2.1 The nervous system: on being Wired

2.1a Neurons: Into the fabulous Forest

A few definitions:

Neurons:

• • Basic building blocks of the nervous system
  • Specialized cellsof the nervous system that receive and pass messages
  • Vary according to function and location
• Most Neurons have:
  • cell body– generates the energy needed to carry out the work of the cell
  • dendrites– receive incoming messages from thousands of adjoining neurons
  • axon– transmits impulses to other neurons

Two types of neurons:

• Afferent Neurons:transmits impulses from the sensory receptors to the spinal cord and brain
  another name Sensory Neurons
• Efferent Neurons:transmits impulses from the spinal cord and brain to muscles and glands
  another name Motor Neurons
• • Example:
    • If you burn your hand, the afferent (sensory) neurons near the surface of your skin will send a message to the brain and spinal cord and you will feel pain. 
    • The efferent (motor) nerves send a messages to your hand to remove it from the heat source.

Glial cells

• • Remove dead neurons and waste products
  • Nourish and insulate neurons
  • Form myelin and play a role in neural transmission of message
    • Myelin – a fatty substance that encases and insulates axons, facilitating transmission of neural impulses.
    • In people with Multiple Sclerosis (MS), this substance is replaced with a hard fibrous tissue that alters the timing of nerve impulses affecting muscle control.

2.1b The Neural Impulse: “The body electric”

• Neural Impulse:
  • Electrochemicalmessages that travel within neurons
  • Neurons contain negatively or positively charged ions
  • Difference in electrical charge polarizesa neuron for firing
    • Firing – Conduction of neural impulse along a neuron

• Resting potential:Potential across the neural membrane when a neuron does not respond to other neurons
• Depolarizationoccurs when area on the resting neuron is stimulated
• Action potential:Provides the basis for the conduction of a neural impulse along an axon

All-or-none principle:

• Action potential always has the same strength and speed
• The neuron either fires, or it doesn’t!

Synapse:

• A junction between the axon terminals of one neuron and the dendrites or the cell bodies of another neuron

2.1c Neurotransmitters: The chemical keys to communication

Neurotransmitters

• • Chemical substances involved in the transmission of neural impulses from one neuron to another
  • Excitatoryand inhibitory functions determine whether a neuron will fire
    • • Excite – Cause other neurons to fire
      • Inhibit – Prevent other neurons from firing

Receptor Site

• • A location on a dendrite of a receiving neuron tailored to receive a neurotransmitter.

Types of Neurotransmitters:

• Acetylcholine – controls muscle contractions
• Dopamine – affects the ability to perceive pleasure, voluntary movement, and learning and memory.
• Serotonin – involved in emotional arousal and sleep
• Norepinephrine – may play a role in depression
• Gamma-aminobutyric acid (GABA)- helps calm anxiety
• Endorphins – they lock into receptor sites for chemicals that transmit pain messages to the brain

2.2 The Divisions of the Nervous System

The Nervous System is divided in:

1. Central Nervous system
   • Brain
   • Spinal Cord
2. Peripheral Nervous system
   • Somatic Nervous System (SNS)
     • Afferent Nerves
     • Efferent Nerves
   • Autonomic Nervous System (ANS)
     • Sympathetic Nervous System
     • Parasympathetic Nervous System
   • The central and peripheral nervous systems make it possible for us to receive information from the world out site and act on it.

2.2a The Peripheral Nervous System: The body’s Peripheral Devices

Somatic nervous system

• • Contains afferent and efferent neurons.  It transmits messages about sights, sounds, smells, temperature, body positions… etc.
  • Messages transmitted from the brain and spinal cord to the somatic nervous system control purposely body movements ( raising hand, winking, or running).

 Autonomic nervous system

• • As in the somatic nervous system it also afferent and efferent neurons.
  • It regulates glands and the muscles of internal organs
  • Automatically controls internal functions like heartbeat, digestion, breathing, etc
  • Further divided into the sympathetic and parasympathetic divisions
    • Sympathetic Nervous System
      • Mobilizes body for emergency
      • Arouses body
    • Parasympathetic Nervous System
      • Calms you down

2.2b The Central Nervous System: The Body’s Central Processing Unit

Spinal Cord

• Transmits signals from the sense organs and muscles that are below the level of the head to the brain
• Distributes signals from the brain to the rest of the body

Spinal Reflex

• A simple, unlearned response to a stimulus that may involve only two neurons: a afferent and efferent neuron
• The image below shows how reflexes work.  The man in the picture is touching something hot. Immediately when he feels the heat his hand moves away from the source. The afferent neurons send the message of heat (pain) to the brain and the efferent neurons send the message to remove the hand form the heat.

• The spinal cord and braincontain gray matter and white matter
• Gray matter consists of non-myelinated neurons and are involved in spinal reflexes.
• White matter consists of myelinated axons and carry messages from and to the brain.

2.4 A Voyage through the brain

Hindbrain is the portion of the brain that contains:

• Medulla: connect the spinal cord to higher levels of the brain. Involved in the regulation of heartbeat, blood pressure, movement and respiration.
• Pons: is a bulge in the hindbrain that lies forward of the medulla. Involved in respiration, attention, sleep and arousal. Transmits information about body movement.
• Cerebellum: involved in maintaining balance and in controlling motor behavior
• Reticular formation:Lower part is within the hindbrain, it ascends through the midbrain into the lower part of the forebrain.
  • Sends messages to the cerebral cortex when stimulated
  • Makes one alert to sensory information
  • Involved in attention and sleep and arousal.

