Neural Anatomy and Physiology

Like all cells neurons have?

• Resting membrane potential

Unlike most other cells, neurons can?

• Rapidly change resting membrane potential

Neurons are highly?

• Excitable

Opposite charges are ______ to each other?

• Attracted

What is required to keep opposite charges separated across a membrane?

• Energy

Voltage

• Measure of potential energy generated by separated charge

What results in potential?

• Charge difference across plasma membrane

Current

• Flow of electrical charge (ions) between two points

What is the flow of electrical charge dependent on?

• Voltage and resistance

Resistance

• Hinderance to charge flow

Insulator

• Substance with high electrical resistance

Conductor

• Substance with low electrical resistance

What are the two main types of ion channels?

• Leakage (nongated) channels; always open

• Gated Channels; protein changes shape to open or close the channel

What are the three main gated channels?

• Chemically gated 

• Voltage gated 

• Mechanically 

How do chemically gated channels open?

• Open only with binding of a specific chemical

How do voltage gated channels open?

• Open and close to changes in membrane potential

How do mechanically gated channels open?

• Open and close in response to physical deformation of receptors 

When channels are open what happens to ions?

• Ions diffuse quickly chemical concentration gradients (from high to low concentration)

• Ions diffuse quickly along electrical gradients toward opposite electrical charge

Electrochemical gradient:

• Electrical and chemical gradients combined

Ion flow creates what?

• Creates an electrical current, and voltage changes across membrane

The resting membrane potential of a resting neuron is approximately?

• -70mV

When a neuron is at resting potential what are the conditions of the inside and outside?

• The inside is negatively charged and the outside is positively charged 

• The membrane is polarized

How is potential generated?

• Differences in ionic composition of Intracellular Fluid and Extracellular fluid

• Differences in plasma membrane permeability

What are the differences between Extracellular fluid and Intracellular fluid in Ionic composition?

• ECF has more Sodium (Na) and is balanced primarily by chloride (Cl) ions

• ICF has higher concentration of potassium (K) ions and is balanced primarily by negatively charged proteins

What ion plays the most important role in membrane potential

• Potassium (K)

What are some differences in plasma membrane permeability with ions (protein, K, Na, Cl)?

• Impermeable to large anionic proteins

• Slightly permeable to Sodium (Na) through leakage channels

• 25X more permeable to potassium than sodium (because more leakage channels)

• Quite permeable to chloride ions

What does the difference in plasma membrane permeability result in?

• More potassium diffuses out that sodium diffuses in

• The inside of the cell is then more negative 

• Establishes the resting membrane potential

What does the Sodium-potassium pump do to the resting membrane potential?

• Na/K ATPase stabilizes the resting membrane potential

• Maintains concentration gradients for Na and K

• 3 sodium out, 2 potassium in

When does membrane potential change?

• Concentrations of ions across membrane change

• Membrane permeability to ions changes

What two signals do changes in membrane potential produce, and how long do they last?

• Graded potentials: Short distances

• Action potentials: Long distances

What are the changes in membrane potential used for?

• Used as signals to receive, integrate and send information

What is depolarization and how is it demonstrated in a cell?

• Depolarization is a decrease in membrane potential (moves towards 0 and up)

• The inside of the cell membrane becomes less negative than Resting membrane potential

• Probability of producing impulse increases

What is hyperpolarization and how is it demonstrated in a cell?

• Increase in membrane potential (away from 0)

• Inside of membrane becomes more negative than resting membrane potential

• Decreased chances of producing impulse

What are graded potentials, how are they triggered and what is the result of this signal?

• A short localized change in membrane potential

• Triggered by stimulus that opens gated ion channels

• Results in depolarization and sometime hyperpolarization

• Depolarization spreads from one area of membrane to next

• Current flows but dissipates quickly and decays

What are action potentials, how are they triggered, where do they occur?

• Main way neurons send signals and do not decay like graded potentials

• Long distance communication

• Occur only in muscle cells and axon of neurons

• Referred to as a nerve impulse in neurons

How is an action potential generated? List the 4 main steps

• Resting state: All gated sodium and potassium channels are closed and only leakage channels are open

• Depolarization: Sodium channels open

• Repolarization: Sodium channels are inactivating and potassium channels open. This returns membrane to resting potential

• Hyperpolarization: Some potassium channels open and sodium channels reset

What is threshold and the all-or-none phenomenon?

