Neurons communicate with each other and other cells at specialised junctions called synapses, a term introduced by Charles Sherrington in 1897. Sherrington was convinced, from his physiological and anatomical studies of reflex functions of the spinal cord, that the transfer of signals between cells is a different type of process from the transfer of signals within a cell. It was not until the electron microscope was developed, 50 years after this, that the structure of synapses could be examined in detail. Synapses are tiny, and also very diverse. Sometimes a functional connection between two neurons is composed of just a single synaptic structure but more commonly there are several. Sometimes thousands of discrete anatomical contacts make up a connection between two neurons. A complex neuron such as a vertebrate motor neuron typically receives several tens of thousands of individual synapses from a variety of sources.

The neuron that is passing a signal on is referred to as presynaptic, and a neuron that is receiving a signal is referred to as postsynaptic. Transmission

Figure2.7 Operation of the three main types of synapse between nerve cells. (a) Schematic arrangement for recording the transfer of signals across a synapse by inserting electrodes close to the presynaptic and postsynaptic sites. (b) Recordings of presynaptic and postsynaptic potentials at the three types of synapse.

Figure2.7 Operation of the three main types of synapse between nerve cells. (a) Schematic arrangement for recording the transfer of signals across a synapse by inserting electrodes close to the presynaptic and postsynaptic sites. (b) Recordings of presynaptic and postsynaptic potentials at the three types of synapse.

across a synapse can be studied by using two microelectrodes, one to record presynaptic potential and the other to record postsynaptic potential (Fig. 2.7). A significant majority of synapses are chemical synapses at which signals are transferred by way of chemicals such as acetylcholine or some amino acids that act as neurotransmitters.

The postsynaptic signal is called the postsynaptic potential, usually abbreviated to PSP. There is a delay, usually of about a millisecond, between the spike and the PSP. Generally, a spike in a presynaptic neuron, about 100 mV in amplitude, causes a PSP that is much smaller, a few millivolts in amplitude, although a few synapses are specialised to act as relays where the PSP is strong enough to trigger a spike in the postsynaptic neuron. At some synapses, the PSP excites the postsynaptic neuron by depolarising it and these PSPs are called excitatory postsynaptic potentials (EPSPs). At other synapses, the PSP hyperpolarises and inhibits the postsynaptic neuron and these PSPs are inhibitory postsynaptic potentials (IPSPs).

When a neuron is excited, it is more likely to release neurotransmitter and so pass signals to its postsynaptic targets. This is because the amount of neurotransmitter that is released depends on the membrane potential at the presynaptic sites. The way that membrane potential is linked to neurotransmitter release is by calcium ions which enter the presynaptic terminal through voltage-sensitive channels. Like the voltage-sensitive sodium channels that cause the upswing of a spike in an axon, these calcium channels open in response to depolarising signals. When a spike invades a pre-synaptic terminal, it depolarises the membrane strongly for a short time, so there is a strong pulse of calcium entry into the terminal and a brief squirt of neurotransmitter is released. Most of the delay in transmission across a chemical synapse is due to the time it takes for the voltage-activated calcium channels to open.

Spikes are not necessary for neurotransmitter release, and the link between membrane potential and the rate of neurotransmitter release is used by many neurons to transmit information in a graded manner. Such neurons have been studied in various visual systems (see section 5.3) and the motor systems of arthropods (see section 8.8), and are called non-spiking neurons because they normally operate without producing spikes. Instead, small variations in membrane potential regulate the amount of neurotransmitter release, probably because the rate of calcium entry into the presynaptic terminals is directly controlled by voltage-sensitive calcium channels. Rather than being released in squirts, neurotransmitter tends to dribble from the presynaptic terminals of these neurons, and some of them sustain a steady leakage.

Neurotransmitter diffuses extremely rapidly across the cleft that separates the membranes of the presynaptic and postsynaptic neurons. Some of its molecules attach to receptor proteins on the postsynaptic membrane and cause the shape of the receptor to alter. Some receptors are parts of ion channels, and a frequent action of neurotransmitter is to cause this type of ion channel to open. As with voltage-sensitive channels, these chemical-sensitive channels allow particular types of ions to pass through when they are open, and the direction and strength of flow are governed by the con centration and electrical gradients across the membrane. Many of these transmitter-activated channels allow sodium ions to pass through, and sodium will tend to enter the neuron, exciting it by causing an EPSP. Other transmitter-activated channels allow chloride ions to pass, and these ions tend to enter the neuron causing an IPSP. Potassium-conducting transmitter-activated channels also mediate IPSPs. Integration involves weighing up the balance of EPSPs and IPSPs within a cell and this determines how excited it is.

At a few synapses, electrical current can flow directly from one neuron to another. These are called electrical synapses (see Heitler, 1990, for a review). Under an electron microscope, an electrical synapse can be recognised as a region where the cell membranes of two neurons come close and touch. Protein channels called connexons link the two cells so that, when the synapse is active, the cytoplasm in the two cells is in direct contact, forming a conductive pathway for the electrical current. Sometimes electrical synapses conduct equally well in both directions, so that each neuron is both presynaptic and postsynaptic. A signal passes from one cell to another with negligible delay at an electrical synapse (Fig. 2.7b), which means that information passes across electrical synapses slightly more rapidly than across chemical synapses. Possibly associated with this, some of the best-known electrical synapses occur in pathways concerned with rapid escape responses (see Chapter 3). Electrical synapses can also help ensure that the neurons they connect are excited synchronously, which is useful for the coordination of some motor activities. This is also useful in some sensory systems such as the retina, in which electrical coupling helps to filter out real signals from extraneous noise. However, compared with chemical synapses, the scope for integration offered by electrical synapses is limited.

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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.

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