Summary and Key Terms

Human Genetics

Genes are sequences of DNA that code for a particular trait. Different versions of a gene are called alleles—sometimes alleles can be classified as dominant or recessive. A dominant allele always results in the dominant phenotype. In order to exhibit a recessive phenotype, an individual must be homozygous for the recessive allele. Genes affect both physical and psychological characteristics. Ultimately, how and when a gene is expressed, and what the outcome will be—in terms of both physical and psychological characteristics—is a function of the interaction between our genes and our environments.

Cells of the Nervous System

Glia and neurons are the two cell types that make up the nervous system. While glia generally play supporting roles, the communication between neurons is fundamental to all of the functions associated with the nervous system. Neuronal communication is made possible by the neuron’s specialized structures. The soma contains the cell nucleus, and the dendrites extend from the soma in tree-like branches. The axon is another major extension of the cell body; axons are often covered by a myelin sheath, which increases the speed of transmission of neural impulses. At the end of the axon are terminal buttons that contain synaptic vesicles filled with neurotransmitters.

Neuronal communication is an electrochemical event. The dendrites contain receptors for neurotransmitters released by nearby neurons. If the signals received from other neurons are sufficiently strong, an action potential will travel down the length of the axon to the terminal buttons, resulting in the release of neurotransmitters into the synaptic cleft. Action potentials operate on the all-or-none principle and involve the movement of Na⁺ and K⁺ across the neuronal membrane.

Different neurotransmitters are associated with different functions. Often, psychological disorders involve imbalances in a given neurotransmitter system. Therefore, psychotropic drugs are prescribed in an attempt to bring the neurotransmitters back into balance. Drugs can act either as agonists or as antagonists for a given neurotransmitter system.

Parts of the Nervous System

The brain and spinal cord make up the central nervous system. The peripheral nervous system is comprised of the somatic and autonomic nervous systems. The somatic nervous system transmits sensory and motor signals to and from the central nervous system. The autonomic nervous system controls the function of our organs and glands, and can be divided into the sympathetic and parasympathetic divisions. Sympathetic activation prepares us for fight or flight, while parasympathetic activation is associated with normal functioning under relaxed conditions.

The Brain and Spinal Cord

The brain consists of two hemispheres, each controlling the opposite side of the body. Each hemisphere can be subdivided into different lobes: frontal, parietal, temporal, and occipital. In addition to the lobes of the cerebral cortex, the forebrain includes the thalamus (sensory relay) and limbic system (emotion and memory circuit). The midbrain contains the reticular formation, which is important for sleep and arousal, as well as the substantia nigra and ventral tegmental area. These structures are important for movement, reward, and addictive processes. The hindbrain contains the structures of the brainstem (medulla, pons, and midbrain), which control automatic functions like breathing and blood pressure. The hindbrain also contains the cerebellum, which helps coordinate movement and certain types of memories.

Individuals with brain damage have been studied extensively to provide information about the role of different areas of the brain, and recent advances in technology allow us to glean similar information by imaging brain structure and function. These techniques include CT, PET, MRI, fMRI, and EEG.

The Endocrine System

The glands of the endocrine system secrete hormones to regulate normal body functions. The hypothalamus serves as the interface between the nervous system and the endocrine system, and it controls the secretions of the pituitary. The pituitary serves as the master gland, controlling the secretions of all other glands. The thyroid secretes thyroxine, which is important for basic metabolic processes and growth; the adrenal glands secrete hormones involved in the stress response; the pancreas secretes hormones that regulate blood sugar levels; and the ovaries and testes produce sex hormones that regulate sexual motivation and behaviour.

Key Terms

action potential

electrical signal that moves down the neuron’s axon

adrenal gland

sits atop our kidneys and secretes hormones involved in the stress response

agonist

drug that mimics or strengthens the effects of a neurotransmitter

all-or-none

phenomenon that incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation

allele

specific version of a gene

amygdala

structure in the limbic system involved in our experience of emotion and tying emotional meaning to our memories

antagonist

drug that blocks or impedes the normal activity of a given neurotransmitter

auditory cortex

strip of cortex in the temporal lobe that is responsible for processing auditory information

autonomic nervous system

controls our internal organs and glands

axon

major extension of the soma

biological perspective

view that psychological disorders like depression and schizophrenia are associated with imbalances in one or more neurotransmitter systems

Broca’s area

region in the left hemisphere that is essential for language production

central nervous system (CNS)

brain and spinal cord

cerebellum

hindbrain structure that controls our balance, coordination, movement, and motor skills, and it is thought to be important in processing some types of memory

cerebral cortex

surface of the brain that is associated with our highest mental capabilities

chromosome

long strand of genetic information

computerized tomography (CT) scan

imaging technique in which a computer coordinates and integrates multiple x-rays of a given area

corpus callosum

thick band of neural fibers connecting the brain’s two hemispheres

dendrite

branch-like extension of the soma that receives incoming signals from other neurons

deoxyribonucleic acid (DNA)

helix-shaped molecule made of nucleotide base pairs

diabetes

disease related to insufficient insulin production

dominant allele

allele whose phenotype will be expressed in an individual that possesses that allele

electroencephalography (EEG)

recording the electrical activity of the brain via electrodes on the scalp

endocrine system

series of glands that produce chemical substances known as hormones

epigenetics

study of gene-environment interactions, such as how the same genotype leads to different phenotypes

fight or flight response

activation of the sympathetic division of the autonomic nervous system, allowing access to energy reserves and heightened sensory capacity so that we might fight off a given threat or run away to safety

forebrain

largest part of the brain, containing the cerebral cortex, the thalamus, and the limbic system, among other structures

fraternal twins

twins who develop from two different eggs fertilized by different sperm, so their genetic material varies the same as in non-twin siblings

frontal lobe

part of the cerebral cortex involved in reasoning, motor control, emotion, and language; contains motor cortex

functional magnetic resonance imaging (fMRI)

