Motor Neurons and Spinal Control of Movement


Motor neurons are among the largest neurons in the central nervous system and have long axons that travel along peripheral nerves to innervate skeletal muscles. They are the final common pathway through which the brain controls bodily movement. Motor neurons receive excitatory and inhibitory synaptic inputs from sensory afferents and from pathways of supraspinal origin either directly or via interneurons. The intrinsic properties of motor neurons determine how these inputs are transformed into a sequence of action potentials that elicit muscle contraction. Motor neuron intrinsic properties are affected by neuromodulatory inputs from supraspinal and spinal pathways. Motor neurons are recruited in ways that make effective use of the muscle fibres they innervate. All motor actions – whether stereotyped behaviours (such as locomotion or withdrawal reflexes), or uniquely specified sequences of muscle contraction – reflect the interaction of supraspinal commands, sensory inputs and spinal cord interneurons.

Key Concepts:

  • Motor neurons are large neurons in the brainstem and spinal cord that innervate skeletal muscle; those innervating the same muscle are grouped in proximity to form a motor neuron pool.

  • The electrophysiological properties of an individual motor neuron are appropriately matched to the contractile properties of the muscle fibres it innervates; together, a motor neuron and its muscle fibres form a motor unit.

  • Motor neurons receive two types of synaptic inputs from descending pathways, spinal interneurons and segmental afferents: (1) excitatory and inhibitory inputs that directly affect membrane potential and (2) neuromodulatory inputs that alter the effects of the excitatory and inhibitory inputs on membrane potential.

  • The intrinsic properties of a motor neuron and the neuromodulatory influences it receives determine how its excitatory and inhibitory synaptic inputs are translated into a firing pattern that elicits muscle contraction.

  • Motor neurons are recruited in a stereotyped order according to the ‘size principle’, in which low‐force motor units are activated first, and sequentially higher force motor units are subsequently added to produce the total muscle force appropriate for the current action.

  • Networks (often called central pattern generators) composed of spinal interneurons support locomotion and other cyclical movement patterns by distributing rhythmic synaptic activity among different motor neuron pools in appropriate temporal sequences.

Keywords: spinal cord; motoneuron; motor neuron; reflex; movement; motor control; neuromodulation

Figure 1.

(a) Motor neuron nuclei of cat (MG) and (SOL) muscles. Right: Dorsal view of spinal cord with positions (dots) of MG (left hemicord) and SOL (right hemicord) motor neurons shown. L6–L7 and L7–S1 segmental boundaries are indicated by horizontal heavy lines. Vertical dashed line indicates midline. Horizontal dashed lines (A–E) indicate levels of cross sections shown on the left. In these sections, heavy lines are the grey–white matter boundaries and light lines are the cord boundaries. Neurons (dots) shown at each level are within 300 μm of that level. Modified from Burke et al. . (b) Reconstruction of cat triceps surae motor neuron. The solid line is the lateral boundary of the spinal cord (ventral is towards the bottom) and the dashed line is the border of the grey matter. Note that numerous dendrites enter the white matter. The dashed line emerging from the soma is the axon. Modified from Brown and Fyffe .

Figure 2.

(a) Tension produced by cat gastrocnemius motor units of three different types during 40 s−1 tetani, each lasting 330 ms and repeated once per second for the time shown. The fast fatiguable unit is strong and fatigues quickly, the slow unit is weak and does not fatigue, and the fast fatigue‐resistant unit has intermediate properties. Modified from Burke et al. . (b) Steady‐state firing rates of human extensor digitorum communis motor units versus total force. As force rises, motor units begin firing and those already firing increase their rates. Modified from Monster and Chan .

Figure 3.

Amplification and prolongation of synaptic input (generated by 1.5 s of high‐frequency activation of monosynaptic muscle spindle Ia afferent input) by the (PIC) in a low‐threshold, slow motor neuron. The current traces in panels A and B were recorded using the voltage‐clamp technique, in which a target voltage level is maintained by injecting (and recording) current through the electrode into the cell; the voltage traces in panel C were recorded under unclamped conditions. (a) At a hyperpolarised holding potential (−90 mV; green trace), this input produced a steady current with a sharp onset and offset. At a depolarised holding potential (∼−55 mV; red trace), the very same input is greatly amplified and prolonged by the PIC. Baseline holding currents are removed to allow the traces to be superimposed. (b) The difference between the currents in (a) reflects the net contribution of the PIC. (c) In unclamped conditions, the same input produces a steady (EPSP) at hyperpolarised levels (∼−90 mV). At a more depolarised level (−70 mV), the same input evokes intense repetitive firing, followed by continued, self‐sustained firing at a lower level when the input is removed. The voltage traces are shown on an expanded vertical scale (which truncates the peaks of the action potentials) to highlight the difference in response at the two voltage levels. Adapted from Heckman CJ, Johnson M, Mottram C and Schuster J (2008) Persistent inward currents in spinal motoneurons and their influence on human motoneuron firing patterns. Neuroscientist 14(3):12. With permission from Sage Publications.

Figure 4.

(a) Triphasic pattern of muscle contraction (i.e. (EMG) activity) associated with a rapid voluntary limb movement. First, activation of the agonist muscle accelerates the limb to maximum velocity. Then, agonist activity ceases and antagonist activation stops the limb. Finally, agonist activation stabilises the limb and maintains the new position. Modified from Ghez . (b) Average rectified EMG response of human wrist extensor muscles to sudden flexion of the wrist (indicated by change in wrist angle trace). Vertical line indicates beginning of flexion torque. The individual had been instructed to return the handle to its initial position as quickly as possible. The series of EMG peaks reflects the succession of monosynaptic segmental excitation of the motor neuron, oligosynaptic segmental and suprasegmental excitation, and purely voluntary excitation. Modified from Lee and Tatton .

Figure 5.

Treadmill location in a cat in which transection of the thoracic spinal cord has isolated the lumbosacral segments. After recovery from surgery, placement on the treadmill induces reciprocal activation of hind limb extensor and flexor muscles which produces relatively normal stance and swing phases of walking. Modified from Pearson .



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Further Reading

Brooks VB (1981) Handbook of Physiology, sect. I: The Nervous System, vol. II: Motor Control, part I. Baltimore: Williams and Wilkins.

Brown AG (1981) Organization in the Spinal Cord. Berlin: Springer.

Kandel ER, Schwartz JH and Jessell TM (1991) Principles of Neural Science. New York: Elsevier.

Rothwell J (1994) Control of Human Voluntary Movement. New York: Chapman and Hall.

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Carp, Jonathan S, and Wolpaw, Jonathan R(Jun 2010) Motor Neurons and Spinal Control of Movement. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000156.pub2]