Virginia Tech BMSP 2135 RAT #17

the location where a presynaptic neuron communicates with its target cell

synapse

the process by which a presynaptic neuron communicates with a postsynaptic neuron through either chemical or electrical synapses; allows voluntary movement, cognition, sensation, emotion, etc.

synaptic transmission

what are the two types of neuronal synapses?

electrical, chemical

which of the 2 types of neuronal synapses is more common?

chemical

synapse where the axon connects to soma

axosomatic

synapse where the axon connects to dendrite

axodendritic

synapse where the axon connects to another axon

axoaxonic

neuron before the synapse, the one sending a message

presynaptic neuron

neuron after the synapse, the one receiving a message

postsynaptic neuron

electrical synapse

- two cells are joined by this that allow the bidirectional flow of ions

- the axolemmas of the two neurons are nearly touching and the _____ ______________ contain precisely aligned channels that form pores through which ions and other small substances may travel, allowing the electrical current to flow directly from the axoplasm of one neuron to that of the next

gap junctions

unique features of electrical synapses

- synaptic transmission is bidirectional

- synaptic transmission is nearly _____________________

instantaneous

instantaneous and bidirectional electrical synapses are beneficial to heart and smooth muscles because it

- allows the activity of a group of cells to be synchronized—when stimulated, the cells will produce action potentials in unison

- they allow those tissues to engage in ____________________ _______________ _______________

coordinated muscle activity

type of synapse in which a presynaptic neuron releases neurotransmitters to trigger a change in the postsynaptic neuron

chemical synapse

chemical synapses

- unidirectional: the message can be sent only by the presynaptic neuron

- the current in electrical synapses eventually becomes (stronger/weaker) as it dissipates into the ECF, but a chemical synapse converts an electrical signal into a controlled chemical signal, so no strength is lost

weaker

chemical synapses

- The chemical signal is reconverted into an electrical signal in the ________________ ________________

postsynaptic neuron

differences between electrical and chemical synapses

synaptic vesicles

- The axon terminal of the presynaptic neuron of every chemical synapse houses synaptic vesicles.

- These vesicles contain chemical messengers called neurotransmitters that transmit signals from the presynaptic to the postsynaptic neuron.

- This is how the electrical signal of the action potential is converted into a _____________ ____________.

chemical signal

differences between electrical and chemical synapses

synaptic cleft

- Whereas the cells of an electrical synapse are electrically connected by gap junctions, the cells of a chemical synapse are separated by a larger but still microscopic space called the synaptic cleft.

- The synaptic cleft measures 20-50 nm and is filled with extracellular fluid and proteins such as _________________.

enzymes

differences between electrical and chemical synapses

neurotransmitter receptors

- In chemical synapses, the postsynaptic neuron must have receptors to which the neurotransmitters released by the presynaptic neuron can bind, or it cannot respond to the signal being transmitted.

- Receptors are generally linked either directly or indirectly to ion channels.

- This is how the chemical signal is converted back into an ____________ ____________.

electrical signal

differences between electrical and chemical synapses

These three features of chemical synapses (synaptic vesicles, synaptic cleft, NT receptors) cause them to transmit signals (faster/slower) than do electrical synapses.

slower

the short delay between the arrival of the AP at the axon terminal of a presynaptic neuron and the postsynaptic potential of a postsynaptic neuron

synaptic delay

With chemical synapses, the signal can vary in size. If more ______________________ are released, then the presynaptic neuron has a greater effect on the postsynaptic neuron. The signal in an electrical synapse, by contrast, will always be the same size.

neurotransmitters

chemical synapse action

(1) An action potential in the presynaptic neuron triggers calcium ion channels in the axon terminal to open.

(2) Influx of calcium ions causes synaptic vesicles to release neurotransmitters into the synaptic cleft.

(3) Neurotransmitters bind to receptors on the postsynaptic neuron.

