Chapter 2Chapter 2Biological Basis of Behaavior

The Nervous SystemThe Nervous System

• Nervous System: transmits messages throughout the body

• Neuron: specialized cell basic (smallest) unit of the nervous system it transmits messages

• Glial cells: provide “support” for neurons form the myelin sheath, remove waste products

Parts of the NeuronParts of the Neuron

dendritesdendritessoma (cell body)soma (cell body)

axonaxon

myelin sheathmyelin sheath

axon axon terminalterminal

nodes of Ranviernodes of Ranvier

Parts of the NeuronParts of the Neuron• Soma (cell body): cell metabolism

• Dendrites: short fibers on cell body, receive “incoming” chemical messages

• Axon: single fiber extending from cell body, carries “outgoing” chemical message

• Axon Terminal: the end of the axon

• Nerve (tract): group or bundle of axons

Parts of the NeuronParts of the Neuron

• Myelin Sheath: white fatty covering on longer axons composed of Glial cells “insulates”the axon helps messages travel further and faster

• Nodes of Ranvier: points where myelin sheath is “pinched” together

3 Types of Neurons3 Types of Neurons

• sensory (afferent) neurons: carry messages from sense organs to spinal cord and brain

• interneurons (association neurons): carry messages from one neuron to another

• motor (efferent) neurons: carry messages from spinal cord or brain to muscles

Communication Within a NeuronCommunication Within a NeuronThe Action PotentialThe Action Potential

The Action PotentialThe Action Potential

• Action Potential: an “electrical” message that travels down the axon causing release of chemicals from axon terminal (neuron “fires”)

• Ions: charged particles that move in and out of the axon sodium “NA” + is most important also Potassium “K” + and Chloride “CL” --

Action PotentialAction Potential

Time (msec)

Mem

bra

ne

Pot

enti

althreshold of excitation

resting potential

absolute refractory period

relative refractory period

1. Resting State1. Resting State

• neuron is at rest: more “NA” + is outside than inside axon

• neuron is “polarized”: difference in electrical potential between inside and outside of axon is -- 70 millivolts

+ + + + + + + + + +

+ + + + + + + + + +

soma + + + terminal

2. Depolarization2. Depolarization

•

depolarization: dendrites are chemically stimulated channels open, positive NA + ions enter axon difference in potential reaches, then exceeds 0 mv message travels down the axon

+ + + + +

soma + + + + + + + + + terminal

+ + + + +

3. Neuron fires3. Neuron fires

• Neuron fires: only if “threshold of excitation” (firing threshold) is reached now more “NA” + inside than outside the axon charge inside is (+ 40 mv) relative to outside signal reaches axon terminal

+ + +

+ + +

soma + + + + + + + + + + terminal

Neuron May Fail to FireNeuron May Fail to Fire

• threshold of excitation: (firing threshold) level of “depolarization” that must be reached for neuron to fire

• graded potential: stimulation of dendrites was too weak to reach threshold and neuron fails to fire (depolarization just “fades away”)

+ + + +

+ + + +

soma + + + terminal

4. Neuron resets4. Neuron resets• return to resting state:

positive ions now flow back out of axon as before, more “NA” + is outside than inside

axon difference in electrical potential between inside

and outside of axon is again - 70 millivolts

+ + + + + + + + + +

+ + + + + + + + + +

soma + + + terminal

4. Neuron resets (cont.)4. Neuron resets (cont.)

• absolute refractory period: right after firing, neuron will not fire again no matter how strong the incoming message

• relative refractory period: after partially “resetting,”neuron will fire again but only if the incoming message is unusually strong

Other facts about the Action Potential Other facts about the Action Potential

• The entire process has taken only a few milliseconds!

• All or None Law: strength of the action potential does not vary. The neuron either fires or it doesn’t.

• The “Rate of firing” is really what changes. The neuron is never really “at rest”

Communication Between Two Communication Between Two NeuronsNeurons

Synaptic Transmission

axon terminal

synapse

neurotransmitter

synaptic vesicle

receptor site

on dendrite

A “Chemical” ProcessA “Chemical” Process

The SynapseThe Synapse

• Synapse has three parts: 1. axon terminal of first neuron 2. synaptic “cleft” or “space” 3. receptor site on dendrite of second neuron

• Neurotransmitters: chemicals released from the axon terminal

• Synaptic Vescicles: sacs in the axon terminal that contain neurotransmitter

1. Release1. Release

• Action potential reaches axon terminal of neuron #1

• Vescicles open and release neurotransmitter in to synaptic space

• neurotransmitter crosses synaptic space

2. Communication2. Communication

• Neurotransmitter attaches to receptor site and “stimulates” neuron #2

• Each specific neurotransmitter and its receptor site fit like a “Lock and Key”

