Chpt 11 Kasp

A typical chemical synapse is made up of two parts, what they be doe?

1) A knoblike axon terminal of the presynaptic neurons, which contains many tiny, membrane-bounded sacs called synaptic vesicles, each containing thousands of neurotransmitter molecules 2) A neurotransmitter receptor region on the postsynaptic neurons membrane, usually located on a dendrite or the cell body.

Steps in Information Transfer Across Chemical Synapses

1) Action potential arrives at axon terminal. 2) Voltage-gated Ca+ channels open and Ca2+ enters the axon terminal. 3) Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis. 4) Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 5) Binding of neurotransmitter opens ion channels, creating graded potentials. 6) Neurotransmitter effects are terminated.

Generation of an Action Potential

1) Resting state: All gated Na+ and K+ channels are closed. 2) Depolarization: Na+ channels open 3) Repolarization: Na+ channels are inactivating, K+ channels open. 4) Hyperpolarization: Some K+ channels remain open, and Na+ channels reset

How can neurotransmitter's effects be terminated?

1) Reuptake by astrocytes or the presynaptic terminal, where the neurotransmitter is stored or destroyed by enzymes, as with norepinephrine 2) Degradation by enzymes associated with the postsynaptic membrane or present in the synapse, as with acetylcholine 3) Diffusion away from the synapse

The neuroepithelial cells three phase process of differentiation

1) They proliferate to produce the appropriate number of cells needed for nervous system development. 2) The potential neurons, neuroblasts, become amitotic and migrate externally into their characteristic positions. 3) The neuroblasts sprout axons to connect with their functional targets and in so doing become neurons.

Depolarization

A decrease in membrane potential: the inside of the membrane becomes less negative than the resting potential.

Summation by the Postsynaptic Neuron

A single EPSP cannot induce an Ap in the postsynaptic neuron. But if thousands of excitatory axon terminals fire on the same postsynaptic membrane, or if a small number of terminals deliver impulses rapidly, the probability of reaching threshold soars. EPSPs can add together, or summate, to influence the activity of a postsynaptic neuron.

Synapse

A synapse is a junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell-its where the action is.

Neuromodulator

A term used to describe a chemical messenger released by a neuron that does not directly cause EPSPs or IPSPs but instead affects the strength of synaptic transmission. Alters the sensitivity of the postsynaptic membrane to a neurotransmitter.

Classification of Neurotransmitters by Chemical Structure

Acetylcholine Biogenic Amines Amino Acids Peptides Purines Gasses and Lipids

Oligodendrocytes

Also branch but have fewer processes than astrocytes. Oligodendrocytes line up along the thicker nerve fibers in the CNS and wrap their processes tightly around the fibers, producing an insulating covering called a myelin sheath.

Nodes of Ranvier

Also called myelin sheath gaps. Adjacent Schwann cells along an axon do not touch one another, forming gaps called nodes of Ranvier. They occur at regular intervals along a myelinated axon.

Synaptic cleft

Although close to each other, presynaptic and postsynaptic membranes are always separated by the synaptic cleft, a fluid filled space.

Propagation of an Action Potential

An AP must be propagated along the axon's entire length. The AP propagates away from its point of origin. In the body, APs are initiated at one end of the axon and conducted away from that point toward the axons terminals.

All-or-None phenomenon

An Ap is an all-or-nothing phenomenon: it either happens completely or doesn't happen at all.

Organelle differences in axons?

An axon contains the same organelles found in the dendrites and cell body with two important exceptions- it lacks rough ER and a Golgi apparatus, so no protein synthesis and packaging. Consequently, an axon depends on its cell body to renew the necessary proteins and membrane components, and on efficient transport mechanisms to distribute them. Axons quickly decay if cut or severely damaged.

Synaptic delay

An impulse may travel at speeds of up to 150m/s down an axon, but neural transmission across a chemical synapse is comparatively slow. It reflects the time required for neurotransmitter to be released, diffuse across the synaptic cleft, and bind to receptors. This is called synaptic delay.

Hyperpolarization

An increase in membrane potential: the inside of the membrane becomes more negative than the resting potential.

Movement away from the cell body is called?