MRI that shows changes in metabolic activity over time

gene

sequence of DNA that controls or partially controls physical characteristics

genetic environmental correlation

view of gene-environment interaction that asserts our genes affect our environment, and our environment influences the expression of our genes

genotype

genetic makeup of an individual

glial cell

nervous system cell that provides physical and metabolic support to neurons, including neuronal insulation and communication, and nutrient and waste transport

gonad

secretes sexual hormones, which are important for successful reproduction, and mediate both sexual motivation and behaviour

gyrus

(plural: gyri) bump or ridge on the cerebral cortex

hemisphere

left or right half of the brain

heterozygous

consisting of two different alleles

hindbrain

division of the brain containing the medulla, pons, and cerebellum

hippocampus

structure in the temporal lobe associated with learning and memory

homeostasis

state of equilibrium—biological conditions, such as body temperature, are maintained at optimal levels

homozygous

consisting of two identical alleles

hormone

chemical messenger released by endocrine glands

hypothalamus

forebrain structure that regulates sexual motivation and behaviour and a number of homeostatic processes; serves as an interface between the nervous system and the endocrine system

identical twins

twins that develop from the same sperm and egg

lateralization

concept that each hemisphere of the brain is associated with specialized functions

limbic system

collection of structures involved in processing emotion and memory

longitudinal fissure

deep groove in the brain’s cortex

magnetic resonance imaging (MRI)

magnetic fields used to produce a picture of the tissue being imaged

medulla

hindbrain structure that controls automated processes like breathing, blood pressure, and heart rate

membrane potential

difference in charge across the neuronal membrane

midbrain

division of the brain located between the forebrain and the hindbrain; contains the reticular formation

motor cortex

strip of cortex involved in planning and coordinating movement

mutation

sudden, permanent change in a gene

myelin sheath

fatty substance that insulates axons

neuron

cells in the nervous system that act as interconnected information processors, which are essential for all of the tasks of the nervous system

neuroplasticity

nervous system’s ability to change

neurotransmitter

chemical messenger of the nervous system

Nodes of Ranvier

open spaces that are found in the myelin sheath that encases the axon

occipital lobe

part of the cerebral cortex associated with visual processing; contains the primary visual cortex

pancreas

secretes hormones that regulate blood sugar

parasympathetic nervous system

associated with routine, day-to-day operations of the body

parietal lobe

part of the cerebral cortex involved in processing various sensory and perceptual information; contains the primary somatosensory cortex

peripheral nervous system (PNS)

connects the brain and spinal cord to the muscles, organs and senses in the periphery of the body

phenotype

individual’s inheritable physical characteristics

pituitary gland

secretes a number of key hormones, which regulate fluid levels in the body, and a number of messenger hormones, which direct the activity of other glands in the endocrine system

polygenic

multiple genes affecting a given trait

pons

hindbrain structure that connects the brain and spinal cord; involved in regulating brain activity during sleep

positron emission tomography (PET) scan

involves injecting individuals with a mildly radioactive substance and monitoring changes in blood flow to different regions of the brain

prefrontal cortex

area in the frontal lobe responsible for higher-level cognitive functioning

psychotropic medication

drugs that treat psychiatric symptoms by restoring neurotransmitter balance

range of reaction

asserts our genes set the boundaries within which we can operate, and our environment interacts with the genes to determine where in that range we will fall

receptor

protein on the cell surface where neurotransmitters attach

recessive allele

allele whose phenotype will be expressed only if an individual is homozygous for that allele

resting potential

the state of readiness of a neuron membrane’s potential between signals

reticular formation

midbrain structure important in regulating the sleep/wake cycle, arousal, alertness, and motor activity

reuptake

neurotransmitter is pumped back into the neuron that released it

semipermeable membrane

cell membrane that allows smaller molecules or molecules without an electrical charge to pass through it, while stopping larger or highly charged molecules

soma

cell body

somatic nervous system

relays sensory and motor information to and from the CNS

somatosensory cortex

essential for processing sensory information from across the body, such as touch, temperature, and pain

substantia nigra

midbrain structure where dopamine is produced; involved in control of movement

sulcus

(plural: sulci) depressions or grooves in the cerebral cortex

sympathetic nervous system

involved in stress-related activities and functions

synaptic cleft

small gap between two neurons where communication occurs

synaptic vesicle

storage site for neurotransmitters

temporal lobe

part of cerebral cortex associated with hearing, memory, emotion, and some aspects of language; contains primary auditory cortex

terminal button

axon terminal containing synaptic vesicles

thalamus

sensory relay for the brain

theory of evolution by natural selection

states that organisms that are better suited for their environments will survive and reproduce compared to those that are poorly suited for their environments

threshold of excitation

level of charge in the membrane that causes the neuron to become active

thyroid

secretes hormones that regulate growth, metabolism, and appetite

ventral tegmental area (VTA)

midbrain structure where dopamine is produced: associated with mood, reward, and addiction

Wernicke’s area

important for speech comprehension