Serotonin molecule -- Will fit >

Dopamine molecule -- Won’t fit >

Serotonin Receptor site

3. Inactivation3. Inactivation

• Neurotransmitter releases from receptor site, moves back into synaptic space, and is either: 1. taken back up into axon #1 terminal for re-

use OR 2. “broken down” into basic components and

carried away for disposal

Neurotransmitters can speed Neurotransmitters can speed OR slow rate of firingOR slow rate of firing

Excitatory Neurotransmitters: “increase”firing rate in the neuron they

attach toInhibitory Neurotransmitters: “decrease” firing rate in the neuron

they attach to(see Summary Table in book)

NeurotransmittersNeurotransmitters

How Drugs WorkHow Drugs Work• Acetylcholine (Ach) is an excitatory

neurotransmitter at muscle synapses

• Botulism: prevents Ach release, result is paralysis

• Black Widow Venom: causes excess Ach release, result is shaking/tremors

• Curare: blocks (occupies) Ach receptros, result is paralysis

• LSD visual hallucinations may be due to blocking of serotonin receptors

Experience and PlasticityExperience and Plasticity

Mark Rosenzweig’s ExperimentMark Rosenzweig’s Experiment

• Two groups of rats one raised in a boring unstimulating

environment one raised in a complex stimulating

environment

• The second group had larger neurons with more synapses

• Plasticity: extent to which the brain will change in response to experience

The Nervous SystemThe Nervous System

Nervous System

Central Nervous System

Brain Spinal Cord

Peripheral Nervous System

Somatic Nervous System

Autonomic Nervous System

Sensory Neurons

Motor Neurons

Sympathetic Nervous System

Parasympathetic Nervous System

Central Nervous System Central Nervous System (2 parts)(2 parts)

• 1. Brain: has 3 divisions. . . . later

• 2. Spinal Cord: large bundle of “nerves” which connects the rest of the body to the brain

Cross-section of spinal cord, with numerous nerves (tracts)

Peripheral Nervous SystemPeripheral Nervous System• Autonomic Division: carries messages

between the internal organs and CNS Sympathetic: arouses, prepares body for “fight or

flight” Parasympathetic: relaxes body

• Somatic Division: carries messages between sense organs/muscles and CNS messages from CNS to muscles “reflex arc” - example of a complete circuit

The Reflexe ArcThe Reflexe Arc

The Reflex ArcThe Reflex Arc

• involves 3 neurons• 1. Afferent (sensory) neuron: carries

sensory information from body to spinal cord (hit with hammer)

• 2. Efferent (motor) neuron: carries motor information from spinal cord to muscles (move leg)

• 3. Association (inter) neuron: connects the two other neurons

Forebrain Cerebral cortex Thalamus Hypothalamus

Midbrain

Hindbrain Cerebellum Pons Medulla

Cross-section of the BrainCross-section of the Brain

1. Hindbrain (Brainstem)1. Hindbrain (Brainstem)• The “oldest” part of the brain (3 parts)• 1. medulla: controls “automatic” functions

such as breathing, heart rate, blood pressure, ALSO place where many axons “cross over” from right to left

• 2. pons: links cerebellum to motor areas of brain and to muscles of body

• 3. cerebellum: balance and movement

Reticular FormationReticular Formation

• Also located in the brainstem, the reticular formation plays an important role in controlling alertness and the “sleep-wake cycle”

2. Midbrain2. Midbrain• A large “relay station”

• Many synapses are located here

• Visual and Auditory information is relayed here

Limbic System

Group (ring) of structures surrounding the midbrain

1. Amygdala - Self preservation (fear and aggression)

2. Hippocampus - Formation of new memories

3. Forebrain (3 basic parts)3. Forebrain (3 basic parts)

• 1. thalamus: “a relay station” for visual and auditory sensory information

• 2. hypothalamus: controls “motivated behaviors”: thirst, hunger, and sexual behavior

• 3. cerebral cortex: see next slide

Forebrain (cont.)Forebrain (cont.)

• 3. Cerebral Cortex: “newest” part of the brain, consists of TWO hemispheres, right and left

• Corpus Callossum: bundle of axons connecting right and left hemispheres

• Convolutions: folds and creases that allow the cortex to fit in the skull

Lobes of the BrainLobes of the Brain

• Each hemisphere is divided into four “lobes”

• 1. Occipital Lobes: interprets visual information

• 2. Parietal Lobes: sense of touch (primary somatosensory cortex)

Lobes of the Brain (cont.)Lobes of the Brain (cont.)