Anterograde movement

Neuron processes

Armlike processes extend from the cell body of all neurons. CNS contain both neuron cell bodies and their processes, PNS consists chiefly of neuron processes. Bundles of neuron processes are called tracts in CNS and nerves in PNS. The two types of neuron processes are dendrites, and axons.

How do neurons use changes in their membrane potential?

As communication signals to receive, integrate, and send information. A change in membrane potential can be produced by anything that alters ion concentrations on the two sides of the membrane, or anything that changes membrane permeability to any ion. Only permeability changes are important for transferring information.

Neurons are amitotic

As neurons assume their roles as communication links of the nervous system, they lose their ability to divide, cannot be replaced if destroyed. Some exceptions include olfactory epithelium and hippocampal regions.

Neuroglia in the CNS

Astrocytes Microglial cells Ependymal cells Oligodendrocytes

Excitatory Synapses

At excitatory synapses, neurotransmitter binding depolarized the postsynaptic membrane, However, in contrast to what happens on axon membranes, chemically gated ion channels open on postsynaptic membranes. Each channel allows Na+ and K+ to diffuse simultaneously through the membrane but in opposite directions.

Axon: Structures

Axon Axon hillock Axon collaterals Terminal branches Axon terminals

Conduction Velocity factors

Axon diameter: The larger the axon diameter, the faster it conducts impulses. Degree of myelination: myelin sheath dramatically increases the rate of AP propagation by acting as an insulator.

Terminal branches

Axon's end that branches profusely

Inhibitory synapses

Binding of neurotransmitters at inhibitory synapses reduces a postsynaptic neuron's ability to generate an AP. Most inhibitory neurotransmitters hyperpolarize the postsynaptic membrane by making the membrane more permeable to K+ or Cl-. Sodium ion permeability is not affected..

Microtubules and neurofibrils in neuron cell bodies

Bundles of intermediate filaments, are important in maintaining cell shape and integrity. They form a network throughout the cell body.

Differences in ionic composition

Cell cytosol contains a lower concentration of Na+ and a higher concentration of K+ than the extracellular fluid. Potassium (K+) plays the most important role in generating membrane potential.

Two divisions of nervous system?

Central nervous system (CNS) Peripheral nervous system (PNS)

Channel-Linked Receptors: Mechanism of Action

Channel-linked receptors are ligand-gated ion channels that mediate direct neurotransmitter action. As the ligand binds to receptor subunits, the proteins change shape. This opens the central channel and allows ions to pass. The membrane potential of the target cell changes.

Somatic nervous system

Composed of somatic motor nerve fibers that conduct impulses from the CNS to skeletal muscles. It is often referred to as the voluntary nervous system because it allows us to consciously control our skeletal muscles.

Myelinated fibers

Conduct nerve impulses rapidly whereas nonmyelinated fibers conduct impulses more slowly. Myelin sheaths are associated only with axons, Dendrites are always nonmyelinated.

Neuron Cell Body

Consists of a spherical nucleus with a conspicuous nucleolus surrounded by cytoplasm. Major biosynthetic center of a neuron, so it contains the usual organelles needed to synthesize proteins and other chemicals.

Sensory division

Consists of nerve fibers (axons) that convey impulses to the central nervous system from sensory receptors located throughout the body.

Central nervous system

Consists of the brain and spinal cord, which occupy the dorsal body cavity. The CNS is the integrating and control center of the nervous system. It interprets sensory input and dictates motor output based on reflexes, current conditions, and past experience.

Autonomic nervous system

Consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands. Autonomic means a law unto itself. Also referred to as the involuntary nervous system.

Somatic sensory fibers

Convey impulses from the skin, skeletal muscles, and joints

Dendrites

Dendrites of motor neurons are short, tapering, diffusely branching extensions. Dendrites, the main receptive or input regions, provide an enormous surface area for receiving signals from other neurons. Dendrites convey incoming messages toward the cell body. These signals are usually not action potentials but are short-distance signals called graded potentials.

What factors generate the resting membrane potential?

Differences in the ionic composition of the intracellular and extracellular fluids, and differences in the permeability of the plasma membrane to those ions.

