---

Recruitment of Motor Units

1. What is a Motor Unit?

• A single alpha (α) motor neuron (in the anterior horn of the spinal cord)
• All the muscle fibers it innervates

"Small muscles that react rapidly and require precise control have more nerve fibers for fewer muscle fibers." - Guyton & Hall, p. 100

---

2. How is Muscle Force Graded? (Two Mechanisms)

A. Multiple Fiber Summation (Spatial Summation / Recruitment)

B. Frequency Summation

---

3. Recruitment of Motor Units - The Core Concept

• When the CNS sends a weak signal - only small motor units are activated first
• As the signal strengthens - progressively larger motor units are recruited
• Muscle force decreases by terminating motor units in reverse order (largest deactivated first)

Each motor unit has a recruitment threshold - the force level at which that motor unit is first activated. - Kandel, Principles of Neural Science, p. 790

---

4. The Size Principle (Henneman's Size Principle, 1957)

Motor units are recruited from smallest to largest - small motor neurons fire first, large motor neurons fire last.

Mechanism - Why are small neurons recruited first?

"Recruitment order is determined by intrinsic differences in the responsiveness of individual motor neurons to relatively uniform synaptic input." - Kandel, p. 790

---

5. Consequences of the Size Principle

1. Fine gradation at low forces: When small motor units (few fibers, low force) are recruited first, force increases in tiny steps - allowing precise, delicate movements.
2. Economy: Slow-twitch, fatigue-resistant fibers are used for sustained low-level work (e.g., maintaining posture). High-force, fast-fatigable units are held in reserve for bursts.
3. The brain cannot selectively activate specific motor units - the recruitment sequence is determined by the spinal cord's intrinsic properties, not supraspinal areas.
4. At about 85% of maximal force, all available motor units have been recruited; beyond that, further force increases come only from increasing firing rate.

---

6. Asynchronous Recruitment - Smooth Contraction

• While some units are contracting, others are relaxing
• This prevents a "jerky" summation and produces smooth, sustained muscle contraction even at low frequencies
• This is essential for fine motor control

"Contraction alternates among motor units one after the other, thus providing smooth contraction, even at low frequencies of nerve signals." - Guyton & Hall, p. 100

---

7. Types of Motor Units (Related to Recruitment Order)

---

8. Summary Diagram (Conceptual)

CNS Signal Strength  →  Weak ──────────────────────────► Strong
                               ↓            ↓             ↓
Motor unit recruited:    Small (Type S) → FR → Fast (Type FF)
Force produced:          Low ──────────────────────────► High
Fatiguability:           Low ──────────────────────────► High
Axon diameter:           Small ─────────────────────────► Large
Conduction velocity:     Slow ──────────────────────────► Fast

---

Key Points to Remember for Exams

1. Motor unit = 1 α motor neuron + all its muscle fibers (Sherrington, 1925)
2. Recruitment = adding more motor units to increase force
3. Size principle (Henneman, 1957) = small units recruited before large
4. Basis: small neurons have high input resistance → larger EPSP → threshold reached first
5. Two ways to grade force: (a) multiple fiber summation (recruitment) and (b) frequency summation
6. Asynchronous firing of motor units ensures smooth contraction
7. Large motor units have the largest force but fatigue the fastest

---