Anatomy 2 Exam 1

Page 1

Day 1

Nervous system: communication and control system

Functions

Collect information

Sensory side

Receptors detect stimuli and send sensory signals to spinal cord and brain

Processes and evaluate information

Brain and spinal cord determine response to sensory input

Initiate response to information

Brain and spinal cord send motor output via nerves to effector

Muscles and glands

Structural organization

Central vs. Peripheral

Central nervous system is made up of brain and spinal cord

Perophrial nervous system is made uo from the nerves and ganglia, basically everything else

Functional organization

Sensory input – detects stimuli and transmits information from receptors to the CNS

Somatic – sensory input from 5 senses

Visceral – input from receptors of internal organs

Motor output – initiates and transmits information from CNS to effectors

Somatic motor – motor output to skeletal muscle

Autonomic motor – motor output to cardiac muscle, smooth muscle and glands

Nerves and ganglia

Nerve: a bundle of parrallel axons in the PNS (Nerves have connective tissue wrappins)

Epineruium: encloses entire nerve

Thick layer of dense irregular connective tissue

Perinerium: wraps fascicle (bundle of axons in nerve)

Layer of dense irregular connective tissue

Endoneurium: wraps an individual axon

Delicate layer of areolar connective tissue

Separates and electrically insulates each axon

Nerves are vascularized

Blood vessels branch through epineurium and perineurium to become capillaries; allow for exchange between axons and blood

Epineurium: encloses entire nerve

Thick layer of dense irregular connective tissue

Perineurium: wraps fascicle (bundle of axons in nerve)

Layer of dense irregular connective tissue

Endoneurium: wraps an individual axon

Structural classification of nerves

Cranial nerves: extend from brain

Spinal nerves: extend from spinal cord

Functional classification of nerves

Sensory nerves contain sensory neurons sending signals to CNS

Motor nerves contain motor neurons sending signals from CNS

Mixed nerves contain both sensory and motor neurons

Most named nerves are in this category

Individual axons in these nerves transmit only one type of information

A ganglion is a cluster of neuron cell bodies in the PNS

What is the function of receptors?

Collect information

Process information

Initiate response

What are the different types of effectors controlled by the nervous system?

Central nervous system

Skeletal muscle

Glands

The neuron is the structural and functional unit of the nervous system

Neuron characteristics

Excitability is responsive to a stimulus

Conductivity is the ability to propagate electrical signal

Secretion release of neurotransmitter in response to a conductive activity (action potential)

Extreme longevity is when a cell can live throughout a person's lifetime

Amitotic happens after fetal development, miotic activity is lost in most neurons

They stop dividing

Know classification of neurons

Unipolar

A single elongated process with the cell body located off to the side

Bipolar

2 processes separated by the cell body

Multipolar

Have more than 2 projections

Functionally

A sensory neuron has sensory input and the skin receptors send info back towards cns

Ganglia collection of cell body in PNS

Then spinal cord there is a inter neuron

Process is happening in spina cord and nervous system

Interneuron

Motor neurons

Motor output

Which term refers to the idea of neuron being responsive to a stimulus?

excitability

conductivity - would be allow signal t g down axon

Secretion – transmitter relating signal to next neuron

Where are interneurons located, and what is their function?

Receive, process, and integrate information from many other neurons

Communicate between sensory and motor neurons

Synapse: place where a neuron connects to another neuron or an effector

Chemical synapse

Pre-synaptic sends a signal and arrives at the bouton (end) which leads to excitation, icrease calcium, dumps into neurotransmitter and leads to post-synaptic cell (look at chart)

What is a chemical synapse within the nervous system, and how does it function?

A gap between 2 neurons

Siganl sent down from pre then sends message to antoer neron adn to anotehr through trasmitter

Glial Cells

Nonexcitable support cells found in CNS and PNS

Appoximately same number of glial cells as ther are neruons

Glial cells account ofr about half of the volume of nervous sytem

Capable of mitosis

Protect and nourish nerons

Provide physical scaffolding for nervous tissue

Guide migrating neruons durin deeoplment

Critical for normal function at neraul synapses

Astrocytes (star-shaped cells)

Have processes that end in perivascular feet

Most abundant glial cell in CNS

Slides 32-37 are important

Which glial cells exist in the CNS? Which in the PNS?