Axon collaterals

Each neuron has only one axon, but axons may have occasional branches along their length. These branches, called axon collaterals, extend from the axon at more or less right angles.

Classification of Neurotransmitters by Function

Effects: Excitatory Versus Inhibitory Actions: Direct Versus Indirect The function of a neurotransmitter is determined by the receptor to which it binds.

Presynaptic Inhibition

Events at the presynaptic membrane can also influence postsynaptic activity. Presynaptic inhibition occurs when the release of excitatory neurotransmitter by one neuron is inhibited by the activity of another neuron via an axoaxonal synapse. End result is less neurotransmitter is released and bound, forming smaller EPSPs.

Dopamine and NE are synthesized from?

From the amino acid tyrosine in a common pathway

The Axon: Functional characteristics

Functionally, the axon is the conducting region of the neuron. It generates nerve impulses and transmits them away from the cell body, along the plasma membrane, or axolemma. In motor neurons, the nerve impulse is generated at the junction of the axon hillock and axon (which is why its called the trigger zone) and conducted along the axon to the axon terminals, which are the secretory region of the neuron.

What are clusters of cell bodies in the PNS called?

Ganglia

Graded potential

Graded potentials are short lived, localized changes in membrane potential that can be either depolarization or hyperpolarizations. These changes cause current flows that decrease in magnitude with distance. Graded potentials are triggered by some change in the neuron's environment that opens gated ion channels.

Changes in membrane potential can produce what two types of signals?

Graded potentials, which are usually incoming signals operating over short distances Action potentials, which are long-distance signals of axons

What is grey matter made of?

Grey matter contains mostly nerve cell bodies and nonmyelinated fibers.

Nerve fibers may be classified according to diameter, degree of myelination and conduction speed. What are those classifications?

Group A fibers Group B fibers Group C fibers

Histamine is synthesized from?

Histidine

Inhibitory Postsynaptic Potentials (IPSPs)

If K+ channels open, K+ moves out of the cell. If Cl- channels open, Cl- moves in. In either case, the charge on the inner face of the membrane becomes more negative. As the membrane potential increases and is driven farther from the axon's threshold, the postsynaptic neuron becomes less and less likely to fire and larger depolarizing currents are required to induce an AP. Hyperpolarizing changes in potential are called inhibitory postsynaptic potentials (IPSPs)

More on Neuronal pools

In a simple type of neuronal pool, one incoming presynaptic fiber branches profusely as it enters the pools and then synapses with several different neurons in the pool. When the incoming fiver is excited, it will excite some postsynaptic neurons and facilitate others. Neurons most likely to generate impulses are those closely associated with the incoming fiber,because they receive the bulk of the synaptic contacts. Those neurons are in the discharge zone of the pool. The peripheral neurons are located in the facilitated zone.

Parallel Processing

In parallel processing inputs are segregated into many pathways and different parts of the neural circuitry deal simultaneously with the information delivered by each pathway. Important in higher order thinking.

Serial processing

In serial processing, the whole system works in a predictable all or nothin manner. One neuron stimulates the next, which stimulates the next, so on. Reflexes are an example.

Myelination in the CNS

In the CNS it is the oligodendrocytes that form myelin sheaths, not Schwann cells. Unlike Schwann cells, which form only one segment of a myelin sheath, an oligodendrocyte has multiple flat processes that can coil around as many as 60 axons at the same time. Doesn't contain an outer collar of perinuclear cytoplasm.

Biogenic Amines

Include the catecholamines such as dopamine, norepinephrine, and epinephrine, and the indolamines, which include serotonin and histamine. Biogenic amine neurotransmitters are broadly distributed in the brain, where they play a role in emotional behavior and help regulate the biological clock. Imbalances in these are associated with mental illness.

Gasses and lipids

Includes Gasotransmitters and endocannabinoids

Amino acids

Includes glutamate, aspartate, glycine, and gamma (y)-aminobutyric acid (GABA).

Types of Circuits

Individual neurons in a neuronal pool both send and receive information, and synaptic contacts may cause either excitation or inhibition. The patterns of synaptic connections in neuronal pools, called circuits, determine the pools functional capabilities. There are four basic circuit patterns; diverging, converging, reverberating, and parallel after discharge circuits.

