Chapter 8

Very few ions 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.

An action potential results from ion movement across the neuron membrane. First Na+ moves into the cell, and then K+ moves out. However it is important to understand that...

Hyperkalemia

An increase in blood K+ concentration-- ______________-- shifts the resting membrane potential of a neuron closer to threshold and causes the cells to fire action potentials in response to smaller graded potentials (figure 8.17c)

Subthreshold; suprathreshold

At normal K+ levels, ____________ graded potentials do not trigger action potentials, and _____________ graded potentials do (Figure 8.17 page 265)

Visceral nervous system

The autonomic division of the PNS is also called the ___________ ____________ ___________ because it controls contraction and secretion in the various internal organs. Autonomic neurons are further divided into sympathetic and parasympathetic branches which can be distinguished by their anatomical organization and by the chemicals they use to communicate with their target cells.

Neuron! (sodium in and potassium out)

The action potential of muscle is identical to the action potential of a...

1.Chemical 2.Neurotransmitter 3.Dendrites and cell body 4. Receptors 5. It becomes activated 6. Chemically-gated

1.If a neuron is receiving a signal from another neuron in the synaptic cleft, what kind of signal is it? Electrical or chemical? 2. What type of signal molecule is it? 3. Where on the neuron is the signal molecule? 4. What must be on the post-synaptic membrane at that location in order to receive the signal? 5. What happens when the NT binds there? 6. What kind of gated-channel will open with this type of stimulus?

Schwann Cells

A single axon may have as many as 500 different _________ _________ along its length. Each ________ _______ wraps around a 1-1.5 mm segment of the axon, leaving tiny gaps, called the nodes of Ranvier, between the myelin-insulated areas. (Figure 8.5c page 247)

All-or-none

Action potentials are sometimes called _____-___-_____ phenomena because they either occur as a maximal depolarization (if the stimulus reaches threshold) or do not occur at all (if the stimulus is below threshold).

Voltage-gated ion channels open,altering membrane permeability (P) to Na+ (PNa+) and K+ (PK+). Figure 8.9 shows the voltage and ion permeability changes that take place in one section of membrane during an action potential. Before and after the action potential, at 1 and 9, the neuron is at its resting membrane potential of -70mV. The action potential itself can be divided into three phases: a rising phase, a falling phase, and the after-hypolarization phase.

Action potentials begin when...

Excitatory; inhibitory

Because depolarization makes a neuron more likely to fire an action potential, depolarizing graded potentials are considered to be _____________. A hyperpolarizing graded potential moves the membrane potential farther from the threshold value and makes the neuron less likely to fire an action potential. Consequently, hyperpolarizing graded potentials are considered to be _____________.

CNS neurons release many different neurocrines, including some polypeptides known mostly for their hormonal activity, such as hypothalamic releasing hormones and oxytocin and vasopressin. In contrast, the PNS secretes only three major neurocrines: the neurotransmitters acetylcholine and norepinephrine, and the neurohormone epinephrine

CNS vs PNS neurocrines

Come in five related subtypes. They are all G protein-coupled receptors linked to second messenger systems. These receptors occur in the CNS and in the autonomic parasympathetic division of the PNS.

Cholinergic muscarinic receptors...

Receptor-channels found on skeletal muscle, in the autonomic division of the PNS, and in the CNS

Cholinergic nicotinic receptors are...

a voltage-gated Na+ channel and a voltage-gated K+ channel, plus some leak channels that help set the resting membrane potential.

Conduction of an action potential requires only a few types of ion channels:

The answer lies with the chemical signals used by the development embryo, ranging from factors that control differentiation of stem cells into neurons and glia to those that direct an elongating axon to its target.

During embryonic development, how can more than 100 billion neurons in the brain find their correct targets and make synapses among more than 10 times that many glial cells? How can a somatic motor neuron in the spinal cord find the correct pathway to form a synapse with its target muscle in the big toe?

Study it on page 256 and read the back of study guide

Figure 8.9

-Channels closed=impermeable -Channels open=permeable -Number of channels open influences extent of permeability and the amount of ions coming through

Gated channels control ion permeability

Memorize Figure 8.5 page 247

Glial Cells

Cell body and dendrites 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.

Graded potentials in neurons are depolarizations or hyperpolarizations that occur in the...

Trigger zone. In efferent neurons and interneurons, the trigger zone is the axon hillock and the very 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.

Graded potentials that are strong enough eventually reach the region of the neuron known as the _________ _________.

The simplest way is to open or close existing channels in the membrane. A slower method for changing membrane permeability is for the cell to insert new channels into the membrane or remove some existing channels.

How does a cell change its ion permeability?

