In one breath
Each spinal segment runs its own motor circuit, with muscle afferents coming in, motoneurons going out and interneurons in between, all acted on from above by the brainstem and forebrain. One α motor neuron and all the extrafusal fibres it supplies form a motor unit, and units are recruited small first, large last (the size principle). γ motor neurons, 30% of the anterior horn output, add no force but set the sensitivity of the muscle spindle. Medial motoneurons and interneurons serve posture through the proximal muscles; lateral ones serve skilled movement through the distal muscles.
Builds on: Movement and organization of the motor system · Afferent fibers, dorsal root and spinal laminae · Leads to: Muscle spindle · Stretch reflex · Upper vs lower motor neuron lesions · Withdrawal reflex
Segmental and suprasegmental control
- The motor system works at three levels: spinal cord, brainstem and forebrain. The spinal level carries the largest share of motor regulation.
- Segmental organization: every segment of the cord has its own circuit for controlling movement.
- Suprasegmental organization: control from above the segment, mainly by the brainstem and forebrain.
Components of the segmental circuit
Each segment is wired to its muscles in both directions:
- In: muscle afferents (Ia, Ib and others) bring signals from the muscle to the cord.
- Out: motoneurons carry the cord’s output to the muscle.
- In between: many interneurons lie between the afferents and the motoneurons.
- From above: descending fibres from suprasegmental centres end on both the interneurons and the motoneurons.
- The set of connections at one segment is one unit of segmental organization.
Its five parts: muscles, afferent neurons, motoneurons, the motoneuron pool and the spinal interneurons.
Draw it: the segmental circuit
- Draw a cross-section of the spinal cord with a muscle beside it.
- Run an afferent (label it Ia) from the muscle through the dorsal root into the grey matter.
- Put an interneuron between that afferent and an anterior horn cell.
- Take the anterior horn cell’s axon out through the ventral root to the same muscle.
- Bring a descending fibre down from above and end it on both the interneuron and the anterior horn cell.
Muscles
- By the movement they produce, muscles are paired as flexors–extensors, adductors–abductors and pronators–supinators.
- Physiologically, the pair that matters most is flexors and extensors:
- Extensors increase the angle at a joint and matter most for posture. The lower-limb extensors work against gravity, so they are the main antigravity muscles, and their tone keeps posture.
- Flexors decrease the angle at a joint and serve the withdrawal reflexes.
- By their electrochemical properties and how long they can keep working, your book lists four types: fast twitch–fatigue resistant, slow twitch–fatigue resistant, fast twitch–fatigable and slow twitch–fatigable.
- Worth knowing, though not in your pages: most texts describe only three motor unit types (slow and fatigue resistant, fast and fatigue resistant, fast and fatigable). A slow, fatigable type is not usually listed.
- Neurophysiologically, the most useful split is medial (proximal) versus lateral (distal).
Medial and lateral groups
| Feature | Medial (proximal) | Lateral (distal) |
|---|---|---|
| Muscles | Axial, girdle, proximal limb | Intrinsic digit muscles, distal limb |
| Movements | Trunk and whole limb | Handling objects |
| Controls | Posture | Skilled voluntary acts |
- Axial muscles attach to the axial skeleton and the girdles. Because they move the trunk and whole limbs, they make the postural adjustments, and the medial extensors do the antigravity work.
- The distal muscles do manipulation: the skilled voluntary movements.
- This medial–lateral split runs through the whole motor system, and motor physiology is easiest to follow through it.
Afferents
- Ia afferents from the muscle spindles are the main afferents for motor activity (Muscle spindle).
- Type II and Ib afferents also take part in motor control (Golgi tendon organ).
- Flexor reflex afferents carry the withdrawal reflexes (Withdrawal reflex).
Motor neurons
Motor neurons (motoneurons) are the neurons that control motor function. They fall into two broad classes.
| Feature | Upper (UMN) | Lower (LMN) |
|---|---|---|
| Cell body in | Motor cortex, brainstem motor nuclei | Anterior horn, cranial motor nuclei |
| Axon ends on | LMNs, directly or via interneurons | Skeletal muscle |
| Examples | Corticospinal, vestibulospinal tracts | α and γ motor neurons |
Upper motor neurons
- They arise in the brain’s motor areas, such as the motor cortex and the brainstem motor nuclei, and end on LMNs in the cord, either directly or through interneurons.
- Their axons form the descending pathways. Key examples: the corticospinal, rubrospinal, vestibulospinal and reticulospinal tracts (Corticospinal (pyramidal) tract, Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts).
Lower motor neurons
- Their cell bodies lie in the ventral (anterior) horn of the spinal grey matter, as the anterior horn cells, or in the motor nuclei of cranial nerves in the brainstem. Their axons supply skeletal muscles.
- Only some descending fibres end on them directly; most relay through interneurons first.
- There are two types: α and γ motor neurons. For what a lesion of each class looks like, see Upper vs lower motor neuron lesions.
α motor neurons
- They are the final common path: every signal the CNS sends to the skeletal muscles has to pass through them.
- They supply the extrafusal fibres, which generate the muscle’s force. One α motor neuron supplies 10 to 1,000 muscle fibres, depending on the muscle.
- Motor unit: one motor neuron with its axon, the axon’s branches, the neuromuscular junction at the end of each branch, and every extrafusal fibre it supplies.
- When the motor neuron fires an action potential, all the fibres of its unit are activated.
