Describe the anatomical and functional divisions of the nervous system. 12-2 Sketch and label the construction of a typical nerve cell. depict the maps of each constituent. and classify nerve cells on the footing of their construction and map. 12-3 Describe the locations and maps of the assorted types of neuroglia. 12-4 Explain how the resting potency is created and maintained. 12-5 Describe the events involved in the coevals and extension of an action potency. 12-6 Discuss the factors that affect the velocity with which action potencies are propagated. 12-7 Describe the construction of a synapse. and explicate the mechanism involved in synaptic activity. 12-8 Describe the major types of neurotransmitters and neuromodulators. and discourse their effects on postsynaptic membranes. 12-9 Discuss the interactions that enable information processing to happen in nervous tissue.

The Nervous System
Includes all nervous tissue in the organic structure
Nervous tissue contains two sorts of cells
Nerve cells
Cells that send and receive signals
Neuroglia ( glial cells )
Cells that support and protect nerve cells

Variety meats of the Nervous System
Brain and spinal cord
Centripetal receptors of sense variety meats ( eyes. ears. etc. )
Nervousnesss connect nervous system with other systems

12-1 Divisions of the Nervous System
Anatomic Divisions of the Nervous System
Central nervous system ( CNS )
Peripheral nervous system ( PNS )

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The Central Nervous System ( CNS )
Consists of the spinal cord and encephalon
Contains nervous tissue. connective tissues. and blood vass Functions of the CNS are to treat and organize:
Sensory informations from inside and outside organic structure
Motor bids control activities of peripheral variety meats ( e. g. . skeletal musculuss ) Higher maps of encephalon intelligence. memory. acquisition. emotion

The Peripheral Nervous System ( PNS )
Includes all nervous tissue outside the CNS
Functions of the PNS
Deliver centripetal information to the CNS
Carry motor bids to peripheral tissues and systems

Nervousnesss ( besides called peripheral nervousnesss )
Packages of axons with connective tissues and blood vass
Carry centripetal information and motor bids in PNS
Cranial nervousnesss — connect to encephalon
Spinal nervousnesss — attach to spinal cord

Functional Divisions of the PNS
Afferent division
Carries centripetal information
From PNS centripetal receptors to CNS
Efferent division
Carries motor bids
From CNS to PNS musculuss and secretory organs

Receptors and effecters of afferent division
Detect alterations or respond to stimuli
Nerve cells and specialised cells
Complex sensory variety meats ( e. g. . eyes. ears )
Respond to efferent signals
Cells and variety meats

Somatic nervous system ( SNS )
Controls voluntary and nonvoluntary ( physiological reactions ) musculus skeletal contractions Autonomic nervous system ( ANS )
Controls subconscious actions. contractions of smooth musculus and cardiac musculus. and glandular secernments Sympathetic division has a stimulating consequence
Parasympathetic division has a relaxing consequence
12-2 Nerve cells
Nerve cells

The basic functional units of the nervous system
The construction of nerve cells
The Cell Body
Neurofilaments and neurotubules in topographic point of microfilaments and microtubules Neurofibrils: packages of neurofilaments that provide support for dendrites and axon Nissl organic structures
Dense countries of RER and ribosomes
Make nervous tissue appear grey ( grey affair )

Highly branched
Dendritic spinal columns
Many all right procedures
Receive information from other nerve cells
80–90 % of neuron surface country
The axon
Is long
Carries electrical signal ( action potency ) to aim

Axon construction is critical to work
Cytoplasm of axon
Contains neurofibrils. neurotubules. enzymes. cell organs
Specialized cell membrane
Covers the axoplasm
Axon knoll

Thick subdivision of cell organic structure
Attaches to initial section
Initial section
Attaches to axon knoll
Branchs of a individual axon
All right extensions of distal axon
Synaptic terminuss
Tips of telodendria

The synapse
Area where a nerve cell communicates with another cell

The synapse
Presynaptic cell
Neuron that sends message
Postsynaptic cell
Cell that receives message
The synaptic cleft
The little spread that separates the presynaptic membrane and the postsynaptic membrane

The synaptic terminus
Is expanded country of axon of presynaptic nerve cell
Contains synaptic cysts of neurotransmitters

