10.4: Synapses

By the end of this section, you will be able to:

• Describe the components of synapses
• Compare electrical and chemical synapses

Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Neurons communicate with other neurons, muscles or glands through the generation and conduction of nerve impulses called action potentials. These electrical impulses propagate from the axon hillock to the axon terminals via changes in the electrical properties of the neuronal cell membrane. All cells have an electrical charge associated with their membrane. However, neurons and other cells are able to change their electrical charge by moving ions across the membrane. At the axon (or synaptic) terminals the action potential needs a way to pass the signal on to the next cell, which they do at structures called synapses.

Synapses

Neurons and their targets form synapses. The neuron that generates and conducts the action potential to the target is called a presynaptic cell. The target cell receiving the action potential is called a postsynaptic cell. While the presynaptic cell is always a neuron (because only neurons have axons and can form a synapse), the postsynaptic cell can be a neuron or another type of cell such as skeletal, cardiac or smooth muscle cells, or glands. In Figure \(\PageIndex{1}\), a presynaptic neuron forms synapses with two postsynaptic neurons. The nerve impulse (or signal) travels from a presynaptic neuron to a postsynaptic cell by crossing the gap between them, the synaptic cleft. The two methods by which the cleft is crossed are 1) electrical and 2) chemical, with chemical being the most common and most diverse.

Figure \(\PageIndex{1}\): Presynaptic and postsynaptic neurons. A presynaptic neuron on top forms synapses with two postsynaptic neurons at the bottom. The parts of the presynaptic neuron are labeled as cell body, nucleus, dendrites and axon covered in a myelin sheath. The direction of signal shows that the electrical signal (action potential) travels from the axon of the presynaptic neuron to the axon terminals of the presynaptic neuron that are in contact with the dendrites and cell body of the postsynaptic neurons. (Image credit: "Neuron Part 1" by BruceBlaus is licensed under CC BY-SA 4.0.)

Chemical Synapses

Chemical synapses involve the transmission of chemical information from one cell to the next and they represent the majority of the synapses found within the nervous system. In a chemical synapse, a chemical signal called a neurotransmitter, is released from the presynaptic cell and it affects the postsynaptic cell. There are many different types of neurotransmitters, for example acetylcholine, serotonin, dopamine, adrenaline, glutamate, etc. Each neurotransmitter has its own specific receptor on the postsynaptic membrane. Chemical synapses can then be classified based on the neurotransmitter that the cells use to communicate (for example glutamatergic synapses use glutamate). Different neurotransmitters and different receptors will determine the overall response to the stimulus. All chemical synapses have common characteristics, which can be summarized in this list and are shown in Figure \(\PageIndex{2}\):

• synaptic end bulb of presynaptic neuron
• neurotransmitter (packaged in vesicles)
• synaptic cleft
• receptors for neurotransmitter
• postsynaptic membrane of postsynaptic neuron

Figure \(\PageIndex{2}\): Chemical Synapse. The axon of the presynaptic neuron terminates at the axon terminal in a synaptic end bulb where the action potential arrives. The arrival of action potential causes the synaptic vesicles containing neurotransmitters to fuse with the membrane and release the neurotransmitters. The neurotransmitters diffuse across the synaptic cleft to bind to its receptor, in this case ligand-gated channels. Neurotransmitters will eventually be cleared from the synapse. (Image credit: "Chemical Synapse" by Young, KA., Wise, JA., DeSaix, P., Kruse, DH., Poe, B., Johnson, E., Johnson, JE., Korol, O., Betts, JG., & Womble, M. is licensed under CC BY 4.0.)

Electrical Synapses

In an electrical synapse, there is a direct connection between the presynaptic and postsynaptic cells and the connection is formed by gap junctions. Thus, the electrical charges of an action potential can pass directly from one cell to the next. If one cell delivers an action potential in an electrical synapse, the joined cell will also generate an action potential because the electrical charges will pass between the cells (Figure \(\PageIndex{3}\)). Although representing the minority of synapses, electrical synapses are found throughout the nervous system. These synapses also occur between excitable cells other than neurons, for example between smooth muscle cells in the intestines and cardiac muscle cells in the heart.

Figure \(\PageIndex{3}\): Electrical Synapse. Two synaptic end bulbs form an electrical synapse where gap junctions allow the passage of action potentials from one cytoplasm to the other, and vice versa. (Image credit: ”Electrical Synapse" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0.)

 

DISORDERS OF THE...

