Lower + Upper Motor System

What are lower motor neurons (LMNs)?

Neurons that directly innervate skeletal muscle fibers to produce muscle contraction.

Where are the cell bodies of spinal lower motor neurons located?

In the anterior (ventral) horn of the spinal cord.

What are upper motor neurons (UMNs)?

Neurons originating in motor-related regions of the cerebral cortex or brainstem that influence lower motor neuron circuits.

Where do cortical upper motor neurons originate?

In motor-related areas of the cerebral cortex, especially layer V.

What is the main function of upper motor neurons?

To send descending motor commands that influence spinal cord and brainstem motor circuits.

What is the major difference between UMNs and LMNs?

UMNs influence motor circuits; LMNs directly innervate skeletal muscle.

Which neurons form the final common pathway to skeletal muscle?

Lower motor neurons.

What are the major types of motor effectors shown in the lecture?

Cilia, glands, and muscles.

What are the two main categories of glands?

Endocrine and exocrine glands.

How do endocrine glands release their secretions?

They release hormones into the bloodstream.

What are examples of endocrine structures mentioned in the lecture?

Neuroendocrine cells, the pituitary gland, and endocrine organs such as adrenal glands.

How do exocrine glands release their secretions?

Through ducts or onto body surfaces.

What are examples of internal exocrine glands or cells?

Goblet cells and digestive glands such as salivary glands.

What are examples of external exocrine glands?

Sweat glands and sebaceous glands.

What are the three main types of muscle tissue?

Smooth muscle, cardiac muscle, and skeletal muscle.

What are visceral smooth muscles associated with?

Internal structures such as the intestine, bronchi, and blood vessels.

What peripheral structure containing smooth muscle is mentioned in the lecture?

The iris.

What are the two major regions of cardiac muscle mentioned?

Atrial and ventricular muscle.

Where is skeletal muscle found according to the lecture?

Trunk, limbs and digits, head, and neck.

What does striated muscle appearance mean?

It shows a repeating banding pattern.

Which two types of muscle are striated?

Cardiac and skeletal muscle.

What does SMS stand for?

Somatic motor system.

What does AMS stand for?

Autonomic motor system.

Which type of muscle is directly controlled by the somatic motor system?

Skeletal muscle.

How many motor neurons connect the CNS to skeletal muscle in the somatic motor system?

One lower motor neuron.

Through which side of the spinal cord do spinal motor axons leave?

The ventral side, through ventral roots.

What are the main effectors of the autonomic motor system?

Smooth muscle, cardiac muscle, and glands.

How many neurons are typically in the autonomic motor pathway from CNS to effector?

Two: a preganglionic neuron and a postganglionic neuron.

What is a preganglionic autonomic neuron?

The first neuron, which carries signals from the CNS to an autonomic ganglion.

What is a postganglionic autonomic neuron?

The second neuron, which carries signals from the autonomic ganglion to the effector.

whats the path for AMS

preganglionic → autonomic → postganglionic → receptor

What is the primary structural difference between somatic and autonomic motor pathways?

Somatic motor output uses one neuron; autonomic motor output generally uses a two-neuron chain.

What structures ultimately generate the physical force for movement?

Skeletal muscles acting on the skeletal system.

What is the role of spinal cord and brainstem motor circuits?

Integrate sensory inputs, coordinate reflexes, and influence lower motor neurons.

What is the role of the motor cortex?

Planning, initiating, and directing voluntary movements.

What is the role of brainstem motor centers?

Basic movements and postural control.

What is the main role of the basal ganglia in motor control?

Gating or regulating the proper initiation of movement.

What is the main role of the cerebellum in motor control?

Sensorimotor coordination and correction of ongoing movement.

What is the role of local circuit neurons?

To integrate motor and sensory information and coordinate motor neuron activity.

What are motor neuron pools?

Clusters of lower motor neurons that innervate a particular muscle.

What is the basic motor control hierarchy shown in the lecture?

Motor cortex and brainstem descending systems → spinal/brainstem local circuits and motor neuron pools → skeletal muscles.

How do sensory inputs contribute to motor control?

They enter local circuits and provide feedback used to adjust muscle activity.

What are the two classes of skeletal muscle lower motor neurons?

Alpha (α) and gamma (γ) motor neurons.

What do alpha motor neurons innervate?

Extrafusal skeletal muscle fibers.

What are extrafusal muscle fibers?

Force-producing skeletal muscle fibers responsible for contraction and movement.

What is the main function of alpha motor neurons?

Generate muscle force for movement and posture.

What do gamma motor neurons innervate?