Are chemical couriers
Are released at presynaptic membrane
Affect receptors of postsynaptic membrane
Are broken down by enzymes
Are reassembled at synaptic terminus

Recycling Neurotransmitters
Axoplasmic conveyance
Neurotubules within the axon
Transport natural stuffs
Between cell organic structure and synaptic terminus
Powered by chondriosome. kinesin. and dynein

Types of Synapsiss
Neuromuscular junction
Synapse between nerve cell and musculus
Neuroglandular junction
Synapse between nerve cell and secretory organ

Structural Classification of Neurons
Anaxonic nerve cells
Found in encephalon and sense variety meats ; little ; all cell procedures look alike Bipolar nerve cells
Found in particular centripetal variety meats ( sight. odor. hearing ) ; little ; one dendrite/ one axon Unipolar nerve cells

Found in centripetal nerve cells of PNS
Besides called pseudounipolar nerve cells
Have really long axons
Fused dendrites and axon
Cell organic structure to one side
Multipolar nerve cells
Park in the CNS
Include all skeletal musculus motor nerve cells ; really long axons ; multiple dendrites ; one axon

Three Functional Classifications of Nerve cells
Centripetal nerve cells – afferent nerve cells of PNS
Motor nerve cells – motorial nerve cells of PNS
Interneurons – association nerve cells

Functions of Sensory Neurons
Monitor internal environment ( splanchnic sensory nerve cells )
Monitor effects of external environment ( bodily sensory nerve cells ) Structures of Sensory Neurons
Cell organic structures grouped in centripetal ganglia
Procedures ( afferent fibres ) extend from centripetal receptors to CNS

Three Types of Sensory Receptors
Monitor internal systems ( digestive. respiratory. cardiovascular. urinary. generative ) Internal senses ( gustatory sensation. deep force per unit area. hurting )
External senses ( touch. temperature. force per unit area )
Distance senses ( sight. odor. hearing )
Monitor place and motion ( skeletal musculuss and articulations )

Motor Nerve cells
Carry instructions from CNS to peripheral effecters
Via motor nerve fibres ( axons )

Two major motor nerve systems
Somatic nervous system ( SNS )
Includes all bodily motor nerve cells that innervate skeletal musculuss Autonomic ( splanchnic ) nervous system ( ANS )
Visceral motor nerve cells innervate all other peripheral effecters Smooth musculus. cardiac musculus. secretory organs. adipose tissue

Preganglionic fibres
Postganglionic fibres

Most are located in encephalon. spinal cord. and autonomic ganglia Between sensory and motor nerve cells
Are responsible for:
Distribution of centripetal information
Coordination of motor activity
Are involved in higher maps
Memory. planning. acquisition

12-3 Neuroglia
Half the volume of the nervous system
Many types of neuroglia in CNS and PNS

Four Types of Neuroglia in the Central Nervous System
Ependymal Cells – extremely branched procedures ; contact neuroglia straight Form epithelial tissue called ependyma
Line cardinal canal of spinal cord and ventricles of encephalon
Secrete cerebrospinal fluid ( CSF )
Have cilia or microvilli that circulate CSF
Monitor CSF
Contain root cells for fix

2. Astrocytes – big organic structure with many procedures
Maintain blood–brain barrier ( isolates CNS )
Create 3-dimensional model for CNS
Repair damaged nervous tissue
Guide nerve cell development
Control interstitial environment

3. Oligodendrocytes – little cell organic structure with few procedures
Additions velocity of action potencies
Myelin insulates myelinated axons
Makes nervousnesss appear white
White affair – Regions of CNS with many medullated nervousnesss
Grey affair – Unmyelinated countries of CNS

Nodes and internodes
Internodes – myelinated sections of axon
Nodes ( besides called nodes of Ranvier )
Gaps between internodes
Where axons may ramify

Microglia – smallest & A ; least legion neuroglia
Migrate through nervous tissue
Clean up cellular dust. waste merchandises. and pathogens

Neuroglia of the Peripheral Nervous System
Multitudes of nerve cell cell organic structures
Surrounded by neuroglia
Found in the PNS

Neuroglia of the Peripheral Nervous System
1. Satellite cells
Besides called amphicytes
Surround ganglia
Regulate environment around nerve cell