Hypokalemia

If blood K+ concentration falls too low-- a condition known as ______________--the resting membrane potential of the cell hyperpolarizes, moving farther from theshold. (figure 8.17d)

The action potential will go in both directions because the Na+ channels around the stimulation site have not been inactivated by a previous depolarization.

If you place an electrode in the middle of an axon and artificially depolarize the cell about threshold, in which direction will an action potential travel: to the axon terminal, to the cell body, or to both?

Gap junctions

In a few pathways, neurons are linked by gap junctions, allowing electrical signals to pass directly from cell to cell.

The loss of myelin from vertebrate neurons can have devastating effects on neural signaling. In the CNS and PNS, the loss of myelin slows the conduction of action potentials. In addition, when current leaks out of the now-uninsulated regions of membrane between the channel-rich nodes of Ranvier, the depolarization that reaches a node may no longer be above threshold, and conduction may fail (Figure 8.16b page264)

In demyelinating diseases...

Neurotransmitters

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

Enteric nervous system

In recent years a third division of the nervous system has received considerable attention. The __________ _________ _________ is a network of neurons in the walls of the digestive tract. It is frequently controlled by the autonomic division of the nervous system, but it is also able to function autonomously as its own integrating center.

(1) Na+ channels (2) K+ channels (3) Ca+ channels (4) Cl- channels

Ion channels are usually named according to the primary ion(s) they allow to pass through them. There are four major types of selective ion channels in the neuron:

The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation. https://quizlet.com/5939811/resting-membrane-potential-flash-cards/ https://quizlet.com/27360644/quiz-2-membrane-potential-flash-cards/

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?

the call repolarizes.

Many channels that open in response to depolarization close only when...

Antagonistic control

Many internal organs receive innervation from both types of autonomic neurons, and it is common for the two divisions to exert _____________ __________ over a single target

Organization of the nervous system

Memorize graph on page 240

1. Mechanically gated ion channels 2. Chemically gated ion channels 3. Voltage-gated ion channels

Most gated channels fall into one of the three categories:

Axon hillock

Most peripheral neurons have a single axon that originates from a specialized region of the body called the ________ __________.

Most common and best-known demyelinating disease. It causes fatigue, muscle weakness, difficulty walking, and loss of vision. Guilllian-barre syndrome is also a demyelinating disease

Multiple sclerosis

Excitable tissues

Nerve and muscle cells are described as ___________ ________ because of their ability to propagate electrical signals rapidly in response to a stimulus. We now know that many other cell types generate electrical signals to initiate intracellular, but the ability of nerve and muscle cells to send a constant electrical signal over long distances is characteristic of electrical signaling in these tissues.

Neurons

Nerve cells or ________, carry electrical signals rapidly and, in some cases, over long distances.

Structure for receiving and spreading signals DENDRITES AND CELL BODY -Receptive area, receive incoming signals AXONS -Send signals -Signal types? At the distal end of the axon, the electrical signal is usually translated into a chemical signal by secretion of a neurotransmitters, neuromodulators, neurohoromone AXON TERMINALS -Send signals - SIgnal type? Neurocrines. The axon terminal contains mitochondria and membrane-bound vesicles filled with neurocrine molecules.

Neuron (Figure 8.2)

Electrical or chemical depending on the type of signal that passes from the presynaptic cell to the postsynaptic one

Neuron-to-neuron synapses are classified as...

Nerve cell body

Neurons are uniquely shaped cells with long processes that extend outward from the _______ _______ _______. These processes are usually classified as either dendrites, which receive incoming signals, or axons, which carry outgoing information.

Interneurons

Neurons that lie entirely within the CNS are know as _______________.

Na+-K+-ATPase (also known as the NA+-K+ pump). The pump uses energy from ATP to exchange Na+ that enters the cell for K+ that leaked out of it. This exchange does not need to happen before the next action potential fires, however, because the ion concentration gradient was not significantly altered by on action potential! A neuron without a functional Na+-K+ pump could fire a thousand or more action potentials before a significant change in the ion gradient occurred. Page 257

Normally, ions that do move into or out of the cell during action potentials are rapidly restored to their original compartments by the ...

a synapse forms. However, synapse formation must be followed by electrical and chemical activity, or the synapse will disappear.

Once an axon reaches its target cell...

number of axons each cell wraps around. In the CNS, one oligodendrocyte branches and forms myelin around portions or several axons (figure 8.5b). In the PNS, one Schwann cell associates with one axon.

One difference between oligodendrocytes and Schwann cells is the...

Getting the critical amount of change in charge to open those voltage-gated channels. THEN you can have ion movement.

Threshold is about...

Efferent neurons subdivide into the:

Somatic motor division, which controls skeletal muscles, and the autonomic division, which control smooth and cardiac muscle, exocrine glands, some endocrine glands, and some types of adipose tissue.