Patterns of Neural Processing

Input processing is both serial and parallel. In serial processing, the input travels along one pathway to a specific destination. In parallel processing, the input travels along several different pathways to be integrated in different CNS regions.

What happens when an impulse reaches the axon terminal?

It causes neurotransmitters to be released into the extracellular space. The neurotransmitters either excite or inhibit neurons with which the axon is in close contact. Because each neuron both receives signals from and sends signals to scores of other neurons, it carrie son conversations with many different neurons at the same time.

Difference in Plasma membrane permeability

K+ is always leaking out of the cell and some Na+ is always leaking in, the sodium potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium.

Neurons

Large, highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another. Neurons have extreme longevity, can function optimally for a 100years. Neurons have a high metabolic rate and require continuous supplies of oxygen and glucose.

Electrical synapses

Less common variety. Consist of gap junctions like those found between certain other body cells. They contain protein channels, called connexons, that intimately connect the cytoplasm of adjacent neurons and allow ions and small molecules to flow directly from one neuron to the next. They provide a simple means of synchronizing the activity of all interconnected neurons. Electrical synapses are far more abundant in embryonic nervous tissue.

Group B fibers

Lightly myelinated fibers of intermediate diameter, transmit impulses at an average rate of 15m/s

Endocannabinoids

Lipid soluble, synthesized on demand. Diffuse freely from the postsynaptic neuron to their receptors on presynaptic terminals where they act as a retrograde messenger to decrease neurotransmitter release. Thought to be involved in memory and learning.

Ependymal Cells

Many are ciliated. They line the central cavities of the brain and the spinal cord, where they form a fairly permeable barrier between the cerebrospinal fluid that fills cavities and the tissue fluid bathing the cells of the CNS. The beating of their cilia helps to circulate the cerebrospinal fluid that cushions the brain and spinal cord.

Converging circuit

Many inputs, one output. A concentrating circuit. Ex: Different sensory stimuli can all elicit the same memory.

Myelin Sheath

Many nerve fibers, particularly those that are long or large in diameter, are covered with segmented myelin sheath. Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses.

Substances moved in the anterograde direction include what?

Mitochondria Cytoskeletal elements Membrane components used to renew the axon plasma membrane, and enzymes needed to synthesize certain neurotransmitters.

Substances transported through the axon in the retrograde direction?

Mostly organelles returning to the cell body to be degraded or recycled. Also serves as an important way for intracellular communication to advise the cell body of conditions at the axon terminals and deliver to the cell body vesicles containing signal molecules.

Group A fibers

Mostly somatic sensory and motor fibers serving the skin, skeletal muscles, and joints. They have the largest diameter and thick myelin sheaths and conduct impulses at speeds ranging up to 150 m/s (over 300mph)

Myelination in the PNS

Myelin sheaths in the PNS are formed by Schwann cells, which indent to receive an axon and then wrap themselves around it in a jelly roll fashion. The Schwann cell cytoplasm is gradually squeezed from between the layers, leaving behind a tight coil of wrapped membranes that is the myelin sheath.

Do small diameter axons have myelination?

Naa, brah brah These are covered by long extensions of adjacent glial cells

Do most neurons function as presynaptic or postsynaptic neurons?

Neither. At a given synapse, the presynaptic neuron sends the information and the postsynaptic neuron receives the information. Most neurons function as both presynaptic and postsynaptic neurons.

Neurons

Nerve cells that are excitable (respond to stimuli) and transmit electrical signals

Whats a long axon called?

Nerve fiber

Nervous tissue is made up of what two principal types of cells?

Neuroglia Neurons

What are the structural units of the nervous system?

Neurons also called nerve cells

Structural classifications of neurons

Neurons are grouped structurally according to the number of processes extending from their cell body. Three major neuron groups make up this classification: multipolar, bipolar, and unipolar neurons.

Actions: Direct vs Indirect

Neurotransmitters that act directly are those that bind to and open ion channels. These neurotransmitters are typically direct acting neurotransmitters. Neurotransmitters that act indirectly promote broader, longer-lasting effects by acting through intracellular second messenger molecules, typically via G protein pathways. In this way their action is similar to that of many hormones. Biogenic amines, neuropeptides, and dissolved gasses are indirect neurotransmitters.