2. Schwann cells
Besides called neurilemma cells
Form medulla sheath ( neurilemma ) around peripheral axons
One Schwann cell sheaths one section of axon
Many Schwann cells sheath full axon

Nerve cells and Neuroglia
Nerve cells perform:
All communicating. information processing. and control maps of the nervous system Neuroglia preserve:
Physical and biochemical construction of nervous tissue
Neuroglia are indispensable to:
Survival and map of nerve cells

Nervous Responses to Injuries
Wallerian devolution
Axon distal to injury perverts
Schwann cells
Form way for new growing
Wrap new axon in medulla

Nerve Regeneration in CNS
Limited by chemicals released by astrocytes that:
Block growing
Produce cicatrix tissue

12-4 Transmembrane Potential
Ion Motions and Electrical Signals
All plasma ( cell ) membranes produce electrical signals by ion motions Transmembrane potency is peculiarly of import to nerve cells

Five Main Membrane Processes in Neural Activities
Resting possible
Graded possible
Action potency
Synaptic activity
Information processing

The Transmembrane Potential – transmembrane potency of resting cell Three of import constructs
The extracellular fluid ( ECF ) and intracellular fluid ( cytosol ) differ greatly in ionic composing Concentration gradient of ions ( Na+ . K+ )
Cells have selectively permeable membranes
Membrane permeableness varies by ion

Passive Forces moving Across the Plasma Membrane
Chemical gradients
Concentration gradients ( chemical gradient ) of ions ( Na+ . K+ ) Electrical
Separate charges of positive and negative ions
Consequence in possible difference

Electrical Currents and Resistance
Electrical current
Motion of charges to extinguish possible difference
The sum of current a membrane restricts

The Electrochemical Gradient
For a peculiar ion ( Na+ . K+ ) is:
The amount of chemical and electrical forces
Acting on the ion across a plasma membrane
A signifier of possible energy
Equilibrium Potential
The transmembrane potency at which there is no net motion of a peculiar ion across the cell membrane Examples: K+ = –90 millivolt ; Na+ = +66 millivolt

Active Forces across the Membrane
Sodium–potassium ATPase ( exchange pump )
Is powered by ATP
Carries 3 Na+ out and 2 K+ in
Balances inactive forces of diffusion
Maintains resting possible ( –70 millivolt )

The Resting Potential
Because the plasma membrane is extremely permeable to potassium ions: The resting potency of about –70 millivolt is reasonably close to –90 millivolt. the equilibrium potency for K+ The electrochemical gradient for Na ions is really big. but the membrane’s permeableness to these ions is really low Na+ has merely a little consequence on the normal resting possible. doing it merely somewhat less negative than the equilibrium potency for K+ The sodium–potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell It serves to stabilise the resting potency when the ratio of Na+ entry to K+ loss through inactive channels is 3:2 At the normal resting possible. these inactive and active mechanisms are in balance The resting possible varies widely with the type of cell

A typical nerve cell has a resting potency of about –70 millivolt

Changes in the Transmembrane Potential
Transmembrane possible rises or falls
In response to impermanent alterations in membrane permeableness
Resulting from opening or shuting specific membrane channels

Sodium and Potassium Channels
Membrane permeableness to Na+ and K+ determines transmembrane possible They are either inactive or active
Passive Channels ( Leak Channels )
Are ever open
Permeability alterations with conditions
Active Channels ( Gated Channels )
Open and near in response to stimuli
At resting possible. most gated channels are closed

Three States of Gated Channels
Closed. but capable of opening
Open ( activated )
Closed. non capable of gap ( inactivated )

Three Classs of Gated Channels
Chemically Gated Channelss
Open in presence of specific chemicals ( e. g. . ACh ) at a binding site Found on nerve cell cell organic structure and dendrites

Voltage-gated Channelss
Respond to alterations in transmembrane potency
Have activation Gatess ( unfastened ) and inactivation Gatess ( near )
Characteristic of excitable membrane
Found in nervous axons. skeletal musculus sarcolemma. cardiac musculus

Mechanically Gated Channels
Respond to membrane deformation
Found in centripetal receptors ( touch. force per unit area. quiver )

Transmembrane Potential Exists Across Plasma Membrane
Cytosol and extracellular fluid have different chemical/ionic balance The plasma membrane is selectively permeable
Transmembrane Potential
Changes with plasma membrane permeableness
In response to chemical or physical stimulations

