Physiology chapter 7

Describe the BBB clinical significance?

An example will be that the BBB presents difficulties in the chemo therapy of the brain diseases because drugs that can enter other organs may not be able to enter the brain. Like patients who have parkinson disease require a chemical called dopamine but are given a precursor molecule called levodopa because this molecule can cross the BBB.

Definition of a neuron

Basic structural and functional unit of the nervous system.

Hyperpolarization of the postsynaptic membrane in response to glycine or GABA is produced by the opening of?

Cl 2 channels. (D)

Hyperpolarization is caused by all of these neurotransmitters except?

Glutamic acid in the CNS. (A)

Functions of Astrocytes?

Help regulate the external environment of neurons in the CNS

Explain the nature of excitatory postsynaptic potentials?

If a released neurotransmitter is excitatory and causes depolarization of post synaptic membrane, then the EPSP is generated (excitatory post synaptic potential). This triggers the action potential generation and allows the transmission of the impulse to the post synaptic neuron.

Describe the structure, locations and functions of gap junctions?

-Gap junctions are water filled channels through which ions can go from one cell to another. -This permits impulses to be conducted directly from one cell to another. -Each gap junction is composed of connexin proteins. -6 connexin proteins in one plasma membrane link up with 6 connexin proteins in the other plasma membrane to form each gap junction.

Function and location of Association Neuron(Interneuron) ?

-Multipolar neuron located within CNS -Serves the associative, integrative, functions of the nervous system.

Identify the myelin sheath and describe how it is formed in the CNS?

-Myelin sheaths are formed by oligodendrocytes -This process occurs postnatal -Each oligodendrocyte has extensions that form a myelin sheath around multiple axons.

Describe how the permeability of the axon membrane to Na+ and K+ is regulated and how changes in permeability to these ions affect the membrane potential?

-Na+ diffuses into cell -Gets depolarized -Voltage regulated Na+ gates open -More Na+ diffuses into the cell -Membrane potential will go from -70mv to +30mv -Then Na+ gate will close -K+ gates open and K+ diffuses out of cell which creates less depolarization and causes cell to repolarize and then hyperpolarize passing the threshold. Then resting potential is achieved.

Two types of Neuroglial cells in the PNS?

-Schwann cells -Satellite cells

Identify the myelin sheath and describe how it is formed in the PNS?

-Schwann cells attach and roll around axon -The cytoplasm is moved into the outer region of cell. -Will leave exposed gaps of axons called nodes of ranvier -Schwann cell provides insulation around the axon.

The supporting cells that form myelin sheaths in the peripheral nervous system are?

Schwann cells (C)

Functions of autonomic motor neurons?

Sends axons to the involuntary effectors - smooth muscle, cardiac muscle,and glands.

As the strength of a depolarizing stimulus to an axon is increased?

The frequency with which action potentials are produced increases. (D)

Functions of motor neurons (Efferent)?

Transmits impulses from the CNS to an effector organ

Explain how ligand gated channels produce synaptic potentials using the nicotinic ACh receptors as an example?

Two of the five polypeptide units of nicotinic ACh receptors contain ACh binding sites, and the channel opens when both sites bind to ACh. The opening of this channel permits the simultaneous diffusion of Na+ and K+ out of the postsynaptic cell. The flow of Na+ predominates because of the steeper electrochemical gradient. This produces the depolarization of an EPSP.

Functions of supporting cells?

-Aids in the function of neurons -Protects and supports neurons -Five time more abundant than neurons

Four Types of Neuroglial cells in the CNS?

-Astrocytes -Oligodendrocytes -Microglia -Ependymal

Explain how ligand-gated channels operate, using muscarinic ACh receptors as an example.

-Some ligand gated channels are actually opened by G-proteins subunits, and are separate proteins from the receptors for the neurotransmitters. That is true for muscarinic Ach receptors, which release G proteins subunits when Ach binds to these receptors. -These subunits diffuse through the membrane to regulate ion channels. In the heart, the beta gamma subunits cause K+ channels to open and K+ to diffuse outwards, increasing the membrane potential and thereby slowing the heart.

Function of a neuron?

-Specialized to respond to physical and chemical stimuli. -Conduct electrochemical impulses. -Release chemical regulators

Two divisions of the autonomic neurons?

-Sympathetic -Parasympathetic

Explain what is meant by the blood brain barrier?