Nervous System: Alzheimer's and Parkinson's Disease

The underlying cause of some neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, appears to be related to proteins—specifically, to proteins behaving badly. One of the strongest theories of what causes Alzheimer’s disease is based on the accumulation of beta-amyloid plaques, dense conglomerations of a protein fragment that aggregates instead of being broken down by microglia. Parkinson’s disease is linked to an increase in a protein known as alpha-synuclein that is toxic to the cells of the substantia nigra nucleus in the midbrain.

Figure \(\PageIndex{4}\): Beta-amyloid plaques form in the brain of Alzheimer's disease patients. These plaques form when a normal process, the slicing of the amyloid precursor protein to remove the beta-amyloid fragment, is disrupted and these fragments are not metabolized normally.  Instead, the stick to each other and disrupt neuronal function.(Image credits: Illustration: "Alzheimers_disease-Beta-amyloid_plaque_formation" by National Institute on Aging is in the Public domain, via Wikimedia Commons; Histology: "Alzheimer_dementia_(3)_presenile_onset " by KGH is licensed under CC BY-SA 3.0, via Wikimedia Commons.)

For proteins to function correctly, they are dependent on their three-dimensional shape. The linear sequence of amino acids folds into a three-dimensional shape that is based on the interactions between and among those amino acids. When the folding is disturbed, and proteins take on a different shape, they stop functioning correctly. But the disease is not necessarily the result of functional loss of these proteins; rather, these altered proteins start to accumulate and may become toxic. For example, in Alzheimer’s, the hallmark of the disease is the accumulation of these amyloid plaques in the cerebral cortex (Figure \(\PageIndex{4}\)). The term coined to describe this sort of disease is “proteopathy” and it includes other diseases. Creutzfeld-Jacob disease, the human variant of the prion disease known as mad cow disease in the bovine, also involves the accumulation of amyloid plaques, similar to Alzheimer’s. Diseases of other organ systems can fall into this group as well, such as cystic fibrosis or type 2 diabetes. Recognizing the relationship between these diseases has suggested new therapeutic possibilities. Interfering with the accumulation of the proteins, and possibly as early as their original production within the cell, may unlock new ways to alleviate these devastating diseases.

CAREER CONNECTIONS

Neurophysiologist

Figure \(\PageIndex{5}\)  A neuroscience student can participate in research projects by monitoring and running various tests during experiments as well as doing data collection and analysis. Student Cammi Borden worked with research psychologist Dr. Kara Blacker at the Naval Medical Research Unit studying low-oxygen conditions impact on brain activity. (Image credit: Public Domain Dedication. Public Use Notice of Limitations: https://www.dvidshub.net/about/copyright.)

Understanding how the nervous system works could be a driving force in your career. Studying neurophysiology (Figure \(\PageIndex{5}\)) is a very rewarding path to follow. It means that there is a lot of work to do, but the rewards are worth the effort.

The career path of a research scientist can be straightforward: college, graduate school, postdoctoral research, academic research position at a university. A Bachelor’s degree in science will get you started, and for neurophysiology that might be in biology, psychology, computer science, engineering, or neuroscience. But the real specialization comes in graduate school. There are many different programs out there to study the nervous system, not just neuroscience itself. Most graduate programs are doctoral, meaning that a Master’s degree is not part of the work. These are usually considered five-year programs, with the first two years dedicated to course work and finding a research mentor, and the last three years dedicated to finding a research topic and pursuing that with a near single-mindedness. The research will usually result in a few publications in scientific journals, which will make up the bulk of a doctoral dissertation. After graduating with a Ph.D., researchers will go on to find specialized work called a postdoctoral fellowship within established labs. In this position, a researcher starts to establish their own research career with the hopes of finding an academic position at a research university.

Other options are available if you are interested in how the nervous system works. Especially for neurophysiology, a medical degree might be more suitable so you can learn about the clinical applications of neurophysiology and possibly work with human subjects. An academic career is not a necessity. Biotechnology firms are eager to find motivated scientists ready to tackle the tough questions about how the nervous system works so that therapeutic chemicals can be tested on some of the most challenging disorders such as Alzheimer’s disease or Parkinson’s disease, or spinal cord injury.

Others with a medical degree and a specialization in neuroscience go on to work directly with patients, diagnosing and treating mental disorders. You can do this as a psychiatrist, a neuropsychologist, or a neurologist, a neurosurgeon, or an anesthesiologist.

Concept Review

Synapses are the contacts between neurons, which can either be chemical or electrical in nature. Chemical synapses are far more common. At a chemical synapse, neurotransmitter is released from the presynaptic neuron and diffuses across the synaptic cleft. The neurotransmitter binds to a receptor protein and causes a change in the postsynaptic cell's membrane (the PSP). 

Review Questions

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology