Integrated Physiology Nervous System and Nervous Excitability

autonomic nervous system is also referred to as the...

visceral nervous system

axon

nerve fiber

axoplasm

cytoplasm of an axon

axonal transport

axoplasmic flow

sensory neuron is also referred to as an...

afferent neuron

interneuron

association neuron

cell membrane of an axon...

axolemma

glial cells also referred to as...

neuroglia, glia

central nervous system consists of

brain and spinal cord

peripheral nervous system consists of

all nerves connecting to the CNS, cranial and spinal

Model neuron (how it works to receive and carry information)

Dendrites receive incoming signals; axons carry outgoing information

3 classifications of neurons

1.) Sensory

2.) Interneurons

3.) Efferent (motor neurons)

anatomy of an axon

composed of...

- soma or cell body

- axons

- carry outgoing information

- dendrites (branches)

- receive incoming signals

Glial cell function

provide physical and biochemical support for neurons in the CNS

10 to 50x more glia cells than neurons

Glial Cells Functions: Oligodendrocytes

form myelin sheaths

Glial Cells Functions: Microglia (modified immune cells)

act as scavengers (phagocytes)

- remove debris, damaged cells, and pathogens

Glial Cells Functions: Astrocytes

- most numerous

- provide substrates for ATP production

- help form blood-brain barrier

- secrete neurotrophic factors

- take up K+, water, neurotransmitters

- source of neural stem cells

Glial Cells Functions: Ependymal cells

- produce and assist in the circulation of CSF

- source of neural stem cells

myelin sheath in the PNS vs CNS

PNS: Schwann cells

CNS: Oligodendrocytes

Functions of the myelin sheath

- electrical insulation

- increases speed of conduction of action potentials

- found in both CNS and PNS

- Schwann cell = PNS

- Oligodendrocyte = CNS

- makes up white matter

- nodes of Ranvier

- gaps in the myelination

Nodes of Ranvier

a section of unmyelinated axon membrane between two Schwann cells

Each Schwann cell forms myelin around a small segment of ______ axon(s).

one

Resting membrane potential

an electrical potential difference across the plasma membrane

Leak channel

ion channels that have gates which randomly alternate between open and closed positions

Ligand-gated channel

ion channel that opens or closes in response to a specific ligand (chemical) stimulus

mechanically gated channel

ion channels that open or close in response to mechanical stimulation, this may take on the forms of touch, pressure, tissue stretching, and vibration

voltage-gated channel

ion channels that opens in response to a change in membrane potential (voltage)

Nernst Equation

describes the membrane potential that a single ion would produce if the membrane were permeable to only that ion

Eion= 61/z *log(ion)out/(ion)in

Membrane potential is influenced by

1.) concentration gradient of ions

2.) membrane permeability to those ions

Goldman-Hodgkin-Katz equation

predicts membrane potential that results from the contribution of all ions that can cross the membrane

see PP slide 32 for formula

Resting membrane potential is primarily determined by...

- K+ concentration gradient (leak channels)

- Cell's resting permeability to K+, Na+, Cl-

depolarizing

(less negative)

makes the membrane potential less polarized

hyperpolarizing

(more negative)

makes the membrane potential more polarized

Graded potentials

in neurons, are "local" changes in membrane potential

graded potentials _______________ in strength as they spread out from the point of origin

decrease

depolarizing graded potentials cause resting membrane potential to ________________

increase

hyperpolarizing graded potentials cause resting membrane potentials to ____________

decrease

Comparison of graded potential vs action potential

graded potential:

- input signal

- occurs usually in cell body and dendrites

- no min level required to initiate

- two signals coming close together in time will sum

action potential:

- regenerating conduction signal

- occurs in trigger zone through axon

- threshold stimulus required to initiate

- refractory period; two signals too close together in time cannot sum

graded potentials can vary in amplitude depending on...

the strength of the stimulus

summation of "strength" graded potentials causes...

changes in the trigger zone of the axon to generate an axon potential

electrical signals: graded potentials

subthreshold and suprathreshold graded potentials in a neuron

Voltage-gated Na+ channels

Na+ channels have activation and inactivation gates

the activation gate closes the channel at...

the resting membrane potential

the activation gate opens when _____________________ ______________ arrives at the channel

depolarizing stimulus

with activation gate open, Na+ enters the cell, inactivation gate closes and Na+ entry stops

the two gates reset to their original positions during...

repolarization caused by K+ leaving the cell

weak stimulus = ________ neurotransmitter

little

strong stimulus = ________ neurotransmitter

more

Trigger Zone: Graded to Action Potentials

Graded potential reaches the trigger zone, opening voltage-gated Na+ channels, causing Na+ to enter the axon. This positive charge spreads to neighboring axon sections, depolarizing them. The refractory period prevents backward conduction, while the loss of K+ helps repolarize the membrane.

local current flow

depolarization section of axon

speed of action potential in neuron is influenced by...

- diameter of axon

- resistance of axon membrane to ion leakage out of the cell

__________ axons are faster along with __________________ axons

larger, myelinated

in demyelinated diseases, conduction slows when...

current leaks out of the previously insulated regions between the nodes

when blood K+ is in the normal range, a subthreshold graded potential....

does not fire an action potential

when blood K+ is in the normal range, a suprathreshold stimulus will...

fire an action potential

increased blood K+ concentration brings the membrane closer to the threshold, a stimulus that would normally be subthreshold can...

trigger an action potential

decreased blood K+ concentration hyperpolarizes the membrane and makes the neuron less likely to...

fire an action potential in response to a stimulus that would normally be above the threshold

chemical synapses use...

neurotransmitters

synapses

pass electrical signals between neurons by neurotransmitters

7 classes of Neurocrines

- acetylcholine

- amines

- amino acids

- purines

- gases

- peptides

- lipids

acetylcholine is synthesized from

choline + acetyl coA

cholinergic

neurons that secrete and receptors that bind acetylcholine

in the synaptic cleft, acetylcholine is rapidly broken down by the enzyme ______________________________

acetylcholinesterase

tyrosine

- dopamine

- norepinephrine is secreted by noradrenergic neurons

- epinephrine

serotonin is made from

Tryptophan

histamine is made from

Histidine

glutamate

excitatory --> CNS

aspartate

excitatory --> brain

GABA

inhibitory --> brain

Glycine

- inhibitory --> spinal cord

may also be excitatory

purines

AMP and ATP

gases

NO and CO

peptides

substances P and opioid peptides

lipids

eicosanoids

cholinergic receptors

- nicotinic on skeletal muscle, in PNS and CNS

- monovalent cation channels --> Na+ and K+

- muscarinic in CNS and PNS

- linked to G proteins

Adrenergic Receptors

- alpha and beta

- linked to G proteins

divergent pathway

one presynaptic neuron branches to affect a larger number of postsynaptic neurons (multiple targets)

convergent pathway

many presynaptic neurons converge to influence a smaller number of postsynaptic neurons (single targets)

inhibitory post-synaptic potential (IPSP)

neurotransmitter release results in a hyperpolarization of the post-synaptic membrane

excitatory post-synaptic potential (EPSP)

neurotransmitter release results in a depolarization of the post-synaptic membrane

If the net summation of EPSPs and IPSPs is a depolarization that reaches a threshold, then...

an action potential occurs at the trigger zone of the postsynaptic neuron

No summation

two subthreshold graded potentials will not initiate an action potential if they are far apart in time

summation causing action potential

if two subthreshold potentials arrive at the trigger zone within a short period of time, they may sum and initiate an action potential

postsynaptic inhibition

1.) one excitatory and one inhibitory presynaptic neuron fire

2.) modulated signal in postsynaptic neuron below threshold

3.) no action potential initiated at trigger zone

4.) no response in any target cell

each cell inhibited equally

presynaptic inhibition

1.) an excitatory neuron fires

2.) an action potential is generated

3.) an inhibitory neuron fires, blocking neurotransmitter release at one synapse