-The brain capillaries impose a very selective BBB. -The structural components consist off.. -Tight junctions between endothelial cells -Restricts paracellular movement of molecules between epithelial cells -It is viewed as a dynamic structure

Explain how action potentials are produced?

1.) Stimulus causes depolarization to occur, allowing Na+ to enter the cells via Na+ channels. 2.) This causes the inside of the cell to be more positive because Na+ rushes down it's electrochemical gradient. 3.)This promotes positive feedback loop, because further depolarization causes more Na+ gates to open this causes entry of Na+ to accelerate. 4.) This reverses the membrane potential from -70 Mv to +30 Mv which closes Na+ channels. 5.) This instead causes K+ channels to open and and K+ will diffuse out of the cell to become more negative restoring the resting membrane back to -70 Mv this is repolarization

How many types of neurons is there?

3 types of neurons -Association (interneuron) -Sensory(Afferent neuron) -Motor(Efferent neuron)

A collection of neuron cell bodies located outside the CNS is called?

A ganglion

The conduction of action potentials in a myelinated nerve fiber is?

A) saltatory. B)Without decrement. C)Faster than in an unmyelinated fiber. D)All of these. ( the answer is all of these options) (D)

Location of autonomic motor neuron cell body?

Located outside the CNS in autonomic ganglia

Function of the axon hillock?

Where action potentials are generated at the initial segment of the axon.

Functions of axons?

Conducts action potentials (impulses) away from the cell body.

Describe the function of acetyl cholinesterase and discuss its physiological significance?

-Acetylcholinesterase is an enzyme that degrades Ach in the synaptic gap into acetate and choline, thereby stopping the ACh from binding to its receptor and regulating the postsynaptic cell. -This provides a way to turn off the effects of Ach, allowing physiological regulation to proceed when cholinergic neurons are stimulated and to cease when these neurons stop releasing further Ach.

Compare EPSP and action potentials, identify where each is produced in a neuron, and explain how action potential can be stimulated by EPSP?

-Action potentials occur in the axons where voltage-regulated channels are located, whereas EPSPs occur in the dendrites and cell body. -Action potentials have a threshold, EPSPs have no threshold, because the ACh released from a single synaptic vesicle produces a tiny depolarization of the postsynaptic membrane. -When more vesicles are stimulated to release ACh, the depolarization is correspondingly greater (meaning it is graded in magnitude). -This is not the case with all-or-none action potentials. Since EPSPs can be graded and have no refractory period, they are capable of summation. Action potentials are incapable of summating due to their all-or-none nature and refractory periods.

Describe the nature and significance of the Blood Brain Barrier?

-Brain cannot obtain molecules from blood plasma by a nonspecific process. -Molecules within brain capillaries must be moved through endothelial cells by diffusion and active transport and endo/exo-cytosis

What are the 3 principle region of a neuron?

-Cell body -Dendrites -Axon

What are the parts of a neuron?

-Cell body -Dendrites -Axon

Function of Nissl bodies?

-Composed of large stacks of rough endoplasmic reticulum -Needed for the synthesis of membrane proteins

Function, location, and structure of Sensory neurons?

-Conducts impulses from sensory receptors into the CNS -also Afferent and are pseudounipolar

Function, location, and structure of Motor neurons?

-Conducts impulses from the CNS to the target organs (muscles or glands) -Efferent and are multipolar

Function of the cell body?

-Contains the nucleus -Nutritional center where macromolecules are produced.

Describe the characteristics of action potentials and how are they conducted in unmyelinated axons?

-Every patch of membrane contains Na+ and K+ channels that can produce and action potential. -The spread of depolarization by the cable properties of an axon is fast compare to how long it takes to produce an action potential. -The more action potentials along a given stretch of axon that have to be produced, the slower the conduction. -Since action potentials must be produced at every fraction of a micrometer in an unmyelinated axon the conduction rate is relatively slow.

Define repolarization and illustrate the process?

-If the gated K+ channels were open, potassium ions will move out of the cell and the membrane potential moves towards the negative side. -Excess efflux of potassium causes hyperpolarziation.

Describe the characteristics of action potentials and how are they conducted in myelinated axons?

