5 topics with brief reviews: definitions, key facts, and class questions.
Nerve endings and synapses are key apparatuses that ensure the connection of the nervous system with target tissues and the transmission of impulses between neurons themselves. In this section, we examine the morphology of the structures that form the terminals of nerve fibers. You will study the functional types of endings, a detailed classification of receptors, and the ultrastructure of interneuronal junctions.
Special attention in the reference guide is paid to proprioceptive sensitivity. We will analyze in detail the structure of neuromuscular spindles — complex encapsulated receptors that control skeletal muscle tone. Neuroeffector synapses (motor end plates), through which the nervous system directly controls muscle fibers and glandular cells, are also described here.
In practical classes and histology examinations, the morphological classification of receptor endings (free, non-free, encapsulated) and the electron microscopic structure of the synapse (pre- and postsynaptic parts, synaptic cleft, vesicles) are most frequently asked. A firm grasp of these topics is an essential foundation for the successful mastering of normal physiology and neurology.
On which processes are receptor nerve endings located?
Receptor endings are localized on the dendrites of sensory (afferent) neurons.
Why are Merkel discs classified as free nerve endings even though they contain a Merkel cell?
The Merkel cell is a specialized epithelial cell, not a glial cell. Because the glial sheath and connective tissue capsule around the dendrite terminal in the epidermis are absent, the receptor is classified as free.
How do intrafusal muscle fibers differ from extrafusal ones?
Intrafusal fibers are located inside the spindle; they are thin and short, and their myofibrils are located only at the ends. Extrafusal fibers are typical contractile elements that form the bulk of the muscle and perform mechanical work.
Which synapses are always inhibitory?
According to the functional classification, axoaxonic synapses always perform only an inhibitory function. Axodendritic and axosomatic can be either excitatory or inhibitory.
What is the difference between a motor end plate and a neuromuscular spindle?
The motor end plate (effector) transmits the motor signal from the nerve to the muscle, causing it to contract. The neuromuscular spindle is a sensory receptor that gathers information about the degree of muscle stretch.
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