• For an axon to fire, depolarization must reach threshold voltage to trigger AP

• All-or-none means an Ap either happens completely of does not happen at all

How is Action potential propagated or spread?

• Sodium comes in through voltage gates in one membrane area that causes other gates in the area to open up (wakes up the whole neighborhood)

• AP is self-propagating once started 

What is frequency and how is it used by the CNS?

• Frequency is the number of APs received per second

• The CNS uses frequencies to differentiate between a weak and strong stimulus

What is a refractory period and what are the two types?

• A refractory period is the time where a neuron cannot trigger another action potential

• The two types are absolute refractory period and Relative refractory period

Describe absolute refractory period

• This is the time from the opening of Na channels until their resetting

• Ensures that each AP is and all-or-none event and enforces one-way transmission of nerve impulses

Describe relative refractory period

• Follows absolute refractory period

• Most Na have returned to resting state and some K are still open

• One exceptionally strong stimulus could stimulate an AP 

Where does AP occur?

• Only in Axon

What factors does the rate of AP propagation depend on?

• The diameter of the Axon

• Degree of Myelination: 

How does Axon diameter affect AP propagation?

• Lager diameter fibers have less resistance to local current flow which increased the impulse conduction 

What are the two types of conduction depending on the degree of myelination?

• Continuous conduction: Slow conduction in unmyelinated areas

• Saltatory conduction: 30x faster in myelinated ares

How does Myelin speed conduction?

• Myelin sheath insulate & prevent leakage of charge

What is Multiple sclerosis, what are its symptoms and what causes it?

• An autoimmune disease that affects mostly young adults 

• Myelin sheaths in CNS are destroyed by the immune systems

• Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence

What happens to Myelin in Multiple Sclerosis?

• Myelin is turned into hardened lesions called scleroses which slows impulse conduction

Treatment for Multiple Sclerosis

• Medications that modify immune system activity

How are nerve fibers classified?

• Diameter

• Degree of myelination

• Speed of conduction

What are the 3 groups of nerve fibers? Describe them

• Group A: largest diameter, myelinated somatic sensory and motor fiber of skin, fastest transmit rate (150 m/s)

• Group B: Intermediate diameter, lightly myelinated, intermediate transmit rate (15 m/s) ANS visceral motor and sensory fibers 

• Group C: Smallest diameter, no myelination, slowest transmit rate ( 1m/s)  ANS visceral motor and sensory fibers 

What are ways that AP propagation can be blocked?

• Local Anesthetics block voltage-gated sodium channels

• Cold temperature or continuous pressure interrupt blood circulation 

What are neurons connected by? What doe these junctions mediate?

• Synapses: help mediate information transfer from neuron to neuron or neuron to effector

Describe presynaptic neurons?

• Neuron conducting impulses toward synapse (sends information)

Describe Postsynaptic neurons?

• Neuron transmitting electrical signal away from synapse (receives information)

What are the synaptic connections? Describe them

• Axodendritic: between axon terminals and dendrites

• Axosomatic: between axon and soma (cell body) 

• Axoaxonal: between axon and axon

• Dendrodendritic: dendrite to dendrite

• Somatodendritic: dendrite to soma

What are the two types of synapses?

• Chemical synapse

• Electrical synapse

Describe the chemical synapse and how it functions

• Composed of two parts: Axon terminal of presynaptic neuron that contains neurotransmitter and receptor region that receives neurotransmitter

• Specialized for release and reception of chemical neurotransmitters

• Chemical event

• Depends on release, diffusion, and receptor binding

What are the 6 steps involved in information transfer across chemical synapses?

• AP arrives at axon terminal of presynaptic neuron

• Voltage-gated calcium channels open and Ca enters axon terminal

• Calcium entry causes synaptic vesicles to release neurotransmitter

• Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane

• Receptor protein changes shape which causes ion channels to open

• Neurotransmitter effects are terminated

What occurs after calcium’s entry into the axon terminal?

