A&P Chapter 8

Postsynaptic cell

The cell that receives the signal

Why do graded potentials lose strength as they move through the cytoplasm?

1) Current leak: the membrane of the neuron cell body has open leak channels that allow positive charge to leak out into the extracellular fluid. Some positive ions leak out of the cell across the membrane as the depolarization wave moves through the cytoplasm, decreasing the strength of the signal down the cell. 2) Cytoplasmic resistance: The cytoplasm provides resistance to the flow of electricity, just as water creates resistance that diminishes the waves from the stone. The combination of current leak and cytoplasmic resistance means that the strength of the signal inside the cell decreases over distance.

3 main types of gated channels

1) Mechanically gated ion channels; found in sensory neurons and open in response to physical forces such as pressure or stretch. 2) Chemically gated ion channels; in most neurons, they respond to a variety of ligands, such as extracellular neurotransmitters and neuromodulators or intracellular signal molecules. (Open only in presence of specific chemicals at a binding site). 3) Voltage-gated ion channels; respond to changes in the cells membrane potential. Voltage-gated Na+ and K+ channels play an important role in the initiation and conduction of electrical signals along the axon.

Two main parameters that influence the velocity of action potential conduction

1) Myelination; the greater the degree of myelination, the faster the conduction 2) Diameter of the axon; The larger the diameter, the faster the conduction

Specificity of neural communication depends on:

1) the signal molecule secreted by neurons 2) the target cell receptors for these chemicals 3) the anatomical connections between neurons and their targets

3 phases of action potential

A change in membrane potential that occurs when voltage-gated ion channels in the membrane open, increasing the cells permeability first to Na+ (which enters) and then to K+ (which leaves). The influx (movement into the cell) of Na+ depolarizes the cell. This depolarization is followed by K+ efflux (movement out), which restores the cell to the resting membrane potential. Rising phase: due to a sudden temporary increase in the cells permeability to Na+. An action potential begins when a graded potential reaching the trigger zone depolarizes the membrane to threshold (-55mV). As the cell depolarizes, voltage-gated Na+ channels open, making the membrane more permeable to Na+. Na+ then flows into the cell, down its concentration gradient and attracted by the negative membrane potential inside the cell. The addition of positive charge to the intracellular fluid further depolarizes the cell. In the top third of of the rising phase, the inside of the cell has become more positive than the outside, and the membrane potential has reversed polarity. As soon as the membrane potential becomes positive, the electrical driving force moving Na+ into the cell disappears, but the Na+ concentration gradient remains, so Na+ continues to move into the cell. As long as Na+ permeability remains high, the membrane potential moves toward the Na+ equilibrium potential (Ena) of +60 mV (Ena is the membrane potential at which the movement of Na+ into the cell down its concentration gradient is exactly opposed by the positive membrane potential). The action potential peaks at +30mV when Na+ channels in the axon close and potassium channels open. Falling phase: This phase corresponds to to an increase in K+ permeability. Voltage-gated K+ channels, like Na+ channels, open in response to depolarization. The K+ channel gates are much slower to open, however, and peak K+ permeability occurs later than peak Na+ permeability. By the time the K+ channels finally open, the membrane potential for the cell has reached +30mV because of Na+ influx through faster-opening Na+ channels. When the Na+ channels close at the peak of the action potential, the K+ channels have just finished opening, making the membrane very permeable to K+. At a positive membrane potential, the concentration and electrical gradients for K+ favor movement of K+ out of the cell. As K+ moves out of the cell, the membrane potential rapidly becomes more negative, creating the falling phase of the action potential. When the falling membrane potential reaches -70mV, the K+ permeability has not returned to its resting state. After-hyperpolarizing: K+ continues to leave the cell through both voltage-gated and K+ leak channels, and the membrane hyperpolarizes, approaching the Ek of -90mV. This after-hyperpolarization is also called undershoot. Finally, the slow voltage-gated K+ channels close, and some of the outward K+ leak stops. Retention of K+ and leak of Na+ into the axon bring the membrane potential back to -70mV.

