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How do axon terminals report to the soma?

How do axon terminals report to the soma?


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It is important to bear in mind that the distance between a neuron's axon terminal and its soma can be extensive, up to about 1m in the human body. The fastest transport along the axon is 400mm/day (For this purpose I suppose Source: Wikipedia).

Of course the axon terminals also need to be supplied with a steady stream of proteins to maintain normal turnover, for example the terminal may need enzymes which metabolise the neurotransmitter, membrane proteins for signalling, vesicle coating and membrane fusion proteins etc.

However, sudden changes are possible in the level available of a certain protein. An example could be sudden depletion of SNAREs (required for vesicle fusion in neurotransmitter exocytosis) caused by botulinum toxin.

My question is, does the terminal somehow report back to the soma in any way if a certain protein needs to be made available more or less? And if so, how? Especially considering that retrograde transport, according to Wikipedia, gets up to 200mm/day only. If that was the mechanism for feedback, it would create a large discrepancy between signals received at the soma and the terminal's actual state.


We can consider the classical example of Long Term Potentiation.

CREB is a transcription factor which is activated during LTP by phosphorylation.

Your reasoning about the delay between signal and transcriptional response is correct and thats why neurons have local translation factories near the dendritic spines and gene regulation (fast responses) is mostly post-transcriptional. (there is no article which says exactly so but you can refer so some articles which report that miRNA-repressed genes are reactivated upon certain signals. They hint that post translational regulation is very important in neurons)

http://www.cell.com/neuron/abstract/S0896-6273%2809%2900939-8

http://nar.oxfordjournals.org/content/40/11/5088


How do axon terminals report to the soma? - Biology

The nervous system is extraordinarily complex, and it is therefore impossible to cover it in its entirety in a single laboratory. This lab will be limited to the study of the basic features of neurons and glial cells - specific organs composed of neurons, including the retina of the eye and the organ of Corti of the inner ear, will be studied in the Sensory Systems lab, in conjunction with the Neuroanatomy course.

The Neuron

An understanding of the nervous system begins with an understanding of its basic morphological and functional unit, the neuron. Neurons are nerve cells that form the conducting system that carries information throughout the central and peripheral nervous systems. Not all neurons look or act the same - they vary in size, shape, and complexity, and the important differences between the various classes of neurons will be of great importance in your study of Neuroanatomy. For now, we will focus on the common structural features that make neurons identifiable at the level of resolution of light and electron microscopes.

Neurons can be divided into four regions:

  • Dendrites. The dendrites make up the receptive portion of the neuron, and receive most synaptic afferent inputs from upstream neurons.
  • Cell body. The cell body, also the soma, is the integrative portion of the neuron, where incoming signals from dendrites are summed together. The neuron will fire or not fire based upon the results of this summation. The soma also contains the nucleus and most of the organelles of the neuron, surrounded by the cytoplasm or perikaryon.
  • Axon. The axon extends away from the soma and is the conductile portion of the neuron. Efferent signals flow down the axon in one direction, toward the terminal branches. Axons can be up to a meter long.
  • Synaptic terminal. At the end of the axon is the synaptic terminal, which is notable for its high concentration of vesicles containing neurotransmitters. This is the effector portion of the neuron when an action potential reaches the terminal, the content of the vesicles is released and either excite or inhibit the next neuron.

While every neuron possesses these four structural features, the relative positions of these features determine the type of neuron. There are three basic neuron types:

  • A multipolar neuron has multiple dendrites extending from the cell body and a single axon extending in the opposite direction.
  • A bipolar neuron has a single dendrite that extends from the cell body, opposite the side from which the single axon extends.
  • A pseudounipolar neuron has a single axon that splits into one branch that runs to the peripheral tissues and a second branch that leads to the spinal cord.

There are a few key points to remember when you are viewing a neuron under the microscope:

  • You should begin by distinguishing the axon from the dendrites. Usually, several short dendrites extend from the cell body. The single axon tends to be longer - while the axon may split into multiple pathways, it typically originates from a single point, the axon hillock.
  • The axon hillock is a conical elevation of the cell body from which the single axon extends.
  • It is not always possible to distinguish dendrites from axons based on their shape and size alone. Instead, you can use Nissl substance to make this easier. Nissl bodies, the equivalent of the rough endoplasmic reticulum in the neuron, are found only in the soma and dendrites of the neuron - never in the axon hillock or axon. By identifying Nissl substance, you can easily distinguish the two processes.
  • Neurofibrils extend from the soma out into the dendrites. These represent aggregates of microtubules and neurofilaments, and can be visualized by EM. Neurofibrils are important because they mediate slow and fast axonal transport, the method by which cytoskeletal elements and membrane-bound organelles move to and from the soma.

