Chapter 37 - Neurons and Synapses

Neurons

basic units of the nervous system that transmit info w/ three main parts. 1) dendrites receive signals 2) cell body processes info 3) axon sends signals.

Ganglia

clusters of neuron cell bodies in the PNS that help relay and process info → mini processing centers outside the brain & spinal cord

Cell body

processes info & keeps the neuron alive

Axon

sends info → long fiber that carries signals away from the cell body to other neurons or muscles

Dendrites

collect info → branched extensions that receive signals from other neurons

Synapse

gap between neurons where signals are passed → uses neurotransmitters to send info

Neurotransmitter

chemical messenger that carries signals across a synapse between neurons by binding to dendrites of next neuron using electrical signal to move

Glial cells

cells that provide support neurons, maintain their environment, and help with communication and waste cleanup

Types of glial cells

Microglia • Ependymal Cells • Astrocytes • Oligodendrocytes • Schwann Cells

Central Nervous System (CNS)

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

Peripheral nervous system (PNS)

everything else that conducts nerve impulses like cranial & spinal nerves & ganglia

Sensory neurons

carry information about any changes to the CNS

Interneurons

connect to brain regions

Motor neurons

signals away from CNS to muscles to cause movement

Neuron ions distribution

unequally distributed between interior and surrounding fluid

Interior cell charge

Negatively charged

Membrane potential

difference in charge across a plasma membrane

Ions x lipid bilayer

Charged ions cannot pass directly through the lipid bilayer

Ion concentrations

Interior = negatively charged organic ions and K+

Exterior = Na+, Ca2+, and Cl-

Resting potential

negative interior electric charge in a neuron when there’s no stimulus and it's relaxed

Sodium-potassium pump

pump transports three Na+ out for every two K+ in

Osmoregulation

ion gradients drive salt secretions → example of oceanic fish using gills, channels and pumps to get salt out of blood into water

Locomotion

using H+ concentration gradient to move flagellum → electron transport system causes more H+ on outside of cell and creates gradient

Gated ion channel

protein that opens and closes with certain stimuli to allow ions through

Voltage-gated ion channel

proteins that are activated by changes in electrical membrane potential which change the shape

Hyperpolarization

membrane potential becomes more negative

Depolarization

membrane potential becomes less negative (more positive)

Graded potential

small change in a neuron’s electrical charge (membrane potential) in response to a stimulus

larger stimulus = greater shifts but decays over time and distance so not big enough to cause action potential

Action potential

huge shift in membrane potential that travels down the axon w/ constant magnitude → triggered when a graded potential reaches a certain threshold, leading to signal transmission (good for long distances)

Generation of action potentials

1. resting state: neuron = more negative inside

2. depolarization: stimulus opens sodium channels, Na+ rushes in → more positive

3. rising phase of action potential: rapid influx of sodium ions → membrane potential to rise sharply, reaching peak

4. falling phase of action potential: potassium channels open, K+ exits, restoring a negative charge inside

5. undershoot: membrane potential becomes more negative than resting state before returning to normal

Rise in membrane potential

depolarization → cell is becoming more positive

Conduction of action potentials

action potential travels along the axon of a neuron, from cell body to synaptic terminals, through rapid depolarization and repolarization → allowing electrical signal to be transmitted

Frequency of action potential

convey information → EX: louder sounds trigger more frequent action potentials in neurons linking ear to brain

Why is it important to have a fast rate that axons within nerves conduct action potential?

The faster axons are able to conduct action potential, the faster your body receives signals that tells them to respond to danger

Natural selection versus axons

NS favors axons that conduct the fastest rate of action potential → evolutionary adaptations to faster rates by using wider axons or insulating axons

Axon structure in invertebrates

Mollusks have giant axons (very wide) that facilitate very fast signaling for behavioral responses like muscle contraction in hunting

Axon structure in vertebrates

Evolutionary adapted to insulated axons (electrical insulation) to help protect and speed up action potential

Myelin sheaths

electrical insulation that surrounds axons

Oligodendrocytes

glia in the CNS that makes myelin sheaths

Schwann cells

glia in the PNS that makes myelin sheaths

Nodes of ranvier

gaps in the myelin sheath on an axon where action potentials jump → speeds up signal transmission

Saltatory conduction

the process where action potentials jump between nodes of Ranvier which speeds up signal transmission

Synapse

place where information is transmitted

Chemical synapse

junction between two neurons where signals are transmitted using neurotransmitters across a synaptic gap

Post-synapse

after response is triggered → synapse returns to its resting state and neurotransmitters are cleared from the synaptic cleft

cleared by diffusion, recapture for recycling, or broken down into inactive fragments by enzymes

Electrical synapse

rely on movement of electrical current moving from one junction to the next

Where do electrical synapses occur?

common in rapid and unchanging pathways, like giant axons in lobsters and mollusks to have quick escapes from predator

also found in vertebrate brains and hearts

EPSP

when depolarization brings membrane potential closer to threshold (more positive) ** ellie

IPSP

when depolarization brings membrane potential farther from threshold (more negative) ** I bring back to normal

Spatial summation

multiple synapses at same time cause an additive effect → trigger action potential

Temporal summation

if single EPSP occurs before the membrane potential rests, repeated synapses add up, increasing effect → shows how frequency of firing neurons works for intense stimuli

Neurotransmitters

chemical messengers that transmit signals across a synapse from one neuron to another

a single neurotransmitter can bind to 12 receptors!!

Aceytolcholine (ACh)

chief neurotransmitter of parasympathetic (PNS)

Glutamate

most common amino acid neurotransmitter in CNS → primary excitatory neurotransmitter by binding to glutamate receptors

What is glutamate good for?

Long term memory

Biogenic amines

synthesized by amino acids like norepinephrine, dopamine, serotonin (all excitatory neurotransmitters)

Importance of biogenic amines

important role in nervous system disorders/treatments like parkinsons = lack of dopamine

Neuropeptides

short chain of amino acids that act like neurotransmitters

EX: endorphins act as natural pain relievers

Opiates function

mimic endorphins and can inhibit dopamine in re-uptake receptors in presynaptic neurons which overloads dopamine into postsynaptic neurons

Gases

vertebrate can release dissolved gas as neurotransmitters

EX: males release nitric oxide (NO) into erect penis

* viagra stops the enzyme that kills action of NO *

What do many toxins, like those from snakes, spiders, and algae, affect?

affect synaptic transmission

How do inhibitors like crotoxin (rattlesnake) and botulinum toxin (botox) affect neurotransmission?

they inhibit the release of acetylcholine (Ach)

What do blocking agents like saxitoxin (red tide algae) and tetrodotoxin (pufferfish) do?

they block ion channels (Na+, K+) and neurotransmitter receptor sites, preventing normal signaling

How does curare (S. American tree) paralyze an organism?

curare competes with acetylcholine (Ach) at neuromuscular junctions, blocking signaling and causing paralysis

How does nerve gas (sarin) affect the nervous system?

it inactivates acetylcholine (Ach), leading to spastic paralysis

What does strychnine (rat poision) do to neurotransmission?

it interferes with IPSPs in the spinal cord, preventing muscles from "turning off" and causing spasms.

How does cocaine affect dopamine in the brain?

blocks the reuptake transporters for dopamine in CNS neurons, leading to overstimulation and eventual depletion of dopamine