Graded Potentials – impermanent
Besides called local potencies
Changes in transmembrane potency
That can non distribute far from site of stimulation
Any stimulation that opens a gated channel
Produces a ranked potency

The resting province
Opening Na channel produces graded possible
Resting membrane exposed to chemical
Sodium channel clears
Sodium ions enter the cell
Transmembrane possible rises
Depolarization occurs

A displacement in transmembrane possible toward 0 millivolts
Motion of Na+ through channel
Produces local current
Depolarizes nearby plasma membrane ( ranked potency )
Change in possible is relative to stimulus

Whether depolarizing or hyperpolarizing. portion four basic features The transmembrane potency is most changed at the site of stimulation. and the consequence decreases with distance The consequence spreads passively. due to local currents

The ranked alteration in transmembrane potency may affect either depolarisation or hyperpolarization The belongingss and distribution of the membrane channels involved determine the nature of the alteration For illustration. in a resting membrane. the gap of Na channels causes depolarisation. whereas the gap of K channels causes hyperpolarization The alteration in transmembrane possible reflects whether positive charges enter or leave the cell The stronger the stimulation. the greater the alteration in the transmembrane potency and the larger the country affected

When the stimulation is removed. transmembrane possible returns to normal Hyperpolarization
Increasing the negativeness of the resting possible
Consequence of opening a K channel
Opposite consequence of opening a Na channel
Positive ions move out. non into cell

Effectss of ranked potencies
At cell dendrites or cell organic structures
Trigger specific cell maps
For illustration. exocytosis of glandular secernments
At centrifugal terminal home base
Release ACh into synaptic cleft

12-5 Action Potential
Action Potentials – electrical urge ; travels along membrane of axon ; initiated by ranked potency Propagated alterations in transmembrane potency
Affect an full excitable membrane
Link graded potencies at cell organic structure with centrifugal terminal home base actions

Originating Action Potential
Initial stimulation
A ranked depolarisation of axon knoll big plenty ( 10 to 15 millivolt ) to alter resting possible ( –70 millivolt ) to threshold degree of voltage-gated Na channels ( –60 to –55 millivolt )

All-or-none rule
If a stimulation exceeds threshold sum
The action potency is the same
No affair how big the stimulation
Action potency is either triggered. or non

Four Stairss in the Generation of Action Potentials
Measure 1: Depolarization to threshold
Measure 2: Activation of Na+ channels
Rapid depolarisation
Na+ ions rush into cytol
Inner membrane alterations from negative to positive

Measure 3: Inactivation of Na++ channels and activation of K+ channels At +30 millivolt
Inactivation Gatess near ( Na+ impart inactivation )
K+ channels open
Repolarization Begins

Measure 4: Tax return to normal permeableness
K+ channels begin to shut
When membrane reaches normal resting possible ( –70 millivolt )
K+ channels finish shutting
Membrane is hyperpolarized to –90 millivolt
Transmembrane possible returns to resting degree
Action potency is over

The Refractory Period
The clip period
From get downing of action potency
To return to resting province
During which membrane will non react usually to extra stimulations Absolute Refractory Period
No action potency possible
Relative Refractory Period
Very big stimulation can originate action potency

Powering the Sodium–Potassium Exchange Pump
To keep concentration gradients of Na+ and K+ over clip
Requires energy ( 1 ATP for each 2 K+/3 Na+ exchange )
Without ATP
Nerve cells stop operation

Propagation of Action Potentials
Moves action potencies generated in axon knoll
Along full length of axon
Two methods of propagating action potencies
Continuous extension ( unmyelinated axons )
Saltatory extension ( medullated axons )

Continuous Propagation
Of action potencies along an unmyelinated axon
Affects one section of axon at a clip
Stairss in extension
Measure 1: Action potency in section 1
Depolarizes membrane to +30 millivolt

Local current
Measure 2: Depolarizes 2nd section to threshold
Second section develops action potency
Measure 3: First section enters stubborn period
Measure 4: Local current depolarizes next section
Cycle repetitions
Action possible travels in one way ( 1 m/sec )

Saltatory Propagation
Action potency along myelinated axon
Faster and uses less energy than uninterrupted extension
Myelin insulates axon. prevents uninterrupted extension
Local current “jumps” from node to node
Depolarization occurs merely at nodes