Conduction of action potentials

A distinguishing characteristic of action potentials is that they can travel over long distances of a meter or more without losing energy, this process is called CONDUCTION. The action potential that reaches the end of an axon is identical to the action potential that started at the trigger zone. The depolarization of a section of axon causes positive current to spread through the cytoplasm in all directions by local current flow. Simultaneously, on the outside of the axon membrane, current flows back toward the depolarized section. The local current flow in the cytoplasm diminishes over distances as energy dissipates. Forward current flow would eventually die out if it weren't for voltage-gated channels. The axon has several voltage-gated Na+ channels. Whenever a depolarization reaches those channels, they open, allowing more Na+ to enter the cell and reinforcing the depolarization. As each segment of axon reaches the peak of the action potential, its Na+ channels inactivate. The section of axon that has just completed an action potential is in its absolute refractory period, with its Na+ channels inactivated. - so action potential cant be moved backward. Depolarization flowing backward from the axon could open voltage-gated channels in the dendrites, making the neuron more excitable Axon isn't directional, so we initiate at axon hillick where it can't move backward (cause there are no voltage dependent gates for it to activate). The refractory period also stops it from going backward and forces it to continue down the axon. Two ket physical parameter influence the speed of action potential conduction in the neuron: 1) The diameter of the axon 2) The resistance of the axon membrane to ion leakage out of the cell (the length constant) The larger the diameter or the more leak-resistant the membrane, the faster an action potential will move. (and lower resistance to ion flow)

Chemical factors that alter electrical activity

A variety of chemicals alter the conduction of action potential by binding Na+, K+ or Ca2+ channels in the neuron membrane. If Na+ channels are not functional (because of NEUROTOXINS), Na+ can't enter the axon, losing strength of signal and then too weal to release a neurotransmitter and the message of the presynaptic neuron is not passed on to the postsynaptic cell and communication fails. Alterations in the ECF concentrations of K+ and Ca2+ are also associated with abnormal electrical activity in the nervous system. The relationship between ECF and K+ levels and the conduction of action potentials is the most straightforward and easiest to understand. The concentration of the K+ in the blood and interstitial fluid is the major determinant of the resting potential of all cells. If K+ concentration in the blood moves out of the normal range, the result is a change in the resting membrane potential of cells. This change can have consequences to the body. At normal K+ levels, subthreshold graded potentials do not trigger action potentials, and suprathreshold ones do. An increase in blood K+ concentration- HYPERKALEMIA- shifts the resting membrane potential of a neuron closer to threshold and causes the cell to fire action potentials in response to smaller graded potentials. If blood concentrations fall too low (HYPOKALEMIA), the resting membrane potential of the cells hyperpolarize, moving farther from the threshold. In this case, a stimulus strong enough to trigger an action potential when the resting potential is a normal -70 does not reach the threshold value. This shows up in muscle weakness because the neurons that control skeletal muscle are not firing normally.

Membrane potential

All living cells have a resting membrane potential difference that represents the separation of electrical charge across the cell membrane. Two factors influence membrane potential: 1) The uneven distribution of ions across the cell membrane. (normally sodium, chloride and calcium are more concenTrated in the ECF than in the cytosol. Potassium, is more concentrated in the cytosol than in the ECF. 2) Differing membrane permeability to those ions. The resting cell membrane is much more permeable to potassium than than to sodium or calcium , making K+ the major ion contributing to the resting membrane potential.

Axon terminal

At this part of the neuron, the electrical signal is translated into a chemical signal by secretion of a neurocrine (neurotransmitter, neurohormone). Store and release neurostransmitters

Efferent neurons (motor)

Axons may divide several times into branches called collaterals. Have enlarged endings called axon terminals. Subdivided into the: 1) somatic motor division, which controls skeletal muscles- voluntary 2) autonomic division, which controls smooth and cardiac muscles, exocrine glands, some endocrine glands, and some types of adipose tissue.

Central nervous system

CNS neurons integrate information that arrives from the sensory division of the PNS and determine whether a response is needed. If a response is needed, the CNS sends output signals that travel through efferent neurons to their targets, which are mostly muscles and glands. -Can initiate activity w out sensory input -CNS need not create any measurable output to the efferent divisions (thinking and dreaming are complex higher brain functions that can take place totally within the CNS)

Afferent neurons (sensory)

Carry information about temperature, pressure, light, and other stimuli from sensory receptors to the CNS. Peripheral sensory neurons are pseudounipolar, with cell bodies located close to the CNS and very long processes that extend out to receptors in the limbs and internal organs. Sensory neurons in nose and eye are much smaller bipolar neurons

Axons

Carry/transmit outgoing information and electrical signals from the integrating center of the neuron to the target cells at the end of the axon. Most peripheral neurons have a single axon and can branch into collaterals, each ending in an axon terminal. At the distal end of the axon the electrical signal usually causes secretion of a chemical messenger molecule.

Integration of neural information transfer

Communication between neurons is not always one to one. The axon of a presynaptic neuron branches, and its collaterals (branches) synapse on multiple target neurons. This pattern is known as DIVERGENCE (spreads information) When a group of presynaptic neurons provide input to a smaller number of postsynaptic neurons, the pattern is CONVERGENCE (allows for multiple sources to influence the output of the postsynaptic cell, increase level of control getting more info before you make a decision). In the brain, there are some synapses where cells on both sides of the synaptic cleft release neurotransmitters that act on the opposite cell.