-In a myelinated axon there basically only Na+ channels concentrated in the nodes of ranvier. -Action potentials only occur at the nodes and leap from node to node (saltatory conduction). -Myelinated axons conduct impulses faster because the voltage gated channels are only at the nodes, and have more cable-like spread of depolarization (which is faster) and fewer sites where action potentials are produced (which is slower) than unmyelinated axons.

Define the structure and function of chemical synapses?

-It is one way and requires the release of neurotransmitter. -The neurotransmitters in the terminal boutons are released from the presynaptic membrane in synaptic vesicles. -They are passed across the synaptic cleft to the post synaptic membrane (which could be another neuron, or an effector organ) bind to receptor proteins and are taken in to the postsynaptic cell through ligand regulated gates which open into ion channels.

Function, location, and structure of Association neurons?

-Located within the CNS and integrates functions of the nervous system -They are multipolar

What are the two types of motor neurons?

-Somatic -Autonomic

Explain how G-protein-coupled channels produce synaptic potentials, using the muscarinic Ach receptor as an example?

-These are opened by the binding of a neurotransmitter to its receptor protein, but the receptor and ion channel are different separate membrane proteins. -The muscarinic ACh receptors are formed from a single subunit, and binds to one ACh molecule. When ACh binds to the receptor it activates G-proteins in the cell membrane (composed of alpha, beta and gamma subunits). Alpha subunit dissociates from the other two subunits (which stick together and form a beta-gamma complex). -Depending on the specific case, either the alpha of the beta-gamma complex then diffuses down the membrane until it binds to an ion channel, causing it to open or close. -This indirectly affects the permeability of K+ channels, causing either a depolarization or hyperpolarization.

Describe the structure of the sheath of Schwann and explain how it promotes regeneration?

-When the axon in a peripheral nerve is cut. The distal portion of the axon that was severed will degenerate and is phgocytosed by Schwann cells -Schwann cells; surrounded the basement membrane, then form a regeneration tube. -Schwann cells of the regeneration tube secrete chemicals that attract growing axon tips -The tube helps guide the regeneration axon to it's destination

Identify the nature of excitatory postsynaptic potentials?

-When the neurotransmitters causes the ion channels to open, it is considered excitatory because it can produce depolarization, which can produce an action potential. -This depends on the summation of ESPS, because if it doesn't produce a high enough depolarization it cant reach threshold and wont cause action potential to fire.

Describe the relationship between axon activity and the amount of neurotransmitters released?

-When there is a greater frequency of action potentials at the axon terminal, there is a greater entry of Ca2+, and thus a greater number of synaptic vesicles undergoing exocytosis and releasing neurotransmitter molecules. -As a result, a greater frequency of action potentials by the presynaptic axon will result in greater stimulation of the postsynaptic axon.

Describe the sequence of events by which action potentials stimulate the release of neurotransmitters from presynaptic axons?

1) Action potential conducted by axon reaches axon terminal 2) Voltage-gated Ca2+ channels open 3) Ca2+ binds to sensor protein in cytoplasm 4) Ca2+-protein complex stimulates fusion and exocytosis of neurotransmitter 5) Neurotransmitters are released from the synaptic vesicles in the synapse.

Function of sensory neuron (Afferent)?

Conducts impulses from a sensory receptor into the CNS

Compare the properties of EPSPs and action potentials, identify where in a neuron these are produced, and explain how EPSPs stimulate action potentials?

A) Action potentials are all or none, have refractory periods and cannot summate, and are conducted by axons without decrement. B) EPSP are graded, have no refractory period, can summate and are local (because they decrease in amplitude as they are conducted). C) EPSP's occur in the dendrites and cell body D)Action potentials begin at the initial segment of the axon. E) EPSP are produced by the binding of a neurotransmitter ligand to its receptor, where areas action potentials are produced by the openings of voltage gated channels in response to a threshold level of depolarization. EPSP serve as the depolarizing stimulus for the production of action potentials at the initial segment of the axon.

Which of these may be produced by the action of nitric oxide?

A) Dilation of blood vessels B) Erection of the penis C) Relaxation of smooth muscles in the digestive tract D) Long-term potentiation (LTP) among neighboring synapses in the brain e)All of these (E) all of these are correct

Describe the mechanism of action of glycine and GABA as neurotransmitters, and discuss their significance?