12-6 Axon Diameter and Speed
Axon Diameter and Propagation Speed
Ion motion is related to cytoplasm concentration
Axon diameter affects action possible velocity
The larger the diameter. the lower the opposition

Three Groups of Axons – classified by diameter. myelination. velocity of action potency

Type A Fibers
Large diameter
High velocity ( 140 m/sec )
Carry rapid information to/from CNS
For illustration. place. balance. touch. and motor urges

Type B Fibers
Medium diameter
Medium velocity ( 18 m/sec )
Carry intermediate signals
For illustration. centripetal information. peripheral effecters

Type C Fibers
Small diameter
Slow velocity ( 1 m/sec )
Carry slower information
For illustration. nonvoluntary musculus. secretory organ controls

“Information” travels within the nervous system
As propagated electrical signals ( action potencies )
The most of import information ( vision. balance. motor bids ) Is carried by large-diameter. myelinated axons

12-7 Synapsiss
Synaptic Activity
Action potencies ( nervus urges ) are transmitted:
From presynaptic nerve cell
To postsynaptic nerve cell ( or other postsynaptic cell )
Across a synapse

Two Types of Synapsiss
Electrical synapses
Chemical synapses

Electrical Synapses – direct physical contact between cells Are locked together at spread junctions ( connexons )
Allow ions to go through between cells
Produce uninterrupted local current and action possible extension Are found in countries of encephalon. oculus. ciliary ganglia

Chemical Synapses – signal transmitted across a spread by chemical neurotransmitters Are found in most synapses between nerve cells and all synapses between nerve cells and other cells Cells non in direct contact

Action potency may or may non be propagated to postsynaptic cell. depending on: Sum of neurotransmitter released
Sensitivity of postsynaptic cell

Two Classs of Neurotransmitters
Excitatory neurotransmitters
Cause depolarisation of postsynaptic membranes
Promote action potencies
Inhibitory neurotransmitters
Cause hyperpolarization of postsynaptic membranes
Suppress action potencies

The Effect of a Neurotransmitter
On a postsynaptic membrane
Depends on the receptor
Not on the neurotransmitter
For illustration. acetylcholine ( ACh )
Normally promotes action potencies
But inhibits cardiac neuromuscular junctions

Cholinergic Synapsiss
Any synapse that releases ACh at:
All neuromuscular junctions with skeletal musculus fibres
Many synapses in CNS
All neuron-to-neuron synapses in PNS
All neuromuscular and neuroglandular junctions of ANS parasympathetic division Events at a Cholinergic Synapse
Action possible arrives. depolarizes synaptic terminus
Calcium ions enter synaptic terminus. trigger exocytosis of ACh ACh binds to receptors. depolarizes postsynaptic membrane
ACh removed by AChE
AChE breaks ACh into ethanoate and choline

Synaptic Delay
A synaptic hold of 0. 2–0. 5 millisecond occurs between:
Arrival of action potency at synaptic terminus
And consequence on postsynaptic membrane
Fewer synapses mean faster response
Reflexs may affect merely one synapse

Synaptic Fatigue
Occurs when neurotransmitter can non recycle fast plenty to run into demands of intense stimulations Synapse inactive until ACh is replenished

12-8 Neurotransmitters and Neuromodulators
Other Neurotransmitters
At least 50 neurotransmitters other than ACh. including:
Biogenic aminoalkanes
Amino acids
Dissolved gases

Norepinephrine ( NE )
Released by sympathomimetic synapses
Excitatory and depolarising consequence
Widely distributed in encephalon and parts of ANS
A CNS neurotransmitter
May be excitant or inhibitory

Involved in Parkinson’s disease and cocaine usage
A CNS neurotransmitter
Affects attending and emotional provinces
Gamma Aminobutyric Acid ( GABA )
Inhibitory consequence
Functions in CNS
Not good understood

Chemical Synapse
The synaptic terminus releases a neurotransmitter that binds to the postsynaptic plasma membrane Produces impermanent. localized alteration in permeableness or map of postsynaptic cell Changes affect cell. depending on nature and figure of stirred receptors

Many Drugs
Affect nervous system by exciting receptors that respond to neurotransmitters Can hold complex effects on perceptual experience. motor control. and emotional provinces