• 3. Temporal Lobes: process auditory information

• 4. Frontal Lobes: higher mental processes language, personality, problem solving, etc. motor projection areas control muscles

Hemispheric SpecializationHemispheric SpecializationRight and left hemispheres are involved

in specific functions

• right hand touch

• right visual field

• language

• logic / math

Left HemisphereLeft Hemisphere

Right HemisphereRight Hemisphere

• left hand touch

• left visual field

• spatial ability

• art

• fantasy

Path to Occipital LobePath to Occipital Lobe

Split Brain PatientsSplit Brain Patients• In 1950s, cutting the Corpus Callosum

reduced siezures in cases of severe epilepsy

• This severs the connection between right and left hemispheres

• Sperry and Gazzaniga discovered some unusual consequences

Split Brain ResearchSplit Brain Research

• Right visual field: what is to the right of the person is directed to left hemisphere

• Left visual field: what is to the left of the person is directed to right hemisphere

• This “crossing over” from right to left takes place at the “optic chiasm”

Split Brain ResearchSplit Brain Research• Right hand touch: what is felt with the

right hand is directed to left hemisphere

• Left hand touch: what is felt with the left hand is directed to right hemisphere

• Remember, language is on the left so the image stored in the right brain cannot be verbally identified (if corpus calossum is cut)

Split Brain ResearchSplit Brain Research

can pick out hammer with left handcan’t pick out hammer with right hand

Split Brain ResearchSplit Brain Research

can pick out ball with right handcan’t pick out ball with left hand

Tools for Studying the Tools for Studying the Nervous SystemNervous System

• microelectrode recording: a needle-like probe records the functioning of a single neuron

• macroelectrode recording (eeg): sensors on scalp measure overall brain activity

Studying Brain StructureStudying Brain Structure

• Computerized Axial Tomography: (CAT scan) a computer combines multiple x ray images forming a 3-D image

• Magnetic Resonance Imaging: (MRI) brain is exposed to radio waves in a magnetic field, release of energy from cells forms a computer image

Studying Brain FunctionStudying Brain Function

• Positron Emission Tomography: (PET scan) active areas absorb more of a radioactive substance, release of particles shows which areas were more active (e.g., while speaking)

• Magnetoencephalography: (MEG) records magnetic energy given off by brain areas which are active

The Endocrine SystemThe Endocrine System

• works closely with the nervous system

• glands: produce and release hormones (e.g., adrenal gland)

• hormones: like neurotransmitters but travel through the bloodstream (e.g., adrenaline)

Behavior Genetics and HeredityBehavior Genetics and HeredityNature-Nurture Debate: is behavior determined by genes or experience?

Behavior Genetics: studies how genes combine with experience to produce behavior

Terminology of GeneticsTerminology of Genetics

• traits: characteristics (e.g., eye color, personality)

• heredity: transmission of traits from one generation to the next

• deoxyribonucleic acid (DNA): complex molecules, building blocks for genes

Genes and ChromosomesGenes and Chromosomes• gene: a chain of DNA molecules that

control a trait long chains of genes make up the chromosomes

• chromosomes: pairs of twisting threadlike structures, human cells have 23 pair (except sperm and

ovum which have only 23) a chromosome looks like a twisting ladder

Laws of InheritanceLaws of Inheritance

• Gregor Mendel: Austrian Monk who discovered laws of dominant and recessive inheritance in 1800s

Genes and InheritanceGenes and Inheritance

• Genes come in pairs “alleles” (one from mom and one from dad)

• Homozygous pair: mom and dad have contributed identical genes (e.g., eye color Brown + Brown)

• Heterozygous pair: mom and dad have contributed different genes (e.g., eye color Brown + blue)

Genotype vs. PhenotypeGenotype vs. Phenotype

• Genotype: underlying genetic code for a trait (a person does not have blue eyes but does carry the gene for them)

• Phenotype: actual outward appearance of a trait (a person has blue eyes)

Dominant and Recessive GenesDominant and Recessive Genes• For some traits, one gene “B” is dominant

over the other “b”

• Dominant: trait will usually appear in the phenotype B + B = Brown eyes B + b = Brown eyes (carries b trait)*

• Recessive: trait will appear in phenotype ONLY if mom and dad both contributed the recessive gene b + b = blue eyes

Studying Genetics in AnimalsStudying Genetics in Animals

• strain studies: “Strains” are almost identical genetically. If two different strains are raised in identical environments, then behavioral differences point to “genetics”

• selection studies: If a trait is genetic, breeding animals with a specific trait should produce many offspring with that trait

Studying Genetics in HumansStudying Genetics in Humans

• family studies: If genes influence a trait, closer relatives should be more similar on that trait than distant relatives.

• twin studies: If a trait is genetic, identical twins (share 100% of genes) should be more similar on the trait than fraternal twins (share 50% of genes).

Studying Genetics in HumansStudying Genetics in Humans

• Adoption Studies: If trait is genetic, adopted children

should be more similar to biological parents

If trait is “learned,” adopted children should be more similar to adoptive parents