α motor neurons come in two sizes, set by the cell body and the axon diameter:
| Feature | Small α | Large α |
|---|---|---|
| Threshold | Low | High |
| Conduction | Slower | Faster |
| Unit size | Small | Large |
| Fibre twitch | Slow | Fast |
| Fibre force | Low | High |
| Fatigue | Resistant | Fatigable |
| Used for | Low-force, tonic work | High-effort force |
- All the fibres of one motor unit are of one type, fast-twitch or slow-twitch.
- The motor neuron decides that type. If a muscle loses its nerve and is then reinnervated by sprouts from a different type of motor neuron, its fibres can change their twitch type.
Recruitment: the size principle
- When synaptic drive is weak, the small, low-threshold motor neurons fire first. They give weak, sustained (tonic) contractions in slow-twitch, fatigue-resistant fibres.
- As the descending drive grows, those same neurons first fire faster.
- Next, more units of the same type join in.
- Only when still more force is needed are the large motor neurons, with their large units, recruited.
- This orderly sequence, small units before large ones, is the size principle.
- It explains why antigravity muscles contain mostly slow-twitch fibres: their job is continuous postural support. Many flexors, by contrast, are fast-twitch muscles that build high forces through rapid recruitment of motor units.
Small units go first, although large ones conduct faster
Recruitment order follows threshold, not conduction speed. Small motor neurons conduct more slowly but have the lowest threshold, so they are recruited first.
γ motor neurons
- They supply the intrafusal fibres, the muscle fibres inside the muscle spindle (Muscle spindle).
- They make up 30% of the fibres leaving the anterior horn cells.
- Their firing adds nothing to muscle force. What they change is how sensitive the spindle is, and that is how they regulate muscle length and tone.
- γ motor neurons together with the intrafusal fibres they supply form the fusimotor system.
- No sensory afferent reaches them; their drive comes from descending pathways. This is the main lever by which higher centres set muscle tone: they raise or lower γ discharge.
Motoneuron pools
- The anterior horn cells, the cell bodies of the spinal motoneurons in the ventral horn, make up the motoneuron pools. As the final common path, they carry all central motor output.
Topographical organization
- Motoneurons are mapped in the cord by the muscles they supply, and the pool has two parts:
- the medial group supplies the axial and proximal muscles, so it controls posture;
- the lateral group supplies the distal muscles, so it controls skilled voluntary movement.
- In the lower cervical and upper thoracic segments (the brachial enlargement), the lateral pool grows to fill about 70% of the ventral horn, because these segments supply the intrinsic muscles of the hand, which do manipulation.
- Your book adds that further down the cord, the pool in the ventral horn is given over to medial-group cells, which supply limb muscles that are progressively more proximal.
- Worth knowing, though not in your pages: the lateral group is found only in the limb enlargements. In the thoracic cord, which supplies no limb muscles, the ventral horn holds mainly the medial group for the trunk.
- The rule to remember: inputs aimed at the medial motoneurons mainly control posture, and inputs spread to the lateral motoneurons mainly control skilled activity.
Spinal interneurons
- They lie between the dorsal and ventral horns.
- Position follows job: motor interneurons cluster nearer the ventral horn, sensory ones nearer the dorsal horn.
- Descending tracts act on motoneurons mostly through interneurons; only some of their fibres end on motoneurons directly. That makes the interneurons the main targets of descending tracts.
Topographical organization
- Interneurons are grouped by position, like the motoneurons:
- the ventromedial group projects mainly to the medial motoneurons and so controls posture;
- the dorsolateral group projects to the lateral motoneurons and so regulates skilled voluntary activity.
| Level | Medial system | Lateral system |
|---|---|---|
| Muscles | Axial, girdle, proximal | Distal, intrinsic digit |
| Motoneurons | Medial group | Lateral group |
| Interneurons | Ventromedial | Dorsolateral |
| Controls | Posture | Skilled movement |
Medial means posture, lateral means skill
At every level (muscles, motoneuron pools, interneurons) medial goes with posture and lateral with skilled movement. The lateral pool is so large in the lower cervical cord because of the intrinsic hand muscles.
Draw it: motoneuron and interneuron topography
- Draw half a cross-section of the cord at the brachial enlargement, with a large ventral horn.
- Shade a small medial group of anterior horn cells and label it “axial and proximal muscles: posture”.
- Shade a much larger lateral group filling about 70% of the ventral horn, and label it “distal and intrinsic hand muscles: skilled movement”.
- Draw a ventromedial cluster of interneurons with an arrow to the medial group, and a dorsolateral cluster with an arrow to the lateral group.
- Mark motor interneurons toward the ventral horn and sensory interneurons toward the dorsal horn.
Exam-answer skeleton: "Motor unit" (short note)
- Definition: one α motor neuron, its axon and branches, the neuromuscular junctions and all the extrafusal fibres it supplies.
- Size: 10 to 1,000 muscle fibres per motor neuron, depending on the muscle.
- One action potential in the neuron activates every fibre of the unit.
- All fibres of a unit are one type, set by the motor neuron; reinnervation by another type can change it.
- Small versus large units, as a table: threshold, conduction, twitch, force, fatigue.
- Recruitment by the size principle: small units first, then faster firing, then more units, then large units.
- Significance: slow-twitch antigravity muscles for steady posture; fast-twitch muscles such as flexors for high force.