Question

1.The central nervous system (CNS) is composed of billions of discrete cells that are not directly connected but are instead connected by synapses, which require neurotransmitters to pass a signal between cells. Why is this an advantage over a CNS in which all the cells are directly connected? That is, what do we gain by having our brains made up of individual cells that don’t automatically fire an action potential when they are stimulated? What are the drawbacks of the chemical synaptic nervous system? Can you think of an idea of how to organize a nervous system that might improve upon the one we already have?

2. What are the consequences of a spinal injury in the thoracic region of the spinal cord if the damage is to the gray matter? What about a complete transection of the white matter in the thoracic region? What body regions and functions that would be disrupted? Why is a white matter injury in the upper cervical region worse than one lower down in the cord? Is the same true of gray matter injuries? How do white matter injuries affect reflexes? How do gray matter injuries affect reflexes? Finally, what is your idea of why the spinal cord is organized in the way it is (white matter surrounding gray matter)?

Answer 1

The term synapse means “coming together.” Where two structures or entities come together, they form a synapse. Although one can use the word synapse to mean any cellular junction, in physiology we traditionally limit its usage to: the junction of two neurons, the junction between a neuron and a target cell (ex. the neuromuscular junction), or the interface between adjacent cardiac muscle cells or adjacent smooth muscle cells. In the nervous system, a synapse is the structure that allows a neuron to pass an electrical or chemical signal to another cell.

Synapse Cells

The cell that delivers the signal to the synapse is the presynaptic cell. The cell that will receive the signal once it crosses the synapse is the postsynaptic cell. Since most neural pathways contain several neurons, a postsynaptic neuron at one synapse may become the presynaptic neuron for another cell downstream.

A presynaptic neuron can form one of three types of synapses with a postsynaptic neuron. The most common type of synapse is an axodendritic synapse, where the axon of the presynaptic neuron synapses with a dendrite of the postsynaptic neuron. If the presynpatic neuron synapses with the soma of the postsynaptic neuron it is called an axosomatic synapse, and if it synapses with the axon of the postsynaptic cell it is an axoaxonic synapse. Although our illustration shows a single synapse, neurons typically have many (even 10,000 or more) synapses.

Synapse Transmission

There are two types of synapses found in your body: electrical and chemical. Electrical synapses allow the direct passage of ions and signaling molecules from cell to cell. In contrast, chemical synapses do not pass the signal directly from the presynaptic cell to the postsynaptic cell. In a chemical synapse, an action potential in the presynaptic neuron leads to the release of a chemical messenger called a neurotransmitter. The neurotransmitter then diffuses across the synapse and binds to receptors on the postsynaptic cell. Binding of the neurotransmitter leads to the production of an electrical signal in the postsynaptic cell.

Why does the body have two types of synapses? Each type of synapse has functional advantages and disadvantages. An electrical synapse passes the signal very quickly, which allows groups of cells to act in unison. A chemical synapse takes much longer to transmit the signal from one cell to the next; however, chemical synapses allow neurons to integrate information from multiple presynaptic neurons, determining whether or not the postsynaptic cell will continue to propagate the signal. Neurons respond differently based on information transmitted by multiple chemical synapses. Let’s take a closer look at the structure and function of each type of synapse.

Electrical synapses transmit action potentials via the direct flow of electrical current at gap junctions. Gap junctions are formed when two adjacent cells have transmembrane pores that align. The membranes of the two cells are linked together and the aligned pores form a passage between the cells. Consequently, several types of molecules and ions are allowed to pass between the cells. Due to the direct flow of ions and molecules from one cell to another, electrical synapses allow bidirectional flow of information between cells. Gap junctions are crucial to the functioning of the cardiac myocytes and smooth muscles.

The process of synaptic transmission at a chemical synapse between two neurons follows these steps:

An action potential, propagating along the axon of a presynaptic neuron, arrives at the axon terminal.

The depolarization of the axolemma (the plasma membrane of the axon) at the axon terminal opens Ca2+ channels and Ca2+ diffuses into the axon terminal.

Ca2+ bind with calmodulin, the ubiquitous intracellular calcium receptor, causing the synaptic vesicles to migrate to and fuse with the presynaptic membrane.

The neurotransmitter is released into the synaptic cleft by the process of exocytosis.

The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic membrane.

Binding of the neurotransmitters to the postsynaptic receptors causes a response in the postsynaptic cell.

The response can be of two kinds:

A neurotransmitter may bind to a receptor that is associated with a specific ion-channel which, when opened, allows for diffusion of an ion through the channel. If Na+ channels are opened, Na+ rapidly diffuses into the postsynaptic cell and depolarizes the membrane towards the threshold for an action potential. If K+ channels are opened, K+ diffuses out of the cell, depressing the membrane polarity below its resting potential (hyperpolarization). If Cl– channels are opened, Cl– moves into the cell leading to hyperpolarization.

The neurotransmitter may bind to a transmembrane receptor protein, causing it to activate a G-protein on the inside surface of the postsynaptic membrane. A cascade of events leads to the appearance of a second messenger (calcium ion, cyclic AMP (cAMP), or IP3) in the cell. Second messengers can have diverse effect on the cell ranging from opening an ion channel to changing cell metabolism to initiating transcription of new proteins.

Chemical synapses have two important advantages over electric ones in the transmission of impulses from a presynaptic cell. The first is signal amplification, which is common at nerve- muscle synapses.