Intrafusal muscle fibers inside muscle spindles.

What are intrafusal muscle fibers?

Specialized muscle fibers within muscle spindles that participate in proprioceptive sensing.

What is the main function of gamma motor neurons?

Regulate tension and sensitivity of muscle spindles.

Where do spinal alpha and gamma motor neurons originate?

In the anterior gray column or ventral horn of the spinal cord.

What is a muscle spindle?

A proprioceptive sensory receptor within skeletal muscle that detects muscle stretch.

What is the difference between extrafusal and intrafusal muscle fibers?

Extrafusal fibers produce force; intrafusal fibers are associated with stretch sensing in muscle spindles.

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Can a motor neuron pool extend over multiple spinal cord segments?

Yes, one muscle's motor pool can span several spinal segments.

Why can motor neuron pools extend across multiple segments?

They allow distributed spinal motor neurons to coordinate activity in the same muscle.

Where are lower motor neurons controlling proximal muscles located in the ventral horn?

More medially.

Where are lower motor neurons controlling distal muscles located in the ventral horn?

More laterally.

What are proximal muscles?

Muscles closer to the body's center or trunk.

What are distal muscles?

Muscles farther from the trunk, such as muscles controlling fingers, hands, feet, and toes.

What is the main function of proximal musculature?

Posture, balance, and large body movements.

What is the main function of distal musculature?

Precise, skilled, fine movements.

What are spinal interneurons?

Neurons within spinal cord circuits that connect and regulate other neurons.

Why are interneurons necessary for movement?

Movement requires coordinated activity across multiple motor neurons and muscles.

Where are many spinal local circuit interneurons located?

In the intermediate zone of the spinal cord.

What does topographical arrangement of spinal local circuits mean?

Their location and connections are organized according to the body regions or muscles they regulate.

What type of interneuron connections are associated with medial/postural circuits?

Long-distance and bilateral connections.

What are commissural axons?

Axons that cross the spinal cord midline to connect the two sides.

Why are commissural interneurons important for posture?

They help coordinate motor activity on both sides of the body.

What are long-distance local circuit neurons?

Interneurons whose axons connect circuits across multiple spinal segments.

What are short-distance local circuit neurons?

Interneurons connecting nearby spinal circuits.

Which spinal circuits use more strictly local interneuron connections?

Lateral circuits involved in fine motor control.

Why do medial motor circuits require broader coordination?

Posture and balance require coordinated activity of muscles on both sides and across multiple segments.

What is a motor unit?

One alpha motor neuron and all the skeletal muscle fibers it innervates.

Can one alpha motor neuron innervate multiple muscle fibers?

Yes, its axon branches to innervate multiple fibers.

Do all motor units have the same number of muscle fibers?

No. Motor unit sizes vary according to their function.

Why are small motor units useful?

They allow more precise and finely graded muscle control.

Why are large motor units useful?

They generate greater muscle force.

What happens to muscle fibers when their only motor neuron is damaged?

They become denervated and lose their normal neural activation.

What is denervation?

Loss of the normal nerve supply to muscle fibers.

What spontaneous electrical activity may occur in denervated muscle fibers?

Fibrillation potentials.

What clinical recording technique is illustrated in the motor-unit slide?

Electromyography (EMG).

What are the three functional classes of motor units?

Fast fatigable, fast fatigue-resistant, and slow.

Which motor unit type has the largest motor neuron?

Fast fatigable.

Which motor unit type has a medium-sized motor neuron?

Fast fatigue-resistant.

Which motor unit type has the smallest motor neuron?

Slow.

Which motor unit type has the highest recruitment threshold?

Fast fatigable.

Which motor unit type has an intermediate recruitment threshold?

Fast fatigue-resistant.

Which motor unit type has the lowest recruitment threshold?

Slow.

Which motor unit type has the fastest conduction?

Fast fatigable.

Which motor unit type has intermediate conduction speed?

Fast fatigue-resistant.

Which motor unit type has the slowest conduction?

Slow.

Which motor unit type produces the greatest force?

Fast fatigable.

Which motor unit type produces intermediate force?

Fast fatigue-resistant.

Which motor unit type produces the smallest force?

Slow.

Which motor unit type fatigues most quickly?

Fast fatigable.

Which motor unit type has intermediate fatigue resistance?

Fast fatigue-resistant.

Which motor unit type is most fatigue resistant?

Slow.

What activities are associated with fast fatigable motor units?

High-force movements such as running or jumping.

What activity is associated with fast fatigue-resistant motor units?

Walking or other intermediate-force activities.

What function is associated with slow motor units?

Sustained postural activity.