Other chemicals released by synaptic terminuss
Similar in map to neurotransmitters
Features of neuromodulators
Effectss are long term. decelerate to look
Responses involve multiple stairss. intermediary compounds
Affect presynaptic membrane. postsynaptic membrane. or both
Released entirely or with a neurotransmitter

Neuromodulators that bind to receptors and activate enzymes
Neuromodulators in the CNS
Bind to the same receptors as opium or morphia
Relieve hurting

Four Classs of Opioids

How Neurotransmitters and Neuromodulators Work
Direct effects on membrane channels
Indirect effects via G proteins
Indirect effects via intracellular enzymes

Direct Effects – Ach. glycine. aspartate
Ionotropic effects
Open/close gated ion channels

Indirect Effects – G Proteins. E. NE. Dopastat. histamine. GABA Work through 2nd couriers
Enzyme composite that binds GTP
Link between neurotransmitter ( first courier ) and 2nd courier Activate enzyme adenylate cyclase
Which produces 2nd courier cyclic-AMP ( camp )

Indirect Effects – Intracellular Receptors
Lipid-soluble gases ( NO. CO )
Bind to enzymes in encephalon cells

12-9 Information Processing
Information Processing
At the simplest degree ( single nerve cells )
Many dendrites receive neurotransmitter messages at the same time Some excitatory. some inhibitory
Net consequence on axon knoll determines if action potency is produced

Postsynaptic Potentials
Graded potencies developed in a postsynaptic cell in response to neurotransmitters Two Types of Postsynaptic Potentials
Excitatory postsynaptic potency ( EPSP )
Graded depolarisation of postsynaptic membrane
Inhibitory postsynaptic potency ( IPSP )
Graded hyperpolarization of postsynaptic membrane

A nerve cell that receives many IPSPs
Is inhibited from bring forthing an action potency
Because the stimulation needed to make threshold is increased Summation
To trip an action potency
One EPSP is non adequate
EPSPs ( and IPSPs ) combine through summing up
Temporal summing up
Spatial summing up

Temporal Summation
Multiple times
Rapid. repeated stimulation at one synapse
Spatial Summation
Multiple locations
Many stimulations. arrive at multiple synapses

A nerve cell becomes facilitated
As EPSPs accumulate
Raising transmembrane possible closer to threshold
Until a little stimulation can trip action potency

Summation of EPSPs and IPSPs
Neuromodulators and endocrines
Can alter membrane sensitiveness to neurotransmitters
Switching balance between EPSPs and IPSPs

Axoaxonic Synapsiss
Synapsiss between the axons of two nerve cells
Presynaptic suppression
Action of an axoaxonic synapse at a synaptic terminus that decreases the neurotransmitter released by presynaptic membrane Presynaptic facilitation
Action of an axoaxonic synapse at a synaptic terminus that increases the neurotransmitter released by presynaptic membrane

Frequency of Action Potentials
Information received by a postsynaptic cell may be merely the frequence of action potencies received Rate of Generation of Action Potentials
Frequency of action potencies depends on grade of depolarisation above threshold Holding membrane above threshold degree
Has same consequence as a 2nd. larger stimulation
Reduces comparative furnace lining period
In the Nervous System
A alteration in transmembrane potency that determines whether or non action
potencies are generated is the simplest signifier of information processing

Information is relayed in the signifier of action potencies
In general. the grade of centripetal stimulation or the strength of the motor response is relative to the frequence of action potencies The neurotransmitters released at a synapse may hold either excitatory or repressive effects The consequence on the axon’s initial section reflects a summing up of the stimulation that arrive at any minute The frequence of coevals of action potencies is an indicant of the grade of sustained depolarisation at the axon knoll Neuromodulators

Can change either the rate of neurotransmitter release or the response of a postsynaptic nerve cell to specific neurotransmitters Nerve cells
May be facilitated or inhibited by extracellular chemicals other than neurotransmitters or neuromodulators The response of a postsynaptic nerve cell to the activation of a presynaptic nerve cell can be altered by: The presence of neuromodulators or other chemicals that cause facilitation or suppression at the synapse Activity under manner at other synapses impacting the postsynaptic cell Modification of the rate of neurotransmitter release through presynaptic facilitation or presynaptic suppression


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