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Spinal Cord: Descending Pathways
Study Guidelines
- 1.
Reproduce the tracts descending the spinal cord and recall that each is strategically placed for access to its particular set of motor neurons, in accordance with the layout in Fig. 16.7 .
- 2.
Identify target neurons selected by the lateral corticospinal tract.
- 3.
Describe how the reticulospinal tracts are concerned with automatic movements and with postural fixation.
- 4.
Summarise the Clinical Panels dealing with upper and lower motor neuron disease and spinal cord injury and contrast upper versus lower motor neuron symptoms.
Anatomy of the Ventral Grey Horn
Cell Columns
Each of the columns of motor neurons in the ventral grey horn supplies a group of muscles with similar functions ( Table 16.1 ). The individual muscles are supplied from cell groups (neurons) within the columns. Axial (trunk) muscles are supplied from the medially placed motor columns, proximal limb segment muscles from the mid-region, and distal limb segment muscles from the lateral columns ( Fig. 16.1 ). Columns supplying extensor muscles lie anterior to those supplying flexors. The autonomic nervous system is represented by the intermediolateral cell column.
Table 16.1
The Somatomotor Cell Columns.
| Cell Column | Muscles |
|---|---|
| Ventromedial (all segments) | Erector spinae |
| Dorsomedial (T1–L2) | Intercostals, abdominals |
| Ventrolateral (C5–C8, L2–S2) | Arm/thigh |
| Dorsolateral (C6–C8, L3–S3) | Forearm/leg |
| Retrodorsolateral (C8, T1, S1–S2) | Hand/foot |
| Central (C3–C5) | Diaphragm |
Cell Types
Large α motor neurons supply the extrafusal fibres of skeletal muscles. Interspersed among them are small γ motor neurons that supply the intrafusal fibres of neuromuscular spindles.
Tonic and Phasic Motor Neurons
The α motor neurons have large dendritic trees receiving some ten thousand excitatory boutons from propriospinal neurons and from supraspinal pathways descending from the cerebral cortex and brainstem. (The term supraspinal refers to any pathway descending to the cord from a higher level.) The somas of α motor neurons receive some five thousand inhibitory boutons, mainly from propriospinal sources.
Two principal types of α motor neurons are recognised: tonic and phasic. Tonic α motor neurons innervate slow oxidative (red) muscle fibres; they are readily depolarised and have relatively slow conducting axons with small spike amplitudes. Phasic α motor neurons innervate fast glycolytic (white) muscle fibres. The phasic neurons are larger, have higher thresholds, and have rapidly conducting axons with large spike amplitudes. Tonic neurons are usually the first recruits when voluntary movements are initiated, even if the movement is to be fast.
Renshaw Cells
The axons of the α motor neurons give off recurrent branches, which form excitatory cholinergic synapses upon inhibitory interneurons called Renshaw cells in the medial part of the ventral horn. The Renshaw cells form inhibitory, glycinergic synapses upon the α motor neurons. This is a classic example of negative feedback , or recurrent inhibition , through which the discharges of α motor neurons are self-limiting (cf. Clinical Panel 8.1 ).
Segmental-Level Inputs to α Motor Neurons
At each segmental level, α motor neurons receive powerful inputs from muscle spindles, Golgi tendon organs, and joint capsules. Note that any inhibitory effect produced by activity in dorsal nerve root fibres requires interpolation of inhibitory interneurons because all primary afferent neurons are excitatory in nature.
Segmental-level inputs to a flexor α motor neuron include the following:
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Type Ia and type II afferents from spindles in the flexor muscles provide the afferent limb of the monosynaptic stretch reflex (e.g. the biceps reflex).
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Type Ia afferents from spindles in extensor muscles exert reciprocal inhibition upon the flexor motor neurons via Ia inhibitory interneurons. Type Ib afferents from Golgi tendon organs in the flexor muscles exert autogenic inhibition upon the flexor motor neurons.
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Type Ib afferents from Golgi tendon organs in extensor muscles exert reciprocal excitation of flexors via excitatory interneurons. Afferents from the flexor aspect of relevant synovial joints are stimulated when the capsule becomes taut in extension. They initiate an articular protective reflex, as described in Chapter 10 .