Gasotransmitters

Nitric oxide, carbon monoxide, and hydrogen sulfide. NO participates in a variety of activities including the formation of new memories by increasing the strength of certain synapses. Excessive release of NO contributes to much of the brain damage seen in stroke patients.

Threshold

No all local depolarization events produce APs. The depolarization must reach threshold values if an axon is to fire. Typically reached when the membrane has been depolarized by 15 to 20mV from resting value.

What are clusters of cell bodies in the CNS called?

Nuclei

Temporal summation

Occurs when one or more presynaptic neurons transmit impulses in rapid-fire order and bursts of neurotransmitter are released in quick succession.

Spatial Summation

Occurs when the postsynaptic neuron is stimulated simultaneously by a large number of terminals from one or, more commonly, many presynaptic neurons. Huge numbers of its receptors bind neurotransmitter and simultaneously initiate EPSPs, which summate and dramatically enhance depolarization.

Acetylcholine (ACh)

Once released by the presynaptic terminal, ACh bind briefly to the postsynaptic receptors. Then it is released and degraded to acetic acid and choline by the enzyme acetylcholinesterase (AChE), located in the synaptic cleft and on postsynaptic membranes. Presynaptic terminals recapture the released choline and reuse it to synthesize more ACH. ACh is released by all neurons that stimulate skeletal muscles and by some neurons of the autonomic nervous system, also found in the CNS.

Diverging circuit

One input, many outputs. An amplifying circuit. Ex: a single neuron in the brain can activate 10 or more motor neurons in the spinal cord.

Sensory neurons

Or afferent neurons, transmit impulses from sensory receptors in the skin or internal organs toward or into the central nervous system. Virtually all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS.

Interneurons

Or association neurons, lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs. Make up over 99% of neurons of the body. Almost all interneurons are multipolar.

Motor neurons

Or efferent neurons, carry impulses away from the CNS to the effector organs (muscles, glands) of the body periphery. Motor neurons are multipolar.

Receptor potential

Or generator potential When the receptor of a sensory neuron is excited by some form of energy, the resulting graded potential is called a receptor potential

Purines

Purines are nitrogen containing chemicals (such as guanine and adenine) that breakdown products of nucleic acids. ATP is now recognized as a major neurotransmitter in the CNS and PNS. Can produce fast excitatory responses or slow, second-messenger responses.

Reflex

Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same response. Retracting your hand from a hot stove.

Reflex arcs

Reflexes occur over neural pathways called reflex arcs that have five essential components- receptor, sensory neuron, CNS integration center, motor neuron, and effector.

What is white matter made of?

Regions of the brain and spinal cord containing dense collections of myelinated fibers are referred to as white matter and are primarily fiber tracts.

Synaptic Potentiation

Repeated or continuous use of a synapse enhances the presynaptic neurons's ability to excite the postsynaptic neuron, producing larger-than-expected EPSPs. This phenomenon is called synaptic potentiation. The presynaptic terminals at such synapses contain relatively high Ca2+ concentration, a condition that triggers the release of more neurotransmitter, which in turn produces larger EPSPs.

Movement towards the cell body is called?

Retrograde movement

Neuroglia in the PNS?

Satellite cells Schwann cells

Nervous system three functions

Sensory input Integration Motor output

PNS two functional subdivisions

Sensory, or afferent division Motor, or efferent division

Astrocytes

Shaped like sea anemones, most abundant and versatile glial cells. Support and brace the neurons and anchor them to their nutrient supply lines. Play a role in making exchanges between capillaries and neurons, helping determine capillary permeability. Guide the migration of young neurons and formation of synapses between neurons. Astrocytes also control the chemical environment around neurons, where their most important job is "mopping up" leaked potassium ions and recapturing and recycling released neurotransmitters. Connected by gap junctions, astrocytes signal each other both by taking in calcium, creating calcium waves, and by releasing extracellular chemical messengers. Fwaaak thats a lot of shit for one thing.