A) Glycine and GABA are inhibitory neurotransmitters that bind to and open Cl- channels. Pumps keep the intracellular Cl- concentration lower than the extracellular concentration, so as long as the membrane potential is less negative than the Cl- equilibrium potential, Cl- will diffuse inwards through the glycine or GABA channels and make the inside more negative, causing inhibition. B) IPSP in the spinal cord produced by glycine are important for the inhibition of antagonistic muscles and inhibitory effects produced by GABA in the cerebellum affect the activity of descending tracts that inhibit unwanted skeletal movements.

Identify two neural pathways in the brain that use dopamine as a neurotransmitter, and explain their significance?

A) Nigrostriatal pathway: Here axons of neurons in substrantia nigrae extends by the MFB (medial forebrain bundle) but ends in corpus striata, the complex called nigrostriatal. This pathway associates with the movements of the individual. Depletion of dopamine signaling to the nigrostriatal complex can lead to Parkinson's disease. B) Tuberoinfundibular pathway: Dopamine produces the neurons in tuberal nucleus i.e specific are of hypothalamus. The axons from this part extends through the system ending in the infundibulum (part of the pituitary gland). Dopamine signaling helps in suppression of the prolactin released in bloodstream of the pituitary gland

Explain how nitric acid is produced in the body, and describe its functions?

A) Nitric oxide (NO) is produced in the body from the amino acids L-arginine by the action of the enzyme nitric oxide synthetase. This enzyme and the production of nitric oxide, occurs in some post synaptic neurons where the NO can function as a neurotransmitter to cause smooth muscle relaxation. It may also be produced by certain post synaptic neurons and function as a retrograde neurotransmitter, affecting the pre synaptic neurons. Nitric oxide is produced in macrophages, where it can help kill bacteria. B)It's also produced in some smooth muscle cells of blood vessels, where it can cause vasodilation and increased blood flow.

Explain the nature of spatial and temporal summation at the synapse?

A) Results from the convergence (a number of axons synapsing onto one) of presynaptic axon terminals on the dendrites and cell body of a postsynaptic neuron. Since synaptic potentials are graded and lack refractory periods (unlike action potentials) this allows them to summate as they are conducted by the postsynaptic neuron. B)The successive activity of a presynaptic axon terminal causes successive waves of transmitter release, resulting in summation of EPSPs in the postsynaptic neuron. This summation helps to determine if the depolarization that reaches the axon hillock will be a sufficient magnitude to generate a new action potential.

Define spatial summation and temporal summation, and explain their functional importance?

A) Spatial summation occurs when many presynaptic axon terminals release neurotransmitters at the same time, so that EPSP are produced at different locations on the postsynaptic neuron. These different EPSP may summate as they converge together at the axon initial segment. B) Temporal summation occurs when a single presynaptic axon undergoes successive burst of activity, releasing neurotransmitters and producing EPSP in succession at the same location in the post synaptic neuron. C) These successive EPSP occur so rapidly that they may summate as they travel to the axon initial segment.

Explain the significance of glutamate in the brain and of NMDA receptors?

A) They function as the major excitatory as neurotransmitters in the CNS. They produce EPSPs and research has revealed that each of the the glutamate receptors encloses an ion channel, similar to the nicotinic ACh receptors. There are 3 subtypes of glutamate receptors, named after the molecules (other than glutamate) that they bind: 1) NMDA receptors 2) AMPA receptors 3) kainate receptors B) The ion channel will not open, simply by the binding of glutamate to its receptor. Two other conditions must be met at the same time: 1) NMDA receptor must also bind to glycine 2) The membrane must be partially depolarized at this time by a different neurotransmitter molecule that binds to a different receptor. Once open, the NMDA receptor channels permit the entry of Ca2+ and Na+ (and exit of K+) into the dendrites of the postsynaptic axon.

Functions of Ependymal cells?

Are epithelial cells that line the ventricles (cavities) of the brain and the central canal of the spinal cord.

Explain how cocaine and amphetamines produce their effects in the brain. What are the dangers of these drugs?

Cocaine and amphetamines block the reuptake if monoamines into presynaptic axon terminals, increasing the amount of these neurotransmitters in the synaptic gap and thereby promoting their stimulation of the postsynaptic neurons. Cocaine blocks the reuptake of a number of monoamines, but its action is primarily derived from its promotion of dopamine action in the brain. Amphetamines mostly block the reuptake of norepinephrine and thereby promote the action of norepinephrine in the brain.

