Chapter 2Neuroscience and Behavior

Neuron

Nerve cells Run through our entire body and

communicate with each other. Neurons are like a tree

› Have branches, trunk, roots and they lie among one another.

3 types

Types of Neurons

Types of Neurons

Sensory Neuron: conveys information to the brain from specialized receptor cells in sense organs and internal organs

Motor Neuron: Signals muscles to relax or contract

Interneuron: communicates information from one neuron to the next› Most neurons in the body are interneurons

The Cell Body (Soma)

Contains the cell’s nucleus› round, centrally located structure › contains DNA› controls protein manufacturing › directs metabolism› no role in neural signaling

Dendrites

• Information collectors• Receive inputs from neighboring

neurons• Inputs may number in thousands• If enough inputs, the cell’s AXON may

generate an output

Dendritic Growth

• Mature neurons generally can’t divide • But new dendrites can grow• Provides room for more connections to

other neurons• New connections are basis for learning

Axon

• The cell’s output structure• One axon per cell, 2 distinct

parts› tubelike structure › branches at end that connect to dendrites

of other cells

Myelin Sheath

• White fatty casing on axon • Acts as an electrical insulator • Not present on all cells• When present, increases the speed of

neural signals down the axon

Glial Cells Support cells that assist neurons by providing structural

support, nutrition, and removal of cell wastes, manufacture myelin.

Glial Cells• Outnumbering brain neurons by about 10 to 1, glial

cells provide support and nutrition for neurons.

• Astrocytes are one type of glial cell that provides connections between neurons and blood vessels in the brain.

• Glial cells are much more actively involved in regulating neuronal communication and activity than previously believed.

How Neurons Communicate

• Neurons communicate by means of an electrical signal called the action potential

• Action potentials are based on the movements of ions between the outside and inside of the cell

• When an action potential occurs, a molecular message is sent to neighboring neurons

Stimulus Threshold: the minimum level of stimulation to activate the neuron.

Polarized Neuron

There is a difference in the electrical charge between the inside and the outside of the axon.

Greater concentration of negative ions inside than out.› Negative inside/positive outside

There are different concentrations of potassium and sodium.› Fluid surrounding the axon contains a larger

concentration of sodium.› The fluid in the neuron has a larger concentration of

potassium.

Communication•Resting Potential: At rest, the inside of the cell is at -70 microvolts

•When sufficiently stimulated by other neurons the neuron depolarizes; starting the action potential.

•Figure shows resting axon being approached by an action potential

Depolarization Ahead of AP•Action potential opens cell membrane, opens for a mere thousandth of a second, to allow sodium (Na+) in•Potassium channels open next allowing potassium to flow out of the axon into the surrounding area. •Inside of cell rapidly becomes more positive than outside to +30 millivolts.•Causes a brief electrical impulse that moves down the axon.

Repolarization Follows

•After depolarization potassium (K+) moves out restoring the inside to a negative voltage•This is called repolarization•The rapid depolarization and repolarization produce a pattern called a spike discharge

Finally, Hyperpolarization

•Repolarization leads to a voltage below the resting potential, called hyperpolarization•Now, neuron cannot produce a new action potential•This is the refractory period

Communication

All-or-none law: either a neuron is sufficiently stimulated and an action potential occurs or a neuron is not sufficiently stimulated and an action potential does not occur.

Communication occurs of speeds up to 270 miles per hour – 2 miles per hour.

Neuron to Neuron

• Synapse: Point of communication between two neurons.

• Synaptic Gap: tiny space between the axon terminal of one neuron and the dendrite of an adjoining neuron.

• Axon terminals: Branches at the end of the axon that contain tiny pouches called synaptic vesicles.

• Neurotransmitters: Chemical messengers manufactured by a neuron.

Neuron to Neuron

Synaptic Transition: Process through which neurotransmitters are released by one neuron, cross the synaptic gap, and affect adjoining neurons.

Reuptake: Process by which neurotransmitter molecules detach from a postsynaptic neuron and are reabsorbed by a presynaptic neuron so they can be recycled and reused again.

Neurotransmitter Release

Action potential causes vesicle to open Neurotransmitter released into synapse Locks onto receptor molecule in

postsynaptic membrane› One way transaction

Neurotransmitter molecules have specific shapes

Receptor molecules have binding sites

Excitatory and Inhibitory Messages

• Excitatory message—increases the likelihood that the postsynaptic neuron will activate and generate an action potential.

• Inhibitory message—decreases the likelihood that the postsynaptic neuron will activate

Neurotransmitters

Disruptions of Acetylcholine Functioning

• Curare—blocks ACh receptors› paralysis results

• Nerve gases and black widow spider venom; too much ACh leads to severe muscle spasms and possible death

Cigarettes—nicotine works on ACh receptors› can artificially stimulate skeletal muscles, leading

to slight trembling movements

Dopamine

• Involved in movement, attention, and learning

• Dopamine imbalance also involved in schizophrenia

• Parkinson’s disease is caused by a loss of dopamine-producing neurons

Parkinson’s Disease

Results from loss of dopamine-producing neurons in the substantia nigra

Symptoms include› difficulty starting and stopping voluntary

movements› tremors at rest› stooped posture› rigidity› poor balance

Serotonin

• Involved in sleep• Involved in depression

› Prozac works by keeping serotonin in the synapse longer, giving it more time to exert an effect

GABA

• Causes inhibitory message to other neurons to help balance and offset excitatory messages.

• Found in brain• Too much impairs learning, motivation and

movement• Too little can lead to seizures• Alcohol makes people feel relaxed partly

because GABA is increased– reduces brain activity.

• Valium and Xanax increase GABA, which inhibits action potentials.

