Explaining behavior at the level of the neuron Your brain and the rest of your

nervous system is made up of neurons.

Neurons are brain cells

All neurons are separated from one another, but communicate electrochemically.

The neuron

The neuron consists of three parts The cell body - contains the nucleus

and much of the machinery that keeps a neuron alive and working.

The dendrites - widely branching structures that receive transmissions from other neurons.

The axon - a single, long, thin, straight fiber with branches near its tip

Myelin sheath The axon is coated in an insulating substance

known as Myelin. Myelin allows for faster transmission of

impulses along an axon. Myelin has breaks in it known as the Nodes of

Ranvier Once an impulse reaches the end of an axon

(the terminal buttons), molecules are released that can either excite or inhibit the receiving cell.

Resting Potential Normally there is an electrical

polarization across the membrane of an axon. This means that there is a negative charge

on the inside of the cell and a positive charge on the outside.

At resting potential the inside of the neuron is at -70 millivolts.

Four Factors Determine the Ionic Distribution That Underlies the

Resting Potential

Differential Permeability of the Membrane

The Sodium/Potassium Pump

Diffusion

Electrostatic Pressure

Differential Permeability of the Membrane Ions pass through membrane at special

pores called ion channels

When neurons are at rest, the membrane is: extremely resistant to the passage of

Sodium (Na+) ions only slightly resistant to the passage of

Potassium (K+) ions and Chloride (Cl-) ions

The sodium potassium pump There are little pumps that pump Sodium

(NA+) out of the cell, and potassium (K+) in. The pumps move 3 Sodium molecules out for every 2

potassium molecules they move in Sodium and potassium both have a +1 charge

more Sodium is being moved out than potassium is being moved in

the build up of Sodium on the outside of the membrane makes it positive and the inside negative.

Diffusion also known as -Random Motion Ions in solution are in random motion Thus, any time that there is an accumulation

of a particular class of ions in one area, the probability is increased that random motion

will move ions out of this area (because there are more ions available to leave)

the probability is decreased that random motion will move more ions into the area (because there are fewer ions available to come in)

Electrostatic Pressure

Like charges repel and opposite charges attract

Therefore electrostatic pressure disperses any accumulation of positive or negative charges in an area

Generation of Action Potentials action potentials (APs; neuron

firing) are triggered at the axon hillock when a neuron is depolarized to the point that the membrane potential at the axon hillock reaches about -65 mV this is the threshold of excitation for

many neurons they are all-or-none (they occur full

blown or not at all)

How does an impulse travel down an axon?

The action potential is an excitation that travels along an axon at a constant strength, no matter how far it must travel.

It is slower then a straight electrical impulse, but has the advantage of maintaining it’s strength no matter how far it must travel.

Travel of the action potential When a neuron fires, certain gates open up

that allow Sodium to flow in. When sodium flows in the electrical charge

is neutralized across the membrane. Then the sodium channels close, and

potassium channels open, allowing potassium to leave the cell. This returns the cell to it’s resting potential (-70

mv).

Travel of the action potential. The axon only has these Sodium gates at

breaks in the myelin sheath called the Nodes of Ranvier.

The sodium gates are voltage dependent - that is they open up when the voltage across the membrane drops Thus, the action potential moves like a wave jumping from one Node of Ranvier to the next

down to the end of the axon. Saltatory Conduction

What happens when an action potential reaches the end of an axon?

The end of an axon has several branching areas called the terminal buttons

Each edge of the terminal button is called the presynaptic membrane.

The presynaptic membrane is separated from the other neuron by what is called the synaptic cleft.

The Synapse The axon that has fired releases a

chemical into the synaptic cleft.

This chemical crosses the gap and binds to what is called the postsynaptic membrane. The chemicals are called neurotransmitters.

They bind at the postsynaptic membrane at what are called receptors.

Neurotransmitters

There are many types of neurotransmitters

Acetylcholine Serotonin Dopamine

Dopamine

Dopamine is one neurotransmitter that has been associated with many neurological disorders Parkinson’s disease.

Muhammed Ali Schizophrenia Sometimes these diseases can be

treated by increasing dopamine levels in the brain

Behavior and the Nervous System

Psychologists distinguish between the central nervous system and the peripheral nervous system.

Central nervous system consists of the brain and the spinal cord

Peripheral nervous system is composed of bundles of axons between the spinal cord and the rest of the body.

Peripheral nervous system

The peripheral system can be further divided

Somatic nervous system = nerves that communicate with the skin and muscles.

Autonomic nervous system = nerves that communicate with the heart, stomach, and other organs.

Autonomic nervous system

The autonomic nervous system is a system that we do not have as much control over.

It largely controls things we wouldn’t want to have to think about breathing heart rate

Divisions of the autonomic nervous system

The sympathetic system - controls fight or flight - increases heart and breathing rate.

The parasympathetic system - decreases heart rate, controls digestion, basically runs the body during normal functioning.

Organization and functioning of the brain

The cerebral cortex - the outer surface of the brain. The wrinkled area.

Right and left hemisphere crosses over - communicates via

the corpus callosum

The four lobes of the cerebral cortex

Frontal Lobe - thought to be involved in planning and working memory primary motor cortex

Parietal Lobe - body sensations primary somatosensory cortex

Occipital lobe - vision Temporal lobe - hearing - advanced

visual processing - emotion

What if we cut the corpus callosum?

Travel from the eye to the brain is divided.

Information from the left side of each eye travels to the left hemisphere of the brain

Information from the right side of each eye travels to the right hemisphere of the brain.

Hemisphere division continued

The left hemisphere of the brain controls speech for most people

Most people are only able to describe information that reaches the left hemisphere.

However information that reaches the right hemisphere quickly crosses the corpus callosum to the left hemisphere, so that it can be described verbally.

What if the corpus callosum is cut

In one experiment researchers showed a woman with a severed corpus callosum a picture of a naked woman to the left field of vision = to the right of her face

When asked what she had seen, she laughed and said a nude picture.

Continued. When the nude picture was shown only in

her right field of vision (left of her face), she laughed and turned a little red in the face.

When asked why she was laughing, she said I don’t know, oh that silly machine.

The right side of the brain knew what it saw and caused her to laugh, the left side of the brain heard the laughter, and tried to interpret why it occurred.

continued

If she had been allowed to point at several alternatives with her left hand (controlled by the right hemisphere), she would have been able to correctly point at the picture she had seen, even though she would say that she didn’t know what she had seen.

Unified consciousness? We all experience a unified consciousness. That is, we experience a single self. The split brain experiments show that that

unified consciousness depends on the two hemispheres being able to communicate.

If the corpus callosum is severed then each hemisphere begins to act and experience things independently of the other.