Nerve Fibers

The nerve fiber consists of a neuron's axon and its myelin sheath, if present. Nerve fibers are found in the peripheral nervous system and central nervous system. In the peripheral nervous system, Schwann cells form the sheath around axons, and each Schwann cell forms the sheath for just one neuron. In the central nervous system, oligodendrocytes form the sheath, and one oligodendrocyte can myelinate multiple neurons. Schwann cells and oligodendrocytes can also associate with axons but not form a myelin sheath around the axon.

Schwann cells, in the peripheral nervous system, and oligodendrocytes, in the central nervous system, wrap around the axons of neurons to form myelin sheaths. Myelin sheaths are electrical insulators and prevent current from leaving axons. The myelin sheath is interrupted at intervals along the axon by the nodes of Ranvier. These nodes represent the points of discontinuity between individual Schwann cells or oligodendrocytes arrayed along the nerve fiber. In myelinated axons, current hops between nodes and travels much faster compared to an unmyelinated axon of similar diameter.

In peripheral nerve fibers, each nerve fiber is enclosed in a delicate connective tissue covering called the endoneurium. Bundles of nerve fibers are surrounded by a more extensive layer of connective tissue called the perineurium surrounds. Finally, the entire peripheral nerve trunk is encapsulated by another connective tissue sheath called the epineurium.

Sensory Nerves

Impulses are carried to the spinal cord by the sensory nerves. Sensory nerves have various types of receptors.

  • Exteroceptors carry sensations of pain, temperature, touch, and pressure from the skin and connective tissue. They may be encapsulated or unencapsulated.
  • Proprioceptors carry impulses of stretch and position from the muscles, tendons, and joints.
  • Visceroreceptors carry stimuli from the internal organs and circulatory system.

Sensory neurons are pseudounipolar.

Motor Nerves

A motor neuron innervates one or many muscle fibers to control muscle contraction. A motor unit is defined as the neuron and the muscle fibers it supplies. Muscles that require fine control have fewer muscle fibers innervated by each neuron muscles that participate in less controlled movements may have many fibers innervated by one neuron. Motor neurons are typically multipolar with an axon that terminates in a neuromuscular junction on the surface of skeletal muscle fibers. The neuromuscular junction will be discussed in the Muscle Lab.

Neurons in Spinal Cord

You should be familiar with the gross structure of the spinal cord from Human Anatomy. The spinal cord is composed of gray matter and white matter. The gray matter, which is shaped like a butterfly, is internal and contains the nerve cell bodies. The white matter is external and contains tracts of nerve fibers. In the center of the cord is the central canal, which is lined by ependyma, epithelial cells that produce cerebrospinal fluid. You will become very familiar with the nuclei and tracts of the spinal cord in Neuroanatomy. For now, it is important to be able to distinguish the two main horns of the spinal cord, along with their associated processes and nuclei.

  • The ventral horn of the spinal cord contains the cell bodies of motor neurons. These neurons extend out of the spinal cord through the ventral root.
  • The dorsal horn of the spinal cord contains the cell bodies of ascending secondary sensory neurons. The primary sensory neurons have their cell bodies outside, but just adjacent to, the spinal cord in the dorsal root ganglion. The sensory neurons of the dorsal root ganglia are pseudounipolar because they send out one process that splits into two branches: one that extends to the periphery (to receive sensory information) and one that extends to the spinal cord (which transmits sensory information). The dorsal root ganglion also contains satellite cells, which provide structural and metabolic support to the sensory neurons.

Neurons in the Brain

In the brain, the positions of the gray and white matter are the reverse of what they are in the spinal cord - the gray matter containing cell bodies is external, and the white matter containing nerve fibers is internal. The gray matter of the cerebral cortex is divided into 6 layers. The characteristic cell type of the cortex is the pyramidal cell, so-called because of their triangular shape. Pyramidal cells have a thick, branching dendrite located at the apex and a long axon that extends toward the white matter.

The cerebellar cortex has three layers: an outer molecular layer with nerve cell processes, a layer of Purkinje cells, and an inner granular layer with several other types of neurons. Purkinje cells are very large neurons that possess a tree of branching dendrites that extend into the molecular layer.

