EEMB 7 chapter 37: neurons and synapses

lines of communication

ganglia, brain, and neurons

structure of a neuron

cellbody → axon → dendrites → synapses → neurotransmitters → glial cells

glial cells

provides physical and chemical support to neurons and maintain their environment

• types: microglial, ependymal cells, astrocytes, oligodendrocytes, schwann cells

central nervous system (CNS)

the brain (cerebrum, cerebellum, brainstem) and the spinal cord

peripheral nervous system (PNS)

everything that conducts nerve impulses outside of the brain and spinal cord (cranial and spinal nerves, ganglia)

information processing cells

sensory neurons, interneurons, and motor neurons

sensory neurons

carries information about changes in external and internal environment to the CNS

interneurons

connect to brain regions

motor neurons

responsible for carrying signals away from the CNS towards muscles to cause movement

ion basics

ions in neurons are unequally distributed between the interior and surrounding fluid

• cell interior is negatively charged (relative), and the difference in voltage is called membrane potential

neuron ion concentrations

ions (charged particles) cannot pass through lipid bilayer

• outside: has more Na+, Ca²+, and Cl-

• inside: organic ions (negative charged proteins) and K+

resting potential

difference in electrical potential in the plasma membrane when the cell is in a state of rest

• between -60 and -80 mV for a neuron

sodium-potassium pump

pump transports three Na+ out of the cell for every two K+ in

• work to reestablish the concentration gradient

locomotion

example of gradient in animals: H+ gradient that powers flagellum in bacteria

• electron transport makes higher concentration H+ outside of cell; proton reenters cell that provides a force that causes flagellar motor to rotate

generation of action potentials

resting state → depolarization → rising phase → falling phase → undershoot

osmoregulation

example of gradient in animals: seen in ocean fish, gradient drives salt secretion. in gills, pumps/channels work to expel salt from blood back into the ocean

gated ion channel

transmembrane proteins that allows ions to pass through membrane in response to specific stimulus

voltage-gated ion channel

class of transmembrane proteins that form ion channels that are activated by changes in electrical membrane potential near channel

hyperpolarization

when membrane potential becomes more negative at particular location on neuron membrane

depolarization

when membrane potential becomes less negative (more positive)

graded potential

shift in membrane potential

• magnitude varies with the strength of the stimulus (larger stimulus causes greater change)

• decay over time and distance from source

action potential

massive change in membrane voltage

• can spread along axons ( transmitting signal over longer distances

• constant magnitude and can regenerate in adjacent regions of membrane

action potential conduction

starts at the beginning of axon and moves towards the synaptic terminal

• the frequency of the AP conveys information too and determines how quick an animal can respond

axon structure adaptation

NS results in adaptations that increase conduction speed

• example: wider axon allows less resistance to the current

invertebrates axon structure

conduction speed varies; giant axons found in animals function in rapid behavioral responses

• example: mollusks → muscle contractions during hunting

vertebrates axon structure

evolutionary adaptation enables fast conduction in axons is electrical insulation (like insulation around wires)

myelin sheath

electrical insulation that surrounds axons

oligodendrocytes

glia in the CNS that produces myelin sheaths

schwann cells

glia in the PNS that produces myelin sheaths

nodes of ranvier

periodic gap in sheath on an axon of certain neurons that serves to facilitate rapid conduction of nerve impulses

saltatory conduction

propagation of AP along the myelinated axons from one node of ranvier to the next, increasing the conduction velocity of APs

synapse

place where information is transmitted between neurons

• most are chemical: presynaptic neuron releases chemical neurotransmitters to transfer information to target cell

neuron communication

synapse returns to resting state after response, and neurotransmitters are cleared from the cleft (diffusion or recaptured)

• some are cleaved by enzyme into inactive fragments

electrical synapses

rely on the movement of an electrical current

• current flows between neurons via junction gaps

• they are common in rapid and unchanging pathways (like giant axons) to facilitate quick escapes from predators

EPSP

when depolarization brings membrane potential toward the threshold

IPSP

moves membrane potential further from he threshold

spatial summation

when several synapses occur at the same time, creating a summative effect

temporal summation

when a single EPSP synapses again before the resting membrane potential is fully restored it can show an additive effect, this is when frequency modulation for intense stimuli come into play

neurotransmitters

chemical messengers that carry messages from one nerve cell to the next nerve, muscle, or gland cell

• a single neurotransmitter can bind to more than 12 receptors!

acetylcholine (ACh)

chief neurotransmitter of parasympathetic nervous system

amino acids

glutamate is the most common nt in the CNS (primary excitatory nt)

• synapses where glutamate is the nt have an important role in the formation of long-term memory

biogenic amines

synthesized from amino acids (norepinephrine)

• norepinephrine: excitatory nt in the autonomic nervous system (PNS)

• dopamine, serotonin

biogenic amines disorders

play an important role in nervous system disorders and treatments

• parkinson’s disease is associated with a lack of dopamine in the brain

neuropeptides

short chain amino acids that act as neurotransmitters (like endorphins)

endorphins (neuropeptide)

act as natural pain killers

• opiates mimic endorphins (and therefore produce many of the same physiological effects)

gases

some vertebrate neurons release dissolved gases as neurotransmitters

• human males: some neurons release nitric oxide (NO) into erectile tissue of penis during sexual arousal

  • viagra: inhibits enzyme that terminates NO

drug effects on synaptic transmission

various toxins abundant in nature, from snakes and spiders to plant resins/algae

• many toxins have their effect on/near synapses

inhibitors

most act by inhibiting the release of nt

• crotoxin from american rattlesnakes: inhibits ACh release

• botox (botulinum toxin): inhibits ACh release

blocking agents

most work by blocking various channel proteins (like saxitoxin from red tide algae and tetrodoxin from pufferfish)

• same block nt receptor sites like curare from american tree that paralyzes by competing with ACh

enzyme destroyers

nerve gas (sarin) kills by spastic paralysis due to it’s inactivation of ACh

strychnine

a common rat poison come from a plant in asia/australia, which interferes with IPSP’s in the spinal chord

• muscles have difficulty “turning off” leading to spastic paralysis and asphyxiation

cocaine

for some dopamine CNS neurons (like brain pleasure centers), cocaine blocks reuptake of transporters

• leads to overstimulation and eventual depletion of dopamine (and other health problems)