Peripheral Motor Endings

Peripheral Motor Endings

In the PNS, motor endings activate effectors via the release of neurotransmitters. Peripheral motor end-ings connect nerves to their effectors. They are related to the innervation of skeletal muscle as well as the innervation of visceral muscles and various glands.

Skeletal Muscle Innervation

Terminals of somatic motor fibers innervating voluntary muscles form complex neuromuscular junctions with their effector cells. The ending of each axon branch, at its target muscle fiber, splits into a cluster of axon terminals, branching over the sarcolemma folds of the fiber. These axon terminals contain mitochondria and synaptic vesi-cles that are filled with acetylcholine (ACh).

As a nerve impulse reaches the axon terminal, ACh is released via exocytosis, diffusing across the synaptic cleft. It attaches to its receptors on the sarco-lemma and the neuromuscular junction. Ligand-gated channels are opened by this ACh binding, allowing sodium and potassium ions to pass. Because sodium enters cells more quickly than potassium is lost, the muscle cell interior becomes depolarized. This graded potential, known as the end plate potential, spreads to nearby areas of the membrane, causing voltage-gated sodium channels to open. An action potential results along the sarcolemma, stimulating contraction of the muscle fiber. At somatic neuromuscular junctions, the synaptic cleft is filled with a basal lamina that is rich in glycoproteins. This basal lamina is not present at other synapses. It contains acetylcholinesterase, which breaks down the ACh.

Visceral Muscle and Gland Innervation

Junctions between autonomic motor endings and their effectors are not as complex as those formed between somatic fibers and skeletal muscle cells. There is repeated branching of autonomic motor axons. Each of these forms synapses en passant (which means “synapses in passing”) with its effec-tor’s cells. An axon serving a smooth muscle or gland has many varicosities, whereas an axon serving a cardiac muscle does not. Varicosities are knob-like swellings that contain mitochondria and synaptic vesicles, resembling a string of pearls. Autonomic synaptic vesicles usually contain ACh or norepi-nephrine (NE). Both act indirectly on their targets, using second messengers. As a result, visceral motor responds are slower than those caused by somatic motor fibers, which cause direct opening of ion channels.

1. Explain the actions of motor endings.

2. Define the term “neuromuscular junction” and the relationship to axon terminals and the end plate potential.

3. Define the term “varicosities.”