HSS 500 Fundamentals of Neural Control of Force Production

Motor neuron (MN)

A neuron specialized in transmitting neural signals that ultimately lead to skeletal muscle contraction; serves as the biological link between the nervous system and the muscular system

• Responsible for producing the electrical activity at the origin of force production

• Typically, one motor neuron innervates a few hundred muscle fibers

• MN receive up to 50,000 synapses, ~95% of these synapses occurring on the dendrites

• The axon of a MN can be very long, extending from the spinal cord to distal muscles of the limbs

Parts of a motor neuron (MN)

Like other neurons, a motor neuron consists of:

• Dendrites – receive synaptic input

• Cell body (soma) – integrates incoming signals

• Axon – conducts action potentials

• Axon terminals – form synapses with muscle fibers

Functions of a motor neuron (MN)

At its most basic level, a motor neuron:

• Receives synaptic input from other neurons

• Integrates these inputs at the soma

• Generates action potentials when threshold is reached

• Conducts the action potential along its axon

• Releases neurotransmitter (acetylcholine) at the neuromuscular junction

• Triggers muscle fiber depolarization and contraction

Upper motor neurons (UMNs)

Neurons whose cell bodies are located in the brain (primarily in the motor cortex and brainstem) and whose axons project downward to synapse onto lower motor neurons in the spinal cord or brainstem

• As they descend, UMN axons pass through the internal capsule, brainstem, and spinal cord, often crossing (decussating) to the opposite side before reaching their target spinal segments

• Once in the spinal cord, they synapse either directly or indirectly (via interneurons) onto lower motor neurons, which then project to skeletal muscle

How do UMNs travel to synapse with LMNs?

Upper motor neuron axons travel in organized bundles called descending tracts; these tracts are collections of parallel axons that run through specific regions of the central nervous system, carrying motor commands from higher centers to the spinal cord

• The most well-known example is the corticospinal tract, which transmits voluntary motor signals from the primary motor cortex to spinal motor neurons

• Upper motor neurons (UMNs) that give rise to the corticospinal tract originate primarily in the primary motor cortex (M1), located in the precentral gyrus of the frontal lobe. However, they also arise from premotor and supplementary motor areas.

Motor homunculus

The organization of the primary motor cortex is not random → it follows a systematic layout known as the motor homunculus, a somatotopic map of the body represented along the primary motor cortex

• “Somatotopic” means that different regions of the cortex correspond to different parts of the body

• Fine motor skills require more precise control → larger representation of the hands & face on the homonculus

Transcranial magnetic stimulation (TMS)

Technique generating a magnetic field (via magnetic coil); positioned in reference to different portions of the brain

• Creates a magnetic field that create a stimulation of UMNs → generation of APs

• Allows mapping of area of the cortex associated with a certain muscle group

Lower motor neurons (LMNs)

Unlike upper motor neurons, which modulate and transmit motor commands, lower motor neurons directly innervate skeletal muscle fibers

• LMNs often referred to as the “final common pathway” of voluntary movement

• Without LMN activation, voluntary muscle contraction cannot occur

• LMNs also receive input from the UMNs and modulate signal based on sensory afferents (e.g., muscle spindles, GTOs) like stretch, tension, pain (involved in reflexes, involuntary control)

Location of LMN cell bodies

• The anterior (ventral) horn of the spinal cord (for body and limb muscles), or

• The motor nuclei of cranial nerves in the brainstem (for muscles of the face and head)

**LMN cell bodies reside in specific spinal cord segments while their axons exit the spinal cord via the ventral roots

Functions of a lower motor neuron (LMN)

Each lower motor neuron:

• Receives input from upper motor neurons

• Integrates signals from spinal interneurons and sensory afferents

• Sends a single axon out of the CNS to innervate multiple muscle fibers

• Forms a motor unit with the fibers it controls

How do LMNs travel to synapse with muscle fibers?

• LMN axons exit the spinal cord via ventral roots and travel within peripheral nerves, which branch to reach specific muscles

• Importantly, the spinal cord is also somatotopically organized, meaning:

  • Neurons innervating proximal muscles are located more medially in the ventral horn

  • Neurons innervating distal muscles are located more laterally

Motor Unit (MU)

A motor unit consists of: one lower motor neuron and all the muscle fibers it innervates

• One motor neuron innervates multiple muscle fibers, BUT one muscle fiber is only under the control of a signal motor neuron → when the motor neuron fires, all fibers within that motor unit contract simultaneously

• All muscle fibers innervated by a single motor neuron have similar contractile properties; The LMN of a given MU either innervates:

  • Type I muscle fibers,

  • Type IIa muscle fibers, OR

  • Type IIx muscle fibers

Innervation number

Refers to the number of muscle fibers innervated by a single motor neuron; typically varies with muscle size

Muscle unit

Consists of the muscle fibers that are innervated by the same MN

• Fibers in a muscle unit generally occupy only a portion of the volume of the muscles

Recruitment threshold of MUs

A MU is recruited when the synaptic input to its MN reaches its recruitment threshold (force level at which a MU begins to discharge APs repetitively to produce a stable force); follows the size principle (type I → type IIa → type IIx fibers)

• Low-threshold motor units: typically smaller, often Type I, recruited at low levels of force demand

• High-threshold motor units: larger, often Type II, recruited at high levels of force demand

Spatial recruitment

How many muscle fibers within the “pool” of total muscle fibers in the muscle are being recruited

Upper limit of recruitment of MUs

Level of force at which the last motor unit is recruited (all motor units are firing); ranges from 60 to 90% of maximal force

• At this point, an increase in force is only associated with an increase in discharge frequency/rate

De-recruitment threshold of MUs

Motor units are also de-recruited as force demand decreases; the de-recruitment threshold is the level of synaptic input/force below which a motor unit stops firing

• Recruitment and de-recruitment thresholds are not always identical

• Some motor units may continue firing slightly below their recruitment threshold (hysteresis effect) for smoother force modulation

Discharge frequency (rate coding) of MUs

Once recruited, the force produced by a motor unit is further modulated by its discharge frequency (firing rate/how many APs are produced by the MU per unit of time); lower discharge rate from motor unit at lower force & as force production increases so does the discharge rate

• Low firing frequency: partial summation of twitches

• High frequency: tetanus

Single twitch

A single twitch corresponds to the mechanical response (force production) of a motor unit following a single action potential

Characterizing a twitch

A twitch can be characterized by three key parameters:

• Contraction time (Time to peak force, t_p): Time from the onset of force development to peak force. Often used as an index of contractile speed.

• Peak twitch force (P_t): Maximum force produced during the twitch.

• Half-relaxation time (t_h): Time required for force to decline from peak force to 50% of peak force. Reflects relaxation properties and calcium reuptake kinetics.

Twitches and summation

• Each action potential produces a twitch

• When action potentials occur in close succession, twitches summate

• Greater discharge frequency → greater force summation

Partial summation

• Twitches overlap

• Force oscillations still visible

Fused tetanus

• Twitches fully overlap

• Oscillations disappear

• Maximal force for that motor unit is reached

**Tetanic force typically achieved around 17–25 pps (fiber-type dependent) & maximal tetanic force is 1.5–10× twitch force (twitch–tetanus ratio)

Double discharge phenomenon

• Brief interspike interval (~10ms) observed at the beginning of a voluntary contraction in humans → substantial increase in force production.

• The force elicited using electrical stimulation of the muscle can also be increased by including a brief interval between 2 stimuli at the start of the stimulation sequence.