Ions in action potential

Conduction of the action potential along the axon requires only a few types of ion channels: voltage gated Na+ channels and voltage gated K+ channels, plus some leak channels that help set the resting membrane potential. Action potentials begin when voltage gated ion channels open, altering membrane permeability (P) to Na+ (Pna) and K+ (Pk).

autonomic neurons

Controls contraction and secretion in the various internal organs. - involuntary further divided into: sympathetic and parasympathetic branches, which can be distinguished by their anatomical organization and the chemicals they use to communicate with their target cells.

Explain the relationship between current flow, conductance, resistance, Ohm's law

Current flow: When ion channels open, ions may move onto or out of the cell. The flow of electrical charge carried by an ion is called the ions current (Iion). The direction of ion movement depends on the elctrochemical gradient of the ion. Potassium ions usually move out of the cell. Na+, Cl-, and Ca2+ usually flow into the cell. The net flow of ions across the membrane depolarizes or hyperpolarizes the cell, creating an electrical signal. Current flow, whether across a membrane or inside a cell, obeys Ohm's Law: -Says that current flow (I) is directly proportional to the electrical potential difference (in volts, V) between two points and inversely proportional to the resistance (R) of the system to current flow: I=Vx1/R or I=V/R -As resistance R increases, current flow (I) decreases Resistance in biological flow is the same as resistance in everyday life; It is a force that opposes flow. Electricity is a form of energy and, and like other forms of energy it dissipates as it encounters resistance In biological electricity, resistance to current flow comes from two sources; the resistance of the cell membrane (Rm) and the internal resistance of the cytoplasm (Ri). The phospholipid bilayer of the cell membrane is normally an excellent insulator, and a membrane with no open ion channels has very high resistance and low conductance. If ion channels open, ions (current) flow across the membrane if there is an electrochemical gradient for them. Opening ion channels therefor decreases the membrane resistance. The internal resistance of most neurons is determined by the composition of the cytoplasm and the diameter of the cell. Cy

The Nernst Equation

Describes the membrane potential that would result if the membrane were permeable to only one ion. For any given ion concentration gradient, this membrane potential is called the equilibrium potential of the ion (Eion) When using the estimated intracellular and extracellular concentrations for K+ in this equation, it predicts a potassium equilibrium potential of -90mV. However an average value for the resting membrane potential of neurons is -70mV (inside the cell relative to outside), more positive than predicted by the potassium equilibrium potential. This means that other ions must be contributing to the membrane potential. Neurons at rest are slightly permeable to Na+, and the leak of positive Na+ into the cell makes the resting membrane potential slightly more positive than it would be if the cell were permeable to only K+.

Synapses

Each synapse has two parts: 1) the axon terminal of the presynaptic cell 2) the membrane of the post synaptic cell In a neural reflex, information moves from presynaptic cell to postsynaptic cell. The postsynaptic cells may be neurons or noneuronal cells. In most neuron-to-neuron synapses, the presynaptic axon terminals are next to either the dendrites or the cell body of the postsynaptic neuron. Postsynaptic neurons with many dendrites also have many synapses. They can also occur on the axon and even at the axon terminal of the postsynaptic cell. Synapses are classified as electrical or chemical depending on the type of signal that passes from the presynaptic cell to the postsynaptic

Action potential

Electrical signals of uniform strength that travel from a neurons trigger zone to the end of its axon. Voltage-gated ion channels in the axon membrane open sequentially as electrical current passes down the axon. As a result, additional Na+ entering the cell reinforce the depolarization, which is why an action potential does not lose strength over distance the way a graded potential does. Instead, the action potential at the end of axon is identical to the action potential that started at the trigger zone: a depolarization of about 100 mV amplitude. The high speed movement of an action potential along the axon is called conduction of the action potential. Action potentials are sometimes called all or none phenomena because they either occur as a maximal depolarization (stimulus reaches threshold) or do not occur at all (stimulus below threshold). The strength of the graded potential that initiates an action potential has no influence on the amplitude of the action potential. There is no single action potential that moves through the cell. The action potential that occurs at the trigger zone is like the movement in domino series. (As first domino falls, it strikes the next, passing on its kinetic energy) In action potential, a wave of electrical energy moves down the axon. Instead of getting weaker over distance, action potentials are replenished along the way so that they maintain constant amplitude. As action potential passes from one part of the axon to the next, the membranes energy state is reflected in the membrane potential of each region. ???