CNS: Astrocytes

Ependymal cells

Microglia

Oligodendrocytes

PNS: Satellite cells

Neurolemmocytes

Neoplasms (tumors) from unregulated cell growth

Sometimes occur in CNS

Tumors originating from the brain, primary brain tumors

Typically originate in supporting tissues

Gliomas, glial cell tumors

May be benign

May be malignant, capable of metastasizing

Day 2

Myelination

Process of wrapping an axon with myelin

Myelin: several layers of membrane of glial cells

High lipid content that gives it a glossy-white appearance and insulates axon

The glia are neurolemmocytes in PNS; oligodendrocytes in CNS

In the PNA

Neurolemmocyte encircles neuron axon and wraps it in layers forming myelin sheath

Neurolemmocyte's cytoplasm and nucleus are pushed to periphery forming neurilemma

A neurolemmocyte can myelinate only 1 mm of axon, so several are needed for one axon

The gaps between the neurolemmocytes are neurofibril node

7 neurolemmocytes

Myelination by oligodendrocytes

In the CNS

One oligodendrocyte can myelinate 1 mm of multiple axons, each at multiple spots

No neurolemma found

Nucleus hangs between axons

Neurofibril nodes between adjacent wrapped segments

Unmyelinated Axons still have glial cells around them, but they are not myelinated

In PNS

Axon sits in depressed portion of neurolemmocyte

In CNS

Unmyelinated axons not associated with oligodendrocytes

Neurolemmocytes- PNS

Oligodendrocyte- CNS

Nervous System disorders affecting myelin

Multiple sclerosis

Progressive demyelination of neurons in CNS

Autoimmune disorder: oligodendrocytes attacked by immune cells

Repeated inflammatory events causing scarring and permanent loss of function

If a person has a brain tumor, is it more likely to have developed from neurons or from glial cells? Why?

- Brain tumors are more likely to develop from glial cells rather than neurons. This is because glial cells can divide and proliferate throughout life, making them more prone to mutations that can lead to tumors. Neurons, on the other hand, typically do not divide after maturing.

If a person suffers from meningitis (an inflammation of the meningeal coverings around the brain), which type of glial cell usually replicates in response to the infection?

- In the case of meningitis, microglia are the type of glial cells that typically replicate in response to the infection. Microglia are the resident immune cells of the central nervous system and play a crucial role in the inflammatory response to infections like meningitis

Which specific type of glial cell forms a myelin sheath associated with axons in the PNS?

- neurolemmocyte

What is the function of the myelin sheath? How does myelination of axons occur in the PNS?

- The myelin sheath insulates nerve axons

In the PNS, neurolemmocyte wrap around axons to form the myelin sheath, starting in utero and continuing through adolescence.

Axon Regeneration

After traumatic injuries, PNS axons can regenerate

Regeneration is possible if

Neuron cell body is intact

Enough neurilemma remains

Where the nucleus is

Regeneration success is more likely if

Amount of damage is less extensive

Distance between site of damage and structure it innervates is shorter

Steps of axon regeneration

Axon severed by trauma

Proximal to the cut: the axon seals off and swells

Distal to the cut: the axon and sheath degenerate (Wallerian degeneration) but the neurilemma survives

Neurilemma and endoneurium form a regeneration tube

Axon regenerates guided by the nerve growth factors released by neurolemmocytes

Axon reinnervates original effector or sensory receptor

CNS axon regeneration is extremely limited

Oligodendrocytes secrete growth-inhibiting molecules; not growth factors

Large number of axons crowd the CNS

Regrowth obstructed by scars from teh astrocytes and connective tissue

What two primary factors determine the effectiveness of PNS axon regeneration?

Amount of damage is less extensive

Distance between site of damage and structure it innervates is shorter

How does the process of nerve regeneration occur in the PNS?

The injury response/site, if new axons form and grow towards the target issue, the forming of new myelin sheaths around them

Types of pumps and channels

Pumps

Membrane proteins that maintain a concentration gradient by moving substance against their concentration gradient

Require cellular energy

Neurons have sodium-potassium pumps and calcium pumps in their membranes

Channels

Protein pores in the membrane that allow ions to move down their concentration gradients (into or out of the cell)

When open they allow a specific type of ion to diffuse

Leak (passive) channels are always open for continuous diffusion

Chemically gated channels are normally closed, but open when neurotransmitter binds

Voltage-gated channels are normally closed, but open when membrane charge changes

