Genetics, Evolution, Methods and Neurons Exam 1

Genotype

: full hereditary information

Phenotype

actual observed properties

Genes

units of heredity (something that can be passed down)

Chromosomes

strands of genes; composed of DNA

DNA

recipe for proteins

What do proteins do and list some examples

• Make structures

  • Receptors are proteins

  • Things that give neurons their structure are proteins

• Control biological reactions

  • Enzymes (make/break down neurotransmitters) are proteins

DNA vs RNA

DNA is a double-stranded molecule that stores genetic information, while RNA is typically single-stranded and plays a role in protein synthesis.

Homozygous

identical pair of genes on 2 chromosomes

Heterozygous

unmatched pair of genes

Dominant

• effect in either homozygous or heterozygous condition

  • One copy of the gene is enough for the trait to be expressed

  • Not more likely to be passed down

Recessive

• effects only in homozygous condition

  • Both genes have to match for the trait to be expressed

What are four important things to remember about genes

• many genes = one outcome

• Genes can be expressed differently in different parts of the body

• Genes can be expressed in some circumstances but not others

• Behaviors in humans are usually a combination of genetic influences and environmental influences

Mutation

heritable change in DNA

What does a change in DNA cause

a change in proteins

Epigenetics

changes in gene expression without changing the DNA

Learning and memory are a result of

changes in gene expression

What is something that can turn genes on and off

experiances

Epigenetic differences are a likely explanation for

differences between monozygotic “identical” twins

How is heritability studied?

• Monozygotic vs. dizygotic twins

• Adopted children

• Biochemical methods

How are Monozygotic vs. dizygotic twins used to study heritability?

• Monozygotic twins - share genes

• Dizygotic twins - do not share more genes than other siblings

• If monozygotic twins show more similarity than dizygotic twins…

How are adopted children used to study heritability

If adopted kids resemble biological parents, rather than parents who raised them, you know that the trait was inhertited.

How are biochemical methods used to study heritability

Take samples from people, sequence a gene of interest, and see if the form of the gene they have predicts the behavior

Describe how genes affect behavior

• Genes do not directly produce behaviors

• Genes produce proteins that increase the probability that a behavior will develop under certain circumstances

• Genes can also have an indirect affect

• Genes can alter your environment by producing behaviors or traits that alter how people in your environment react to you

Evolution

• change over generations in the frequencies of various genes in a population

• Genes that are associated with reproductive success will become more prevalent in successive generation

artificial selection

Breeders chose plants or animals with desirable trait and make them parents of next generation

What are four things needed to qualify something as artificial selection?

• Requires that variation exists

• Requires that trait is heritable

• Breeder ensures that some individuals reproduce more successfully

• Gene becomes more prevalent with each generation

Natural selection

• Popularized by Charles Darwin

• Gradual process by which traits become more or less common in population as function of reproductive success

Identify/describe techniques used to study brain structure

• Brain damage

• Record activity during behavior

• Correlate brain anatomy with behavior

Brain Damage (example of how brain damage can be used to study brain structures)

Broca (1861): Patient with damage to left frontal cortex lost ability to speak

• Pattern across patients

• Brain damage can result in very specialized behavioral impairments

Humans rarely have damage to just one area, and site of lesion varies

Ablation

removal of brain area

Lesion

damage to brain area

What is an advantage of studying brain damage

Can be controlled in lab animals

Sham lesion

everything but current

How is a Stereotaxic instrument used: device precisely places electrode in brain

• Anesthetize animal, drill hole in skull, insert electrode, put it into position, pass current to damage area

• Sham lesion: everything but current

Hypothalamus

a region of the forebrain below the thalamus which coordinates both the autonomic nervous system and the activity of the pituitary, controlling body temperature, thirst, hunger, and other homeostatic systems, and involved in sleep and emotional activity

Transcranial magnetic stimulation (TMS) this isn’t your kind at work it actually does stimulate the neurons and generates action potentials

Apply intense magnetic field to part of scalp, which temporarily inactivates neurons

Measure behavior before, during, and after stimulation (before, during, and after neuron inactivation)

Ex: During inactivation of visual cortex, no conscious perception of stimuli, but still eye movements

List non-invasive ways to record brain activity

• Electroencephalograph (EEG)

• Magnetoencephalograph (MEG)

• Positron-emission tomography (PET)

• Functional magnetic resonance imaging (fMRI)

Electroencephalograph (EEG)

pros of EEG

• Excellent temporal resolution: millisecond-bymillisecond

• Electrical signal is direct measure of brain activity

Cons of EEG

• Limited spatial resolution:

