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Neuroscience revision notes: Chapter 1: Studying the nervous system Neurons: A neuron (also known as nerve cell) is an electrically excitable cell that takes up, processes and transmits information through electrical and chemical signals. It is one of the basic elements of the nervous system Neurons are the cells that form a framework for communication throughout the nervous system. Neurons are the cells that form a framework for communication throughout the nervous system. They can come in several shapes and sizes depending on their specialized functions but all neurons will have axons and dendrites that produce from the cell body. Based on their roles, the neurons found in the human nervous system can be divided into three classes: sensory neurons, motor neurons and interneurons.  Sensory neurons get information about what's going on inside and outside of the body and bring that information into the CNS so it can be processed.  Motor neurons get information from other neurons and convey commands to the muscles, organs and glands.  Interneurons which are found only in the CNS, connect one neuron to another. They receive information from other neurons (either sensory neurons or interneurons) and transmit information to other neurons (either motor neurons or interneurons). The basic functions of a neuron: 1. Receive signals (or information) 2. Integrate incoming signals (to determine whether or not the information should be passed along) 3. Communicate signals to target cells (other neurons or muscles or glands) Neurons, like other cells, have a cell body called the soma. The nucleus of the neuron is found in the soma. Neurons need to produce a lot of proteins, and most neuronal proteins are synthesized in the soma as well. Various processes extend from the cell body. These include many short, branching processes, known as dendrites, and a separate process that is typically longer than dendrites, known as the axon. Dendrites: Dendrites are the primary targets for synaptic input from the axon terminals of other neurons and are distinguished by their high content of ribosomes, as well as by specific cytoskeletal proteins. Dendrites are the structures on the neuron that receive electrical messages. Incoming signals/ messages can either be excitatory- which means they tend to make the neuron fire (generate an electrical impulse), they increase the stimulation of a neuron. Or inhibitory- which means that they tend to keep the neuron from firing, they decrease the activity of the neuron. These signals will accumulate in the cell body, or soma, of the neuron after being received by the dendrites. Once action potentials are received by the dendrites, they will be sent to a portion of the soma known as the axon hillock. Most neurons receive many input signals throughout their dendritic trees. A single neuron may have more than one set of dendrites, and may receive many thousands of input signals. Whether or not a neuron is excited into firing an impulse depends on the sum of all the excitatory and inhibitory signals it receives. If the neuron does end up firing, the nerve impulse, or action potential, is conducted down the axon. Also, the dendrites tend to taper and are often covered with little bumps called spines. Axons: Most neurons have a single axon that typically sends electrical impulses outwards away from the cell body. Axons can vary in length from extremely short to over 1m to reach from the base of your spine to your ankle. The axon arises from the cell body at a specialized area called the axon hillock. Many axons are covered with a special insulating substance called myelin, which helps them convey the nerve impulse rapidly. Myelin is never found on dendrites. Towards its end, the axon splits up into many branches and develops bulbous swellings known as axon terminals or nerve terminals. These axon terminals make connections on target cells. Synapses: Neuron to neuron connections are made onto the dendrites and cell bodies of other neurons. These connections, known as synapses, are the sites at which information is carried from the first neuron, the presynaptic neuron, to the target neuron the postsynaptic neuron. A small gap exists at this membrane-to membrane junction point called a synapse. It contains molecular structures, or machines, that control energy by allowing electrical or chemical signals to be rapidly transmitted. The synaptic connections between neurons and skeletal muscle cells are generally called neuromuscular junctions. At most synapses and junctions, information is transmitted in the form of chemical messengers called neurotransmitters. When an action potential travels down an axon and reaches the axon terminal, it triggers the

release of neurotransmitter from the presynaptic cell. Neurotransmitter molecules cross the synapse and bind to membrane receptors on the postsynaptic cell, conveying an excitatory or inhibitory signal. Thus, the third basic neuronal function- communicating information to target cells- is carried out by the axon and the axon terminals. Most synapses are chemical; these synapses communicate using chemical messengers. Other synapses are electrical; in these synapses, ions flow directly between cells. At a chemical synapse, an action potential triggers the presynaptic neuron to release neurotransmitters. These molecules bind to receptors on the postsynaptic cell and make it more or less likely to fire an action potential. Synaptic transmission:

Chemical and electrical process by which the information encoded by action potentials is passed on at synaptic contacts to the next cell in a pathway • Presynaptic terminals (axon terminals) and their postsynaptic specializations are typically chemical synapses – the most abundant type of synapse in the NS

• Electrical synapses (facilitated by the gap junctions) are more rare Chemical and electrical process by which the information encoded by action potentials is passed on at synaptic contacts to the next cell in a pathway • Presynaptic terminals (axon terminals) and their postsynaptic specializations are typically chemical synapses – the most abundant type of synapse in the NS

