Neurons and Neurotransmitters

Neurons

Neurons:  individual cells in the nervous system that receive, integrate, and transmit information

-  Process information

-  Convey information to other neurons in the brain

-  Ultimately generate behavior and experience

-  Evolution

-  First developed

-  Sea anemone, jellyfish.

-  Adaptive

-  Allowed them to move under their own power
-  Became fixed in evolution
-  The same electrochemical mechanisms exist for  conducting information in all nerves

-  Special Features

-  Dendrite

Dendrite:  Parts of the neuron that are specialized to receive information
-  Combine into feeler-like structures known as dendritic trees
-  Receive information from other neurons

-  Axon

Axon:  A long, thin fiber that transmits signals away from the cell body to other neurons or to muscles or glands.

-  Myelin sheath

Myelin sheath:  Insulating material, derived from glial cells, that encase some axons
-  Speeds up the transmission of signals along the axon.
-  Insulates the signals of one axon from the axons of another neuron
-  Multiple sclerosis

-  Synaptic buttons (axon terminals)

Synaptic button: Small knobs that secrete neurotransmitters located on the end of the axon.

-  Synapse

Synapse: A junction where information is transmitted from one neuron to another
-  The functional connection that permits one neuron to communicate with another
-  The most important part of a neuron
-  Vesicles
-  Clustered at the very end of the axon
-  Contain the neurotransmitter
-  Synaptic cleft
Synaptic cleft:  a microscopic gap between the terminal button of one neuron and the cell membrane of another neuron

Nerve Cell Communication

-  Within Cell

-  Action potential

-  All or nothing
-  Same in all axons
-  Speed of transmission differs
2 mph - 200 mph
-  Intensity
-  The number of times the neuron fires
-  The number of neurons affected

-  Between Cells

-  Synaptic transmission

-  Involves the release of chemical transmitter molecules
-  Graded

-  Factors affecting amount of transmission

-  Number of molecules released
-  Number of synapses active

-  Action Potential

-  Axon Interior

-  Many protein molecules
-  Very little sodium
-  Negative charge

-  Exterior Tissue Fluid

-  Lots of Sodium
-  Very little protein
-  Positive charge

-  Inside negative relative to the outside

-  Resting potential

Resting potential: the stable, negative charge when neuron is inactive

-  Nerve Impulse

-  Involves the movement of sodium molecules across the axon membrane
Nerve impulse: a brief, local inrush of positively charged sodium ions
-  Occurs only on a little region of the axon
-  Where the nerve impulse is at a given point in time
-  Nerve cell at rest
-  Sodium channels are closed
-  Activated nerve cell
-  Sodium channels pop open in response to a nerve impulse
-  Results in an action potential
Action potential: A brief change in a neuron’s electrical charge
-  Sodium ions enter the cell
-  Voltage at that place shifts from negative to positive
-  Process continues in a sequence from top of axon to bottom

-  Synaptic Transmission

-  Set off by action potential

-  Vesicles release a shower of chemical transmitter molecules
-  Molecules diffuse across the synaptic cleft
-  Attach to receptor molecules on the outside of the target cell membrane
-  Effects
-  No effect at all
-  Excite neuron
-  Increase rate of firing
-  Inhibit neuron
-  Decrease rate of firing

Neurotransmitters

-  75 different neurotransmitters types

-  Specific type of neuron can only manufacture one type of neurotransmitter

-  Specific neurotransmitters work on specific kinds of synapses

-  Lock and key

-  Binding triggers receptor molecules into action

-  Endorphins (endogenous morphines)

Endorphins: the family of internally produced chemicals that resemble opiates in structure and effects

-  Opium

-  Extract of poppy plant
-  Relieves pain
-  Induces feelings of pleasure

-  Morphine

-  The major active ingredient of opium
-  Simple molecule with a particular shape
-  Led to the discovery of the endorphins
-  Best understood of all drugs that act on the brain
-  Morphine antagonists
Antagonist: a chemical that opposes the action of a neurotransmitter
-  Can be made in the lab
-  Naloxone
-  Rapidly and completely reverses the effects of morphine

-  Morphine-related drugs and their antagonists

-  Chemically and structurally similar
-  Suggests existence of opiate receptor system

-  Opiate Receptors in Brain

-  Discovered by Solomon Snyder and Candace Pert (1974)

-  Suggests brain must produce own opiates

- Endorphine

-"endogenous morphine"

-  Natural brain opioids (Endorphines)

-  Several types exist in the brain

- Peptide compounds

-  Relieve pain and induce feelings of pleasure

-  Highly addictive

-  Purpose of Endorphins

-  Released during stress

-  Counter pain and suffering induced by stress

-  Minor injuries
-  Childbirth

-  Addictiveness of stressful activities

The Brain: Structure and Function

Tools of Discovery

Clinical observations

-Oldest method

-Methodically recording effects of damage to the brain

- Still in use



Manipulating the brain

Stimulating specifica areas of the brain

Lesions


Recording electrical activity

Microelectrodes

Electroencephalogram (EEG)

Noninvasive

Computed Tomography (CT) scans

•Using x-rays to photograph the brain

•Benefits

–Pictures of bone, soft tissue and internal organs

–Quick

Positron-Emission Tomography

•Diagnostic technique

–Uses radioactive tracers

•Measures blood flow in different parts of the brain

–Consumption of glucose

Magnetic Resonance Imaging (MRI)