Distinguish between chemical and electrical synapses

Electrical synapses: pass an electrical signal, or current, directly from the cytoplasm of one cell to another through the pores of gap junction proteins. Information can flow in both directions through most gap junctions, but in some current can flow in only one direction. Electrical synapses occurs mainly in neurons of the CNS. They are also found in glial cells, in cardiac and smooth muscle, and in nonexcitable cells that use electrical signals, like the pancreatic beta cell. The primary advantage of electrical synapses is rapid and bidirectional conduction of signals from cell to cell to synchronize activity within a network of cells. Gap junctions also allow chemical signal molecules to diffuse between adjacent cells Chemical synapses: The vast majority of synapses in the nervous system are chemical synapses, which use neurocrine molecules to carry information from one cell to the next. At chemical synapses, the electrical signal of the presynaptic cell is converted into a neurocrine signal that crossed the synaptic cleft and binds to a receptor on its target cell. A chemical synapse is a gap between two neurons where information passes chemically, in the form of neurotransmitter molecules. An electrical synapse is a gap which has channel proteins connecting the two neurons, so the electrical signal can travel straight over the synapse.

Sympathetic

Fight or flight

Synaptic vesicles

Filled with neurotransmitter that is released on demand. Some vesicles are docked at active zones along the membrane closest to the synaptic cleft, waiting for a signal to release their contents. Others act as a reserve pool, clustering close to the docking sites. Axon terminals also contain mitochondria to produce ATP for metabolism and transport.

Neurons

Function unit of nervous system carry/ conduct electrical signals rapidly and in some cases, over long distances. They are uniquely shaped cells, most having long, thin extensions, or processes from cell body, that can extend up to a meter in length.

Slow axonal transport

Moves material by axoplasmic or cytoplasmic flow from the cell body to the axon terminal. Can be used only for components that are not consumed rapidly by the cell, like enzymes and cytoskeleton proteins.

Explain how the GHK equation relates to the membrane potential of a cell

GHK equation predicts resting membrane potentials based on given ion concentrations and membrane permeabilities and can be used to predict what happens to membrane potential when ion concentrations or membrane permeabilities change. Several different ions contribute to the membrane potential of cells. The Goldman-Hodgkin-Katz (GHK) equation calculates the membrane potential that results from the contribution of all ions that can cross the membrane. This equation includes membrane permeability values because the permeability of an ion influences its contribution to the membrane potential. (If it isn't permeable, it doesnt affect membrane permeability.) Na+, K+, and Cl- are the three ions that influence membrane potential in resting cells. Each ions contribution to the membrane potential is proportional to its ability to cross the membrane. Resting membrane potential (Vm) is determine by the combined contributions of the (concentration gradient X membrane permeability) for each ion. Ca2+ isn't permeable and therefor not part of the equation.

Graded potential

Graded potentials in neurons are depolarizations or hyperpolarizations that occur in the dendrites and cell body or, less frequently, near the axon terminals. These changes in membrane potential are called "graded" because their size, or amplitude, is directly proportional to the strength of the triggering event. A large stimulus causes a strong graded potential, and a small stimulus results in a weak graded potential. In neurons in the CNS and the efferent division, graded potentials occur when chemical signals from other neurons open chemically gated ion channels, allowing ions to enter or leave the neuron. Mechanical stimuli (like stretch) or chemical stimuli open ion channels in some sensory neurons. Graded potentials may also occur when an open channel closes, decreasing the movement of ions through the cell membrane. (ex. if K+ leak channels close, fewer K+ leave the cell. The retentions ok K+ depolarizes the cell). The strength of the initial depolarization in a graded potential is determined by how much charge enters the cell. If more Na+ channels open, more Na+ enters, and the graded potential has a higher initial amplitude. The stronger the initial amplitude, the farther the graded potential can spread through the neuron before it dies out.

Trigger zone

Graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone. In efferent neurons and interneurons, the trigger zone is the axon hillock and they first part of the axon, a region known as the initial segment. In sensory neurons, the trigger zone is immediately adjacent to the receptor, where the dendrites join the axon. The trigger zone is the integrating center of the neuron and contains a high concentration of voltage-gated Na+ channels in its membrane. If graded potentials reaching the trigger zone depolarize the membrane to the threshold voltage, voltage-gated Na+ channels open, and an action potential begins. If depolarization does not reach threshold, the graded potential simply dies out as it moves into the axon. Because depolarization makes a neuron more likely to fire an action potential, depolarizing graded potentials are considered to be excitatory. A hyperpolarizing graded potential moves the membrane potential farther from the threshold value and makes the neuron less likely to fire an action potential, making then inhibitory. A stronger initial stimulus on the cell body initiates a stronger depolarization and current flow. The graded potential still diminishes, its higher initial strength ensures that it is above threshold at the trigger zone

Gated channels

How does a cell change its ion permeability? The simplest way is to open or close existing channels in the membrane. Neurons contain a variety of gated ion channels that alternate between open and closed states, depending on intracellular and extracellular conditions. A slower method for changing membrane permeability is for the cell to insert new channels into the membrane or remove some existing channels. Ion channels are usually names according to the primary ions they allow to pass through them. 4 major types: 1) Na+ channels 2) K+ channels 3) Ca2+ channels 4) Cl- channels Others are less selective that allow both Na+ and K+ to pass. Some channels, like the K+ leak channels that are the major determinant of resting membrane potential, spend most of their time in an open state. Other channels have gates that open or close in response to particular stimuli.

