Physiology Lecture 4: Motor Neurons

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Questions and Answers

Where do upper motor neurons primarily reside?

Upper motor neurons primarily reside in the motor region of the brain stem or cerebral cortex.

What role do upper motor neurons in the pyramidal tract play?

Upper motor neurons in the pyramidal tract are involved in controlling conscious movements.

What is the function of the rubrospinal tract?

The rubrospinal tract is involved in involuntary movements that improve and maintain balance.

What types of fibers do alpha motor neurons innervate?

Alpha motor neurons innervate extrafusal muscle fibers. Signup and view all the answers

What distinguishes the function of extrapyramidal tracts from pyramidal tracts?

Extrapyramidal tracts are involved in subconscious motor processes, while pyramidal tracts are responsible for conscious movements. Signup and view all the answers

What roles do general visceral motor neurons play in the autonomic nervous system?

They contribute to both sympathetic and parasympathetic functions, controlling involuntary actions of the heart and other viscera. Signup and view all the answers

What are the key processes that occur at the neuromuscular junction?

The motor neuron releases acetylcholine (Ach) which binds to post-synaptic receptors, leading to muscle contraction. Signup and view all the answers

What are the clinical findings associated with upper motor neuron lesions?

Upper motor neuron lesions lead to hyperreflexia, spasticity, and a positive Babinski reflex. Signup and view all the answers

How do lower motor neuron lesions differ in their clinical presentation from upper motor neuron lesions?

Lower motor neuron lesions cause hyporeflexia, flaccid paralysis, and atrophy, unlike the spasticity seen in upper motor neuron lesions. Signup and view all the answers

What is the consequence of motor neuron axon degeneration in conditions like spinal muscular atrophy?

It impairs signals from the brain to the muscles, leading to movement difficulties and weakness. Signup and view all the answers

Where are upper motor neurons located?

Upper motor neurons are located in the cerebral cortex of the brain or the brain stem. Signup and view all the answers

What is the primary function of lower motor neurons?

Lower motor neurons extend from the spinal cord to the muscle fibers and tendons, facilitating muscle contraction. Signup and view all the answers

What role do upper motor neurons play in voluntary motor responses?

Upper motor neurons are consulted during voluntary motor responses to coordinate thought and action. Signup and view all the answers

How does the nervous system respond during a reflex action?

In reflex actions, the nervous system executes automatic responses without involving upper motor neurons. Signup and view all the answers

What is a reflex arc?

A reflex arc is a neural pathway that controls reflex actions, involving afferent and efferent nerves. Signup and view all the answers

What is the primary function of the lower motor neurone?

To transmit signals from the upper motor neurone to the effector muscle. Signup and view all the answers

What are the three broad types of lower motor neurons?

Somatic motor neurons, special visceral efferent motor neurons, and general visceral motor neurons. Signup and view all the answers

Where are alpha motor neurons primarily located?

In the brainstem and spinal cord, specifically in the anterior horn. Signup and view all the answers

What is the role of gamma motor neurons?

To innervate muscle spindles and regulate their sensitivity to stretch. Signup and view all the answers

What do branchial motor neurons innervate?

Muscles of the head and neck that derive from branchial arches. Signup and view all the answers

What functions do general visceral motor neurons contribute to?

Both sympathetic and parasympathetic functions of the autonomic nervous system. Signup and view all the answers

What characterizes the neuromuscular junction?

It is a specialized synapse between a motor neuron and a muscle fiber. Signup and view all the answers

How do beta motor neurons differ from alpha motor neurons?

Beta motor neurons innervate both extrafusal and intrafusal fibers, while alpha motor neurons primarily innervate extrafusal fibers. Signup and view all the answers

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Study Notes

Motor Neurons Overview

Motor neurons are critical for transmitting signals from the central nervous system to muscles, enabling movement.
Upper motor neurons are situated in the cerebral cortex or brainstem; lower motor neurons are found in the spinal cord, extending to muscle fibers and tendons.

Upper and Lower Motor Neurons Interaction

Lower motor neurons rely on upper motor neurons for complex motor tasks, aiding coordination between thoughts and actions for voluntary movements.
In reflex activity, lower motor neurons operate independently from upper motor neurons, resulting in faster responses.

Types of Lower Motor Neurons

Three main types of lower motor neurons:
- Somatic motor neurons: Control skeletal muscles.
- Special visceral efferent motor neurons: Innervate head and neck muscles from branchial arches.
- General visceral motor neurons: Involved in autonomic functions, influencing involuntary muscle contractions.

Somatic Motor Neurons

Classified into alpha, beta, and gamma motor neurons:
- Alpha motor neurons: Innervate extrafusal muscle fibers, primarily responsible for skeletal muscle contraction; cell bodies in the anterior horn of the spinal cord.
- Beta motor neurons: Less understood, innervate both extrafusal and intrafusal muscle fibers.
- Gamma motor neurons: Innervate muscle spindles, regulating their sensitivity and assisting in the fine-tuning of muscle contractions.

Neuromuscular Junction

Specialized synapse where motor neurons communicate with muscle fibers through the release of acetylcholine (Ach), facilitating muscle contraction.

Lesions and Their Effects

Upper motor neuron lesions lead to:
- Hyperreflexia, spasticity, and a positive Babinski reflex (toe fanning upward).
Lower motor neuron lesions cause:
- Hyporeflexia, flaccid paralysis, and muscle atrophy.

Pathways of Upper Motor Neurons

Upper motor neurons are divided into:
- Pyramidal tracts: Control conscious movements, originating in the motor cortex.
- Extrapyramidal tracts: Involved in subconscious processes such as posture and balance (e.g., rubrospinal and tectospinal tracts).

Overall Function of Motor Neurons

Motor neurons are essential in converting electrical impulses from the brain into coordinated muscle actions, enabling voluntary and reflexive movements.