week 1 motor units

Questions and Answers

What is the primary function of upper motor neurons located in the cerebral cortex?

Regulation of muscle tone

Initiation of voluntary movements (correct)

Activation of α-motor neurons

Mediating sensory-motor reflexes

Which structure plays a key role in maintaining posture and balance in response to sensory inputs?

Cerebellar neurons

Upper motor neurons in the brainstem (correct)

Local circuit neurons

α-motor neurons

Which type of neurons are primarily responsible for activating α-motor neurons?

Local circuit neurons (correct)

Interneuron pathways

Sensory neurons

Upper motor neurons

What capability do local circuit neurons possess even without inputs from the brain?

Coordinate highly coordinated limb movements Signup and view all the answers

Where are local circuit neurons primarily located?

Spinal cord or motor nuclei of brainstem cranial nerves Signup and view all the answers

Which of the following best describes the role of upper motor neurons in skilled movements?

They are essential for complex spatiotemporal sequences of skilled movements. Signup and view all the answers

What sources do upper motor neurons in the brainstem respond to for their regulatory functions?

Vestibular, auditory, visual, and somatic sensory inputs Signup and view all the answers

What is a critical characteristic of local circuit neurons in terms of their operation?

They maintain interconnections for rhythmical and stereotyped behavior. Signup and view all the answers

What role does the cerebellum play in relation to motor control?

It detects and corrects motor errors through feedback control. Signup and view all the answers

Which type of motor unit has the lowest activation threshold and is the first to be recruited?

Slow (S) motor units Signup and view all the answers

Which statement accurately describes the basal ganglia's function?

It prepares circuits for initiating voluntary movements. Signup and view all the answers

Which motor unit type is associated with the MyHC-I isoform?

Slow (S) motor units Signup and view all the answers

What is true about motor units in muscle contraction?

Motor units represent the smallest force the muscle can exert. Signup and view all the answers

What is a characteristic of fast fatigable (FF) motor units?

Generate high levels of force in brief contractions Signup and view all the answers

How do cerebellum and basal ganglia differ in their functions regarding movement?

The cerebellum helps in long-term motor learning and error correction. Signup and view all the answers

What can result from a malfunction in the basal ganglia?

Development of motor control disorders, such as Parkinson's disease. Signup and view all the answers

Which type of muscle fibres are fast fatigue-resistant (FR) motor units associated with?

Fast Oxidative/Glycolytic (FOG) type IIa muscle fibres Signup and view all the answers

What defines a motor neuron pool?

A group of motor neurons that collectively innervate a single muscle. Signup and view all the answers

Which motor unit type tends to innervate a larger number of powerful muscle fibres?

Fast fatigable (FF) motor units Signup and view all the answers

Which of the following is NOT characteristic of the cerebellum?

Directly initiates movements through lower motor neurons. Signup and view all the answers

What distinguishes slow (S) motor units from fast motor units?

Rich in myoglobin and mitochondria Signup and view all the answers

What is the main structural feature of a lower motor neuron axon?

It branches within the muscle to impact multiple muscle fibres. Signup and view all the answers

What method can be used for classifying muscle fibre types?

SDS-PAGE electrophoresis or ATPase histochemistry Signup and view all the answers

What is a key feature of fast fatigable (FF) motor units regarding their mitochondria?

Sparse mitochondria, contributing to easy fatigue Signup and view all the answers

What is the primary focus of ATPase histochemistry in muscle analysis?

Identifying muscle fiber types through staining Signup and view all the answers

Which MyHC isoform is added to the classical types in the electrophoretic SDS-PAGE method?

Type IIx Signup and view all the answers

What is an average innervation ratio for the soleus muscle?

180 muscle fibers per motor neuron Signup and view all the answers

Which of the following muscles has the highest innervation ratio?

Gastrocnemius muscle Signup and view all the answers

What phenomenon describes the change in muscle fiber phenotype due to electrical nerve stimulation?

Motor unit plasticity Signup and view all the answers

How many muscle fibers can the extraocular muscles control per motor unit on average?

3 fibers Signup and view all the answers

What is a significant outcome of chronic electrical nerve stimulation on muscle fibers after 56 days?

Acquisition of the histochemical phenotype of slow oxidative fibers Signup and view all the answers

Which of these muscle fibers types may correspond to both MyHC isoforms IIb and IIx?

Type IIb fibers Signup and view all the answers

What does the size principle in motor unit recruitment explain?

Smaller neurons can reach threshold voltage more easily. Signup and view all the answers

According to Ohm's Law, which formula is correctly matched with its components?