Cell body

The _____ ______ of a neuron resembles a typical cell, with a nucleus and all organelle needed to direct cellular activity. An extensive cytoskeleton extends outward into the axon and dendrites. Despite its small size, the cell body with its nucleus is essential to the well-being of the cell because its contains DNA that is the template for protein synthesis.

Absolute

The ________ refractory period is the period during which you can't generate another action potential because you can't open those sodium channels again.

Postsynaptic cell

The cell that receives the signal

Refractory period The adjective refractory comes from the Latin word "stubborn". The "stubbornness" of the neuron refers to the fact that once an action potential has begun, a second action potential cannot be triggered for about 1-2 msec, no matter how large the stimulus. This delay, which represents the time required for the Na+ channel gates to reset to their resting positions, is called the absolute refractory period. Because of the absolute refractory period, a second action potential cannot occur before the first has finished. (Figure 8.12 page 260). Consequently, action potentials moving from trigger zone to axon terminal cannot overlap and cannot travel backwards.

The double gating of Na+ channels plays a major role in the phenomenon known as the ___________ __________.

Electrical signals to be conducted in only one direction.

The double-gating mechanism found in axonal voltage-gated Na+ channels allows...

Motor neurons

The expression ________ _________ is sometimes used to refer to all efferent neurons. However, clinically, the term motor neuron is often used to describe somatic motor neurons that control skeletal muscles.

Ependymal cells

The final class of glial cells is the _____________ _____, specialized cells that create a selectively permeable epithelial layer, the ependyma, that seperates the fluid compartments of the CNS. The ependyma is one source of neural stem cells, immature cells that can differentiate into neurons and glial cells.

The neuron, or nerve cell

The functional unit of the nervous system.

Microglia

The glial cells known as ____________ are specialized immune cells that reside permanently in the CNS. (Figure 8.5a,b). When activated, they remove damaged cells and foreign invaders.

Conduction

The high-speed movement of an action potential along the axon is called ____________ of the action potential.

Synaptic cleft

The narrow space between cells is called the __________ __________.

The central nervous system (CNS) which consists of the brain and the spinal cord and the peripheral nervous system (PNS) which consists of sensory (afferent) neurons and efferent neurons.

The nervous system can be divided into two parts:

Neurons-- the basic signaling units of the nervous system-- and support neurons known as glial cells.

The nervous system is composed primarily of two cell types:

Presynaptic cell

The neuron that delivers a signal to the synapse

Transmit outgoing electrical signals from the integrating center of the neuron to the end of the axon. At the distal end of the axon, the electrical signal is usually translated into a chemical message by secretion of a neurotransmitter, neuromodulator, neurohormones. Neurons that secrete neurotransmitters and neuromodulators terminate near their target cells, which are usually other neurons, muscles, and glands.

The primary function of an axon is to...

Synapse

The region where an axon terminal meets its target cell is called a ___________.

K+ concentration gradient and the cell's resting permeability to K+, Na+, and Cl-. A change in either the K+ concentration gradient or ion permeabilities changes the membrane potential.

The resting membrane potential of living cells is determined primarily by the...

The rising phase is due to a sudden temporary increase in the cell's permeability to Na+. An action potential begins when a graded potential reaching the trigger zone depolarizes the membrane to threshold (-55mV). Read the rising phase and falling phase on page 256-257

The rising phase of the action potential

Satellite Cells

The second type of PNS glial cell, the ___________ ______. is a nonmyselinating Schwann cell (Figure 8.5a). _____________ _________ form supportive capsules around nerve bodies located in ganglia. A ganglion is a collection of nerve cell bodies found outside the CNS.

activation

The speed with which a gated channel opens and closes differs among different types of channels. Channel opening to allow ion flow is called channel ____________. For example, Na+ channels and K+ channels of axons are both activated by cell depolarization. The Na+ channels open very rapid, but the K+ channels are slower to open. The result is an initial flow of Na+ across the membrane, followed later by a flow of K+

Neurotrophic factors

The survival of neuronal pathways depends on _____________ ____________ secreted by neurons and glial cells.

Integrating center

The trigger zone is the ___________ _________ of the neuron and contains a high concentration of voltage-gated Na+ channels in its membrane.

Chemical

The vast majority of synapses in the nervous system are ___________ synapses, which use neurocrine molecules to carry info from one cell to the next. At these synapses, the electrical signal of the presynaptic cell is converted into a neurocrine signal that crosses the synaptic cleft and bind to a receptor on its target cell.

Change in membrane potential that occurs when voltage-gated ion channels in the membrane open, increasing the cell's 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 of the cell), which restores the cell to the resting membrane potential.

To summarize the action potential is a...