Parallel after-discharge circuit

Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell. Impulses reach output cell at different times, causing a burst of impulses called an after-discharge. Ex: May be involved in exacting mental processes such as mathematical calculations.

Reverberating circuit

Signal travels through a chain of neurons, each feeding back to previous neurons. An oscillating circuit. controls rhythmic activity. Ex: involved in breathing, sleep0wake cycle, and repetitive motor activities such as walking.

Unipolar neurons

Single short process that emerges from the cell body and divides T-like into proximal and distal branches. The more distal process, the peripheral process, is often associated with sensory receptor. The central process enters the CNS.

Microglial Cells

Small and ovoid with relatively long thorny processes. They monitor the health of nearby neurons, then migrate to the ones that are in trouble. Transform into a special type of macrophage that phagocytizes the microorganisms or neuronal debris of invading microorganisms or dead neurons.

Group C fibers

Smallest diameter, nonmyelinated. Consequently incapable of saltatory conduction and conduct impulses slowly.

Motor divisions two main parts

Somatic nervous system Autonomic nervous system

Effects: Excitatory vs Inhibitory

Some neurotransmitters are excitatory (cause depolarization). Some are inhibitory (cause hyperpolarization). Others exert both effects, depending on the specific receptor types with which they interact. GABA=inhibitory, glutamate=excitatory, ACh=depends

Chemical Synapses

Specialized to allow the release and reception of chemical neurotransmitters.

Coding for Stimulus Intensity

Strong stimuli generate nerve impulses more often in a given time interval than do weak stimuli. Stimulus intensity is coded for by the number of impulses per second-that is, by the frequency of action potentials- rather than by increases in the strength of the individual APs

Neuroglia

Supporting cells, small cells that surround and wrap the more delicate neurons

Schwann cells

Surround all nerve fibers in the PNS and form myelin sheaths around thicker nerve fibers. Similar to Oligodendrocytes. Schwann cells are vital to regeneration of damaged peripheral nerve fibers.

Satellite cells

Surrounds neuron cell bodies located in the peripheral nervous system and are though to have many of the same functions in the PNS as astrocytes do in the CNS

Axosomatic synapses

Synapses between axon endings of one neuron and cell bodes of other neurons are axosomatic synapses

Axodendritic synapses

Synapses between the axon endings of one neuron and the dendrites of other neurons are axodendritic synapses.

What are the two types of summation?

Temporal summation Spatial summation

Organization of Neurons: Neuronal pools

The billions of neurons in the CNS are organized into neuronal pools. These functional groups of neurons integrate incoming information received from receptors or different neuronal pools and then forward the processed info to other destinations.

How does a neuroblasts growing axon know where to go?

The growing tip of an axon called a growth cone,gives the axon the ability to interact with its environment. Extracellular and cell surface adhesions and nerve cell adhesion molecules (N-CAM) provide anchor points for the growth cone. Neurotropins signal the growth cone to come this way, such as Netrin. Ephrin, or Slit tell it to go way, Semaphorin tell it to stop here.

Axon hillock

The initial region of the axon arises from a cone shaped area of the cell body called the axon hillock and then narrows to form a slender process that is uniform in diameter.

Relative Refractory Period

The interval following the absolute refractory period. During the relative refractory period, most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarization is occurring. Requires exceptionally strong stimulus to generate an AP.

Axon terminals

The knob like distal endings of the terminal branches. Also called terminal boutons.

Motor output

The nervous system activates effector organs-the muscles and glands- to cause a response, called motor output

How do neurons originate?

The nervous system originates from a dorsal neural tube and the neural crest formed from surface ectoderm. The neural tube, whose walls begin as a layer of neuroepithelial cells, becomes the CNS.

Integration

The nervous system processes and interprets sensory input and decides what should be done at each moment-a process called integration

Sensory input

The nervous system uses its millions of sensory receptors to monitor changes occurring both inside and outside the body. The gathered information is called sensory input

Presynaptic neuron

The neuron conducting impulses toward the synapse is the presynaptic neuron.

Postsynaptic neuron

The neuron transmitting the electrical signal away from the synapse is the postsynaptic neuron.