Neurotransmitters

In most pathways, neurons release chemical signals, called neurotransmitters, into the extracellular fluid to communicate with neighboring cells.

Explain the changes in ion permeability and ion flow that take place during an action potential

In order for an action potential to be initiated, permeability of Na+ must be increased. This increase through Na+ voltage-dependent gates is done by depolarization of the cell, which allows for these gates to open. ..... Very few ions actually move across the membrane in a single action potential, so that the relative Na+ and K+ concentrations inside and outside the cell remain essentially unchanged. (only 1 in every 100,000 K+ must leave the cell to shift the membrane potential from +30 to -70mV, equivalent to the falling phase of the action potential. The tiny number of ions that cross the membrane during an action potential does not disrupt the Na+ and K+ concentration gradients. Usually the ions that so move in/out are rapidly restored to their original compartments by Na+-K+-ATPase (Na+-K+ pump), which uses energy from ATP to exchange Na+ that enters the cell for K+ that leaked out of the cell. This exchange doesn't need to happen before the next action potential fires because the ion concentration gradient was not significantly altered by one action potential.

Describe the role of the following in synaptic communication:

Ionotropic and metabotropic receptors: neurotransmitters and neuromodulators: 3) Fast and slow synaptic potentials: Neurotransmitters that bind to G protein coupled receptors linked to second messenger systems initiate slow postsynaptic responses. Changes in membrane potential resulting from these alterations (open/closing channels) in ion flow are called SLOW SYNAPTIC POTENTIALS because the response of the second messenger pathway takes longer than the direct opening or closing of a channel. The response also lasts longer. Slow responses aren;t limited to altering the open state of ion channels, may also modify existing cell proteins or regulate the production of new cell proteins. Fast synaptic responses are always associated with the opening of ion channels. The neurotransmitter binds to and opens a receptor-channel on the postsynaptic cell and the ECF. The resulting change in membrane potential is a FAST SYNAPTIC POTENTIAL because it begins quickly and last not long. 4) excitatory and inhibitory postsynaptic potentials:

Fast axonal transport

Moves organelles at rates of up to 400mm per day. The neuron uses stationary microtubules as tracks along which transported vesicles and mitochondria "walk" with the aid of attached foot-like motor proteins. These motor proteins alternately bind and unbind to the microtubules with the help of ATP, stepping their organelles along the axon in a stop-and-go fashion. Can go in two directions, either forward transport (anterograde) that moves vesicles and mitochondria from from the cell body to the axon terminal, or backward transport (retrograde) returns old cellular components from the axon terminal to the cell body for recycling.

Mixed nerve

Nerves that carry signals in both direction

Neurocrines

Neurocrine chemical composition is varied, and these molecules may function as neurotransmitter, neuromodulators, or neurohormones. neurotransmitters and neuromodulators act as paracrine signals, with target cells located close to the neuron that secretes them. The difference between these two depends on the receptor to which the chemical is binding, as many neruocrine molecules can act in both roles. If a molecule primarily acts at a synapse and elicits a rapid response, it is a neurotransmitter. Neuromodulators act as both synaptic and nonsynaptic sites and are slower acting. Some neurotransmitters and modulators act on the cell that secretes them, making them autocrine signals as well. Neurohormones in contrast are secreted into the blood and distributed throughout the body.

