Ch 7 - Neuronal Excitability

ganglia

cluster of cell bodies in the PNS

nucleus

cluster of cell bodies in the CNS

nerve

bundle of axons in PNS

trace or tract

bundle of axons in CNS

Cell body

Input portion of neuron, contains organelles

Dendrites

Input portion of a neuron, extend out from soma

axon hillock

Where axon meets cell body. If threshold is reached, AP originates here

axon terminal

contains synaptic end bulbs, which are filled with synaptic vesicles containing neurotransmitters

anterograde axonal transport

Uses kinesin proteins to carry organelles and synaptic vesicles from cell body to axon terminal.

retrograde axonal transport

Uses Dynein motor proteins to transport membrane vesicles and other materials from axon terminal to cell body to be degraded.

afferent division of PNS

Somatic and special senses

efferent division of PNS

Somatic and autonomic

somatic nervous system

skeletal muscle

divisions of autonomic PNS

Sympathetic, parasympathetic, enteric

enteric nervous system

smooth muscle and glands of GI tract

sympathetic nervous system

Fight or flight. Acts on smooth muscle, cardiac muscle, and glands

parasympathetic

Rest and digest, feed and breed. Acts on smooth muscle, cardiac muscle, and glands

astrocytes

maintain extracellular Na+/K+ balance, maintain BBB, guide neurons during development and plays role in synapse formation

oligodendrocytes

Form and maintain multiple myelin sheaths per cell in CNS

microglia

phagocytes - remove debris, damaged cells, pathogens in CNS

ependymal cells

simple cuboidal epithelium that produces and circulates CSF in CNS

schwann cell

Form myelin sheath and contribute to axon regeneration in PNS. 1 cell per sheath

Satellite cells

surround and structurally support PNS cell bodies; regulate exchange of materials between cell bodies and interstitial fluid

white matter

myelinated axons only

gray matter

cell bodies, dendrites, unmyelinated axons, SYNAPSES, neuroglia

voltage-gated channel location

down all axons

mechanically-gated channel location

dendrites and cell body of sensory receptors

ligand-gated channel location

dendrites and cell bodies of most axons to recieve signals from other neurons

graded potential

Small deviation from resting membrane potential - depolarizing is excitatory, hyperpolarizing is inhibitory. Occur at dendrites and cell bodies of neurons

postsynaptic potential

Change in membrane potential of postsynaptic neuron after NT binds to its receptors. Excitatory or inhibitory

Receptor potential

Initial electric change generated in sensory touch receptors due to stimulus

End-plate potential

Depolarization of the membrane of a muscle fiber caused by Acetylcholine from a neuron binding to its receptors

increasing stimulus strength

increases the frequency of AP firing

resting state

All V.G. K+/Na+ channels are closed. RMP but negative charge builds up on inside of membrane, positive on outside.

depolarization (to +30mV)

When threshold reached, V.G. Na+ channel activation gates open causing Na+ influx and buildup of positive charge along inner membrane surface. Outside becomes negatively charged.

Beginning of repolarization

V.G. Na+ channel inactivation gates close and K+ channels open. As K+ ions leave neuron, negative charge begins building on inner membrane surface.

End of repolarization

K+ efflux continues and more negative charge builds and eventually restores RMP. V.G. Na+ channel activation gates close and inactivation gates open. Return to resting state when K+ gates finally close.

saltatory conduction

Ions flow through channels only at nodes of ranvier, AP skips from node to node.

continuous conduction

Ions flow through channels at each adjacent region of unmyelinated axon.

Extracellular K+ effect on neuronal excitability

Increase = Less K+ leaves, causing neuron to depolarize

Decrease = More K+ leaves, causing neuron to hyperpolarize

Extracellular Na+ effect on neuronal excitability

Increase = more Na+ enters, causing neuron to depolarize

Decrease = less Na+ enters, causing neuron to hyperpolarize

Extracellular Na+ effect on neuronal excitability

Increase = V.G. Na+ channels require higher voltage to open

Decrease = V.G. Na+ channels open at lower voltage

electrical synapse

Uses gap junctions - enables faster communication and synchronization

chemical synapse

Most common. Chemical messengers (NTs) diffuse across synaptic cleft.

Signal transmission at chemical synapse

AP arrives, opening V.G. Ca2+ channels to let calcium in. Ca2+ binds to synaptotagmin and synaptotagmin-Ca2+ complex binds to SNARE proteins causing synaptic vesicles to fuse with presynaptic membrane. NTs diffuse across and bind/open L.G. Na+ channels on postsynaptic cell to initiate AP.

Inhibitory post-synaptic potential

NT release hyperpolarizes post-synaptic membrane through opening of Cl- or K+ ion channels.

Excitatory post-synaptic potential

NT release depolarizes post-synaptic membrane through opening of Na+ channels

ionotropic receptors

Receptor itself is a ligand-gated ion channel. Eg. Ach or GABA-mediated.

metabotropic receptors

Receptor is coupled to G-protein that directly or indirectly (through 2nd messenger) opens/closes ion channel

methods of NT removal

diffusion, enzymatic degradation, reuptake by cells or astrocytes

summation determines response of postsynaptic neuron

If the sum of PSPs is overall excitatory by +15 mV or more, an AP will fire (-70mV + 15 mV = -55 mV)

main types of NTs

Neuropeptides and small-molecule NTs

small-molecule NTs

Acetylcholine, amino acids, biogenic amines, purines, gases, endocannabinoids

Acetylcholine (ACh)

Most common, only NT at neuromuscular junction. Released by cholinergic neurons in CNS and PNS. Removed from cleft by acetylcholinesterase (AChE)

cholinergic receptors

can be nicotinic or muscarinic

nicotinic cholinergic receptor

Ionotropic receptor, results in EPSP. Present in some CNS neurons, autonomic ganglia and neuromuscular junctions.

muscarinic cholinergic receptor

metabotropic receptor, results in IPSP or EPSP. Present in some CNS neurons, and autonomic effector organs (smooth/cardiac muscle, glands)

glutamate

Amino acid, powerful excitatory NT. Most excitatory neurons and half of synapses communicate with it.

glutamate receptors

Ionotropic, mainly Na+ channels. Also AMPA and NMDA receptors. Inactivated via reuptake.

aspartate

Amino acid, excitatory NT in CNS. Activates NMDA receptors.

GABA

Amino acid, most common inhibitory NT in CNS. (1/3 of all brain synapses, 1/2 of all spinal) Derived from glutamate. Receptors can be ionotropic or metabotropic.

Ionotropic GABA receptors

Open Cl- ion channels to hyperpolarize cell - GABAa, GABAc

Metabotropic GABA receptors

Activates G-proteins to directly or indirectly open K+ ion channels to hyperpolarize cell - GABAb

glycine

common inhibitory NT in spinal cord (1/2 of all spinal synapses). Ionotropic receptor that opens Cl- channels.

biogenic amines

Derived from modified and decarboxylated amino acids.