• Over a population of cells

• Scalp/skull/brain tissue blur electrical signal

• Signal varies with cortical depth and orientation of neurons

• Magnetoencephalograph (MEG)

• Measures tiny magnetic fields generated by brain activity

• Magnetic field less impacted by scalp - better spatial resolution

• Positron-emission tomography (PET)

• Inject radioactive chemical used by brain, typically glucose

• As it decays, it emits gamma rays

• Scanner picks up on where those rays come from

• Areas with most radiation are presumably area with most brain activity

Functional magnetic resonance imaging (fMRI)

• Measures changes in blood flow and oxygen content in blood

  • Scanner is strong magnet

  • Magnetic properties of blood let you see where brain is active

• When brain area is more active:

  • More blood flow

  • Less oxygen in blood

Pros of fMRI

• Good spatial resolution – millimeters

Cons of fMRI

• Poor temporal resolution: Hemodynamic response starts 1-2 s after neurons fire, and peaks after ~6 s

• Remember that neural responses on are on the order of milliseconds

• MRI scanner is strong magnet: puts limits on who can participate

• Scanner is noisy and small

• You can’t move: limitations on tasks/responses

CAT

• Dense structures (e.g., bone) appear white, while less dense materials (e.g., air) appear dark

• Hyperdense = brighter than brain

• Hypodense = darker than brain

• Grey and white matter are more dense than CSF

• Fat is less dense than water

• White matter has higher fat content (myelin) than gray matter, so it appears darker

MRI

• Uses powerful magnetic field (same machine as fMRI)

• Tissues with different water content react differently to magnet

• CSF - lots of water

• Gray matter - some water

• White matter - least water (myelin)

Diffuser Tensor Imaging

• White matter connectivity

• Looks at direction of water movement

• Water more easily flows down axon than across it

Ramon y Cajal

found that the brain is composed of single cells separated by small gaps

• Used staining (Golgi method) in late 1800s

• Stains whole cells, but only some of them

Neurons

cells that receive information and transmit it to other cells

Glia

support cells with other functions

Synapses

gaps between neurons

Two types of brain cells

neurons, glia

Three major parts of the neuron

Dendrites, soma, and axon

Dendrites

receive information

Neuron can have any number

• Can have many branches

• More surface area = more receptors

• Some dendrites have dendritic spines

Soma (cell body)

Metabolic work: reactions that maintain cells

• Has basic cell parts (nucleus, ribosomes, mitochondria)

• Controls metabolic function

– Breaking down molecules to get energy

– Making compounds the cell needs

• DNA here; some proteins made here

• Many cell bodies have synapses, like dendrites do

Axon

sends information towards other neurons/organ/muscle

• Presynaptic terminal/end bulb/bouton

• One per neuron (but it can branch)

• can be meter or more in length

• In vertebrates, can be covered in myelin sheath – Makes communication faster

• Presynaptic terminal at end of each branch: releases chemicals into synapse

Name the different types of glia

Astrocytes – Microglia – Oligodentrocytes/Schwann cells – Radial glia

Which are more numerous, neurons or glia? Which are bigger

1. glia

2. neurons are normally bigger

Astrocytes

• • Star shaped (like an asterisk)

• Dilate blood vessels – More activity in neurons leads to need for nutrients

• Wrap around presynaptic terminals of groups of related axons – Take up chemicals and re-release them to synchronize activity – Allow neurons to send messages in waves

Microglia

• Very small

• Immune function - Remove waste, viruses, fungi

• Other immune cells can’t usually get to the brain/spinal cord, so microglia recognize foreign bodies and deal with them

Oligodentrocytes

brain

Schwan cells

periphery

Radial glia

During development, guide neurons and axons and dendrites

• After development, most radial glia differentiate into neurons, and some into astrocytes and oligodendrocytes

Oligodentrocytes/ Schwan cells

Both build myelin – White matter – Insulates some vertebrate axons • Increases signal speed – Each segment wraps 30-50 axons

Vertebrate neurons depend almost entirely on

Glucose

Electricity

Flow of charge

Electrical potential

energy that results from storing

charges

– Measured in volts

– For neurons, we’ll be talking about millivolts (mV)

membrane potential

Difference in voltage between inside and outside of

neuron

Threshold of excitation

Action potentials happen when stimulation

is beyond threshold

– Subthreshold stimulation - small response

proportional to amount of current

– Beyond threshold, regardless of how far

beyond, same big response = all-or-none law

• Rapid depolarization, followed by reversal, then

back to baseline

At rest sodium channels are (1) and potassium channels are (2)

1) closed

2)almost closed

When membrane reaches the excitation threshold what happens to the channels for sodium and potassium

Both open

What percentage of sodium ions flood the cell when sodium channels are open?

