Med. Sci. 532

EFFECTORS
Our nervous system allows us to respond to sensory information as well as to do behaviors that may be somewhat independent of the sensory information our CNS receives. This motor or output or effector part of our nervous system works by: i.   neural activation of skeletal muscle (skeletomotor innervation), ii.  neural activation of smooth and cardiac muscle and exocrine gland cells (autonomic innervation, including the enteric nervous system), and iii. by the humoral response carried out with endocrine glands (neuroendocrine system).
Skeletomotor innervation The axon of an alpha motor neuron innervates extrafusal muscle fibers in skeletal muscle at sites called a neuromuscular junction (NMJ) or motor endplate. The cell bodies of alpha motor neurons are located in lamina IX in the spinal cord (see picture at right) and in certain cranial nerve-related motor nuclei (see separate section on cranial nerves).  One alpha motor neuron innervates from as few as 5-15 skeletal muscle fibers or up to as many as 150-200.  A motor unit is defined as an alpha motor neuron and all of the extrafusal muscle fibers it innervates. Thus motor units vary in size.
There is normally only one NMJ on each skeletal muscle fiber in adulthood.  Examples of two NMJs are in focus in the picture to the right.  Notice how the preterminal part of an axon divides into many terminals at each NMJ, and that the myelin sheath ceases to exist around the axon somewhat proximal to all of the terminal branching.
Viewed at higher power, each terminal is unmyelinated but capped by a Schwann cell. The terminal has mitochondria and clusters of synaptic vesicles. The latter contain acetylcholine. Basal lamina is interposed between the nerve terminal and the sarcolemma, and it extends into the grooves (also called 'junctional gutters') along the muscle surface at the NMJ.
	Neurocytology

Med. Sci. 532