Functional Anatomy of Synapses

There are two types of synapses: electric and chemical. At electric synapses, the plasma membranes of the pre- and postsynaptic cells are joined by gap junctions (Chapter 3). These allow the local currents resulting from arriving action potentials to flow directly across the junction through the connecting channels in either direction from one neuron to the neuron on the other side of the junction, depolarizing the membrane to threshold and thus initiating an action potential in the second cell. Although numerous in cardiac and smooth muscles, electric synapses are relatively rare in the mammalian nervous system, and we shall henceforth discuss only the much more common, chemical synapse.

Figure 8-25 shows the structure of a single typical chemical synapse. The axon of the presynaptic neuron ends in a slight swelling, the axon terminal, and the postsynaptic membrane under the axon terminal appears denser. Note that in actuality the size and shape of the pre- and postsynaptic elements can vary greatly (Figure 8-26). A 10- to 20-nm extracellular space, the synaptic cleft, separates the pre- and postsynaptic neurons and prevents direct propagation of the current from the presynaptic neuron to the postsynaptic cell. Instead, signals are transmitted across the synaptic cleft by means of a chemical messenger—a neuro-transmitter—released from the presynaptic axon terminal. Sometimes more than one neurotransmitter may be simultaneously released from an axon, in which case the additional neurotransmitter is called a co-transmitter. These neurotransmitters have different receptors in the postsynaptic cell.

The neurotransmitter in terminals is stored in membrane-bound synaptic vesicles, some of which are docked at specialized regions of the synaptic membrane. When an action potential in the presynaptic neuron reaches the end of the axon and depolarizes the axon terminal, voltage-gated calcium channels in the membrane open, and calcium diffuses from the extracellular fluid into the axon terminal near the docked vesicles. The calcium ions induce a series of reactions that allow some of the docked vesicles to fuse with the presynap-tic plasma membrane and liberate their contents into the synaptic cleft by the process of exocytosis.

Direction of action potential transmission

Synaptic vesicle

—Terminal of presynaptic axon

Mitochondrion

Direction of action potential transmission

—Terminal of presynaptic axon

Mitochondrion

Synaptic vesicle

Physiologic Anatomy Synapse

Postsynaptic density

FIGURE 8-25

(a) Diagram of a synapse. Some vesicles are docked at the presynaptic membrane ready for release. The postsynaptic membrane is distinguished microscopically by "postsynaptic density," which contains proteins associated with the receptors. (b) An enlargement showing synaptic specialization.

Part b redrawn from Walmsley et al.

Postsynaptic density

FIGURE 8-25

(a) Diagram of a synapse. Some vesicles are docked at the presynaptic membrane ready for release. The postsynaptic membrane is distinguished microscopically by "postsynaptic density," which contains proteins associated with the receptors. (b) An enlargement showing synaptic specialization.

Part b redrawn from Walmsley et al.

Once released from the presynaptic axon terminal, neurotransmitter and cotransmitter, if there is one, diffuse across the cleft. A fraction of these molecules bind to receptors on the plasma membrane of the postsyn-aptic cell (the fate of the others will be described later). The activated receptors themselves may contain an ion channel, or they may act indirectly, via a G protein, on separate ion channels. In either case, the result of the binding of neurotransmitter to receptor is the opening or closing of specific ion channels in the postsynaptic plasma membrane. These channels belong, therefore, to the class of ligand-sensitive channels whose function is controlled by receptors, as discussed in Chapter 7, and are distinct from voltage-gated channels. (Exceptions to this generalization—the activation of metabolic pathways rather than ion channels—will be discussed later.)

Although Figure 8-26 shows a few exceptions, in general the neurotransmitter is stored on the presyn-aptic side of the synaptic cleft, whereas receptors for the neurotransmitters are on the postsynaptic side. Therefore, most chemical synapses operate in only one direction. One-way conduction across synapses causes action potentials to be transmitted along a given multineuronal pathway in one direction.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

Postsynaptic Membrane
Postsynaptic cell

Microtubule Vesicle attachment site

Synaptic vesicles

Microfilament

Presynaptic membrane

Vesicle docking site

Synaptic cleft

Postsynaptic membrane

Receptor

Postsynaptic density

Because of the sequence of events involved, there is a very brief synaptic delay—as short as 0.2 sec— between the arrival of an action potential at a presyn-aptic terminal and the membrane-potential changes in the postsynaptic cell.

Neurotransmitter binding to the receptor is a transient event, and as with any binding site, the bound ligand—in this case, the neurotransmitter—is in equilibrium with the unbound form. Thus, if the concentration of unbound neurotransmitter in the synaptic cleft is decreased, the number of occupied receptors will decrease. The ion channels in the postsynaptic membrane return to their resting state when the neu-rotransmitter is no longer bound. Unbound neurotransmitters are removed from the synaptic cleft when they (1) are actively transported back into the axon terminal or, in some cases into nearby glial cells; (2) diffuse away from the receptor site; or (3) are enzy-matically transformed into ineffective substances, some of which are transported back into the axon terminal for reuse.

The two kinds of chemical synapses—excitatory and inhibitory—are differentiated by the effects of the neurotransmitter on the postsynaptic cell. Whether the effect is inhibitory or excitatory depends on the type of signal transduction mechanism brought into operation when the neurotransmitter binds to a receptor and the type of channel the receptor influences.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • mauno
    How are unbound neurotransmitters removed from the synaptic cleft?
    8 years ago
  • Helen
    What is the technical name of a “docking site” on a postsynaptic cell membrane?
    8 years ago
  • reijo keskitalo
    What is the "docking site" on a postsynaptic cell membrane called?
    8 years ago
  • ralph
    How is it action potential generated synapses transmission?
    7 years ago

Post a comment