Voltage-gated Na+ channels have activation and inactivation gates

Three states of voltage-gated Na+ channels

Resting state

Activation gate closed; inactivation gate open

Entry of Na+ prevented

Activation state

Activation gate open (due to voltage change); inactivation gate open

Na+ moves through channel

Inactivation state

Activation gate open; inactivation gate closed

Entry of Na+ prevented

This state lasts a short time – the channel quickly resets to resting state

Modality gated channels

Normally closed, but open in response to specific type of sensory stimulus

For example, change in temp, pressure, light

Found in membranes of sensory neurons that respond to changes in their environment

For example, some receptor neurons of the skin have modality gated channels that open in response to mechanical pressure

Entire plasma membrane of neuron

Leak channels (for example, K+ leak channels)

Na+/K+ pumps

Maintain resting membrane potential

Plasma membrane of functional segments in a neuron

Receptive segment (dendrite and cell body)

Chemically gated channels (for ex. Chemically gated Cl- channels)

Initial segment (axon hillock)

Voltage-gated Na+ channels and voltage-gated K+ channels

Conductive segment (axon and its branches)

Voltage-gated Na+ channels and voltage-gated K+ channels

Transmissive segment (synaptic knobs)

Voltage-gated Ca2+ channels and Ca2+ pumps

Release of neurotransmitter

Describe the three states of voltage-gated Na+ channels in neurons.

Closed (Resting): No sodium ions pass through.

Open (Activated): Sodium ions flow in, causing depolarization.

Inactivated: Stops sodium flow, ending the action potential.

Which functional segment of a neuron contains chemically gated channels? Which functional segments contain voltage-gated channels?

Chemically gated channels are in the receptive segment (dendrites and cell body).

Voltage-gated channels are in the initial segment (axon hillock), conductive segment (axon), and transmissive segment (synaptic knobs).

Neurons and Ohm’s Law

Electrical energy

Movement of charged particles

Neuron activity dependent upon electrical current

Voltage (V)

Amount of difference in electrical charge between two places, represents potential energy

Measured in volts or millivolts

Current (I)

Movement of charged particle across barrier separating them

Can be harnessed to do work

Transmit action potential, stimulate one cell downstream another cell

Resistance (R)

Opposition to movement of charged particle (that is, the barrier)

An increase in resistance lowers the current

Plasma membrane

Olm’s law

Current= voltage/resistance (I=V/R)

Current increases with larger voltage and smaller resistance

As applied to neurons

Charged particles are ions, and current is generated when ions diffuse through channels

Voltage exists across the membrane due to unequal distribution of ions

The membrane resists ion flow, and this resistance changes due to the actions of gated channels

Resistance decreases when channels open

Current is generated when ions diffuse through channels

Voltage exists across membrane due to unequal distribution of ions

Membrane resists ion flow and this resistance changes due to if the gate is open or close

Closed- high resistance

Open- low resistance

Neurons at rest

Characteristics of resting neurons

Ions are unevenly distributed across the plasma membrane due to the actions of pumps

Higher concentration of K+ in cytosol verses interstitial fluid (IF)

Higher concentrations of NA+,Cl-, CA2+ in IF than in cytosol

Calcium concentration gradient exists at synaptic knob

Gated channels are closed in the functional segments of the cell

There is an electrical charge difference across the membrane (called a membrane potential)

Cytosol is relatively negative compared to IF

Resting membrane potential (RMP) is typically –70mV

This difference can be measured with microelectrodes (one inside cell; one outside) and a voltmeter

Resting membrane potential (RMP)

K+ diffusion is the most important factor in setting RMP

K+ diffuses out of the cell due to its concentration gradient

K+ diffusion out is limited by the electrical gradient (the pull of the negative RMP on the positive ion)

If K+ were the only ion that leaked, RMP would be where the K+ concentration and electrical gradients are at equilibrium (-90mV)

Since there are a few Na+ leak channels, Na+ also influences RMP

Na+ diffuses in due to its concentration gradient and the electrical gradient

This small Na+ leakage means RMP is less negative (-70mV)

Resting membrane potential

The role of Na+/K+ pumps

By pushing 3 positive charges out and pushing in only 2, the pump contributes about –3mV (of the –70 mV total)

More importantly it remains the concentration gradients for these ions

What is the role of ions, the phospholipid bilayer, and plasma membrane channels in neurons relative to the concepts of current, voltage, and resistance?

Ions: Create electrical gradients.