Glial Cell in the Central Nervous System

Neuroglia are the main non-nervous cells of the central nervous system. The are present in the extracellular space of nervous tissue, or neuropil. You will observe four types of CNS neuroglia in this lab:


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1 INTRODUCTION

Compared to other cell types, neurons display enormously high levels of morphological complexity and a relatively large intracellular volume. Because of their size and intricate morphology, neurons heavily rely on highly complex intracellular membrane transport mechanisms (Reck-Peterson et al., 2018 Südhof, 2018 ). These membrane trafficking mechanisms are not only required for generating morphological complexity and to mediate neuronal growth, they also equip neurons with tools to mediate long-distance intercompartmental signaling, as well as various forms of molecular recycling pathways to maintain long-term viability (Hirokawa & Tanaka, 2015 Reck-Peterson et al., 2018 ). The axon, given its length and its crucial roles in brain development and connectivity, is the compartment in neurons that takes the most advantage of these complex intracellular membrane transport mechanisms. Anterograde membrane transport from the neuronal soma toward the distal axon is essential for biosynthetic pathways that mediate axon growth, pathfinding, and synaptic connectivity by delivering essential components required for growth cone expansion and presynaptic assembly. Retrograde membrane transport pathways serve as mediators for growth cones and synapses to report on their status to the neuronal soma. Moreover, retrograde traffic is required for the delivery of damaged or dysfunctional macromolecules and organelles to the neuronal soma, where the bulk of degradative lysosomes are located, to promote neuronal health and to prevent neurodegeneration (Maday et al., 2014 Sleigh et al., 2019 ).

The two primary retrograde trafficking routes that have been discovered to operate in neuronal axons are the endolysosomal pathway and macroautophagy (hereafter referred to simply as autophagy). Here we provide an overview of the current state of knowledge on these pathways, for example, as to how endolysosomal and autophagosomal vesicles emerge, how their cargos (i.e., dysfunctional proteins or entire organelles) are selected and transported, and how they converge and join forces to mediate axonal signaling and recycling.


Parts of a Neuron

Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.

It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.

Art Connection

Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.

Which of the following statements is false?

  1. The soma is the cell body of a nerve cell.
  2. Myelin sheath provides an insulating layer to the dendrites.
  3. Axons carry the signal from the soma to the target.
  4. Dendrites carry the signal to the soma.

Soma (biology)

The soma, or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. The word soma is Greek, meaning "body" the soma of a neuron is often called the "cell body". There are many different specialized types of neurons and the size of the soma can range from about 5 micrometres to over 1 millimetre for some of the largest neurons of invertebrates.

The cell nucleus is a key feature of the soma. The nucleus is the source of most of the RNA that is produced in neurons and most proteins are produced from mRNAs that do not travel far from the nucleus. This creates a challenge for supplying new proteins to axon endings that can be several feet away from the soma. Axons contain microtubule-associated motor proteins that allow for the transport of protein-containing vesicles from the soma to the distant ends of axons. Such transport of molecules away from the soma allows the nucleus to help maintain critical cell functions in all parts of the neuron.

The survival of some sensory neurons depends on axon endings making contact with sources of survival factors that prevent apoptosis. The survival factors are molecules such as nerve growth factor (NGF). NGF interacts with receptors on axon endings, and this produces a signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors are endocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon. [1]


What Are Terminal Buttons? (with pictures)

Terminal buttons are structures on the end of the axon, the trailing part of a neuron, that carry signals to neighboring neurons, glands, or muscles. When electrical signals enter a neuron, they travel down the length of the axon, which branches out to create a number of terminal buttons. Small sacs known as vesicles at each button fill with neurotransmitters and burst open when triggered by a signal from the neuron. This releases chemicals that can leap to a neighboring cell to excite or inhibit it, depending on the neurotransmitter involved.

Also known as end bulbs, terminal buttons are a key component of the anatomy of the neuron. In cases where they communicate with other neurons and gland cells, a small space known as the synapse provides room for the chemical signal to travel. At the neuromuscular junction, a neuron fires neurotransmitters across a synapse to a muscle cell, which can trigger a movement. The number of terminal buttons on any given axon can vary, and they may contain large numbers of vesicles to provide ample supplies of neurotransmitters.

Neurons have different functions in the body that can lead to variations in structure. A motor neuron, for example, is involved in the regulation of movement, and thus works differently than a sensory neuron. In all cases, these specialized cells rely on the ability to communicate with extreme rapidity across a synapse. Neurons can fire so quickly that a response appears almost instantaneous, when in fact it may involve a long relay of signals from cell to cell.