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In execution of the withdrawal reflex described in Chapter 14 , large numbers of excitatory ‘flexor reflex’ interneurons are activated over several spinal segments on the same side as the stimulus, as well as inhibitory interneurons supplying motor neurons to antagonist muscles.
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Renshaw cells.
A reciprocal list can be drawn up for extensor motor neurons, with substitution of extensor thrust inputs for flexor reflex interneurons.
Descending Motor Pathways
Important pathways descending to the spinal cord are:
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Corticospinal (pyramidal tracts)
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Reticulospinal (extrapyramidal tracts)
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Vestibulospinal
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Tectospinal
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Raphespinal
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Aminergic
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Autonomic
Corticospinal Tract
The corticospinal tract is the great voluntary motor pathway. About 30% of its fibres take their origin from the primary motor cortex in the precentral gyrus. Other sources include the supplementary motor area on the medial side of the hemisphere. About 30% originate from the premotor cortex on the lateral side and 40% of the corticospinal tract originate from the somatosensory cortex and the parietal lobe ( Fig. 16.2 ). The contributions from the two sensory areas mentioned terminate in the sensory nuclei of the brainstem and spinal cord, where they modulate sensory transmission.
The corticospinal tract descends through the corona radiata and posterior limb of the internal capsule to reach the brainstem. It continues through the crus (cerebral peduncle) of the midbrain and the basilar pons to reach the medulla oblongata ( Fig. 16.3 ). Here it forms the pyramid (hence the synonym, pyramidal tract ).
During its descent through the brainstem, the corticospinal tract gives off fibres that activate motor cranial nerve nuclei, notably those serving the muscles of the face, jaw, and tongue. These fibres are called corticobulbar ( Fig. 16.4 ). (The term ‘corticonuclear’ is also used because the term ‘bulb’ is open to different interpretations.)
At the spinomedullary junction ( Fig. 16.5 ):
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About 80% (70% to 90%) of the fibres cross the midline in the pyramidal decussation .
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These fibres descend on the contralateral side of the spinal cord as the lateral corticospinal tract (crossed corticospinal tract).
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About 10% of the fibres that do not decussate enter the ventral corticospinal tract/anterior corticospinal tract , which occupies the ventral/anterior funiculus and terminate at cervical and upper thoracic levels. These fibres cross in the white commissure and supply motor neurons serving muscles in the anterior and posterior abdominal walls.
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The other 10% of uncrossed fibres join the lateral corticospinal tract on the same side.
The corticospinal tract is stated to have about one million nerve fibres. The average conduction velocity is 60 m/s, indicating an average fibre diameter of 10 μm (see ‘rule of six’ in Chapter 9 ). About 3% of the fibres are extra-large (up to 20 μm); they arise from giant neurons (cells of Betz) , located mainly in the leg area of the motor cortex (see Chapter 29 ). All corticospinal fibres are excitatory and appear to use glutamate as their transmitter substance.
Targets of the Lateral Corticospinal Tract
Distal limb motor neurons
In the ventral grey horn, lateral corticospinal tract axons may directly synapse upon the dendrites of α and γ motor neurons supplying limb muscles, notably in the upper limb, but typically do so via interneurons within the spinal cord grey matter. Individual axons within the lateral corticospinal tract may activate either ‘large’ or ‘small’ motor units. A motor unit consists of a ventral horn cell and all the muscle fibres it innervates. Neurons of small motor units selectively innervate a small number of muscle fibres and play a role in performing delicate and precise movements such as those required to play the piano. Ventral horn cells controlling a muscle such as the gluteus maximus individually excite hundreds of muscle cells at once because this muscle is responsible for gross, unrefined movements.
A unique property of these corticomotoneuronal fibres of the lateral corticospinal tract is the concept of fractionation, relating to the variable activity of interneurons, whereby small groups of neurons can be selectively activated to perform a specific function. It is most obvious in the case of the index finger, which can be flexed or extended quite independently, although three of its long tendons arise from muscle bellies devoted to all four fingers. Fractionation is essential for the execution of skilled movements such as buttoning a coat or tying shoelaces. Following damage to the corticomotoneuronal system anywhere from the motor cortex to the spinal cord, skilled movements are lost and seldom recover completely.
As mentioned already in Chapter 10 , the α and γ motor neurons are coactivated by the lateral corticospinal tract during a given movement, so that spindles in the prime movers signal active stretch while those in the antagonists signal passive stretch.
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