Biological Bases of Behavior Unit III

Neural processing and the endocrine system

Neurons: The Origin of Behavior Neurons: cells in the nervous system that communicate with one another to perform information-processing tasks Components of neurons: Cell body – coordinates information-processing tasks and keeps the cell alive Dendrites – receive information from other neurons Axon – transmits information to the other neurons, muscles and glands

Glial cells – support cells found in the nervous system Some digest parts of dead neurons Some provide physical and nutritional support for neurons Myelin sheaths – an insulating layer of fatty material around the axons of some neurons Formed by some glial cells Synapse: junction or region between the axon of one neuron and the dendrites or cell body of another

The synapse

Types of Neurons Sensory neurons – receive information from the external world and convey it to the brain via the spinal cord Motor neurons – carry signals from the spinal cord to the muscles to produce movement Interneurons – connect sensory neurons, motor neurons and other interneurons Most common

Communicating information within a neuron Communication happens in two stages: Conduction of an electric signal over relatively long distances within neurons Transmission of electric signals between neurons over the synapse

Communication -cont- Neuron’s cell membrane is porous and allows ions (small electrically charged molecules ) to flow in and out of the cell When at rest, channels that allow small, positively charged potassium ions (K+) to pass are open Channels that allow the flow of other molecules are normally closed There are more potassium ions inside the neuron, so some K+ ions flow out leaving the neuron with fewer positively charged molecules on the inside relative to the outside Resting potential – difference in electric charge between the inside and outside of a neuron’s cell membrane

Communication -cont- Action potential – electric signal that is conducted along the length of a neuron’s axon to the synapse Occurs only when an electric shock reaches a certain level, or threshold When an electric charge is raised to the threshold value, the K+ channels briefly shut down and other channels that allow the flow of sodium (Na+), another positively charged ion, are opened When an electric current passes down the length of a myelinated axon, the charge “jumps” from node to node instead of travelling down the entire axon

Refractory period – time following an action potential during which a new action potential can’t be initiated After action potential reaches it’s maximum, K+ flows out until the axon returns to resting potential Eventually an active chemical “pump” in the cell membrane moves Na+ back outside the axon and K+ inside

Action Potential

The Action Potential

Synaptic transmission between neurons Terminal buttons – knoblike structures that branch out from most axons When the action potential reaches the terminal button, it stimulates the release of neurotransmitters – chemicals that transmit information across the synapse to a receiving neuron’s dendrites Neurotransmitters float across the synapse and bind to sites on the dendrites of the receiving neuron called receptors – parts of the cell membrane that receive the neurotransmitter and initiate a new electric signal Activation of receptors on the receiving neuron can cause a new electric potential to be initiated, called synaptic transmission

Neurotransmitters left in the synapse are cleared up through one of three processes: Reuptake – neurotransmitters are reabsorbed by the terminal buttons of the presynaptic neuron’s axon Enzyme deactivation – specific enzymes break down and destroy specific neurotransmitters Autoreceptors – detect how much of a neurotransmitter has been released into a synapse and signal the neuron to stop releasing the neurotransmitter when too much is present

Synaptic Transmission

Types of Neurotransmitters

How Drugs Mimic Neurotransmitters Agonists – drugs that increase the action of a neurotransmitter Prozac blocks reuptake of serotonin

Antagonists – drugs that block the function of a neurotransmitter

The Organization of the Nervous System

Made up of two divisions: Nervous system – interactive network of neurons that conveys electrochemical information throughout the body Made up of two divisions: Central nervous system (CNS) – brain and the spinal cord Peripheral nervous system (PNS) – connects the CNS to the body’s organs and muscles Also composed of two parts:

Somatic nervous system – set of nerves that conveys information into and out of the CNS Autonomic nervous system (ANS) – set of nerves that carries involuntary and automatic commands that control blood vessels, body organs and glands Sympathetic nervous system – set of nerves that prepares the body for action in threatening situations Parasympathetic nervous system – helps the body return to a normal resting state

The Human Nervous System

Sympathetic and parasympathetic systems

Components of the CNS: The Spinal Cord Some behaviors do not require input from the brain to the spinal cord Spinal reflexes – simple pathways in the nervous system that quickly generate muscle contractions