Phospholipid Bilayer: Acts as a barrier, providing resistance.

Plasma Membrane Channels: Regulate ion flow, altering resistance and generating current

Describe the conditions of a neuron at rest in terms of the RMP; concentration gradients for Na+, K+, Cl– along the entire neuron and Ca2+ at the synaptic knob; and the state of the gated channels.

At rest, a neuron has:

RMP: ~-70 mV.

Concentration Gradients:

Na+: Higher outside.

K+: Higher inside.

Cl–: Higher outside.

Ca2+: Higher outside (synaptic knob).

Gated Channels: Most are closed, some K+ leak channels are open.

Explain how an RMP is established and maintained in neurons.

Ion Gradients: Differences in Na+ and K+ concentrations.

Selective Permeability: The membrane allows certain ions to pass more easily.

Na+/K+ Pump: Moves 3 Na+ out and 2 K+ in, maintaining gradients.

Leak Channels: K+ leak channels contribute to the negative charge inside.

Physiologic Events in the Neuron Segments

Different physiologic events occur at each neuron segments

Work in sequence from initial stimulation of dendrites/cell body to the release of neurotransmitter from the synaptic knob

Receptive Segment

Graded potentials are small, short-lived changes in the RMP with the following characteristics:

They are established in the receptive segment by the opening of chemically gated ion channels

Local current changes are short-lived (a few ms or less)

They vary in degree of change and direction of change of the RMP

They can be large or small

They can cause a depolarization or hyperpolarization

Graded potentials in a postsynaptic neuron = postsynaptic potentials

Postsynaptic potential resulting in depolarization is called an excitatory postsynaptic potential (EPSP)

Postsynaptic potential resulting in hyperpolarization is called an inhibitory postsynaptic potential (IPSP)

Numerous postsynaptic potentials can be generated because postsynaptic neuron can bind many neurotransmitter molecules simultaneously

Generation of an EPSP

Excitatory postsynaptic potentials (EPSPs) are depolarizations caused by Na+ entry

Inhibitory postsynaptic potentials (IPSPs) are hyperpolarization caused by K+ exit or Cl− entry

Initial Segment

Summation of EPSPs and IPSPs occurs at axon hillock

Voltage changes from the dendrites and soma are added

The sum may or may not reach threshold membrane potential for initiating an action potential

Threshold is the minimum voltage change required

Typically, threshold is about –55 mV

Generally, multiple EPSPs must be added to reach threshold

If threshold is reached at the axon hillock (initial segment)

Voltage-gated channels open, and an action potential is generated

Summation occurs across space and time

Spatial summation

Multiple locations on cell’s receptive regions receive neurotransmitter simultaneously and generate postsynaptic potentials

Temporal summation

A single presynaptic neuron repeatedly releases neurotransmitter and produces multiple EPSPs within a very short period of time

All-or-none law

If threshold reached, action potential generated and propagated down axon without any loss in intensity

If threshold not reached, (stimulus is subthreshold), voltage-gated channels stay closed, no action potential

The axon shows same intensity of response to values greater than threshold

Like what occurs with a gun

With sufficient pressure on trigger, gun fired

With insufficient pressure on trigger, not fired

Firing is the same even if trigger squeezed very hard

Conductive Segment

The axon (axolemma) conducts action potentials

Action potential involves depolarization and repolarization

Depolarization is gain of positive charge as Na+ enters through voltage-gated Na+ channels

Repolarization is return to negative potential as K+ exits through voltage-gated K+ channels

Action potential is propagated down axon to synaptic knob

Voltage-gated channels open sequentially down axolemma

Propagation is called a nerve signal or nerve impulse

Generation of an Action Potential: Depolarization and its Propagation

Action potential: steps in depolarization

As Na+ enters from adjacent region, voltage-gated Na+ channels open

Na+ enters the axon causing the membrane to have a positive potential

Action potential: steps in depolarization (continued )

Inactivation state: Na+ channels close becoming inactive (unable to open) temporarily

Steps 1-2 repeat in adjacent regions and the impulse moves toward synaptic knob

Action potential: steps in repolarization

Depolarization slowly opens K+ channels, and K+ diffuses out, causing negative membrane potential

K+ channels stay open for a longer time, so K+ exit makes cell more negative than RMP (hyperpolarization)

