Human Anatomy: Lower Motor Neurons Quiz

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Questions and Answers

What do the axons of lower motor neurons form?

Peripheral nerves

Dorsal roots

Cranial nerves

Ventral roots (correct)

How many spinal nerves are present in humans?

30 (correct)

Which spinal segments are responsible for the innervation of the arm muscles?

T1–T12

L1–L5

C1–C8

C3–T1 (correct)

What type of spinal nerves contain both sensory and motor fibers?

Mixed spinal nerves Signup and view all the answers

Which segment does not contribute to any spinal nerves in humans?

C10 Signup and view all the answers

What defines the spinal segments?

The vertebra where the nerve originates Signup and view all the answers

Which of the following correctly describes the distribution of lower motor neurons?

Distributed unevenly across different segments Signup and view all the answers

Which of the following pairs are correctly matched?

S1–S5: pelvic muscles Signup and view all the answers

What is defined as the combination of one alpha motor neuron and all the muscle fibers it innervates?

Motor unit Signup and view all the answers

Which term refers to the collection of alpha motor neurons that innervate a single muscle?

Motor neuron pool Signup and view all the answers

Why is it important to exert just the right amount of force during movements?

To avoid excessive energy waste and injury Signup and view all the answers

What happens if too much force is applied during a movement?

Crushing objects and wasting energy Signup and view all the answers

Which of the following statements is true about muscle contractions?

Muscle contraction relies on the combined actions of motor units. Signup and view all the answers

What does the term 'graded control' in muscle contraction refer to?

The ability to adjust force according to task demands. Signup and view all the answers

The actions of which type of neurons are primarily responsible for muscle contractions?

Alpha motor neurons Signup and view all the answers

What could happen if too little force is applied during a movement?

Inconsistent movement and possible inability to complete a task Signup and view all the answers

What characterizes red (dark) muscle fibers?

They have a large number of mitochondria and enzymes for oxidative energy metabolism. Signup and view all the answers

Why are fast (F) fibers more prone to fatigue compared to slow (S) fibers?

F fibers contain fewer mitochondria. Signup and view all the answers

Which muscles primarily contain slow (S) fibers?

Antigravity muscles of the leg and torso. Signup and view all the answers

How do pale (white) muscle fibers primarily generate energy?

By relying on anaerobic metabolism. Signup and view all the answers

What is the primary difference in the contraction speed of slow (S) and fast (F) muscle fibers?

Fast fibers contract rapidly and more powerfully than slow fibers. Signup and view all the answers

What type of muscle fibers would you expect to find in the flight muscles of birds that fly?

Mostly fast (F) fibers for quick movements. Signup and view all the answers

What is a common characteristic of white muscle fibers?

They possess fewer mitochondria than red fibers. Signup and view all the answers

Which of the following best describes the physical appearance of dark meat in chicken?

It is rich in mitochondria, giving it a darker color and richer flavor. Signup and view all the answers

What is the role of gamma motor neurons in the motor system?

To regulate muscle tone and monitor stretch Signup and view all the answers

Which organ is primarily responsible for proprioception related to tendon tension?

Golgi tendon organ Signup and view all the answers

What type of input do spinal interneurons primarily provide?

Inhibitory and excitatory input for reflexes Signup and view all the answers

What aspect of proprioception is associated with joint receptors?

Motion and position information from joints Signup and view all the answers

What is a key function of inhibitory inputs in the motor system?

Preventing excessive muscle activation Signup and view all the answers

Which statement accurately describes the generation of spinal motor programs for walking?

It involves a complex interplay of excitatory and inhibitory inputs. Signup and view all the answers

Which of the following best defines proprioception?

The sense of body position and movement Signup and view all the answers

How did Charles Sherrington summarize the importance of the motor system?

By emphasizing that moving things defines human capability. Signup and view all the answers

What happens to muscle fibers after prolonged inactivity?

Muscle fibers undergo atrophy Signup and view all the answers

What is the primary cause of death in ALS patients?

Respiratory muscle failure Signup and view all the answers

Which of the following symptoms is NOT typically associated with ALS?

Cognitive decline Signup and view all the answers

Who was the famous victim of ALS, known for his baseball career?

Lou Gehrig Signup and view all the answers

What is the estimated number of Americans currently diagnosed with ALS?

30,000 Signup and view all the answers

How quickly do ALS symptoms typically progress over time?

1–5 years Signup and view all the answers

Which type of neuron is closely related to muscle fibers in ALS?

Lower motor neuron Signup and view all the answers

What is a common consequence of immobilizing joints in a cast after an injury?

Muscle atrophy Signup and view all the answers

What characterizes fast fatigable (FF) muscle fibers?

They generate the strongest contractions. Signup and view all the answers

Which type of muscle motor unit is associated with small-diameter axons?

Slow motor units Signup and view all the answers

What type of muscle fibers are typically found in FF motor units?

Fast fatigable white fibers Signup and view all the answers

Which characteristic differentiates the alpha motor neurons of FF units from those of FR units?

FF neurons are generally the biggest. Signup and view all the answers

In how many types of motor units can muscle fibers coexist?

Three types Signup and view all the answers

How do the contractions of fatigue-resistant (FR) fibers compare to those of fast fatigable (FF) fibers?

FR fibers contract slower but are more sustainable. Signup and view all the answers

Which statement is true about the relationship between motor units and muscle fibers?

Each motor unit contains muscle fibers of only a single type. Signup and view all the answers

What feature distinguishes the motor neurons of different motor units?