Less than 1% or outside concentration of sodium

Where does the refractory period come from?

Sodium channels that

opened to let sodium in are inactivated

– Sodium channels remain inactivated until membrane

hyperpolarizes

– Once hyperpolarized, channels de-inactivate, and regain ability to

open in response to stimulus

Propagation of Action Potential

At axon hillock, sodium ions enter

• That area positively charged in comparison with

neighboring portion of axon

• Positive ions flow within axon to neighboring region

• Positive charge depolarizes next portion of

membrane, causing next portion to reach threshold

and open voltage-gated channels

• Action potential generated in each section of axon at

same strength

Electrical charge flows in both directions, so

why doesn’t the action potential?

Areas just passed are in refractory period

Charles Scott Sherrington (1906)

communication between neurons differs from communication along an axon

–Communication along an axon = action potential

•Electrical in nature: charged ions flow

–Specialized gap between neurons = synapses

•How does message get across synapse?

–Based on observations of reflexes

Reflexes

Automatic muscular response to a stimulus

Reflex arc

circuit from sensory neuron to muscle response

–Sensory neuron senses touch

–Excites second neuron (intrinsic neuron)

–Intrinsic neuron excites motor neuron

–Motor neuron excites muscle

Why are reflexes slower than conduction along an axon?

What happens between neurons is slower than what happens within a neuron

Cumulative effect of stimuli

Several weak stimuli at different times or locations produce reflex together that they don’t produce individually

Inhibition

When one set of muscles is excited, different set relaxed

Stimuli that happen near each other at the same time can have a (1) effect

cumulative

Neurons can only tell muscles to contract, not to

relax

Excitatory Post Synaptic Potentials

If stimulate twice with large gap of time in between, two EPSPs

•If time interval between stimulation is short enough, the second adds to the first = temporal summation

Not an action potential

If stimulate twice with large gap of time in between, two EPSPs

•If time interval between stimulation is short enough, the second adds to the first = temporal summation

Input from axon can hyperpolarize postsynaptic cell

–Increases the negative charge inside cell

•Further from threshold, less likely to fire action potential

•Synaptic input opens channels for potassium (positively charged) to leave cell or chloride (negatively charged) to enter cell

•Like EPSPs, IPSPs decay over time and space

–Don’t travel like action potentials

Excitatory

–Depolarizing - closer to threshold of excitation

–Make action potentials in post-synaptic cell more likely

–Increase firing rate over spontaneous rate

–Synaptic input opens sodium channels

•Na+ enters cell

Inhibitory

–Hyperpolarizing - further from threshold of excitation

–Make action potentials in post-synaptic cell less likely

–Decrease firing rate below spontaneous rate

–Synaptic input opens potassium gates or chloride gates

•K+ leaves cell

•Cl- enters

Otto Lowei

nerves stimulate muscle by releasing chemicals

–Experiment:

•Stimulate vagus nerve of Frog 1 to decrease heart rate

•Collect fluid from around Frog 1’s heart and transfer to Frog 2’s heart

•Frog 2’s heart also decreases rate, even though you haven’t touched its nerves

–Conclusion: Something collected in the fluid (chemical) controls heart rate

•When you stimulate the vagus nerve, chemicals get released into the fluid around the heart

•Those chemicals change the heart rate of Frog 2

Chemical transmission

Neurotransmitter

chemical, released at a synapse, that affects another neuron

–Can be excitatory or inhibitory

Glutamate

most plentiful excitatory neurotransmitter

•Associated with learning and memory, excitotoxicity

GABA (gamma-aminobutyric acid)

typically inhibitory

•Some sedatives act on GABA receptors

Acetylcholine

Loss in Alzeheimer’s disease, neuromuscular junction

Dopamine

Movement, reward

Norepinephrine

Mood, arousal, fight or flight

Serotonin

Sleep state, mood

Chemical Events at Synapse

Neurons make chemicals that serve as neurotransmitters

•Action potential at presynaptic terminal lets calcium enter cell, which cause release of neurotransmitter into synaptic cleft

•Released neurotransmitter molecules attach to receptors on postsynaptic neuron and alter its activity

•Neurotransmitter separates from receptors and can be taken back into presynaptic neuron or may diffuse away

•Some postsynaptic cells send reverse message to control release of neurotransmitter

How do neurons make neurotransmitters

Amino acids, which come from diet

Where are most neurotransmitters made

Pre-synaptic terminal

Where are neurotransmitters packaged into

Vesicles