K+ channels eventually close and RMP is reestablished

Steps 3-5 repeat in adjacent regions as the impulse moves toward synaptic knob

Refractory period

Period after start of action potential when it is impossible or difficult to fire another action potential

Absolute refractory period (about 1 ms)

No stimulus can initiate another action potential

Na+ channels are open, then inactivated

Ensures propagation goes toward synaptic knob; doesn’t reverse direction

Relative refractory period (just after absolute)

Another action potential is possible (Na+ channels have reset) but the minimum stimulus strength is now greater

Some K+ channels are still open; cell is slightly hyperpolarized and further from threshold

Continuous vs. saltatory conduction

Continuous conduction occurs on unmyelinated axons

Charge opens voltage-gated channels, which allows charge to enter, which spreads to adjacent region and opens more channels, sequentially

Action potential occurs along entire length of axon

Saltatory conduction occurs on myelinated axons

Action potential occurs only at neurofibril nodes, which is where the axon’s voltage-gated channels are concentrated

After Na+ enters at a node it starts a rapid positive current down the inside of the axon’s myelinated region

The current becomes weaker with distance, but still strong enough to open voltage-gated channels at the next node

Action potential occurs at the node, and the process repeats at the next node; impulse appears to jump from node to node

Saltatory conduction is much faster than continuous conduction and myelinated cells use less ATP to maintain resting membrane potential

Transmissive Segment

Activity at the synaptic knob

Arrival of action potential opens voltage-gated Ca2+ channels

Ca2+ diffuses into knob (Ca2+ pumps had established gradient)

Ca2+ binds to proteins associated with synaptic vesicles and triggers exocytosis

Vesicles fuse with membrane, neurotransmitter released

Neurotransmitter binds to ligand receptors

Neurons can synthesize more than one neurotransmitter

Only one type is released at a time

Dependent on frequency of action potentials the reach the synaptic knob

How are EPSPs and IPSPs established in the receptive segment of a neuron?

What is the significance of the threshold membrane potential in the initial segment of a neuron?

How do depolarization and repolarization occur in the conductive segment of a neuron?

Explain propagation of an action potential – in both an unmyelinated axon and a myelinated axon.

What is the sequence of events from the arrival of the propagated action potential (a nerve signal) at the synaptic knob until the release of neurotransmitter into the synaptic cleft?

Graded Potentials Versus Action Potentials

Graded potentials

Occur in neuron’s receptive region due to ion flow through chemically gated channels

Can be positive or negative changes in charge

Are graded: have larger potential change to stronger stimulus

Are local (travel only a short distance)

Action potentials

Occur on neuron’s conductive region (axon) due to ion flow through voltage-gated channels

Involve depolarization (Na+ in) then repolarization (K+ out)

Are all or none once threshold is reached

Propagate down entire axon to synaptic knob

Velocity of Action Potential Propagation

Conduction speed depends on axon diameter and myelination

Thicker fibers conduct faster than thin ones

Thick axons offer less resistance to current flow down the axon

Myelinated fibers conduct much faster than unmyelinated ones

Current flow under myelin (between nodes) is very fast

Nerve fiber groups

Nerve fiber: an axon and its myelin sheath

Group A: conduction velocity as fast as 150 m/sec

Large diameter, myelinated fibers

For example, most somatic sensory neurons; all somatic motor neurons

Group B conducts at 15 m/sec; Group C at 1 m/sec

Small diameter and/or unmyelinated

For example, some visceral neurons; some somatic sensory neurons from skin

Frequency of Action Potentials

Impulse frequency (action potentials per second) varies

Although impulse amplitude (change in voltage) is constant, firing frequency varies with stimulus strength

Bright lights cause faster firing frequency on the optic nerve than dim lights

When motor nerves fire at faster frequency it causes muscle to generate more tension

For neurons that can use different neurotransmitters, the firing frequency can influence the type of transmitter released

Explain how action potentials differ from graded potentials.

What are the general characteristics of group A nerve fibers, and what functions do they normally serve?

Explain how frequency of action potentials differs from the velocity of an action potential propagation.