List and give examples of the seven groups of neurocrine secretions

Neurocrines can be grouped according to structure: 1) Acetylcholine; synthesized from choline and acetyl coenzyme A. The synthesis from these two precursors is a simple enzymatic reaction that takes place in the axon terminal. Neurons that secrete ACh and receptors that bind ACh are CHOLINERGIC. Cholinergic receptors come in two main subtypes: - Nicotinic, an agonist, receptor-channels found on skeletal muscle in the autonomic division of the PNS and in the CNS. These receptors are monovalent cation channels through which both Na+ and K+ can pass. Na+ entry exceeds K+ entry and depolarizes the postsynaptic cell, making it more likely to fire an action potential - Muscarinic receptors come in five related subtypes. They are all G protein coupled receptors linked to second messenger systems. The tissue response to activation of a muscarinic receptor varies with the receptor subtype. These receptors occur in the CNS and on targets of the autonomic parasympathetic division of the PNS. (Receptors located - smooth and cardiac muscle, endocrine/exocrine glands,CNS) 2) Amines; the amine neurotransmitters are all active in the CNS. Like the amine hormones, these neurotransmitters are derived from single amino acids. (These are all released by adrenergic neurons that bind to adrenergic receptors) -Seratonin, made from amino acid tryptophan -Dopamine (CNS) derived from tyrosine -Norepinephrine (smooth/cardiac muscle, CNS), makor neurotransmitter of PNS autonomic sympathetic division. -Epinephrine (smooth/cardiac muscle, CNS) Adrenergic receptors are divided into two classes; -Alpha -Beta Linked to G protein coupled receptors Work through different second messenger pathways 3) Amino acids; excite or inhibit a postsynaptic cell -Glutamate is the primary excitatory neurotransmitter of the CNS, and -aspartate is an excitatory neurotransmitter in selected regions of the brain. Excitatory neurotransmitters depolarize their target cells, usually by opening ion channels that allow flow of + ions into the cell. -Gamma-aminobutyric acid is the main inhibitory neurotransmitter in the brain.Do this by hyperpolarizing their target cell by opening Cl- channels and allow it to enter the cell. 4) Peptides - Substance P, involved in pain pathways - Opioid peptides, mediate pain relief 5) Purines - AMP -ATP (G protein receptors) 6) Gases -Nitric oxide (NO), unstable gas -CO -H2S 7) Lipids - eicosanoids, endogenous ligands for cannabinoid receptors (G- protein)

Interneurons

Neurons that lie entirely within the CNS.

Describe different patterns for neurotransmitter synthesis, recycling, release, and termination of action

Neurotransmitter synthesis: Takes place both in the nerve cell body and in the axon terminal. Polypeptides must be made in the cell body because axon terminals do not have organelles needed for protein synthesis. The large propeptide that results from synthesis is packaged into vesicles along with the enzyme needed to modify it. The vesicles then move from the cell body to the axon terminal by fast axonal transport. Inside the vesicle, the propeptide is broken down into smaller active peptides. Smaller neurotransmitters like acetylcholine, amines, and purines, are synthesized and packaged into vesicles in the axon terminal. The enzymes needed for their synthesis are made in the cell body and released into the cytosol. The dissolved enzymes are then brought to axon terminals by slow axonal transport Neurotransmitter release: Neurotransmitters in the axon terminal are stored in vesicles, so their release into the synaptic cleft takes place by exocytosis. This is similar to exocytosis in other cells, but much faster. Neurotoxins that block neurotransmitter release exert their action by inhibiting specific proteins of the cells exocytosis apparatus. When depolarization of an action potential reaches the axon terminal, the change in membrane potential sets off a sequence of events; The axon terminal membrane has voltage gated Ca2+ channels that open in response to depolarization. Calcium ions are more concentrated in ECF, so they want to move inside the cell. Ca2+ entering the cell binds to regulatory proteins and initiates exocytosis. The membrane of the synaptic vesicle fuses with the cell membrane and the fused area opens and neurotransmitters inside vesicle move to synaptic cleft. These molecules diffuse across the gap to bind with membrane receptors on the postsynaptic cell. When they bind to their receptors, a response is initiated in the postsynaptic cell. The membrane of the vesicle then becomes part of the axon terminal membrane. Termination of neurotransmitter activity: If unbound neurotransmitter is removed from the synapse, the receptors release bound neurotransmitter, terminating its activity, to keep the ratio of unbound/bound transmitter constant. Removal of unbound neurotransmitter from the synaptic cleft can be done in several ways. Some diffuse away from the synapse, become separate from receptor. Others are inactivated by enzymes in the synaptic cleft (ACh in ECF is rapidly broken down into choline and acetyl coA by the enzyme AChE in the extracellular matrix and the membrane of the postsynaptic cell.) Many are removed from the ECF by transport back into the presynaptic cell or into adjacent neurons or glia, which metabolizes it.

Describe and compare absolute and relative refractory periods ???

Once an action potential has begun, a second action potential cant be triggered for about 1-msec, no matter how large the stimulus. This delay, called the ABSOLUTE REFRACTORY PERIOD, represents the time required for the Na+ channel gates to reset to their resting positions. Because of this, a second action potential cant occur before the first has finished. A RELATIVE REFRACTORY PERIOD follows the absolute refractory period. During the relative refractory period, some but not all Na+ channel gates have reset to their original positions. In addition, during the relative refractory period, K+ channels are still open. A stronger than normal depolarizing graded potential is needed to bring the trigger zone up to threshold because both types of gates are open, and a stronger potential is needed to create and action potential since the threshold has temporarily moved closer to 0. Although Na+ enters through newly reopened Na+ channels, depolarization due to Na+ entry is offset by K+ loss through still opened K+ channels. As a result, any action potentials that fire during the relative refractory period will be of smaller amplitude than normal. Refractory periods limit the rate at which signals cab be transmitted down neuron. The absolute refractory period also ensures one-way travel of an action potential from cell body to axon terminal by preventing the action potential from traveling backward.