(4) Ion channels open, leading to a _________ _________________ and possibly an action potential.

local potential

Neuromuscular junctions are always excitatory, they always moved the muscle closer to having an action potential. Is this true of local potentials?

no

A positive or negative change in the membrane potential of a neuron as a result of synaptic transmission

postsynaptic potential

a change in the membrane potential of a neuron in which it becomes less negative due to the influx of positive ions; the membrane potential at the trigger zone of the postsynaptic neuron moves closer to threshold

EPSP

a change in the membrane potential of a neuron in which it becomes more negative due to the outflow of positive ions or the influx of negative ions; small, local hyperpolarization, moves the membrane potential at the trigger zone farther away from threshold; inhibits an AP from firing

IPSP

importance of terminating synaptic transmission

After presynaptic neurons have generated a specific response in the postsynaptic neuron, the response cannot be initiated again until the _________________ _______________ stops being stimulated. In other words, it has to be "turned off" in order to be "turned on" again.

postsynaptic neuron

what are the 3 ways of terminating synaptic transmission?

diffusion and absorption, degradation in the synaptic cleft, reuptake into the presynaptic neuron

terminating synaptic transmission: 3 methods

diffusion and absorption

- Some neurotransmitters simply diffuse away from the synaptic cleft through the extracellular matrix, where they diffuse through the plasma membrane of a neuron or _________________ and are then returned to the presynaptic neuron.

astrocyte

the process by which a neuron integrates all of the postsynaptic potentials from multiple presynaptic neurons

neural integration

neural integration

- combination of all input from presynaptic neurons to have one cumulative effect on the what?

- process of putting together all the excitatory and inhibitory stimuli that determine whether a neuron will or won't fire an action potential

postsynaptic neuron

Adding the input from several postsynaptic potentials to affect the membrane potential at the trigger zone

summation

the additive effect of EPSPs triggered by a single presynaptic neuron that fires APs in rapid succession; occurs when NTs are released repeatedly from the axon terminal of a single presynaptic neuron

temporal summation

the additive effect of EPSPs triggered by multiple presynaptic neurons firing APs simultaneously; simultaneous release of NTs from the axon terminals of multiple presynaptic neurons

spatial summation

Local potentials can summate and depolarize the trigger zone to threshold, triggering an __________ _______________ in the postsynaptic neuron

action potential

The Big Picture of Chemical Synaptic Transmission

(1) action potential: an AP reaches the ________ _______________ of the presynaptic neuron

axon terminal

The Big Picture of Chemical Synaptic Transmission

(1) action potential: an AP reaches the axon terminal of the presynaptic neuron

(2) synaptic transmission: calcium channels open in the presynaptic neuron; NTs are released from __________ _______________ and bind to receptors on the postsynaptic neuron

synaptic vesicles

The Big Picture of Chemical Synaptic Transmission

(1) action potential: an AP reaches the axon terminal of the presynaptic neuron

(2) synaptic transmission: calcium channels open in the presynaptic neuron; NTs are released from synaptic vesicles and bind to receptors on the postsynaptic neuron

(3) postsynaptic potentials: NTs trigger an IPSP or EPSP, moving the membrane potential of the postsynaptic neuron either farther from or closer to __________________

threshold

The Big Picture of Chemical Synaptic Transmission

(3) postsynaptic potentials: NTs trigger an IPSP or EPSP, moving the membrane potential of the postsynaptic neuron either farther from or closer to threshold

(4) summation: multiple neurons trigger an IPSP and/or EPSP in the postsynaptic membrane. If enough local potentials summate at the trigger zone to reach threshold, voltage-gated ___________ channels open and the axon of the postsynaptic neuron will generate an AP.

sodium

The Big Picture of Chemical Synaptic Transmission

(4) summation: multiple neurons trigger an IPSP and/or EPSP in the postsynaptic membrane. If enough local potentials summate at the trigger zone to reach threshold, voltage-gated sodium channels open and the axon of the postsynaptic neuron will generate an AP.

(5) termination of synaptic transmission: NT concentration in the __________ ___________ decreases and synaptic transmission is terminated.

synaptic cleft

Nearly all neurotransmitters induce _______________ _________________ by binding to their receptors in the postsynaptic membrane

postsynaptic potentials

What are the two classes of neurotransmitter receptors?

ionotropic, metabotropic

2 classes of NT receptors

ionotropic receptors

- receptors that are part of what?

- called ionotropic because they directly control the movement of ions into or out of the neuron when bound by a neurotransmitter.

- neurotransmitters that bind ionotropic receptors have very rapid but short-lived effects on the membrane potential of the postsynaptic neuron

ligand-gated ion channels

2 classes of NT receptors

metabotropic receptors

- receptors within the plasma membrane that are connected to a separate ion channel in some fashion

- they are called metabotropic because they are directly connected to metabolic processes that begin when they are bound by neurotransmitters

- most metabotropic receptors are connected through a group of intracellular enzymes called what?