• Ca causes synaptotagmin protein to react with the SNARE protein that control’s fusion of synaptic vesicles with axon membrane

• Fusion results in exocytosis of neurotransmitter into synaptic cleft

• Higher frequency causes more vesicles to exocytose

Describe electrical synapses

• Joined by gap junctions that connect cytoplasm of adjacent neurons

• Communication is rapid

Where are electrical synapses most abundant?

• Embryonic nervous tissue

Neurotransmitter receptors cause graded potentials that vary in strength based on what factors?

• Amount of neurotransmitter released

• Time neurotransmitter stays in cleft

Based on the effect of chemical synapse, what are the two types of postsynaptic potentials?

• EPSP: Excitatory postsynaptic potentials→ sodium intake greater than K outtake

• IPSP: Inhibitory postsynaptic potentials→ K moves out and CL moves in causing hyperpolarization

What are the two types of summation? Describe them

• Temporal summation: One or more presynaptic neurons transmit impulse in a rapid-fire order

• Spatial summation: Postsynaptic neuron is stimulated by large number of terminals simultaneously

What is the result of repeated use of synapse (synaptic potentiation)?

• Increases the ability of presynaptic cell to excite postsynaptic neuron

• Ca concentration increases in the presynaptic terminal which releases more neurotransmitter

• Long-Term potentiation: learning and memory

Describe presynaptic inhibition

• Release of excitatory neurotransmitter is inhibited by another via axoaxonal synapse 

How are neurons classified?

• Chemical structure

• Function

Describe Acetylcholine 

• Released at neuromuscular junctions

• Used by many ANS neurons and CNS neurons

• Degraded by acetylcholinesterase

What neurotransmitters are classified as Biogenic amines?

• Catecholamines: dopamine, norepinephrine, and epinephrine all made from the amino acid tyrosine

• Indolamines: serotonin (amino acid tryptophan) and histamine (amino acid histidine)

What neurotransmitters are classified as Amino acids?

• Glutamate

• Aspartate

• Glycine

• GABA

What neurotransmitters are classified peptides?

• Substance P

• Endorphins

• Gut-brain peptides

What neurotransmitters are classified purines?

• Adenosine: A potent inhibitory neurotransmitter

• ATP

• Monomers of nucleic acids that have effects in both PNS and CNS

What neurotransmitters are classified as gases and lipids?

• Gasotransmitters: Nitric Oxide, Carbon monoxide, hydrogen sulfide 

• Lipid soluble and are synthesized on demand

What are the two classifications of neurotransmitter functions?

• Effects

• Actions

Describe the effects of neurotransmitters?

• Neurotransmitters can be either excitatory (depolarizing) or inhibitory (hyperpolarizing)

• Effect determined by the receptor to which it binds

Describe the direct action of neurotransmitters

• Neurotransmitters binds directly to and opes ions channels

• Promotes rapid responses by altering membrane potential

Describe the indirect action of neurotransmitters

• Neurotransmitters Acts through intracellular second messengers

What is a neuromodulator

• A chemical messenger released by neuron that does not directly cause EPSPs or IPSPs but affects the strength of synaptic transmission

What are channel-linked receptors? Describe them

• Ligand gated ion channels

• Action is immediate and brief

Describe  G protein-linked receptors

• Involves transmembrane protein complexes 

• Responses are indirect, and often prolonged 

• Causes widespread metabolic changes

What is neural integration?

• Neurons functioning together in groups

What are neuronal pools? Describe their functions

• Functional groups of neurons

• Integrate incoming information received from receptors and forward process to other destinations

What is a discharge zone?

• Neurons closer to incoming fiber are more likely to generate impulse

What is a facilitated zone?

• Neurons on periphery pool are farther away from incoming fiber

What is Serial processing?

• Input travels along one pathway to a specific destination

What is parallel processing?

• Input travels along several pathways

• One stimulus promotes numerous responses

What are the 4 types of circuits?

• Diverging

• Conversing

• Reverberating

• Parallel after-discharge

Where does the nervous system originate from?

• Neural tube

Where is the neural crest formed from?

• Ectoderm

What does learning in early childhood do to synapses?

• Reinforces certain synapses and prunes away others

How do astrocytes aid in neural development?

• Provide physical supper and the cholesterol needed for construction of synapses