Dendrites

Processes that extend outward from nerve cell body, which receive incoming signals and pass them along to an integrating center within the cell Increases surface area for communication and can communicate with multiple neurons Within in the CNS, dendrites function as independent compartments, sending signals back and forth with other neurons in the brain. Can change size and shape in response to input from neighboring cells

Neuron cell body

Resembles a typical cell, with a nucleus and all organelles needed to direct cellular activity. An extensive cytoskeleton extends outward into the axon and dendrites. Contains DNA that is the template for protein synthesis.

Parasympathetic

Rest and digest

Excitability

The ability of a neuron to respond to a stimulus and fire an action potential

Synapse plasticity

The ability of the nervous system to change activity at synapses. Short term plasticity may enhance activity at the synapse or decrease it.

Saltatory conduction

The apparent jump of the action potential from node to node. This is an effective alternative to large diameter axons and allows for rapid action potentials through small axons

Axonal transport

The axon cytoplasm is filled with many types of fibers and filaments but lack ribosomes and endoplasmic reticulum. So, proteins destined for the axon or axon terminal must be synthesized on the rough endoplasmic reticulum in the cell body. These proteins are moved by way of axonal transport.

Nervous system

The brain and the spinal cord are integrating centers for homeostasis, movement and many other body functions. They are the control center for the nervous system, a network of billions of nerve cells linked together in a highly organized manner to form the rapid control system of the body.

Explain the role of myelin in the conduction of action potentials

The conduction of action potentials down an axon is faster in axons with high-resistance membranes so that current leak out of the cell is minimized. Unmyelinated axons have low resistance to current leak because the entire axon membrane is in contact with the extracellular fluid and it has ion channels through which current can leak. Myelinated axons limit the amount of membrane in contact with the ECF. Small sections of bare membrane- the nodes of Ranvier- alternate with longer segments wrapped in multiple layers of membrane (the myelin sheath). This sheath creates a high-resistance wall that prevents ion flow out of the cytoplasm. The conduction process of myelinated axons is similar to unmyelinated, except that it occurs only at the nodes in myelinated axons. Each node has a high concentration of voltage gated Na+ channels, which open with depolarization and allow Na+ into the axon. Sodium ions entering at a node reinforce the depolarization and restore the amplitude of the action potential as it passes from node to node. What makes them more rapid? Channel opening slows conduction slightly. In unmyelinated axons, channels must open sequentially all the way down the axon membrane to maintain the amplitude of the action potential. But in myelinated axons, only the nodes need Na+ channels because of the insulating properties of the myelin membrane. As the action potential passes along myelinated segments, conduction is not slowed by channel opening. If it can't go out of the cell it is going to go down the cell.

Conductance

The ease with which ions flow through a channel is called the channels conductance. It varies with the gating state of the channel and with the channel protein isoform. Some channels, like the K+ leak channels that are the major determinant of resting membrane potential, spend most of their time in an open state. Other channels have gates that open or close in response to particular stimuli.

Internal resistance

The internal resistance of most neurons is determined by the composition of the cytoplasm and the diameter of the cell. Cytoplasmic composition is relatively constant. Internal resistance decreases as cell diameter increases. The membrane resistance and internal resistance together determine how far current will flow through a cell before the energy is dissipated and the current dies. The combination of the two resistances is called the length constant for a given neuron.

Nerves

The long axons of both afferent and efferent peripheral neurons are bundled together with connective tissue into cordlike fibers, nerves, that extend from CNS to targets of the component neurons.

Synaptic cleft

The narrow space between two cells (between the post and presynaptic cell). This "space" is filled with extracellular matrix whose fibers hold the presynaptic and postsynaptic cells in position.

Neurocrine receptors

The neurocrine receptor found in chemical synapses can be divided into two categories: 1) receptor-channels, which are ligand-gated ion channels (mediate rapid responses by altering ion flow across the membrane, so they are also called ionotropic receptors. some are specific for a certain ion) 2) G protein-coupled receptors (mediate slower responses because the the signal must be transduced through a second messenger system.)

Presynaptic cell

The neuron that delivers a signal to the synapse

Synapse

The regions where an axon terminal meets its target. Consists of a presynaptic cell, a postsynaptic cell and a narrow gap between them called the synaptic cleft.