Components of the CNS: The Brain

The Hindbrain Hindbrain – area of the brain that coordinates information coming into and out of the spinal cord Sometimes called the brain stem – includes: Medulla – extension of the spinal cord into the skull that coordinates heart rate, circulation and respiration Reticular formation – brain structure that regulates sleep, wakefulness and levels of arousal Cerebellum – large structure that controls fine motor skills Thalamus – directs messages to the sensory receiving areas in the cortex and transmits replies to the cerebellum and medulla Pons – relays information from the cerebellum to the rest of the brain

The Hindbrain

The MIdbrain

The forebrain Highest level of the brain Controls complex cognitive, emotional, sensory and motor functions Divided into two main parts: Cerebral cortex – outermost layer of the brain Subcortical structures – areas of the forebrain housed under the cerebral cortex near the center of the brain

Subcortical structures Limbic system – located below the cerebral hemispheres Associated with emotions and drives Amygdala – involved in many emotional processes, particularly the formation of emotional memories Hippocampus – plays a role in the storage of memories

Subcortical structures -cont- Thalamus – sits on top of the brain stem and serves as a relay station Like a server in a computer network Hypothalamus – regulates body temperature, hunger, thirst and sexual behavior Basal ganglia – large neuron clusters that work with the cerebellum and cerebral cortex to control and coordinate voluntary movements

The forebrain

The cerebral cortex Divided into two hemispheres Each controls the opposite side of the body Corpus callosum – thick band of nerve fibers that connects large areas of the cerebral cortex on each side of the brain and supports communication of information across the hemispheres Hemispheres are subdivided into four areas/lobes Occipital lobe – processes visual information Parietal lobe – processes information about touch Temporal lobe – hearing and language Frontal lobe – has specialized areas for movement, abstract thinking, planning, memory and judgment

Motor cortex – area at the back of the frontal lobe that controls voluntary movements Sensory cortex – area at the front of the parietal lobes that registers and processes body touch and movement sensations Association areas – areas of the cerebral cortex that are composed of neurons that help provide sense and meaning to information registered in the cortex Are usually less specialized and more flexible than neurons in the primary areas

The brain and language Aphasia – impairment of language, usually caused by left hemisphere damage either to Broca’s area or to Wernicke’s area Broca’s area – controls language and expression Usually in the left frontal lobe Directs muscle movements involved in speech Wernicke’s area – controls language reception Usually in left temporal lobe

Plasticity – the brain’s ability to change, especially during childhood, by reorganizing after damage or by building new pathways based on experience

Our divided brain Split brain – results from surgery that isolates the brain’s two hemispheres by cutting the fibers connecting them Left visual field Right half of each eye Right hemisphere Right visual field Left half of each eye Left hemisphere

Some things to remember about the brain’s hemispheres: There is no activity to which only one hemisphere makes a contribution Logic is not confined to the left hemisphere and creativity and intuition are not exclusive properties of the right hempishere It is impossible to educate one hemisphere at a time There is no evidence that people are purely “left-brained” or “right-brained”

How we study the brain Brain lesioning – abnormal disruption in the tissue of the brain resulting from injury or disease Is done with laboratory animals to determine the function of different parts of the brain Strokes and brain injuries create lesioned areas in the brain

Electrical activity within the brain is measured with an electroencephalogram (EEG) Multiple electrodes are attached to the outside of the head Advantages: Can detect very rapid changes in electrical activity, allowing analysis of stages of cognitive activity Disadvantages: Provides poor spatial reasoning of the source (cells, brain region) of electrical activity

Neuroimaging techniques PET (positron mission tomography) scan – indicates specific changes in neuronal activity by detecting where a radioactive form of glucose goes while the brain performs a given task Advantage – provides visual image corresponding to anatomy Disadvantages: Requires exposure to low levels of radioactivity Not as clear as an MRI Cannot follow changes faster than 30 seconds

CT (computed tomography) scan – series of X-rays taken from different angles and combined by computer into a composite representation of a slice through the body MRI (magnetic resonance imaging) – show brain anatomy fMRI (functional MRI) – can reveal brain’s functioning and structure

fMRI scan of a concussion

Genetics, evolutionary psychology and behavior

Behavior genetics Chromosome – strands of DNA wound around each other in a double-helix configuration DNA – complex molecule containing the genetic information that makes up the chromosomes Genes – biochemical units of heredity that make up the chromosomes

Twin and adoption studies Identical twins – develop from a single fertilized egg that splits in two Also called monozygotic twins Share 100% of their genes Fraternal twins – develop from separate fertilized eggs Also called dizygotic twins Share about 50% of their genes Likelihood increases with use of fertility treatments and genes