

• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
• Each muscle fiber is innervated by only one motor neuron.
• The muscle fibers within a motor unit are usually dispersed throughout the muscle.
• When a motor neuron is activated, all its fibers contract in unison.

Motor Neuron Pathways

• Dendrites and cell bodies are located in the spinal cord.
• Axons travel the length of the cord in bundles (ventral root).
• The axons enter peripheral nerves.
• The terminal endings of axons branch to many muscle fibers within the entire muscle.

Muscles of the Head and Neck

• Muscles of the head and neck often have motor neurons originating in the brainstem.
• The nerves carrying these axons are "cranial nerves."

Damage to Lower Motor Neurons

• Damage leading to paralysis, loss of reflexes, decreased muscle tone or atrophy.

Diseases of the Lower Motor Neurons

• Poliovirus can impact lower motor neurons.
• Acute flaccid myelitis (AFM) is rare and can damage lower motor neurons, causing weakness.
• Amyotrophic lateral sclerosis (ALS) is a degenerative disease, impacting both lower and upper motor neurons.

Disease of the NMJ

• Myasthenia gravis (MG) is an autoimmune disease that impacts the NMJ.
• Antibodies impair ACh receptors, leading to reduced strength of muscle contraction.

Muscle Spindles and Proprioception

• Muscle spindles are sensory receptors within muscle tissue.
• They are arranged in parallel with the muscle fibers.
• Muscle spindles provide the CNS with length and rate information of muscles.
• Contain sensory nerve endings, and motor gamma neurons that alter responsiveness.
• Different types of sensory endings are stimulated based on the length and rate of stretch, providing information about muscle position and movement.
• Gamma motor neurons help maintain sensitivity during muscle contraction.
• Muscle spindles help us sense position and movement, as well as causing reflexes.

Golgi Tendon Organs (GTOs)

• GTOs are sensory receptors in tendons (arrange in series with fibers).
• GTOs are sensitive to changes in muscle contraction; provide information about force and tension.
• Trigger inhibitory interneurons.

Autogenic Inhibition (Inverse Myotactic Reflex)

• GTOs are sensitive; during active contraction; activate interneurons, preventing further contraction of the same muscle
• GTO activates inhibitory responses to prevent over-contraction.

Joint Receptors

• Joint receptors help maintain movement and provide a protective function.
• Different types of joint receptors are sensitive to different joint motions. (e.g., Golgi-Mazzoni, Ruffini, etc)

Proprioception pathways

• Includes information from muscles (sensory endings in muscles), and joints.
• Conscious proprioception pathways project to higher cortical centers.
• Unconscious pathways go to the cerebellum.

Use of Proprioceptive Information in Central Processing

• Cerebellum acts in co-ordination of movement; learns skills such as typing and sports.
• Information gathered by proprioception is vital for proper movement planning and execution, particularly at extremes of range of motion.

Acute vs. Persistent Pain

• Acute pain is a warning signal during tissue damage and is essential for survival.
• Persistent pain results when nociceptors remain triggered inappropriately, or there are changes in CNS processing.
• Peripheral and central sensitization may lead to chronic pain.

Nociceptive Pain

• Nociceptors are activated by actual or potential tissue damage.
• Nociceptors and nerve fibers convey pain stimuli to the spinal cord.
• Different types of nociceptors exist, responding to different types of stimuli.
• Pain information converges upon specific neurons, and is conveyed to higher brain regions.
• Pain processing can be modified by other inputs and systems (e.g., gate control mechanisms, descending pain pathways).

Pain Neuromatrix

• The neuromatrix is a distributed network of neurons that contribute to the perception of pain.
• It is affected by sensory inputs from the body, cognitions, and emotions.
• Inputs and outputs influence the perception of pain (and the affective-motivational response to pain).

Mechanisms of Persistent Pain

• Persistent pain may occur even after tissue is repaired.
• Sensitization of the nervous system at peripheral and spinal cord levels occurs.
• Central sensitizations can change higher level pain centers to process input differently.
• Increased sensitization and decreased inhibiting neural systems contribute to persistent pain.

Changes to the CNS with Chronic Pain

• Structural and functional changes can occur with chronic pain.
• Brain activity and response to stimuli can alter in response to chronic pain.

Other Changes Related to Chronic Pain

• Sympathetic, parasympathetic and endocrine systems can be altered by chronic pain states.
• Cytokines can contribute to chronic pain.
• Cognitive factors (thoughts and feelings) play a significant role in processing pain.

Electromyography (EMG)

• EMG involves recording electrical activity from muscles.
• A motor unit is comprised of a motor neuron and the fibers it innervates.
• Various factors can affect EMG signals, including electrode placement and cross-talk.

Nerve Conduction Velocity (NCV) Testing

• NCV assesses the speed of nerve impulse transmission from stimuli to innervated muscles.
• Testing different portions of the nerve can detect if there is damage with NCV.

H-Reflexes and F-Waves

• The H-reflex measures the strength of monosynaptic stretch reflexes; assesses the integrity of the pathways; detects possible spinal compression.
• The F-wave, a different response, assess axon health and speed in a second portion of the peripheral nerve.

Summary of Techniques

• Various techniques may differ in their use.
• Clinical evaluation of NCV, and H-reflex data can help clinicians locate specific areas of concern.
• EMG, NCV, H-reflex and F-wave results are interpreted in the context of patient history and other clinical data.

Additional Information

• Various conditions can cause abnormal responses to testing.