Describe the relationship between dopaminergic neurons, Parkinson's disease, and schizophrenia?

Drugs used to treat schizophrenia act against the D2 subtype of dopaminergic receptor, which can cause side effects resembling Parkinson's disease. This suggests that overactivity of the mesolimbic dopamine pathways contributes to schizophrenia, which also helps explain why people suffering from Parkinson's disease could develop schizophrenia if they are treated with too much L-dopa (which increases dopaminergic transmission).

Explain how refractory periods affect the frequency of action potentials?

During refractory period, the nerve fiber does not respond to the stimuli because sodium channels are in closed state. If the refractory period is more means, the frequency of action potentials decrease and vice versa.

Functions of Schwann Cells?

Form myelin sheath (neurilemma) around peripheral axons

Function of Oligodendrocytes?

Form myelin sheaths around axons of the CNS

Which of these statements about ACh receptors is false?

G-proteins are needed to open ion channels for nicotinic receptors. (C)

What is the white matter part of the brain?

High concentration of axons that have myelin sheaths around them

What is the grey matter of the CNS composed of?

High concentrations of cell bodies and dendrites, which lack myelin sheaths.

Postsynaptic inhibition is produced by?

Hyperpolarization of the postsynaptic membrane. (B)

Define depolarization and illustrate the process?

If the gated Na+ channels open, sodium ions move into cell and the membrane potential moves to the positive side.

Explain the all-or-none law of action potentials?

If the membrane potential reaches a threshold level, then the action potential is generated. If the threshold potential is not reached, no action potential is generated, this is known as "all or none principle"

Explain the nature of inhibitory postsynaptic potentials?

If the neurotransmitter is inhibitory and causes hyperpolarization of post synaptic membrane, then IPSP (inhibitory post synaptic potential) is generated. This inhibits the action potential generation and does not allow the transmission of the impulse to the post synaptic neuron

Describe the effect of increased stimulus strength on action potential production?

If the stimulus strength is increased, the threshold potential difference can be achieved . This triggers the opening of fast sodium channels, so that a sharp depolarization phase is observed.

Define voltage regulated channels?

Voltage regulated channels respond towards the changes in the membrane potential. -For ex.) the voltage gates of calcium channels open as soon as the action potential reaches the axon terminal.

A drug that inactivates acetylcholinesterase?

Increases the ability of ACh to stimulate muscle contraction. (C)

Depolarization of an axon is produced by?

Inward diffusion of Na+

Describe the action and significance of acetyl cholinesterase?

It is an enzyme present on the post-synaptic membrane, or immediately outside the membrane with its active site facing the synaptic cleft. AChE hydrolyzes ACh into acetate and choline, inactivating ACh, and stopping the activity in the postsynaptic cell. Acetate and choline can reenter the presynaptic axon terminals and then be resynthesizes into ACh.

Describe long-term potentiation, explain how it is produced, and discuss its significance?

Long term potentiation (LTP) occurs when the excitability of a synapse increases due to high frequency stimulation of the pre synaptic neuron. This may be the result of increased insertion of AMPA receptors for glutamate into the post synaptic membrane. Increased AMPA receptors increase the ability of glutamate to depolarize the postsynaptic membrane, thereby increasing the ability of glutamate to stimulate its NMDA receptors. LTP is believed to represent a mechanism of learning at the individual synapse.

Functions of Microglia?

Migrate through the CNS and phagocytose foreign and degenerated material

Explain how monoamines are inactivated at the synapse and how this process can be clinically manipulated?

Monoamines are mostly inactivated by their reuptake, using membrane transporters, from the synaptic gap into the presynaptic axon terminals, followed by their inactivation by monoamine oxidase (Mao). Clinically, MAO inhibitors are used to treat the symptoms of Parkinson's disease by promoting the effects of dopamine, and MAO inhibitors are also used to reduce clinical depression.

List the monoamines and indicate their chemical relationships.

Monoamines are regulatory molecules derived from amino acids. Dopamine, norepinephrine, and epinephrine are derived from tyrosine and are in the subcategory of catecholamines. Serotonin is a monoamine derived from tryptophan, and histamine is a monoamine derived from histidine.

Which of these is not a characteristic of action potentials?

Na 1 and K 1 gates open at the same time. (C)

Define chemically regulated channels?

