NEUS 609 - Cells of the Nervous System

Neurons

functional unit of the nervous system

complex cells with numerous processes forming complex circuits

electrically excitable cells

100 billion

There are ____ neurons in the CNS

resting membrane potential

unequal distribution of charge/ions across a membrane

-70

___ mV is the resting membrane potential for a neuron

Sodium-potassium ATPase

the resting potential is mainly established and maintained by ____

• pumps 3 Na+ out

  • pumps 2 K+ in

• uses energy from ATP hydrolysis aka energy dependent

• uses 50% of energy in the brain

transporters

establish and maintain resting membrane potentials by separation of ionic charges

ion channels

permit ions to flow down their concentration gradients (inward OR outward) thereby changing the membrane potential

depolarization

inside becomes less negative

hyperpolarization

inside become more negative

graded potential

• aka receptor potential

• neurotransmitters/ligands binding to their receptors on dendrites and cell bodies

• ligand binding causes the receptor to open, allowing ions to flow down their gradient, thereby changing resting membrane potential

• Na+ depolarizes while Cl- hyperpolarizes

• happens in dendrites and cell bodies

action potenital

• all or nothing depolarization occurring only in axons

• channels open in response to membrane voltage

• opening occurs rapidly with a predictable change in membrane voltage after reaching depolarizing/excitatory threshold

• once triggered it propagates down axons and end up in neurotransmitter release at the synapse

neuronal transmission

___ is just graded potentials and action potentials alternating and triggering the others

cell body

aka soma aka perikaryon

• where the nucleus is

• major synthetic center of the cell

• polygonal/round/oval depending on the type of neuron

• has Nissl bodies to synthesize proteins

dendrites

major site of synaptic input

• usually, multiple and highly branched

• form somatodendritic compartment along with cell body

• majority of neuron’s receptive field aka antenna

• receive majority of synaptic input

Axon

• generates and propagates the action potential

• may extend long distances from the cell body

• usually one per neuron

• originate from ___ hillock of cell body

• lacks Nissl body but still able to synthesize proteins somehow

• may be myelinated or not

Chemical synapse

Close apposition of the membrane of the presynaptic terminal with the membrane of the next neuron in the circuit

• terminal bouton aka presynaptic terminal

• contain synaptic vesicles that transport neurotransmitter to membrane and release them in synaptic cleft

• NO membrane contact ~20-30 nm separation

• transmitter diffuse across synaptic cleft leading to 0.5ms delay in signaling

• asymmetrical and thus UNIDIRECTIONAL

Nissl bodies

site of protein synthesis in the cell soma made of stacked rough ER and polysomes\

• unique to neurons

Golgi complexes

site of proteins packing and trafficking to the cell membrane

Mitochondria

powerhouse of the cell

provides energy

Cytoskeleton

made of filaments (intermediate & actin) for movement and support and microtubules for transport of material inside the cell as well as support

Axon hillock

• part of the cell body

• site where axon originates

• appear pale because of its lack of Nissl bodies

• sorting region between axon and somatodendritic compartments

• synaptic inputs summate at the ___ which determine whether an action potential is triggered based on if it reaches threshold or not

dendrite morphology

___ is characteristics for specific neuron subtypes, reflecting connectivity and functions

dendritic spine

small projections (<1micron) from the surface of dendrites only

• receive excitatory synaptic input making cell more likely to respond

• highly dynamic (change shape, retract, re-extend) aka structural component of plasticity (learning and memory)

• mature when mushroom shaped

Initial segment

the first portion of the axon ~20-60 microns long

NON-MYELINATED region that precedes myelin sheath

site of initiation of action potential

Electrical synapse

• membranes touch each other

• 6 proteins assemble to form a connexon

• connexons of each membrane connect with the ones on the other forming ion channels

• GAP JUNCTIONS

• BIDIRECTIONAL

• less common

• ‘Found in retina for example

• FAST

• minimal plasticity

axo-dendritic

axon synapsing on a dendrite (spine or shaft)

axo-somatic

axon synapsing on the cell body

axo-axonic

axon synapsing on another’s initial segment OR terminal bouton

• never synapses in the middle

neuromuscular jucntion

aka NMJ

axon synapsing on a motor plate in a muscle

farther

The ___ from the cell body a synapse is, the LESS impact it has on the potential as it fades when it travels

afferrent

connect peripheral sensory receptors to central circuits

efferent

project motor info from CNS to muscle glands

multipolar neuron

usually carry motor or integration functions

• complex dendrites provide for multiple synaptic input and functional integration

• control motor behavior

• process, integrate and store info

  • > 95% of neurons

bipolar neuron

usually carry sensory functions

convey special sensory information

• gustation

• audition

• vision

• olfaction

• balance

pseudounipolar

usually carry sensory functions

convey general sensory information

• pain/nociception

• pressure

• heat

• cold

• body position/proprioception

neuroglia, Schwann cells

Supporting cells of the nervous system are usually divided into ___ (CNS) and ___ (PNS)

• ratio of 10:1 to 1:1 in CNS based on brain regions

Astrocytes

• structural support

• homeostasis

• stellate morphology (fibrous aka white matter or protoplasmic aka gray matter)

• physical support

• nutritional support

• ion homeostasis

• transmitter uptake

• scar formation

• induce BBB

• modulate synapses

Oligodendrocytes

myelin formation in CNS

• 1 oligodendrocyte for up to 40 internodes on different axons

• this ratio is why demyelination in CNS is more severe than in PNS

Microglia

= brain macrophages, highly phagocytic, main inflammatory cell in the CNS, NON-NEURAL origin

• respond to photogenic insults like trauma or inflammation

• eliminate synapses (pruning) as a normal part of refining neural circuitry

• when ACTIVATING release adaptive cytokines to clear debris and stimulate repair

• oscillating between activated and inactivated form

Ependymal cells

line ventricles

ensheathing schwann cells

surrounding peripheral axons by wrapping processes around axon and growing one of them internally, creating compact myelin, creating a hydrophobic region preventing ion exchange

• 1 Schwann cell for 1 internode

• in PNS

Satellite Schwann cells

surround neuronal cell bodies in ganglia in PNS

surround autonomic ganglia

Blood brain barrier

BBB

highly selective semipermeable barrier formed by cells lining blood vessels (endothelial cells)

endothelial cells form tight junctions with each other, induced by astrocyte contact

tight junctions prevent material in the circulating blood from non-selectively entering CNS

tripartate synapse

consists of pre- and postsynaptic elements plus astrocyte processes

astrocytes take up glutamate and recycle back to the neuron

myelin

discontinuous, lipid rich sheath around axons only

• internodes = myelinated segments

• node of Ranvier = short section of axon that is unmyelinated and has a high concentration of Na+ voltage gated channels

• act as insulator speeding up conduction

paranode

the end of myelin internodes fusing to axonal membrane by tight junctions which restrict voltage gated Na+ channels to the node of Ranvier and prevent conduction failure

diameter

the thickness and length are proportional to the ___ of the axon

unmyelinated

___ axons are still surrounded by Schwann cells that do not grow lamellae