V = I x R Signup and view all the answers

How does the size of a neuron's resistance affect its voltage change with the same synaptic input?

Larger neurons have greater resistance but a smaller voltage change. Signup and view all the answers

Why can smaller motoneurons reach action potential even with smaller voltage changes?

They are more sensitive to synaptic inputs. Signup and view all the answers

Which of the following statements about motoneuron recruitment is true?

Smaller motoneurons are recruited before larger ones during muscle force regulation. Signup and view all the answers

If a motoneuron has higher resistance, what effect does this have on the current it can generate with a given voltage?

It will generate a smaller current. Signup and view all the answers

Which equation correctly reflects the relationship between current, voltage, and resistance for a neuron?

I = V / R Signup and view all the answers

What happens to the voltage change across a small motoneuron as compared to a large motoneuron when subjected to the same synaptic input?

Small motoneurons exhibit a larger voltage change. Signup and view all the answers

Study Notes

Upper Motor Neurons (UMNs)

Cell bodies are localized in the cerebral cortex or brainstem.
UMNs in the cortex initiate voluntary movements and manage complex skilled sequences.
Axons of UMNs synapse with local circuit neurons and sometimes directly with lower motor neurons for distal muscles.
UMNs in the brainstem regulate muscle tone and control posture and balance influenced by sensory inputs (vestibular, auditory, visual, somatic).

Local Circuit Neurons

Interneurons responsible for activating α-motor neurons.
Located near corresponding α-motor neurons in the spinal cord or cranial nerve motor nuclei.
Receive descending projections from higher centers and mediate sensory-motor reflexes.
Maintain interconnections for rhythmic and stereotyped behaviors.
Can coordinate involuntary movements like walking independently from brain inputs.

Cerebellum and Basal Ganglia

Complex circuits that regulate upper motor neurons without containing motor neurons themselves.
The cerebellum detects and corrects motor errors and is essential for motor learning.
The basal ganglia suppress unwanted movements and prepare UMNs for movement initiation, with malfunctions linked to Parkinson’s and Huntington’s diseases.

Motor Neuron – Muscle Relationship

Each lower motor neuron innervates muscle fibers within a single muscle and can branch to synapse on multiple fibers.
Each muscle fiber is innervated by one α-motor neuron, which activates all fibers it innervates upon reaching an action potential threshold.
A motor unit consists of a motor neuron and the muscle fibers it innervates, ensuring even distribution of contractile force across a muscle.

Motor Unit Types

Motor units vary in size, with size correlated to the number of muscle fibers innervated and the force generated.
Small α-motor neurons innervate fewer muscle fibers for low force, while larger neurons innervate more fibers for greater force.
Three major motor unit types include:
- Slow (S) Motor Units: Small size, low activation threshold, high endurance, important for sustained contractions.
- Fast Fatigue-Resistant (FR) Motor Units: Intermediate size, generates higher force than S units, fatigue-resistant.
- Fast Fatigable (FF) Motor Units: Largest size, highest force output, easily fatigable.

Muscle Fiber Typing

Muscle fiber types can be classified via ATPase histochemistry or SDS-PAGE electrophoresis.
ATPase histochemistry identifies fiber types (Type I, IIa, IIb) based on staining.
SDS-PAGE separates proteins by mass to determine MyHC protein types, including additional type IIx.
Potential overlap in fiber types due to MyHC isoform variations (e.g., IIb may correspond to MyHC-IIb or IIx).

Muscle Unit Distribution

Different muscles have varying innervation ratios; for example, the soleus has 180 fibers per motor neuron, while gastrocnemius has 1000-2000.
Extraocular muscles have a low innervation ratio of 3 fibers per unit, allowing for fine control.
Use-dependent motor unit plasticity allows for fiber phenotype shifts after chronic electrical stimulation, which has implications for therapies in paralyzed patients.

Regulation of Muscle Force

Recruitment follows the size principle; smaller motor neurons are activated first with lower thresholds.
Describes the relationship defined by Ohm’s Law (I = V / R), with regard to current, voltage, and resistance.
Smaller neurons have higher resistance and lower voltage change for action potential induction, maintaining the same synaptic input efficacy across neuron sizes.

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Description

Explore the functions and roles of upper motor neurons (UMNs) located in the cerebral cortex and brainstem. This quiz covers their significance in the initiation of voluntary movements and the regulation of muscle tone. Understand the connections UMNs make with local circuit neurons and lower motor neurons for skilled movements.