Forebrain is the portion of the brain that contains:

• • Thalamus: Relay station for sensory stimulation to the cortex and in the functions of sleep and attention
  • Hypothalamus: Regulates body temperature, concentration of fluid, storage of nutrients, motivation, and emotion
    • Involved in hunger, thirst, sexual behavior, caring for offspring, and aggression

Limbic system: a group of structures involved in memory. motivation and emotion that forms a fringe along the inner edge of the cerebrum.

• • Amygdala: Connected with aggression, fear, vigilance, emotions, learning, and memory
  • Hippocampus: involved in memory formation. People with damage in this area can recall old events, but can’t store new information.

Cerebrum: Responsible for thinking and language

Cerebral cortex: Surface of the cerebrum

• • Wrinkled or convoluted with ridges and valleys (fissures)
  • Connected with cognitive abilities

Corpus callosum: Bundles of some 200 million nerves that connects the two hemispheres of the brain.

2.5 The Cerebral Cortex

Before we start this section I recommend watching this Video: “Neuroanatomist Jill Bolte Taylor had an opportunity few brain scientists would wish for: One morning, she realized she was having a massive stroke. As it happened — as she felt her brain functions slip away one by one, speech, movement, understanding — she studied and remembered every moment. This is a powerful story about how our brains define us and connect us to the world and to one another.”  (source TED.com (Links to an external site.))

The brain is divided in:

• Two Hemispheres
  • Left and Right Hemispheres
• Four Lobes
  • Occipital lobe – deals with vision
  • Temporal lobe – deals with hearing and auditory functions
  • Parietal lobe – contains the somatosensory cortex
  • Frontal lobe – contains the motor cortex

2.5a The Structure of the cerebral cortex

Somatosensory Cortex: receives messages from skin senses all over the body (temperature, touch, pain, and movement)

Receives signals from skin senses

Amount of cortical area determined by how sensitive that particular body part is – NOT by the size of the body part 
.

Motor Cortex: linked to muscular responses throughout the body


Sends motor commands to muscles

Amount of cortical area determined by how precise movements are for that part of the body – NOT by the size of the body part.

• Senses and motor control are contralateral
• Left side of body sensed/controlled by right hemisphere
• Right side of body sensed/controlled by left hemisphere

• Brain hemispheres mirror and differ in function
• Left hemisphere contains language functions for nearly all right-handed people
• If damaged before the age of 13, speech functions can transfer to the right hemisphere

2.5b Thinking, Language, and the cerebral cortex

Key areas for language are usually in left hemisphere.

• Broca’s area is associated with production of language. Damage in this area results in slow and laborious speech.  This video shows an example of a person with damage in this area of the brain.

• Wernicke’s area is associated with comprehension of language.  Damage in this area results lack of coherent speech. The next video shows an example of a person with damage in this area of the brain also known as Fluent Aphasia.

2.5c Left Brain, Right Brain

The left-brain vs right-brain idea is exaggerated.  Research does show that for most right handed people: the left side is more involved in intellectual undertakings such as language, math and problem solving while the right side is usually superior in visual-spatial functions, recognitions of faces, emotional responses.  However, both hemispheres do not act independently! Their functions overlap and they respond simultaneously.

2.6 The Endocrine System

The endocrine system comprises ductless glands that release hormones into the bloodstream.

• Hormones: Regulate growth, metabolism, and some behaviors. They also maintain steady bodily states.
• • Negative feedback loop – Gland is signaled to stop when the right amount of a hormone has been secreted
• Pituitary Gland
  • Lies below the hypothalamus
  • Labeled as the master gland
  • Secretes hormones that regulate the functioning of other glands
  • Growth hormone, prolactin, vasopressin, and oxytocin
  • Hypothalamus regulates pituitary activity

Please watch this video.

It summarizes and describes the main functions of the Endocrine System and it compares it with the Nervous System.

2.7 Evolution and Heredity

2.7b Heredity, Genetics, and Behavioral Genetics

• Heredity: Biological transmission of traits from parent to offspring through genes
• Genetics: Subfield of biology that studies heredity
  • Behavioral genetics – Focuses on the contributions of genes to behavior
• Genotype: Individual’s genetic makeup (nature)
• Phenotype: Actual appearance, based on genotype and environmental influences (nature and nurture)

2.7c Genes and Chromosomes

Gene

• Basic unit of heredity
• Regulates the development of specific traits

Chromosomes

• Made up of strings of genes
• Humans have 46 chromosomes (23 pairs)
• Formed by DNA (deoxyribonucleic acid)
  • Double helix – Contains the genetic code
• Below is Figure 2.15: which shows a schematic representation of the cells, chromosomes and DNA.

NOTE that the extent of influence of genes on complex psychological traits is debated.

One receives 23 chromosomes from each parent: 23rd pair of chromosomes are sex chromosomes.

• XX – Female
• XY – Male

2.7d Kinship Studies

Kinship Studies: They focus on the presence of traits and behavior patterns in people who are or are not related biologically.

• Twin studies:
  • These studies are conducted to determine the role of genetic factors
  • Research have found that identical twins who are reared apart are about as similar as identical twins reared together on a variety of measures of intelligence, personality, temperament, occupational and leisure-time interests, and social attitude!
• Adoption studies
  • Psychologists look for similarities between children and their adoptive and natural parent.
  • When children reared by adoptive parents are more similar to their biological parents in a particular trait, strong evidence exists for a genetic role in the development of that trait.