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Innervation of Muscles and Joints
Study Guidelines
- 1.
Be able to outline the differences in motor units with respect to movements of all kinds and with respect to the functional significance of their sizes and muscle chemistries.
- 2.
Sketch a motor end plate, indicating the locations of transmitter, receptor, and hydrolytic enzyme.
- 3.
Sketch an intrafusal muscle fibre, indicating the locations of two motor and three sensory nerve endings.
- 4.
Describe the functional significance of coactivation of α and γ motor neurons during voluntary movements.
- 5.
With the essentials of this chapter still in mind, consider a first read through the Electromyography section of Chapter 12 .
In gross anatomy the nerves to skeletal muscles are branches of mixed peripheral nerves. The branches enter the muscles about one-third of the way along their length, at motor points ( Fig. 10.1 ). Motor points have been identified for all major muscle groups for the purpose of functional electrical stimulation by physical therapists, to increase muscle power.
Pattern of innervation of skeletal muscle.
Only 60% of the axons in the nerve to a given muscle are motor points to the muscle fibres that make up the bulk of the muscle. The rest are sensory in nature, although the largest sensory receptors—the neuromuscular spindles—have a motor supply of their own.
Motor Innervation of Skeletal Muscle
The nerve of supply branches within the muscle belly, forming a plexus from which groups of axons emerge to supply the muscle fibres ( Figs. 10.1 and 10.2 ). The axons supply single motor end plates placed about halfway along the muscle fibres ( Fig. 10.3A ).
A motor unit comprises a motor neuron in the spinal cord or brainstem together with the squad of muscle fibres it innervates. In large muscles (e.g. the flexors of the hip or knee) each motor unit contains 1200 muscle fibres or more, whereas in small muscles (e.g. the intrinsic muscles of the hand) each unit contains 12 muscle fibres or fewer. Small units contribute to the finely graded contractions used for delicate manipulations.
There are three different types of skeletal muscle fibres.
- 1.
Slow-twitch, oxidative fibres are small and rich in mitochondria and blood capillaries (hence, red). They exert small forces and are fatigue resistant. They are deeply placed and suited to sustained postural activities, including standing. (Also called type I or slow-twitch, fatigue resistant. )
- 2.
Fast, glycolytic (FG) fibres are large, mitochondria-poor, and capillary-poor (hence, white). They produce brief, powerful contractions. They predominate in superficial muscles. (Also called type IIb or fast-twitch, fatigable. )
- 3.
Intermediate (fast, oxidative–glycolytic, FOG) fibres have properties intermediate between the other two. (Also called type IIa or fast-twitch, fatigue resistant .)
Every muscle contains all three types of muscle fibres, and the proportion of each within a muscle reflects its function. A given motor unit contains only one type of fibre but the fibres of each motor unit interdigitate with those of other units. A given muscle may be referred to as ‘slow’ or ‘fast’ based on the type of muscle fibres it contains.
Motor End Plates
At the myoneural junction the axon divides into a handful of branchlets that groove the surface of the muscle fibre ( Fig. 10.3B ). The underlying sarcolemma is thrown into junctional folds . The basement membrane of the muscle fibre traverses the synaptic cleft and lines the folds. The underlying sarcoplasm shows an accumulation of nuclei, mitochondria, and ribosomes known as a sole plate .
Each axonal branchlet forms an elongated terminal bouton containing thousands of synaptic vesicles loaded with acetylcholine (ACh). Synaptic transmission takes place at active zones facing the crests of the junctional folds ( Fig. 10.3C ). Vesicular ACh is extruded at great speed by exocytosis into the synaptic cleft. The ACh diffuses through the basement membrane to bind with ACh receptors in the sarcolemma.
Activation of the receptors leads to depolarisation of the sarcolemma. The depolarisation is led into the interior of the muscle fibre by T tubules . The sarcoplasmic reticulum liberates Ca 2+ ions that initiate contraction of the sarcomeres.
The enzyme acetylcholinesterase is concentrated in the basement membrane, and about 30% of released ACh is hydrolysed without reaching the postsynaptic membrane. Following hydrolysis, the choline moiety is actively taken back up and returned to the axoplasm.
Terminal boutons also have some dense-cored vesicles containing one or more peptides ( Fig. 10.3C ). The best known of these is calcitonin gene-related peptide , a potent vasodilator.
Details of the muscle fibre contraction process are shown in Fig. 10.4 .
Motor Units in the Elderly
The loss of muscle mass and function associated with ageing (sarcopenia) has anatomical sequelae both at cellular and subcellular levels associated with the individual motor unit. The progressive loss of motor neurons with aging is associated with reduced muscle fibre number and size. This is mainly due to loss of motor neurons from the spinal cord and brainstem, often due in part to low-grade peripheral neuropathy arising from vascular disease and/or nutritional deficiency.
Electromyographic records taken from contracting muscles during the seventh and eighth decades of life show giant motor unit potentials. The extra-large potentials result from the takeover of vacated motor end plates of lost motor neurons by collateral sprouts from the axons of adjacent healthy motor units. Details of electromyography and clinical neuromuscular disorders are in Chapter 12 . However, the progressive loss of muscle function is related to incomplete reinnervation of the units.
At the intracellular level, key factors include changes in muscle proteins and the loss of the coordinated control between contractile elements. This is coincident with altered protein expression associated with mitochondrial and sarcoplasmic reticulum function.
The changes in skeletal muscle during the process of aging also present in the pathogenesis of acquired and hereditary neuromuscular disorders.
Studies on specific intervention strategies in experimental models have shown promising results, and it is hoped that these advances can soon be translated to clinical practice.
Sensory Innervation of Skeletal Muscle
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