1. The uneven distribution of ions across the cell membrane. - Sodium, chloride and calcium (Ca^2+) are more concentrated in the extracellular fluid than in the cytosol. Potassium is more concentrated in the cytosol than in the extracellular fluid. 2. Differing membrane permeability to those ions. The resting cell membrane is much more permeable to K+ than to Na+ or Ca^2+. This makes K+ the major ion contributing to the resting membrane potential.

Two factors influence the membrane potential:

less likely to occur natuarally. CNS glial cells tend to seal off and scar the damaged region, and damaged CNS cells secrete factors that inhibit axon regrowth.

Under some conditions, axons in the PNS can regenerate and re-establish their synaptic connections. Schwann cells secrete neurotrophic factors that keep the cell body alive and stimulate regrowth of the axon. Regenerating of axons in the CNS is...

Graded potentials and action potentials

Voltage changes across the membrane can be classified into two basic types of electrical signals:

Chemically-gated because it's where signal molecules come over and cause channels to open.

What kind of gated channels do you have on the cell body and dendrites?

Voltage-gated

What kind of gated channels on the axon hillock and all the way down the axon?

1. Current leaks- 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, diminishing the strength of the signal inside 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.

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

These voltage-gated Na+ channels have two gates to regulate ion movement rather than a single gate. The two gates, known as activation and inactivation gates, flip-flop back and fourth to open and close the Na+ channel.

Why do voltage-gated Na+ channels close at the peak when depolarization was the stimulus for Na+ channel opening?

Glial cells

________ ______ are the unsung heroes of the nervous system, outnumbering neurons by 10-50 to 1. For many years scientists thought that the primary function of ________ ______ was physical support, and that ______ ______ had little influence on information processing. That view has changed. Although _______ ______ do not participate directly in the transmission of electrical signals over long distances, they do communicate with and provide important biochemical support to neurons. The PNS has two types of glial cells-- Schwann cells and satellite cells--and the CNS has four types: oligodendrocytes, microglia, astrocytes, and ependymal cells.

Sensory neurons

_________ ________ carry information about temperature, pressure, light, and other stimuli from sensory receptors to the CNS.

Action potentials

_________ __________, also known as spikes, are the electrical signals of uniform strength that travel from a neuron's trigger zone to the end of its axon. In _________ __________, ion channels in the axon membrane open sequentially as electrical current passes down the axon.

Action potential

_________ ___________ are very brief, large depolarizations that travel for long distances through a neuron without losing its strength. Their function is rapid signaling over long distances, such as from your toe to your brain.

Electrical

__________ synapses pass an electrical signal, or current, directly from the cytoplasm of one cell to another through the pores of gap junction proteins. These synapses occur mainly in neurons of the CNS. They are also found in glial cell, in cardiac and smooth muscle, and in nonexcitable cells that use electrical signals, such as pancreatic beta cells. The primary advantage of electrical synapses is rapid conduction of signals from cell to cell that synchronizes activity within a network of cells. Gap junctions also allow chemical signal molecules to diffuse between adjacent cels.

Graded potentials

___________ _____________ are variable-strength signals that travel over short distances and lose strength as they travel through the cell. They are used for short-distance communication. If a depolarizing __________ ___________ is strong enough when it reaches an integrating region within a neuron, the graded potential initiates an action potential.

Dendrites

___________ are thin, branched processes that receive incoming information from neighboring cells. They increase the surface area of a neuron, allowing it to communicate with multiple other neurons. The primary function of the ___________ in the PNS is to receive incoming information and transfer it to an integrating region within the neuron. Within the CNS, ___________ function is more complex.

Mechanically

___________ gated ion channels are found in sensory neurons and open in response to physical forces such as pressure or stretch.

Schwann cells; oligodendrocytes

____________ ______ in the PNS and __________________ in the CNS support and insulate axons by forming myelin, a substance composed of multiple concentric layers of phospholipid membrane. In addition to providing support, the myelin acts as insulation around axons and speeds up their signal transmission. Myelin forms when the glial cells wrap around an axon, squeezing out the glial cytoplasm so that each wrap becomes two membrane layers.

Sensory receptors

____________ _____________ throughout the body continuously monitor conditions in the internal and external environments. These sensors send information along sensory neurons to the CNS, which is the integrating center for neural reflexes. 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

Chemically

____________ gated ion channels in most neurons respond to a variety of ligands, such as extracellular neurotransmitters and neuromodulators or intracellular signal molecules.

Voltage

____________- gated ion channels respond to changes in the cells membrane potential. __________-gated Na+ and K+ channels play an important role in the initiation and conduction of electrical signals along the axon.

Astrocytes

______________ make up about half of the cells in the brain. They are a source of neural stem cells. The take up K+, water neurotransmitters. They secrete neurotrophic factors. They help form the blood-brain barrier, and they provide substrates for ATP production.