• Electrical synapses (facilitated by the gap junctions) are more rare Chemical and electrical process by which the information encoded by action potentials is passed on at synaptic contacts to the next cell in a pathway Chemical and electrical process by which the information encoded by action potentials is passed on at synaptic contacts to the next cell in a pathway. Presynaptic terminals (axon terminals) and their postsynaptic specializations are typically chemical synapses- the most abundant type of synapse in the nervous system. Electrical synapses (facilitated by the gap junctions) are more rare Synaptic vesicles: spherical structures filled with neurotransmitter molecules  the positioning of synaptic vesicles at the presynaptic membrane and their fusion to initiate neurotransmitter release is regulated by a number of proteins within or associated with the vesicle.  The neurotransmitter released from synaptic vesicles modify the electrical properties of the target cell by binding to neurotransmitter receptors localized primarily at the postsynaptic specialization  The intricate and concerted activity of neurotransmitters, receptors, related cytoskeletal elements, and signal transduction molecules are the basis for nerve cells to communicate with one another, and with effector cells in muscles and glands. Glial cells: Neuroglial cells are more numerous than neurons in the brain, but don’t participate directly in synaptic interactions and electrical signalling The nervous system was made up of two types of cells, neurons and glia, with the neurons acting as the basic functional unit of the nervous system and the glia playing a supporting role. The glia are essential to nervous system function and there are many more glial cells in the brain than neurons. But, their supportive functions help define synaptic contacts and maintain the signalling abilities of neurons. Roles:  Maintaining the ionic milieu of nerve cells  Modulating the rate of nerve signal propagation  Modulating synaptic action by controlling the uptake of neurotransmitter at or near the synaptic cleft  Providing a scaffold for some aspects of neural development  Aiding recovery from neural injury Types of glial cells:  Astrocytes: -

the most numerous type of glial cell and are found only in the central nervous system. They come in different types and have a variety of functions. o They help to regulate blood flow in the brain, maintain the composition of the fluid that surrounds neurons, and regulate communication between neurons at the synapse. o During development, astrocytes help neurons find their way to their destinations and contribute to the formation of the blood-brain barrier, which helps isolate the brain from potentially toxic substances in the blood Oligodendrocytes: o The Oligodendrocytes of the CNS and the Schwann cells of the PNS share a similar function. o Both of these types of glial cells produce myelin, the insulating substance that forms a sheath around the axons of many neurons. o Myelin dramatically increases the speed with which an action potential travels down the axon, and it plays a crucial role in nervous system function. Ependymal cells: o Ependymal cells which line the ventricles of the brain and the central canal of the spinal cord, have hairlike cilia that beat to promote circulation of the cerebrospinal fluid found inside the ventricles and spinal canal. Schwann cells: o The Oligodendrocytes of the CNS and the Schwann cells of the PNS share a similar function. o Both of these types of glial cells produce myelin, the insulating substance that forms a sheath around the axons of many neurons. o Myelin dramatically increases the speed with which an action potential travels down the axon, and it plays a crucial role in nervous system function. o

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Satellite cells: satellite glial cells cover the cell bodies of neurons in PNS ganglia. Satellite glial cells are thought to support the function of the neurons and might act as a protective barrier, but their role is still not well understood. Microglial cells: o microglia are related to the macrophages of the immune system and act as scavengers to remove dead cells and other debris. o o

o hare many properties with macrophages and are primarily scavenger cells that remove cellular o debris from sites of injury or normal cell turnover Cellular diversity in the nervous system Although the cellular constitutes of the human nervous system and are in many ways similar to those of other organs, they are unusual in their extraordinary diversity. The human brain is estimated to contain about 86 billion neurons and at least that many glia. Among these two overall groups, the nervous system has a greater range of distinct cell types- whether categorized by morphology, molecular identity, or physiological role- other than any other organ system.

Neural circuits: Neural circuits - process specific kinds of information and provide the foundation of sensation, perception and behaviour. Neurons never function in isolation; they are organized into ensembles called neural circuits that process specific kinds of information. The synaptic connections that underlie neural circuits are typically made in a dense range of dendrites, axon terminals, and glial cell processes that together constitute what is called neuropil. The neuropil constitutes the regions between nerve cell bodies where most synaptic connectivity occurs. Basic constituents of neural circuits:  Afferent neurons- carry information from periphery toward the brain or spinal cord  Efferent neurons- carry information away from the brain or spinal cord away from the circuit  Interneuron- local circuit neurons, only participate in the local aspects of a circuit, based on the short distances over which their axons extend A simple example of a neural circuit is one that mediates the Myotatic reflex, commonly known as the knee-jerk reflex.