Peptides

The neuropeptides, essentially strings of amino acids, include a broad spectrum of molecules with diverse effects. For example, a neuropeptide called substance P is an important mediator of pain signals. By contrast, endorphins, which include beta endorphin, dynophin, and enkaphalins act as natural opiates.

Action Potential

The principal way neurons send signals over long distances is by generating and propagating (transmitting) action potentials. Only cells with excitable membranes, neurons and muscle cells, can generate action potentials. AP is a brief reversal of membrane potential (from -70mV to +30mV). Depolarization of followed by repolarization and often a short period of hyperpolarization. Do not decay with distance as graded potentials do.

Functional Classification of neurons

This scheme groups neurons according to the direction in which the nerve impulse travels relative to the central nervous system. There are: Sensory neurons Motor neurons Interneurons

Multipolar neurons

Three or more processes- one axon and the rest dendrites. They are the most common neuron type in humans, with more than 99% of neurons belonging to this class. Multipolar neurons are the major neuron type in the CNS.

Transport along the axong

Through the cooperative efforts of motor proteins and cytoskeletal elements (microtubules and actin filaments) substances travel continuously along the axon away and towards the cell body.

Difference between electrical synapses and chemical synapses?

Transmission along an axon and across an electrical synapse is a purely electrical event. However, chemical synapses convert the electrical signals to chemical signals that travel across the synapse to the postsynaptic cells, where they are converted back into electrical signals

Visceral sensory fibers

Transmit impulses from the visceral organs (organs within the ventral body cavity)

Motor division

Transmits impulses from the CNS to effector organs, which are the muscles and glands. These impulses activate muscles to contract and glands to secrete. They effect a motor response.

Serotonin is synthesized from?

Tryptophan

Bipolar neurons

Two processes- an axon and a dendrite- that extend from opposite sides of the cell body. These rare neurons are found in some of the special sense organs. Ex: retina of the eye, olfactory mucosa.

Excitatory Postsynaptic Potentials (EPSPs)

Unlike axons which have voltage-gated channels that make an AP possible, postsynaptic membranes generally do not generate APs. The dramatic polarity reversal seen in axons never occurs in membranes containing only chemically gated channels because the opposite movement of K+ and Na+ prevent excessive positive charge from accumulating inside the cells. For this reason, instead of APs, local graded depolarization events called excitatory postsynaptic potentials occur at excitatory postsynaptic membranes. The only function of EPSPs is to help trigger an AP distally at the axon hillock of the postsynaptic neuron. If currents reaching the hillock are strong enough to depolarize the axon to threshold, axon voltage-gated channels open and an AP is generated.

G Protein-Linked Receptors: Mechanism of Action

When a neurotransmitter binds to a G Protein-linked receptor, the G protein is activated. Activated G proteins typically work by controlling the production of second messengers such as cyclic AMP, cyclic GMP, or Ca2+.These second messengers, in turn, act as go-betweens to regulate ion channels or activate kinase enzymes that initiate a cascade of enzymatic reactions in the target cells.

Absolute Refractory Period

When a patch of neuron membrane is generating an AP and its voltage-gated sodium channels are open, the neuron cannot respond to another stimulus, no matter how strong.

Saltatory conduction

When an AP is generated in a myelinated fiber, the local depolarizing current does not dissipate through the adjacent membrane regions, which are nonexcitable. Instead the current is maintained and moves rapidly to the next myelin sheath gap, where it triggers another AP. Consequently, APs are triggered only at the gaps, a type of conduction called saltatory conduction because the electrical signal appears to jump from gap to gap along the axon.

Postsynaptic potential

When the stimulus is a neurotransmitter released by another neuron, the graded potential is called a postsynaptic potential, because the neurotransmitter is released into a fluid filled gap called a synapse and influences the neuron beyond the synapse.

Lipofuscin

harmless by-product of lysosomal activity in neuron cell body. Sometimes called the aging pigment because it accumulates in neurons of elderly individuals.

Peripheral nervous system

part of the nervous system outside the CNS. Consists mainly of the nerves (bundles of axons) that extend from the brain and spinal cord. Spinal nerves carry impulses to and from the spinal cord, cranial nerves carry impulses to and from the brain. These serve as communication lines that link all parts of the body to the CNS.