Endorphins

• Control pain and pleasure• Released in response to pain• Morphine and codeine work on

endorphin receptors• Runner’s high— feeling of pleasure

after a long run is due to heavy endorphin release

Drugs and Synaptic Transmission

Drugs increase or decrease the amount of neurotransmitters released.

Can block the reuptake of neurotransmitters.› Antidepressants SSRIs: Selective serotonin

reuptake inhibitors. Prozac, Zoloft, Paxil Drugs inhibit the reuptake of serotonin in certain

neurons. Increases the availability of serotonin in the brain.

› Cocaine interferes with the reuptake of dopamine.

Drugs and Receptors

Some drugs are shaped like neurotransmitters and mimic them.

Antagonists: fit the receptor and block the NT› Naloxone block opioid

receptor sites. Use for heroin overdose

Agonists: fit receptor well and act like the NT› Nicotine similar to

acetylcholine

Parts of the Nervous System

Central nervous system (CNS)› Brain and spinal

cord Peripheral nervous

system (PNS)› Carries messages

to and from CNS

Endocrine System• System of glands located throughout the body

that secrete hormones into the bloodstream• Hormones: Stimulate growth and different

reactions, such as activity levels and mood.› Have specific receptor sites just like

neurotransmitters.• Hypothalamus: Structure of the brain that

regulates the release of hormones from the pituitary gland.

Endocrine System

• Pituitary gland—attached to the base of the brain, secretes hormones that affect the function of other glands

• Adrenal glands—hormones involved in human stress response• Adrenal Cortex: outer portion of the adrenal glands;

secrete hormones that interact with the immune system.

• Adrenal Medulla: inner portion of the adrenal glands; secretes epinephrine and norepinephrine.

• Gonads (sex organs)—hormones regulate sexual characteristics and reproductive processes; testes in males, ovaries in females

Plasticity and Neurogenesis

Neuroplasticity: the brains ability to change function and structure.

Functional Plasticity: Brain’s ability to shift functions from damaged to undamaged brain areas.› Stroke victims relearn speaking, walking, or

reading. Undamaged areas assume the ability to process

and execute tasks. Structural Plasticity: Brain’s ability to change

its physical structure in response to learning, active practice, or environmental influence.

Neurogenesis

Development of new neurons. Stress, exercise, environment and

social status all affect the rate of neurogenesis.

New neurons are incorporated into the existing neural networks in the brain.

Brainstem

Region of the brain made up of the hindbrain and the midbrain.

Hindbrain: Region at the base of the brain that contains several structures that regulate basic life functions.

Hindbrain The Hindbrain:

› Made up of the Medulla, Pons, and the Cerebellum.

› Medulla: Involved in vital functions such as heart rate, blood pressure and breathing.

› Pons: connects the medulla to the two side of the cerebellum. Regulates body movement,

attention, sleep and alertness.› Cerebellum: two-sided structure at

the back of the brain. Involved in balance and

coordination.› Reticular Formation: nerve fibers

located in the center of the medulla that helps regulate attention, arousal, and sleep (reticular activating system)

Midbrain Located between the

hindbrain and the forebrain. Involved in vision and

hearing.› EX: eye movement

Substantia Nigra: motor control and contains a large concentration of dopamine-producing neurons.

Reticular activating system.› Important for attention,

sleep, and arousal.› Begins in the hindbrain and

rises through the midbrain into the lower part of the forebrain.

› Some drugs, such as alcohol reduce its activity.

Forebrain

Four key areas of the forebrain.› Thalamus› Hypothalamus› Limbic System› Cerebral

Cortex

Cerebral Cortex

Wrinkled outer portion of the forebrain, which contains the most sophisticated brain centers.› Composed of glial cells and neuron cell bodies and

axons- gives it a grayish color (gray matter) and white myelinated axons (white matter).

Cerebral Hemispheres: Nearly symmetrical left and right halves of the cerebral cortex.

Corpus Callosum: thick band of axons that connects the two cerebral hemispheres and acts as a communication link between them.

Lobes of the Cortex

• Frontal lobe—largest lobe, produces voluntary muscle movements; involved in thinking, planning, and emotional control

• Temporal lobe—primary receiving area for auditory information

• Occipital lobe—primary receiving area for visual information

• Parietal lobe—processes somatic information

Limbic System

Forms a fringe along the inner edge of the cerebrum.

Involved in learning and memory, emotion, hunger, sex, and aggression.

If damaged people can recall old memories, but cannot create new ones.

Damage can also make people act aggressively or passive.

The Limbic System

Hippocampus: Curved forebrain structure that is part of the limbic system and is involved in learning and forming new memories.

Amygdala: almond shaped cluster of neurons in the brain’s temporal lobe.› Involved in memory and emotional

response; especially fear.

Thalamus

Latin meaning “inner chamber”

Serves as a relay station for sensory stimulation.

Transmits sensory information to the parts of the brain that respond and interpret the information.

Relays sensory input from the eyes and the ears to the appropriate parts of the brain.

Hypothalamus

Contains nuclei involved in a variety of behaviors› sexual behavior› hunger and thirst› sleep› water and salt

balance› body temperature

regulation› circadian rhythms› role in hormone

secretion

Cortical Specialization

• Localization—notion that different functions are located in different areas of the brain

• Lateralization—notion that different functions are processed primarily on one side of the brain or the other

Language and the Brain

• Aphasia—partial or complete inability to articulate ideas or understand language because of brain injury or damage

• Broca’s area—plays role in speech production

• Wernicke’s area—plays role in understanding and meaningful speech

Cutting the Corpus Callosum

Split-brain operation: surgical procedure that involves cutting the corpus callosum.› Used to stop or reduce recurring seizures.› Contains seizures to one hemisphere.