G-proteins

intracellular enzyme complexes involved in a variety of second-messenger systems

G-proteins

G-proteins

- when the neurotransmitter molecule binds to the receptor, it activates one or more G-proteins and begins a cascade of enzyme-catalyzed reactions

- the end result of the cascade is the formation of a compound inside the postsynaptic neuron called a ___________ _________________ (in this system the neurotransmitter molecule is considered the "first messenger").

second messenger

a chemical formed inside a cell that triggers some sort of change within the cell

second messenger

The second messenger then opens or closes a this in the plasma membrane of the postsynaptic neuron

ligand-gated ion channel

Which neurotransmitter have we already learned about? Is it typically excitatory or inhibitory?

acetylcholine, excitatory

What are synapses that have ACh called?

cholinergic synapses

What enzyme breaks down ACh?

acetylcholinesterase (AChE)

name 2 catecholamines

epinephrine, norepinephrine

catecholamines are called amines because they are synthesized from what?

amino acids (specifically tyrosine)

are catecholamines typically excitatory or inhibitory?

excitatory

Is GABA typically excitatory or inhibitory?

inhibitory

groups of interneurons in the CNS that process specific types of stimuli

neuronal pools

neuronal pools

- networks of neurons that perform a common function

- typically are a tangled mat of neuroglial cells, dendrites, and axons in the brain, whereas their ________ __________ may lie in other parts of the CNS

cell bodies

neuronal pools are organized into functional groups known as __________ ______________ (neural networks)

neural circuits

Each neuronal pool begins with one or more neurons called ________ ____________ that initiate the series of signals.

input neurons

The input neuron branches repeatedly to serve multiple neurons in the pool; however, it may have different effects on different neurons.

For some neurons, it may generate EPSPs that trigger an action potential, and for others, it may simply bring the trigger zone closer to threshold.

This difference is determined by the number of contacts the input neuron makes with the _______________ _____________

postsynaptic neuron

specific patterns of connection between neuronal pools

neural circuits

Each neuronal pool in a circuit receives input from other pools, and then produces __________ that travels to additional pools

output

what are the 2 types of neural circuits?

diverging, converging

neural circuits

diverging circuits

- a single input neuron contacts several output neurons

- begins with one axon of an input neuron branching to make contacts with multiple ________________ ___________

postsynaptic neurons

neural circuits

diverging circuits

- The axons of these postsynaptic neurons then branch to contact more neurons, which in turn make contact with yet more neurons, and so on.

- Diverging circuits are critical because they allow (multiple/a single) neuron(s) to communicate with multiple parts of the brain and/or body.

a single

neural circuits

converging circuits

- axon terminals from multiple input neurons converge onto a single postsynaptic neuron, allowing for __________ _________________ of synapses

spatial summation

neural circuits

converging circuits

- converging circuits are important for control of skeletal muscle movement—the interneurons in the spinal cord receive input from neurons in different regions of the brain, which then converges to synapse on the motor efferent neurons that stimulate skeletal muscle contraction.

- converging circuits also allow the ___________ __________ to respond to the sensory stimuli that it collects and processes

nervous system

To prevent electrical activity in the brain from becoming chaotic and overly excitatory, the CNS has two basic mechanisms to stabilize neural circuits.

what are the 2 mechanisms?

inhibitory circuits, synaptic fatigue

preventing neural circuits from becoming overly excitatory

inhibitory circuits

- most neural circuits have an intrinsic ____________ _____________ _________________ mediated by neuromodulators that inhibits either the input neurons or the postsynaptic neurons of their pools if they become overly excited.

- additionally, some neuronal pools consist largely of neurons that release neuromodulators that control the activity of other neural circuits.

negative feedback mechanism

_____________ refers to the fact that synaptic transmission becomes progressively weaker with prolonged and intense excitation due in part to a gradual decrease in the sensitivity of the postsynaptic neurons in the circuit to neurotransmitters when they are overused

Fatigue

preventing neural circuits from becoming overly excitatory

synaptic fatigue

- over the long term, the number of neurotransmitter receptors in the plasma membrane of the postsynaptic neuron actually (increases/decreases) when high levels of neurotransmitters are present for extended periods

- this "downregulation" of postsynaptic receptors is thought to be why people develop a tolerance to certain medications that modulate neurotransmitter release.

decreases

____________ is characterized by recurrent episodes of abnormal, disorganized electrical activity in the brain called seizures.

