Tag Archives: neurons

Neuroscience: The Nervous System

he nervous system is the body’s speedy, electrochemical communication system. It consists of all the nerve cells of the peripheral and central nervous systems.

The central nervous system consists of the brain and spinal cord. The peripheral nervous system is the sensory and motor neurons that connect the central nervous system to the rest of the body. Nerves are the neural cables of the nervous system containing many axons. They are part of the peripheral nervous system. They are connected to the central nervous system by muscles, glands, and sense organs.

Sensory neurons carry incoming information from the sense receptors to the central nervous system. Interneurons are part of the central nervous system. They internally communicate and intervene between the sensory inputs and motor inputs. They are the most common type of neuron. Motor neurons carry outgoing information from the central nervous system to the muscles and glands.

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– The Peripheral Nervous System –

The peripheral nervous system is made up of the somatic nervous system and the autonomic nervous system. The somatic nervous system controls the body’s skeletal muscles. The autonomic nervous controls the glands and muscles of the internal organs. It is broken down into the sympathetic and the parasympathetic nervous system. The sympathetic nervous system arouse the body, mobilising its energy in stressful situations (fight or flight response). The parasympathetic nervous system calms the body, conserving its energy.

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– The Central Nervous System –

The spinal cord is the information highway connecting the peripheral nervous system to the brain. Ascending neural tracts send up sensory information. Descending neural tracts send down motor control information. Reflexes are the body’s autonomic response to stimuli controlled by the spinal cord. They are composed of 1 sensory neuron and 1 motor neuron that communicate through one interneuron. Because they only run through your spinal cord, they react automatically without your brain being involved in the process. The spine sends information back to the brain. Bodily pain or pleasure is controlled by the brain.

The brain receives information, interprets it and then decides on a response. It functions like a computer, receiving slightly differing images of an object from the eyes, it computes the differences and infers how far the object must be to project such a difference.

Neural networks are interconnected neural cells which, with experience, can learn, as feedback strengthens or inhibits connections that produce certain results. Stephen Kosslyn and Oliver Koening proposed to think of neural networks as networks of people. Neuron network with nearby neurons with which the can have short, fast connections. Each layer of a neural connects with various cells in the next layer. Learning occurs as feedback strengthens connections that produce certain results. New computer models simulate this process plus the excitatory and inhibitory conniptions to mimic the brain’s capacity for learning.

 

Neuroscience: The Action Potential

Action potential is something I find a lot of people who study psychology or neuroscience struggle with. For me, I just could never remember where it was sodium or potassium on the inside or outside of the cell membrane. Luckily, action potential is not as difficult to understand when you break it down into its major subsections.

Introduction

Action potentials are generated by neurons and are always consistent in size and duration; their strength does not diminish as it propagates down the axon. The strength of the action potential is determined by the frequency it propagates down the axon; the frequency and pattern of the action potentials generate a code used by neurons to transfer information appropriately from one location to another. Neuroscientists use an oscilloscope to study action potential as it sensitive to changes in voltage over time. The action potential works its way down the axon until it reaches the axon terminal. At the axon terminal, a synaptic transmission is initiated.

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Generating the Action Potential and All-or-None 

The perception of any sensation such as warmth or pain as well as any movement is the direct result of action potential. Depolarization of the neuron is what is necessary for an action potential to be generated, and it can arise in a variety of ways depending on the kind of neuron. The most common way to depolarize a neuron is by entry of sodium ions. The important thing is that the stimulus is strong enough to depolarize the neuronal membrane. Action potential works on what is known as all-or-none response, meaning it works on a threshold basis. As it is consistent in size, it relies on the stimulus exceeding a threshold called generator potential in order to fire, rather than the strength of the stimulus itself. This may seem confusing because yes, the strength of the stimulus must exceed the threshold , so in a way strength of the stimulus matters. However, what is important to remember is that once the threshold has been exceeded, the strength of the stimulus has no importance or effect on the strength of the action potential whatsoever.

Properties of Action Potential 

The membrane of a neuron at rest is around -65mV or -65 milli-volts with the regard to the outside. Volts is a good indicator that we are dealing with electrical current. During action potential, however, the membrane potential becomes briefly positive. This change from a negative to positive membrane potential is depolarization. Depolarization, a very rapid process, will continue until the membrane potential reaches 40 mV. To give you an idea how rapid the entire process of action potential including reaching back to -65mV it takes 2 milliseconds.

An action potential will only ever travel in one direction because of the refractory period. When the action potential travels away from the soma or cell body towards the axon terminals it is called orthodromic conduction. When the action potential travels towards the soma, the action potential is called antidromic conduction. Regardless of the direction it is traveling, action potential conductance rate varies, but the typical rate is 10m/s. The average length of an axon is about 2cm.

The Process 

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1. Resting Potential: As stated before, the resting membrane potential is around -65 mV but it ranges depending on the neuron. Different textbooks will tell you different resting membrane potentials, but it’s not something you need to worry too much about. It is also important to note than Vm or the membrane action potential is equal to 0. Vm=0.

Threshold: The membrane potential at which there stimulus is strong enough to open the sodium channels. The opening of the voltage-gated sodium channels changes the ionic permeability of the membrane in favour of sodium.

