Motor Unit Structure and Function

Motor Unit

• A motor unit consists of a lower motor neuron and all the skeletal muscle fibers it innervates
• Lower motor neurons are efferent neurons that transmit signals from the central nervous system to skeletal muscles
• The term "motor unit" refers to a single lower motor neuron and all the muscle fibers it controls
• A single lower motor neuron can innervate multiple muscle fibers, and a single muscle fiber can receive signals from multiple lower motor neurons

Structure of a Motor Unit

• A lower motor neuron has a soma, an axon, and terminal buttons
• The axon terminals release neurotransmitters that bind to receptors on the surface of muscle fibers
• The connection between the axon terminal and the muscle fiber is called a neuromuscular junction (NMJ)

Function of a Motor Unit

• When a lower motor neuron is stimulated, it releases neurotransmitters that bind to receptors on muscle fibers, causing them to contract
• The coordinated contraction of multiple muscle fibers innervated by a single lower motor neuron results in movement
• Motor units are the basic functional units of the motor system

Types of Motor Units

• Small motor units: consist of a small number of muscle fibers and are responsible for fine movements, such as those of the eyes and fingers
• Large motor units: consist of a large number of muscle fibers and are responsible for gross movements, such as those of the limbs and torso

Abnormalities of Motor Units

• Weakness: can result from damage to lower motor neurons or muscle fibers
• Atrophy: a decrease in the size of muscle fibers, often due to lack of stimulation by lower motor neurons
• Fasiculations: abnormal contractions of muscle fibers, often due to damage to lower motor neurons
• Hypotonia: decreased muscle tone, often due to damage to lower motor neurons
• Hyporeflexia: decreased reflexes, often due to damage to lower motor neurons

Motor Unit

• A motor unit consists of a single lower motor neuron and all the skeletal muscle fibers it innervates.
• Lower motor neurons transmit signals from the central nervous system to skeletal muscles.
• A single lower motor neuron can innervate multiple muscle fibers, and a single muscle fiber can receive signals from multiple lower motor neurons.

Structure of a Motor Unit

• A lower motor neuron has a soma, an axon, and terminal buttons.
• The axon terminals release neurotransmitters that bind to receptors on the surface of muscle fibers.
• The connection between the axon terminal and the muscle fiber is called a neuromuscular junction (NMJ).

Function of a Motor Unit

• When a lower motor neuron is stimulated, it releases neurotransmitters that bind to receptors on muscle fibers, causing them to contract.
• The coordinated contraction of multiple muscle fibers innervated by a single lower motor neuron results in movement.
• Motor units are the basic functional units of the motor system.

Types of Motor Units

• Small motor units consist of a small number of muscle fibers and are responsible for fine movements, such as those of the eyes and fingers.
• Large motor units consist of a large number of muscle fibers and are responsible for gross movements, such as those of the limbs and torso.

Abnormalities of Motor Units

• Weakness can result from damage to lower motor neurons or muscle fibers.
• Atrophy is a decrease in the size of muscle fibers, often due to lack of stimulation by lower motor neurons.
• Fasiculations are abnormal contractions of muscle fibers, often due to damage to lower motor neurons.
• Hypotonia is decreased muscle tone, often due to damage to lower motor neurons.
• Hyporeflexia is decreased reflexes, often due to damage to lower motor neurons.

Motor Unit

• Consists of a lower motor neuron and all the skeletal muscle cells it contacts and controls
• Lower motor neurons are efferent neurons of the peripheral nervous system, carrying information away from the central nervous system
• Controls skeletal muscle, which is the main muscle type of our body, connected to our skeleton to move us around

Neuromuscular Junction

• The synapse between a lower motor neuron and a skeletal muscle cell
• Lower motor neurons synapse with multiple skeletal muscle cells, forming multiple neuromuscular junctions

Function of Motor Unit

• When a lower motor neuron fires an action potential, it causes all the skeletal muscle cells in its unit to contract
• This is why the motor unit is called a unit, as the skeletal muscle cells function together as a single unit

Organization of Motor Neurons

• Soma of motor neurons are in the spinal cord or brain stem
• Axons pass out through cranial nerves or spinal nerves into the peripheral nervous system
• Axons branch out and synapse on skeletal muscle cells in their motor unit

Types of Motor Units

• Small motor units: found in small muscles that need rapid precise control (e.g., eye movement, finger movement)
• Large motor units: found in large muscles that do not need rapid precise control (e.g., limb and trunk muscles)

Abnormalities of Motor Units

• Abnormalities can cause weakness or loss of strength of contraction of skeletal muscle
• Abnormalities of the lower motor neuron can cause:
  • Atrophy of skeletal muscle (decreased bulk or size)
  • Fasciculations (involuntary twitches of skeletal muscle)
  • Hypotonia (decrease in the tone of skeletal muscle)
  • Hyporeflexia (decreased muscle stretch reflexes)

Motor Unit

• Consists of a lower motor neuron and all the skeletal muscle cells it contacts and controls
• Lower motor neurons are efferent neurons of the peripheral nervous system, carrying information away from the central nervous system
• Controls skeletal muscle, which is the main muscle type of our body, connected to our skeleton to move us around

Neuromuscular Junction

• The synapse between a lower motor neuron and a skeletal muscle cell
• Lower motor neurons synapse with multiple skeletal muscle cells, forming multiple neuromuscular junctions

Function of Motor Unit

• When a lower motor neuron fires an action potential, it causes all the skeletal muscle cells in its unit to contract
• This is why the motor unit is called a unit, as the skeletal muscle cells function together as a single unit

Organization of Motor Neurons

• Soma of motor neurons are in the spinal cord or brain stem
• Axons pass out through cranial nerves or spinal nerves into the peripheral nervous system
• Axons branch out and synapse on skeletal muscle cells in their motor unit

Types of Motor Units

• Small motor units: found in small muscles that need rapid precise control (e.g., eye movement, finger movement)
• Large motor units: found in large muscles that do not need rapid precise control (e.g., limb and trunk muscles)

Abnormalities of Motor Units

• Abnormalities can cause weakness or loss of strength of contraction of skeletal muscle
• Abnormalities of the lower motor neuron can cause:
  • Atrophy of skeletal muscle (decreased bulk or size)
  • Fasciculations (involuntary twitches of skeletal muscle)
  • Hypotonia (decrease in the tone of skeletal muscle)
  • Hyporeflexia (decreased muscle stretch reflexes)