Epilepsy

A seizure results from a sudden burst of excitatory electrical activity within a ___________ _________, which may be triggered by instability in the membrane potential of a single neuron.

neuronal pool

epileptic seizures

- the excess excitation overwhelms the inhibitory circuits that would normally prevent overexcitation.

- once the inhibitory mechanisms are lost, a continuous wave of ____________ spreads over part of the brain (a partial seizure) or the entire brain (a generalized seizure)

excitation

what percentage of our brain do we use?

100%

halves of the cerebrum = __________________

hemispheres

the deep groove that separates the two cerebral hemispheres

longitudinal fissure

shallow grooves that separate the superficial parts of the cerebrum

sulci (sulcus)

deep grooves between major brain structures or lobes of the cerebrum

fissures (fissure)

elevated ridges between the sulci

gyri (gyrus)

our brain surface has some many wrinkles because, if our brain was smooth, we would require much larger ____________ to hold them

skulls

The cerebral hemispheres house 2 ventricles, the what?

the right and left lateral ventricles

what are the 5 lobes of the cerebrum?

frontal, parietal, occipital, temporal, insulas

lobes of the cerebrum

frontal lobes

- two anterior lobes of the cerebral hemispheres; responsible for planning and executing movement and complex mental functions

- posterior boundary is the _________ _____________

central sulcus

lobes of the cerebrum

frontal lobes

- just anterior to the central sulcus is the _______________ __________ = a gyrus located anterior to the central sulcus of each cerebral hemisphere in the frontal lobes; contains the primary motor cortex

precentral gyrus

lobes of the cerebrum

parietal lobes

- lobes of the cerebral hemispheres located posterior to the frontal lobes; responsible for processing and integrating sensory information, and also function in attention

- the major gyrus of each of these lobes is the ________________ ___________, which sits just posterior to the central sulcus

postcentral gyrus

lobes of the cerebrum

parietal lobes

- postcentral gyrus = a gyrus located posterior to the central sulcus of each cerebral hemisphere in the parietal lobes; contains the what?

primary somatosensory cortex

lobes of the cerebrum

temporal lobes

- cerebral lobes located on the lateral surface of the cerebrum; perform functions related to hearing, language, memory, and emotions

- separated from the frontal and parietal lobes by the ____________ _____________

lateral fissure

lobes of the cerebrum

occipital lobes

- the posterior lobes of the cerebrum; process all information relating to vision

- separated from the parietal lobe by the what?

parieto-occipital sulcus

lobes of the cerebrum

insulas

- deep lobes of the cerebrum; have functions relating to taste and to viscera

- only visible when you pry the frontal, parietal, and temporal lobes apart at the __________ _____________

lateral fissure

the ____________ _________ covers the cerebral hemispheres in the same way that bark covers a tree

cerebral cortex

Most of the cerebral cortex is _______________, or "new cortex," so named because it is the most recent part of our brains to have evolved.

neocortex

neocortex

- the neocortex is composed of _______ layers of neurons and neuroglia and varies in thickness from about 1.5 to 4.5 mm.

- the many convolutions of the gyri give the neocortex quite a large surface area—about 0.25 square meter (2.5 square feet).

6

Other components of the cerebrum and the remainder of the brain function largely in ___________________ _______________, that is, those carried out below the level of conscious thought

examples: control of BP and body temp

subconscious processes

subconscious processes are also called _______________ because they do not directly involve the cerebral cortex

subcortical

areas of the cerebral cortex that integrate different types of information

association areas

some association areas integrate only one type of information ("______________"), and others integrate several different types of stimuli ("multiple-task")

"single-task

interneurons located in the frontal lobe of the cerebral cortex that are involved in the conscious planning of movement; initiate movement via lower motor neurons of the PNS

upper motor neurons

this is located in the precentral gyrus of the frontal lobe and functions in conscious planning of movement

primary motor cortex

this is located anterior to the primary motor cortex in the frontal lobe and is involved in planning, guidance, coordination, and execution of movement

premotor cortex