Ion movement

The resting membrane potential of living cells is determined primarily by the K+ concentration gradient and the cells resting permeability to K+, Na+, and Cl-. A change in either K+ concentration gradient or ion permeabilities changes the membrane potential. At rest, the cell membrane of a neuron is only slightly permeable to Na+. If membrane suddenly increases Na+ permeability, Na+ enters the cell, moving down its electrcochemical gradient. This addition of positive Na+ to the intercellular fluid DEPOLARIZES the cell membrane and creates an electrical signal. Movement of ions across a membrane can also HYPERPOLARIZE a cell. if the cell membrane suddenly becomes more permeable to K+, positive charge is lost from inside the cell, and the cell becomes more negative (hyperpolarizes) A cell may also hyperpolarize if negatively charged ions, like Cl-, enter the cell from the ECF. A significant change in membrane potential occurs with the movement of very few ions (change in potential by 100mV, only 1 in every 100,000 K+must enter or leave the cell). This small fraction shows that the ICF concentration of K+ remains essentially unchanged even though potential has changed by 100. However, the electrical signal created by moving a few K+ has a significant effect on the cells membrane potential.

Channel activation

The speed with which gated channels open and close differ among different types of channels. Channel opening to allow ion flow is called channel activation. (ex. Na+ channels and K+ channels of axons are both activated by cell depolarization. The Na+ channels open very rapidly, but the K+ channels are slower to open. The result is an initial flow of Na+ across the membrane, followed later by K+ flow). Many channels that open in response to depolarization close only when the cell repolarizes. The gating portion of the channel protein has an electrical charge that moves the gate between open and closed positions as membrane potential changes. Some channels also spontaneously inactivate. Even though the activating stimulus that opened them continues, the channel "times out" and closes. (like an automatic door). An inactivated channel returns to its normal closed state shortly after the membrane repolarizes. Channel activity can also be modulated by chemical factors that bind to the channel protein, such as phosphate groups

Electrical synapses

Where the presynaptic and postsynaptic cells are connected by gap junction channels. Gap junctions allow electrical current to flow directly from cell to cell.

Chemical synapses

The vast majority of synapses in the body are chemical ones, where the presynaptic cell releases a chemical signal that diffuses across the synaptic cleft and binds to a membrane receptor on the postsynaptic cell.

Local current flow

The wave of depolarization that move through the cell. Current in biological systems is the net movement of positive electrical charge.

Na+ Channels ???

These channels have two gates to regulate ion movement rather than a single gate. The two gates: 1) activation 2) inactivation These flip flop back and forth to open and close the Na+ channel. When a neuron is at its resting membrane potential, the activation gate of the Na+ channel closes and no Na+ can move through the channel. The inactivation gate, an amino acid sequence behaving like a ball and chain on the cytoplasmic side of the channel, is open. When the cell membrane near the channel depolarizes, the activation gate swings open, opening the channel and allowing Na+ to move into the cell down its electrochemical gradient. The addition of positive charge further depolarizes the inside of the cell and starts a positive feedback loop. More Na+ channels open, and more Na+ enters, further depolarizing. As long as the cell remains depolarized, activation gates in Na+ remain open. Positive feedback loops require outside intervention to stop them. In axons, the inactivation gates in the Na+ channels are the outside intervention that stops the escalating depolarization of the cell. Both active and inactivation gates move in response to depolarization, but the inactivation gate delays its movement for .5msec. During that delay, the Na+ channel is open, allowing enough Na+ influx to create the rising phase of the action potential. When the slower inactivation gate finally closes, Na+ influx stops, and the action potential peaks. This double gating plays major role in the refractory period

Compare and contrast graded potentials and action potential

Voltage changes across the membrane can be classified into two basic types of electrical signals; 1) Graded potentials are variable-- strength signals that travel over short distances and lose strength as they travel through the cell. Used for short distance communication. If a depolarizing graded potential is strong enough when it reaches an integrating region within a neuron, the graded potential initiates an action potential 2) Action potentials are very brief, large depolarizations that travel for long distances through a neuron without losing strength. Their function is rapid signaling over long distances (toe to brain).

Emergent properties

complex processes, such as consciousness, intelligence, and emotion that cannot be predicted from what we know about the properties of individual nerve cells and their specific connections.

organization of nervous system

divided into two parts: 1) The central nervous system consisting of the brain and the spinal cord (acts as the integrating center for neural reflexes) 2) The peripheral nervous system consisting of sensory (afferent) neurons (sends info to the CNS through sensory neurons) and efferent neurons (takes info from the CNS to target cells via efferent neurons).

Collateral

part of axon that ends in a bulbous axon terminal that contains mitochondria and membrane-bound vesicles filled with neurocrine molecules.