2. Depolarisation

3. Rising Phase: As the membrane potential is negative at the resting phase, the opening of the sodium channels creates a great pull. This is because the sodium ions (Na+) are positive. The result is sodium ions rushing into the cell causing rapid depolarisation. This changes the membrane action potential towards the equilibrium value of sodium or ENa.

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4. Falling Phase: The voltage-gated sodium channels become inactivated, and in turn the voltage-gated potassium channels open. This time around, there is a great pull on the potassium ions (K+, also positive) outside the cell as the sodium ions have made the membrane potential very positive. Potassium ions rush outside the cell, resulting in a negative membrane potential once more, close to  Eor the equilibrium value of potassium.

Undershoot: The membrane potential is more negative than -65 mV.

Absolute Refractory Period: The cell cannot fire during this period as there is a gradual restoration of the membrane potential back to -65mV and the Vm to 0. During this time, even if the stimulus is strong enough to exceed the threshold, the membrane will not fire. It lasts about 1ms.

Relative Refractory Period: The membrane potential stays hyper-polarised or at the undershoot until the potassium channels close. It can be difficult to initiate another action potential even after the absolute refractory period, longer than 1 ms.

Factors Influencing the Conduction Velocity 

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The rate of the action potential depends on the strength of the depolarising current; specifically, it is reflected in firing frequency of the action potential. The maximum frequency, however, is 1000 Hz. Generally, axon potential conduction velocity increases diameter of the axon as well as the size of the axon itself and the number of ion channels. Smaller axons tend to require a much greater stimulus to reach the threshold necessary for depolarisation, as such as they are more sensitive to local anaesthetics.

Methods in Systems Neuroscience: Histological Stains and Tract Tracing

Histological Stains

Cells and major fibre tracts make up the basic structure of the brain and are observed using histological procedures. The most common histological procedures involve stains such as the Nissl and Weil stains. This particular cell stain stains the Nissl substance (granular bodies) found in neuronal cytoplasms. The Nissl bodies are composed of rough endoplasmic reticulum and free polyribosomes and as such are the site of protein synthesis. For a Nissl stain, neurones are held in a parofromaldehyde or formalin fixed tissue. The selective stain uses aniline dye, which colours the somas and dendrites of neurones blue, or more specifically their ribosomal RNA. The Weil fibre stains is regressive, requiring differentiation. Weil fibre stains use a hematoxylin based stain, which dyes myelin and red blood cells dark red. Weil is unique in that is can be used on a frozen section of tissue.

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Tract Tracing

Anterograde tract tracing is used to identify projections than run from cell bodies to axon terminals. Various tracer molecules are used including but not limited to the green fluorescent protein, lipophilic dyes and radioactively tagged amino acids. Genetic tracers are also used including viruses and proteins. The most common genetic tracers are the Herpes complex virus type 1 (HSV) and the Rhabdoviruses. Lipophilic dyes are commonly used in electrophysiology as anterograde tracer; however, they are not selectively unidirectional nor are they actively transported across the synaptic cleft. An example of an anterograde projection is the transmission of visual information from the superior colliculus to the substantial nigra.

Retrograde tract tracers are used to identify projections from axonal terminals to somas. The most common retrograde tracers are viral stains such as the modified rabies virus or pseudorabies virus (PRV) or Batha stain. The PRV infection spreads upstream through a pathway of linked neurones. An example of retrograde projections is the transmission of nociceptive information from the parabrachial nucleus to dopaminergic neurones in the midbrain. 

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Also, bidirectional tracers, as the name suggests, can work both in an anterograde and retrograde fashion. Common bidirectional tracers include WGA-HRP, biotinylated dextran and cholera toxin subunit b. A major pitfall of bidirectional tracers or dyes is that they can move retrograde then anterograde along branches axon collateral falsely indicating an anterograde tracing. In other words, one might falsely observe that A projects to C (see diagramme above). It is important to correctly identify the direction of projections as it helps identify the morphology of a cell. Cells with different morphologies, unsurprisingly, have different processing capacities. An example of a branched projection is the tectonigral projection.

Finally, simultaneously anterograde and retrograde tracers do exist; however, not as common as one or the other. The use of tracers can be accompanied by TH immunochemistry to help identify the locations of certain neurones.

Bibliography:

Anterograde tracing. (n.d.). Retrieved January 13, 2015, from http://en.wikipedia.org/wiki/Anterograde_tracing

Coizet, V., et al. (2010). “The parabrachial nucleus is a critical link in the transmission of short-latency nociceptive information to midbrain dopaminergic neurons.” Neuroscience 168(1): 263-272.

Comoli, E., et al. (2003). “A direct projection from superior colliculus to substantia nigra for detecting salient visual events.” Nat Neurosci 6(9): 974-980.

Fung, K. (n.d.). Stains in Neuropathology. Retrieved January 15, 2015, from http://moon.ouhsc.edu/kfung/JTY1/NeuroHelp/ZNEWBS12.htm

QBI Histology and Microscopy. (n.d.). Retrieved January 15, 2015, from http://web.qbi.uq.edu.au/microscopy/cresyl-violet-staining-nissl-staining/

Redgrave, P. (Director) (2014, November 11). Methods in Systems Neuroscience. BMS224 Brain and Behaviour 1 . Lecture conducted from University of Sheffield , Sheffield.

Retrograde tracing. (n.d.). Retrieved January 13, 2015, from http://en.wikipedia.org/wiki/Retrograde_tracing