For example, when a sensory neuron is stimulated by the experience of pain, it can send a signal along the nerve pathways to the spinal cord, relaying a signal to the brain. The brain can signal motor neurons to tell the body to change position, thereby avoiding the sensation. Time elapsed between an experience like touching a hot pan and jerking back in pain can seem instant, illustrating how quickly the brain can respond to a situation. Large supplies of vesicles at the terminal buttons allow neurons to fire, relax, and fire again very quickly.

Research on the structure of neurons provides important information about what occurs when there are problems with the brain and nervous system. People with demyelinating diseases, for instance, experience a gradual erosion of the protective sheath which covers nerves. This degrades efficiency, making it harder for nerves to send and control signals. As a result, the patient can develop symptoms like weakness and tremors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.


Type of Neurons

Sensory neurons

These run from the various types of stimulus receptors, e.g., to the central nervous system (CNS), the brain and spinal cord.
Link to CNS

The cell bodies of the sensory neurons leading to the spinal cord are located in clusters, the dorsal root ganglia (DRG), next to the spinal cord. Their axon extends in both directions: a peripheral axon to receptors at the periphery and a central axon passing into the spinal cord. The latter axon usually terminates at an interneuron.

The diagram is a simplified view of the relationship between sensory and motor neurons running to and from the spinal cord.

Interneurons

Interneurons are also called association neurons.

It is estimated that the human brain contains 100 billion (10 11 ) interneurons averaging 1000 synapses on each that is, some 10 14 connections.

The term interneuron hides a great diversity of structural and functional types of cells. In fact, it is not yet possible to say how many different kinds of interneurons are present in the human brain. Certainly hundreds perhaps many more.

Motor neurons

Most motor neurons are stimulated by interneurons, although some are stimulated directly by sensory neurons.

Synapses

Differentiation: one axon but many dendrites

Recent work (Shelly, M., et al., Science, 327:547, 29 January 2010) has provided a clue to the mechanism by which a neuron precursor cell develops only one axon but many dendrites. Working with isolated hippocampal neuron precursors (from rat embryos), these workers showed that the cyclic nucleotide cAMP accumulates at one spot on the developing neuron, and it is here that the axon sprouts. However, the rising level of cAMP at that spot suppresses cAMP elsewhere in the cell, allowing the related cyclic nucleotide, cGMP, to accumulate. cGMP drives the formation of dendrites.

Still to be discovered is the signal (or signals) that cause the localized accumulation of cAMP at one spot in the cell. Perhaps it is a cell-intrinsic signal (see other examples), or a cell-extrinsic signal (see other examples), or perhaps both.


Axon – Structure and Functions

  • Electrochemical events in the cell body summate in the axon hillock, and the effect are directly passed to the attached axon.
  • If an action potential is generated, the axon conducts it away from the cell body.
  • The axon attaches directly to the dendrites in some neurons. In this arrangement, the axon conducts action potential toward the cell body.
  • Several types of proteins in the plasma membrane of an axon (axolemma) allow it to produce and conduct action potentials.

  • The initial segment of an axon has the highest concentration of these reactive proteins. this region is an axon’s trigger zone, where action potentials are first produced if electrochemical events in the dendrites and/or cell body reach a threshold level.
  • Axons vary in length from a few millimeters to just over a meter. They are less branched than most dendrites, but often give off one or more large collateral branches that lead to other cells.

  • Before ending, an axon and its collaterals split into clusters of small, thin terminate branches (telodendria).

  • The rounded tips of the terminal branches are called synaptic knobs, (end bulbs, axon terminals, synaptic terminals, terminal boutons or buttons).

Continue expanding your knowledge on nervous system anatomy with these practice quizzes and labelling exercises.

  • Inside the synaptic knobs are vesicles (synaptic vesicles) that contain neurotransmitter molecules.

  • A synaptic knob usually lies adjacent to another cell. the neighboring cells and the gab that separates them is called a synapse.
  • The plasma membrane of postsynaptic cell has receptors for the neurotransmitters in the vesicles of the synaptic knob.
  • When an action potential is generated, it travels (propagates) the length an axon to synaptic knobs, where it signals the synaptic vesicles to release some of the stored neurotransmitters.

Acknowledgements

We thank members of our groups for helpful discussions, Henry Roehl for a lyn-Cherry construct, Myriam Roussigne and Patrick Blader for sharing data prior to publication, Vladimir Korzh and other colleagues for transgenic lines, Tiago Branco for MATLAB programming, colleagues in the field for reagents and Fish Facility personnel for care of the fish. This study was supported by programme grant support from the Wellcome Trust, project grant support from MRC and BBSRC and a European Communities grant entitled 'Evolution and Development of Cognitive, Behavioural and Neural Lateralisation' to SW and a Wellcome Trust studentship to IB.