Neurotransmitters are the chemical substance released by the axon terminals. They bind to receptors present on the Pre/Post synaptic neurons of the effector organs. Then a cellular response is produced.

The absolute refractory period of a neuron?

Occurs during depolarization and the first part of the repolarization phase. (D)

Give examples of endogenous opioid polypeptides, and discuss their significance?

Opioids are endogenous polypeptide neurotransmitters that act like opium or morphine and include B-endorphin, enkephalins, and dynorphin. There is evidence that these neurotransmitter help reduce the transmission of pain, and that they induce feelings of well-being or even euphoria

Repolarization of an axon during an action potential is produced by?

Outward diffusion of K+

Explain how postsynaptic inhibition is produced and how IPSPs and EPSPs can interact?

Postsynaptic inhibition occurs as a result of the production of inhibitory postsynaptic potentials, which are hyperpolarization's of the postsynaptic membrane in response to inhibitory neurotransmitters. Other pre synaptic axons may release excitatory neurotransmitters at the same time, producing EPSP. The EPSP and IPSP produced at different synapse on a postsynaptic period, and decrease in amplitude as they are conducted. IPSP reduce the degree of depolarization of the EPSP, reducing or abolishing their ability to stimulate the postsynaptic neuron to produce action potentials.

Describe the mechanism of presynaptic inhibition?

Presynaptic inhibition of a neuron occurs when a different neuron makes an axoaxonic synapse with it. If this second neuron releases an inhibitory neurotransmitter, the hyperpolarization produced can abolish the regeneration of the action potential by the presynaptic axon. If the second neuron releases an excitatory neurotransmitter near the synapse. This can inactivate Ca2+ channels in the axon terminal and reduce its ability to release neurotransmitters.

Functions of node of ranvier?

Produces nerve impulses.

Function of dendrites?

Receives impulses and conducts graded impulses toward the cell body

Functions of somatic motor neurons?

Responsible for both reflex and voluntary control of skeletal muscles.

Which of these neurons are pseudounipolar?

Sensory neurons

Identify the nature of inhibitory postsynaptic potentials?

Sometimes ligand regulated channels produce a hyperpolarization , where the inside of the postsynaptic membrane becomes more negative, this inhibits the production of action potentials.

The summation of EPSPs from numerous presynaptic nerve fibers converging onto one postsynaptic neuron is called

Spatial summation. (A)

Functions of Satellite cells (ganglionic gliocytes)

Support neuron cell bodies within ganglia of the PNS

Describe where stimulatory and inhibitory effects of muscarinic ACh receptors occur and how these effects are produced?

The muscarinic Ach receptors in the heart cause an increase in the membrane potential, which slows the rate of depolarization required to produce the heartbeat, thereby slowing the heart rate. In the gastrointestinal tract, Ach receptors cause the alpha subunits of G-proteins to diffuse through the membrane and open channels for Na+ and Ca+, causing depolarization and contraction of smooth muscles and the secretion of glands

Describe the location of neurotransmitters?

The neurotransmitters are present in the storage synaptic vesicles of the axon terminals.

Explain how the monoamine neurotransmitters are inactivated at the synapse?

The stimulatory effects of the monoamines, like those of ACh, must be quickly inhibited as to maintain proper neural control. It is stopped by 1) reuptake of the neurotransmitter molecules from the synaptic cleft into the presynaptic axon terminal, then 2) degradation of the monoamine by an enzyme within the axon terminal called monoamine oxidase (MAO)

Explain how ligand gated channels are opened, using nicotinic ACh receptors as an ex?

The structure of nicotinic type of acetylcholine receptors contains 5 peptide subunits, which together from a central pore. If the acetylcholine binds to the receptor, the channel pore opens and allows the influx if sodium ions and simultaneous efflux of potassium ions. This results in depolarization of an excitatory postsynaptic potential (EPSP)

Which of these statements about catecholamines is false?

Their effects are increased by action of the enzyme catechol-O methyltransferase. (B)

Which of these is not a characteristic of synaptic potentials?

They are all-or-none in amplitude. (A)

Define the structure and function of electrical synapses?

They are synapses between two cells that are electrically coupled and joined by gap junctions. They are present in cardiac and some smooth muscle (ex.uterus) where they allow action potentials to spread from cell to cell, so the organ can contract as a unit.