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11.4: Neuronal Communication

  • Page ID
    22338
  • By the end of this section, you will be able to:

    • Explain the events that occur during the conduction of nerve impulses
    • Differentiate the nerve impulse propagation in saltatory and continuous conduction
    • Describe the components of synapses and 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. These nerve impulses represent 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. In this section, you will look at the basics of neuronal communication, mainly focusing on the conduction of the nerve impulse.

    Conduction of Nerve Impulses

    Neurons possess electrical excitability, which is the ability to respond to a stimulus by generating a nerve impulse. In the majority of cases, the dendrites of a neuron are the place where local changes in the membrane electrical properties happen through synapses. Dendrites receive stimuli from the external environment (e.g. somatic sensory neurons) or internal environment (e.g. visceral sensory neurons, motor neurons or interneurons). The amount of change in the membrane electrical charge is determined by the strength of the stimulus that causes it. For example, a needle pricking a fingertip will result in a bigger stimulus compared to a blunt object touching the same fingertip. Once a stimulus (or multiple stimuli) produces a significant change in the membrane electrical properties of a dendrite and reaches a predetermined threshold, then a nerve impulse (also called action potential) occurs. An action potential is generated at the axon hillock of a neuron and progresses rapidly along the axon's plasma membrane to reach the targets (a second neuron, a muscle or a gland). This movement of an action potential along the axon is called propagation. While the stimuli can be weak or strong, the action potential follows a All-or-None Law by which it always has the same strength (referred to as amplitude) independently of the stimulus. This minimizes the possibility that information will be lost along the way. The only way to modulate the response is through the frequency of action potentials - how many action potentials reach the target in a set amount of time. A bigger stimulus will produce a series (or train) of action potentials that are close together, while a weak stimulus will produce sparse action potentials.

    The speed of an action potential is influenced by the diameter of the axon and by its myelination. The larger the diameter of the axon, the faster the action potential will be conducted. Myelinated axons are able to carry action potentials faster than unmyelinated axons. As discussed in the previous section, myelinating cells (oligodendrocytes in the CNS and Schwann cells in the PNS) wrap around axons forming the myelin. Nodes of Ranvier are gaps between segments of myelin. The electrical charges of the action potential can "jump" from one gap to another one, thus allowing a faster speed of the action potential. This progression of a nerve impulse is called saltatory conduction. However, in unmyelinated axons, one side of the axon is not covered by myelin and the electrical charges move along the entire axonal membrane, thus taking longer to reach their target. This progression of a nerve impulse is called continuous conduction. Once the action potential reaches the axon terminal, it is either transported as electrical charge into the next cell or transformed into a chemical signal, depending on the type of synapse that the synaptic end bulb is forming with its target.

    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. If the postsynaptic cell is a neuron, a new action potential might be generated in the postsynaptic neuron and reach its postsynaptic targets.

    One multipolar neuron with multiple dendrites and one long axon branching to reach 2 other neurons.
    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)

    Synapses

    There are two types of connections between electrically active cells: electrical synapses and chemical 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 \(\PageIndex{2}\)). 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.

    Two synaptic bulbs face each other, with channels between them where action potentials pass through.
    Figure \(\PageIndex{2}\): 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 viceversa. (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)

    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 \(\PageIndex{3}\):

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

    The synaptic end bulbs of chemical synapses are filled with vesicles containing one type of neurotransmitter. When an action potential reaches the axon terminals, the vesicles merge with the cell membrane at the synaptic end bulb, releasing the neurotransmitter through exocytosis into the small gap between the cells, known as the synaptic cleft. Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event. The binding of the neurotransmitter to its receptor causes a brief electrical change across the postsynaptic membrane. The change depends on the type of neurotransmitter receptor. Changes in the postsynaptic cell membrane can cause a nerve impulse to begin in the postsynaptic cell or inhibit the generation of an action potential. The flow of information is unidirectional: from the presynaptic cell to the postsynaptic cell. After its release in a chemical synapse, neurotransmitters need to be removed from the synaptic cleft to ensure the propagation of new synaptic signals.

    Circles inside a bulb are vesicles filled with dots for neurotransmitters which then are pushed out
    Figure \(\PageIndex{3}\): 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)

    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 that is not functioning correctly. 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.

    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. 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

    Understanding how the nervous system works could be a driving force in your career. Studying neurophysiology 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, a neuroscience nurse, or a neurodiagnostic technician, among other possible career paths.

    Concept Review

    The basis of the electrical signal within a neuron is the action potential that propagates down the axon. For a neuron to generate an action potential, it needs to receive input from another source, either another neuron or a sensory stimulus. That input will result in a change in the electrical properties of the cell membrane, based on the strength of the stimulus. Once the stimulus is strong enough, it will generate an action potential that travels along the axon to the synaptic end bulb.

    The diameter of the axon and the presence or absence of myelin determines how fast the action potential is conducted down the axon. A larger diameter and the presence of a myelin sheath will ensure a fast propagation of the action potential.

    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 element and diffuses across the synaptic cleft. The neurotransmitter binds to a receptor protein and causes a change in the postsynaptic membrane (the PSP). The neurotransmitter must diffuse, be inactivated or removed from the synaptic cleft so that the stimulus is limited in time.

    Review Questions

    Q. At an electrical synapse, what do the presynaptic and postsynaptic cells communicate through?

    A. neurotransmitters

    B. neurotransmitter receptors

    C. nodes of Ranvier

    D. gap junctions

    Answer

    Answer: D

    Q. Which of the following axons would propagate an action potential faster than the others?

    A. myelinated, large diameter, axons

    B. myelinated, small diameter, axons

    C. unmyelinated, large diameter, axons

    D. unmyelinated, small diameter, axons

    Answer

    Answer: A

    Glossary

    action potential
    change in the electrical properties of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
    All-or-None Law
    the principle that the strength by which a neuron responds to a stimulus is not dependent on the strength of the stimulus;
    chemical synapse
    connection between two neurons, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance
    continuous conduction
    slower propagation of the action potential along an unmyelinated axon
    electrical synapse
    connection between two neurons, or any two electrically active cells, where an action potential can flow across gap junctions into the adjacent cell
    neurotransmitter
    chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell
    postsynaptic cell
    cell receiving a synapse from another cell
    presynaptic cell
    cell forming a synapse with another cell
    propagation
    movement of an action potential along the length of an axon
    saltatory conduction
    faster propagation of the action potential in a myelinated axon, from one node of Ranvier to the following node
    synaptic cleft
    small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element
    threshold
    membrane voltage at which an action potential is initiated

    Contributors and Attributions