Chapter 2: Electrical signals of Nerve cells Electrical potentials across nerve cell membranes: Resting membrane potential: The resting membrane potential is when a neuron establishes and maintains a stable voltage across its membrane. The resting membrane potential Is determined by the uneven distribution of ions (charged particles) between the inside and the outside of the cell, and by the different permeability of the membrane to different types of ions. Neurons have a resting membrane potential of about -30mV to -90mV. If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized. If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized. All of the electrical signals that neurons use to communicate are either depolarizations or hyperpolarization's from the resting membrane potential. Receptor potentials: From the activation of sensory neurons by external stimuli such as light, sound, heat. E.g. touching skin activates Pacinian corpuscles, receptor neurons that sense mechanical disturbances of the skin Amplitudes are graded in proportion to the magnitude of the sensory stimulus Synaptic potentials: Synaptic potential refers to the potential difference across the postsynaptic membrane that results from the action of neurotransmitters at a neuronal synapse. Allow transmission of information from one neuron to another Synaptic potentials serve as the means of exchanging information in the complex neural circuits found in both the central and peripheral nervous systems. Action potentials: Action potentials are responsible for long range transmission of Information within the nervous system and allow the nervous system to transmit information to its target organs, such as muscle. One way to elicit an action potential is to pass an electrical current across the neuron membrane.  In normal circumstances, this current would be generated by receptor potentials or by synaptic potentials. In the laboratory, however, electrical current suitable for initiating an action potential can be readily produced by inserting a micro- electrode into a neuron and then connecting the electrode to a battery. o

Amplitude of the action potential is independent of the magnitude of the current

Hyperpolarization: If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized. If current delivered in this way makes the membrane potential more negative (hyperpolarization), nothing very dramatic happens. The membrane potential simply changes in proportion to the magnitude of the injected current. Depolarization: If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized. If current of the opposite polarity is delivered, so that the membrane potential of the nerve cell becomes more positive than the resting potential. In this case, at a certain level of membrane potential, called the threshold potential, action potentials occur.

How ions movements produce electrical signals: Electrical potentials are generated across the membranes of neurons- and indeed, across the membranes of all cells- because:  There are differences in the concentrations of specific ions across nerve cell membranes  These membranes are selectively permeable to some of these ions Active transporters: A protein in the plasma membrane of the neuron that actively moves ions in and out of the cell against their concentration gradient Primary active transport: the ion concentration gradients established by proteins which actively move ions into or out of cells against their concentration gradients. Secondary active transport: use an electrochemical gradient-generated by active transport as an energy source to move molecules against their gradient, and thus does not directly require a chemical source of energy such as ATP. Ion channels: proteins that allow only certain kinds of ions to cross the membrane in the direction of their concentration gradients. Because they are charged, ions can't pass directly through the hydrophobic lipid regions of the membrane. Instead, they gave to use specialized channel proteins that provide a hydrophilic tunnel across the membrane. Some ion channels are highly selective for one type of ion, but others let various kinds of ions pass through. Ion channels that mainly K+ to pass are called potassium channels, and ion channels that mainly allow Na+ to pass are called sodium channels. Electrochemical equilibrium: -

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+ there is an exact balance between two opposing forces (1) the concentration gradient that causes K to move from inside to outside, taking along positive charge, and (2) an opposing electrical gradient that increasingly tends + to stop K from moving across the mem- brane.  The number of ions that needs to flow to generate this electrical potential is very small (approximately –12 + 2 + 10 moles of K per cm of membrane, or less than one millionth of the K ions present on each side). This means that the concentrations of permeant ions on each side of the membrane remain essentially constant, even after the flow of ions has generated the potential. The tiny fluxes of ions required to establish the membrane potential do not disrupt chemical electro neutrality because each ion has an oppositely charged counter ion to maintain the neutrality of the solutions on each side of the membrane.

Forces that create membrane potentials: Equilibrium potential:  The equilibrium potential is conventionally defined in terms of the potential difference between the outside and inside compartments.  Electrical potential generated across the membrane at electrochemical equilibrium  Can be predicted by a simple formula called the Nernst equation where: o Where Ex is the equilibrium potential for any ion X, R is the gas content, T is the absolute temperature, z is the valence (electrical charge) of the permeant ion, and F is the Faraday constant (the amount of electrical charge contained in one mole of a univalent ion) o When the concentration of K+ is higher inside than outside, an inside negative potential is measured across the K+ permeable neuronal membrane o The balance of chemical and electrical forces at equilibrium means that the electrical potential can determine ion fluxes across the membrane, just as the ionic gradient can determine the membrane potential. Electrochemical Equilibrium in an Environment with More Than One Permeant Ion: Goldman’s Equation:  V= the voltage across the membrane  P= permeability of the membrane to each ion of interest  Extended version of the Nernst equation that takes into account the relative permeability of each of the ions involved. The Ionic basis of the resting membrane potential: Action of ion transporters creates substantial transmembrane gradients for most ions

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There’s much more K+ inside the neuron than outside, and much more Na+ outside than inside These transporter-dependant concentration gradients are, indirectly, the source of the resting neuronal membrane potential and the action potential. Hodgkin and Katz experiment: Assuming that the internal K+ concentration is unchanged during the experiment, a plot of membrane potential vs log (external K+ concentration) = straight line with slope of 58mV per tenfold change in external K+ concentration at room temperature.  Value not exactly 58mV because other ions are also slightly permeable (Cl-, Na+) Showed that the inside negative resting potential arises because:  The membrane of the resting neuron is more permeable to K+ than other ions  There is more K+ inside the neuron than outside The selective permeability to K+ is caused by K+ permeable membrane channel that are open in resting neurons and the large K+ concentration gradients is produced by membrane transporters that selectively accumulate K+ within neurons. The Ionic basis of action potentials: If the membrane were to become highly permeable to ...