Only FF neurons have the fastest conducting axons. Signup and view all the answers

Study Notes

Spinal Control of Movement

The motor system consists of all the muscles and the neurons that control them.
The sole executant of movement is muscle.
Behavior requires the coordinated action of almost 700 muscles.
The spinal cord has circuitry for coordinated movement, especially repetitive behaviors.
Motor control can be divided into two parts:
- Spinal cord's command and control of coordinated muscle contraction
- Brain's command and control of motor programs within the spinal cord

The Somatic Motor System

Muscles can be classified as striated or smooth
Smooth muscle is found in the digestive tract, arteries, and is innervated by the autonomic nervous system, responsible for peristalsis and blood pressure regulation.
Striated muscles include cardiac (heart) and skeletal muscle.
Cardiac muscle contracts rhythmically, even without innervation. It is controlled by the autonomic nervous system.
Skeletal muscle is found throughout the body and is responsible for movement of bones, eyes, respiration, facial expressions, and speech.
Skeletal muscle is enclosed in tendons at the ends of the muscle.
Each muscle fiber is innervated by a single axon from the central nervous system.

The Lower Motor Neuron

Somatic musculature is innervated by the somatic motor neurons in the ventral horn of the spinal cord. These cells are sometimes called lower motor neurons.
These neurons directly command muscle contraction (the final common pathway).

Segmental Organization of Lower Motor Neurons

Axons of lower motor neurons bundle together to form ventral roots.
Each ventral root joins with a dorsal root to form a spinal nerve that exits the cord through notches in the vertebrae.
Spinal nerves are mixed, containing sensory and motor fibers.
Motor neurons that supply a particular spinal nerve belong to a corresponding spinal segment (cervical, thoracic, lumbar, or sacral).
Distribution of skeletal muscles and lower motor neurons is uneven throughout the body and the spinal cord

Alpha Motor Neurons

Alpha motor neurons directly trigger the generation of muscle force.
An alpha motor neuron and all the muscle fibers it innervates collectively constitute a motor unit.
The collection of alpha motor neurons that innervates a single muscle is a motor neuron pool.

Graded Control of Muscle Contraction

The CNS controls force of contraction by:
- Varying the firing rate of motor neurons.
- Recruiting additional synergistic motor units.
Motor units vary in size, and are recruited based on the amount of force needed.

Muscle Fiber Structure

Muscle fibers are formed early in fetal development by the fusion of muscle precursor cells (myoblasts), resulting in multinucleated cells.
Each muscle fiber is enclosed in an excitable cell membrane called the sarcolemma.
Inside the muscle fiber, there are cylindrical structures called myofibrils which contract in response to an action potential.

Muscle Contraction

Myofibrils are divided into segments by z-lines.
A segment is a sarcomere.
Thin filaments slide along thick filaments.
Myosin heads bind actin, pivot, and disengage, causing the filaments to slide.

Excitation-Contraction Coupling

ACh is released from the axon terminals of alpha motor neurons, causing an action potential in the muscle fiber.
The action potential triggers the release of Ca2+.
The release of Ca2+ leads to contraction.
Relaxation occurs when Ca2+ levels are lowered.

Myasthenia Gravis

Myasthenia gravis is an autoimmune disease causing muscle weakness and fatigability, typically of the facial muscles.
It's characterized by antibodies against nicotinic ACh receptors, interfering with normal actions of ACh at the neuromuscular junctions.

ALS: Glutamate, Genes, and Gehrig

ALS is a progressive neurodegenerative disease affecting motor neurons.
Symptoms include muscle weakness and atrophy, swallowing and breathing difficulties.
A suspected cause is excitotoxicity (overstimulation by glutamate and other neurotransmitters).
Several genes are associated with ALS, including defects in the enzyme superoxide dismutase.

Proprioception

Proprioception is sensory information about where the body is positioned.
Muscle spindles are specialized structures within skeletal muscles.
Spindles are stretch receptors that indicate muscle length.
Group Ia sensory axons wrap around the muscle fibers in the middle third of the spindle capsule.
The stretch reflex causes a muscle to contract when it is stretched.

Golgi Tendon Organs

Golgi tendon organs are strain gauges measuring muscle tension.
They are series, not parallel.
These proprioceptors sense and communicate muscle force information to the spinal cord.

Proprioception from the Joints

Joints contain mechanosensitive axons that react to joint angle changes, velocity, and direction.

Spinal Interneurons

Interneurons mediate many spinal reflexes.
They receive input from sensory axons, descending pathways (brain), and lower motor neuron axons (collaterals).
Interneurons can be excitatory or inhibitory, coordinating motor programs. Reciprocal inhibition is an example of how interneurons work to enable coordinated movement.

Excitatory Input

The flexor reflex (e.g. stepping on a tack) involves coordinated activation of flexor muscles and inhibition of their antagonists.

The Generation of Spinal Motor Programs for Walking

Central pattern generators (CPGs) are spinal circuits generating rhythmic motor activity.
CPGs can function without brain input, even if the spinal cord is severed.
NMDA receptors are crucial for initiating complex locomotor patterns like walking in lamprey fish.

Concluding Remarks

Spinal cord control of movement involves intricate interactions across different levels, from biochemistry to behavior.
Sensory feedback (like muscle spindles and Golgi tendon organs) and descending pathways from the brain influence spinal activity and motor control.

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Description

Test your knowledge on the anatomy and functions of lower motor neurons in humans. This quiz covers the spinal nerves, spinal segments, and the mechanisms of muscle innervation and contraction. Determine how these concepts apply to muscle actions and force generation during movement.