•Head placed in strong magnetic field

•Affects spin of atoms

•Produces detailed picture of brain soft tissue

Functional Magnetic Resonance Imaging (fMRI)

Used to locate mental functions in brain

Brain

-  Part of the central nervous system

-  Encased in bone

-  Bathed in cerebrospinal fluid

Cerebrospinal fluid (CSF): a fluid that bathes, nourishes, and provides protective cushioning for the brain

- Three pound organ

-  Contains billions of interacting cells

Brain stem

-  Oldest part of brain

-  Function

- Life support

-  Alertness

-  Incoming information

-  Maintains necessary bodily functions

Reticular Formation

-  Inside brain stem

- Extends from the spinal cord to the thalamus

- Filters incoming information

- Used for reflexes

- Essential for forebrain activity

- Moruzzi & Magoun (1949)

- Damage causes sleep in cat

- Stimulation causes awakening in cat

-  "Power Supply" determining level of brain activity

Cerebellum

-  Attached to rear of brainstem

- Posture

-  Body position

-  Coordinating muscular movements

-  Memory for simple learned responses

-  In newer sections

-  Expanded to handle new functions

-  Tripled in size over last 1 million years

Limbic System

Between brainstem and cortex

"Mammalian brain"

Highly developed in mammals

In humans

Dwarfed by more recent structures

Cerebral cortext

Assumed different functions

Functions

Maintain homeostasis

Emotional reactions for survival

Sexual desire

Feeding

Self protection

Components

Pituitary gland

Amygdala

Hypothalamus

Maintains body systems through feedback

Eating, drinking, sleeping, waking, body temp, chemical balances, hormones...

Emotional states

Ability to handle stress

Acting through pituitary gland

Reward centers

Olds & Milner (1954)

Implanted electrode in rat hypothalamus

Hypothetically a "pain center"

Control behavior through punishment

Appeared to be rewarding

Olds (1958)

Frantically self stimulate

Drop from exhaustion

In humans

Involved in pleasurable feelings

Other areas of brain invoved in pleasure

Implicated in addictive disorders

-Reward-deficiency syndrome

Hippocampus

Hippocampus: a limbic structure that has a special role in the storage of memories

-  Involved in storing new memories

-  Sharp wave event

-  Storage memories in cortex

-  Severe damage causes amnesia


Cerebral Cortex

Convoluted outer layer of the cerebrum

-  Folded and bent so large surface area can fit inside skull

Cerebral hemispheres: right and left halves of the cerebrum

Corpus callopsum: the structure that connects the two cerebral hemispheres

Frontal Lobe

-  Located at front of skull

-  Contains primary motor cortex

-  Control of movement

-  Topographic mapping

-  Method used to determined the mapping between the motor cortex and the body.

-  Stimulation of different brain areas

-  Subsequent behavior is observed

-  One-to-one mapping

-  Stimulation results in movement

-  Lower extremities

-  Upper extremities

-  Space correlated with use

-  Involved in intelligence

-  Personality

-  Phineas T Gage

Parietal Lobe

-  Sensory and perceptual activity

-  Somatosensory Cortex

-  Behind motor cortex

-  Stimulation results in sensations of touch

-  One-to-one mapping

Temporal Lobe

-  Contains auditory cortex responsible for hearing

-  Complex auditory analysis

-  Specialized

Occipital Lobe

-  Back of head

-  Visual cortex

-  Primary visual center

-  Stimulation results in the perception of random flashes of light


The Brain - Cerebral Specialization

Hemisphere-specific deficits

Split Brain Research

Roger Sperry

1960’s

Resulted in Nobel prize for Sperry in 1981

Severed corpus callosum in cats

Bundle of nerve fibers connecting hemispheres of brain

Relays information between the hemispheres

Severing connection between hemispheres should disable communication between the hemispheres
Each hemisphere should work independently

After operation, cats were trained on visual problem solving tasks

One eye blindfolded

Taught individual hemispheres

During testing, alternate eye blindfolded

Cats were unable to perform visual task

Epilepsy Patients

Corpus callosum severed to reduce severity of seizures

Patients appear normal

Nature of the operation apparent only in specialized circumstances

Testing Split-Brain Patients

Giving patient object to hold in left hand

Left hand connected to right hemisphere
Right hemisphere has limited capacity for speech

Patient can not name object held in the left hand

Patient can use left hand to pick out the object it was previously holding

Important lesson

Usually allow verbal apparatus to determine knowledge of an individual
Frequently know much more than what we can verbalize

Visual Tests

Use chimeric images

Half of visual input presented to right hemisphere only
Half of visual input presented to left hemisphere only

Verbal

Report input presented to left hemisphere
Evidence that left hemisphere processes verbal information

Spatial

Report input presented to right hemisphere
Evidence that right hemisphere processes spatial information

Patient unaware of conflicting nature of stimuli

Spatial Tests

Block patterns

Sperry

Further demonstration of right hemisphere dominance in processing spatial information

Cerebral Specialization and Pop Psychology

Hemispheres of brain do serve specific functions

Do not serve the functions attributed to them by pop psychology

Hemispheres responsible for certain skills but not all facets of that skill

Brandi Binder

Right side of cortex removed at age six

Lost almost all control over muscles on right side of body

Now, at age 13, is an A student

Excels in many areas thought to be supported by right hemisphere