Neuroscience: Lower Motor Neurons and Spinal Motor Control Units

The somatic motor system represents the final execution level of the motor hierarchy, responsible for carrying out commands generated in the brain to facilitate muscle contraction.
Motor control is organized into a general scheme involving various motor centers that work in a coordinated fashion to produce movement.
Understanding motor units and the mechanisms by which the brain activates them is essential for determining the appropriate force of muscle contraction.
Specific spinal circuits, such as Central Pattern Generators (CPGs), play a critical role in rhythmic locomotion.

Movements are roughly categorized into four distinct types based on their complexity and the neural circuits involved:
Reflex Movements
- These are the simplest forms of movement.
- An example is the knee-jerk reflex.
Postural Movements
- These are complex refractory movements designed to keep the body upright during various activities.
Voluntary Movements
- These are the most complex types of movements.
- They require planning and integration within the cerebral cortex.
Rhythmic Movements
- Examples include walking and running.
- These movements utilize specialized spinal cord circuits known as Central Pattern Generators.

Motor centers consist of sets of neurons (ranging from $100,000$ to $1,000,000$ ) located in specific parts of the brain.
The system is organized into three strong hierarchical levels: strategy, tactics, and execution.
The Highest Level: Strategy
- Involves the association areas of the neocortex.
- Responsible for the abstract design, planning, and goal-setting of a movement.
The Middle Level: Tactics (Motor Programming)
- Involves the primary motor cortex, the cerebellum, and the basal ganglia.
- Responsible for the sequences of muscle contraction arranged in space and time to achieve a specific goal.
- The basal ganglia are critical for choosing and initiating the planned movement.
- There are two major loops at this level: one through the basal ganglia and thalamus back to the cortex, and another through the cerebellum.
The Lowest Level: Execution
- Involves lower motor neurons in the brainstem and the spinal cord.
- Brainstem neurons are primarily involved in maintaining posture.
- The spinal cord level includes circuits of spinal interneurons as well as alpha and gamma motor neurons.
Sensory-Motor Integration
- Motor centers require continuous sensory input to perform appropriately.
- Information follows a "bottom-up" path (sensory inputs from the external world or intermediate levels to higher levels) and a "top-down" path (motor information traveling from the highest levels down to the muscles).

The human body contains approximately $750$ different muscles, which can be categorized by location:
- Axial Muscles: Responsible for trunk movement.
- Proximal Muscles: Responsible for shoulder, elbow, and knee movements.
- Distal Muscles: Provide fine-tuned control of the hands, feet, fingers, and toes.
Muscles are also divided into four functional groups:
- Prime Movers (Agonistic Muscles): The primary muscles responsible for a specific movement.
- Antagonistic Muscles: Muscles that oppose or reverse a particular movement.
- Synergetic Muscles: Muscles that assist the prime movers.
- Fixators: Specialized synergists that immobilize a bone or a muscle's origin point.
Action Pairings
- Agonistic and antagonistic muscles are located on opposite sides of the joint across which they act.
- The biceps brachii acts as a flexor (reducing the angle at the joint).
- The triceps brachii acts as an extensor (increasing the angle at the joint).
- Some muscles, like the gastrocnemius, can serve multiple roles: it acts as a flexor for the knee joint but as an extensor for the ankle joint.

The somatic motor system explicitly controls skeletal muscles, including those involved in respiration (diaphragm and intercostal muscles) and eye movement (muscles in the eye sockets).
Lower Motor Neurons (LMNs)
- Movement contraction is initiated by alpha motor neurons.
- These neurons are physically situated in the ventral horn of the spinal cord.
Spinal Cord Enlargements
- The spinal cord features two significant enlargements to accommodate the high density of alpha motor neurons required for limb control.
- Cervical Enlargement: Spans from segment $C_3$ to $T_1$ . It contains a large mass of neurons to control the muscles of the hands and arms.
- Lumbar Enlargement: Contains additional alpha motor neurons dedicated to controlling the muscles of the legs.
Upper Motor Neurons (UMNs)
- Neurons located in the brainstem and cerebral cortex are called upper motor neurons.
- They communicate commands to the lower motor neurons either directly or via interneuronal networks.

Single alpha motor neurons in the ventral horn send axons to contact specific muscle fibers, forming synaptic connections.
Neurotransmission
- The primary neurotransmitter used is acetylcholine.
- Acetylcholine acts on nicotinic acetylcholine receptors in the muscle fibers.
- This produces a synaptic potential called the end plate potential (EPP).
- In the neuromuscular system, even a single spike of acetylcholine release produces an EPP amplitude that is well beyond the threshold needed to generate an action potential in the muscle fiber. This ensures a $100\%$ success rate for action potential generation per spike.
Key Definitions
- Motor Unit: Consists of one motor neuron and all the skeletal fibers it innervates.
- Motor Neuronal Pool: The collection of all motor neurons that innervate all the muscle fibers within a single particular muscle.

There is a highly orderly, spatial relationship between the location of motor neuronal pools and the muscles they innervate:
- Medial to Lateral Axis: Motor neurons innervating axial (trunk) muscles are positioned medially in the ventral horn, while those innervating distal muscles are positioned laterally.
- Dorsal to Ventral Axis: Motor neurons for flexor muscles are located more dorsally, while motor neurons for extensor muscles are located more ventrally.
Spatial Clustering
- Motor neuronal pools innervating a single muscle occupy a rod-shaped cluster that runs across several spinal cord segments.
- For example, neurons for the arm are found in segments $C_3$ to $T_1$ .
- Scientific studies (such as those involving tracers in cats) have mapped specific pools, such as the medial gastrocnemius and soleus muscles, by observing alpha motor neurons pick up tracers at terminals and transport them back to the cell bodies in the spinal cord.