Classification of Neurotransmitters

Neurotransmitters:

Are synthesized by neurons and stored within vesicles in synaptic knobs

Are released from the vesicles when action potential triggers calcium entry into synaptic knob

Bind to a receptor in a target cell

Trigger a physiologic response in the target cell

Approximately 100 different ones have been named and classified

Four main chemical classes of neurotransmitters:

Acetylcholine

Structure differs substantially from other transmitters

Biogenic amines (monoamines)

An amino acid is slightly modified to synthesize the transmitter

Catecholamines (For example, dopamine) are made from tyrosine

Indolamines (For example, serotonin) are made from histidine or tryptophan

Amino acids

Include common transmitters glutamate, glycine, GABA

Neuropeptides (peptides)

Chains of amino acids (2 to 40 amino acids long) including endorphins, substance P

Neurotransmitters are also classified by function

Classification by effect on membrane potential

Excitatory transmitters cause EPSPs; inhibitory transmitters cause IPSPs

But some transmitters can excite some targets and inhibit others depending on the specific receptor present on the target cell

Classification by action

Direct transmitters bind to receptors that are chemically gated channels (immediate postsynaptic potential)

Indirect transmitters bind to receptors that involve G-proteins and second messengers

More diverse effects possible

Features of Neurotransmitters

Acetylcholine (ACh) is best characterized neurotransmitter

Used in PNS to stimulate skeletal muscle; used in the CNS to increase arousal

Synthesized from acetate and choline; stored in synaptic vesicles

Action potential triggers its release into cleft

Some ACh attaches (briefly) to postsynaptic receptor

ACh is cleared from cleft by being broken down to acetate and choline by acetylcholinesterase

Acetate and choline are taken up by presynaptic cell for recycling

Effect on target cell depends on receptor present

Nicotinic vs. muscarinic receptors

Removal of neurotransmitter from the synaptic cleft can occur in varied ways

Enzymes might degrade neurotransmitter

Presynaptic transporters might import neurotransmitter (“reuptake”)

Some neurotransmitter diffuses away from synapse, reabsorbed by glia

Some drugs have their effect by influencing transmitter removal

For example, selective serotonin reuptake inhibitors treat depression

For example, galantamine hydrobromide is an acetylcholinesterase inhibitor used to treat Alzheimer disease

Interaction with target cells

Effect of ACh on target cell depends on type of receptor embedded within membrane of target cell

ACh binds nicotinic receptors (inotropic receptors) to open ion channels and cause EPSP

ACh binds muscarinic receptors (metabotropic receptors) to activate second messenger system involving G protein

Can result in EPSP or IPSP

Neuromodulation

Neuromodulators—chemicals that alter responses of local neurons

Facilitation

Modulation that causes greater response in postsynaptic neuron

May increase amount of neurotransmitter in cleft or number of postsynaptic receptors

Inhibition

Modulation that causes weaker response

May decrease amount of neurotransmitter in cleft or number of postsynaptic receptors

Nitric oxide

Might be a neurotransmitter or a modulator

Is a short-lived, nonpolar gas

Made and released by postsynaptic neurons in brain where it is believed to strengthen memory by affecting presynaptic cells

Effects in the PNS include blood vessel dilation

Endocannabinoids

Influence same receptors that marijuana does

Small, nonpolar molecules

Made and released by postsynaptic neurons

Have effects on presynaptic neurotransmitter release

Influence memory, appetite

Describe how neurotransmitters are classified based upon structure and function.

How is it possible for acetylcholine to generate either an EPSP or an IPSP?

How does nitric oxide act as a neuromodulator?

Neural Integration and Neuronal Pools of the CNS

Neuronal pools (neuronal circuits)

Groups of neurons arranged in specific patterns

Four types of circuits

Converging

Diverging

Reverberating

Parallel-after-discharge

Pool may be localized or distributed in several regions of CNS

Converging Circuit

Input converges at a single postsynaptic neuron

For example, multiple sensory inputs synapse on neurons in the salivary nucleus. Sights, sounds, and smells of cooking lead to one output: salivation.

Diverging Circuit

Spreads information from one presynaptic neuron to several postsynaptic neurons

For example, neurons in the brain that control walking send commands to several different muscles for proper balance, posture, and motion

Reverberating Circuit

Use feedback to produce repeated, cyclical activity

Once started, it stays active until there is an inhibitory stimulus or synaptic fatigue

For example, circuits that keep us breathing regularly during sleep

Parallel-After-Discharge Circuit

Input transmitted simultaneously along several paths to a postsynaptic cell

Since paths vary in number of synapses, signal arrives at postsynaptic cell at various times

Believed to be involved in higher-order thinking

How are neurons arranged in a converging circuit?

What are the differences between a reverberating circuit and a parallel-after-discharge circuit?