Chapter 2: The Brain and the Nervous System

Nervous System

• Central Nervous System: brain and spinal cord
  • Spine connects brain with body.
  • Spinal cord also produces immediately responses (e.g. touching hot things).
• Peripheral Nervous System: nerves that connect to nervous receptors, muscles, and glands
  • (Sensory nerves carry messages from skin and muscles to the spinal cord. Motor nerves carry orders from Central Nervous System to muscles and glands (which control hormones).)
  • Somatic Nervous System: muscles, skin
  • Autonomic Nervous System: involuntary actions (e.g. breathing)
    • Sympathetic System: emergency responses; hyping up
    • Parasympathetic system: non-emergency responses; calming

Types of Cells

• Neurons are cells that transmit into the Central Nervous System
• Glia cells hold neurons in place, provide nutrients, and help neurons grow

Neurons

Neurons	Body (Soma)	Axon
Receive messages	DNA	Communicates with neurons, muscles, and glands
Transmit messages to body	Growth and reproduction	Insulated by fatty myelin sheath (insulates against interferences, makes communication faster)

• 100 billion neurons in the brain
• Stem cells are a type of immature cell with the potential to be any type of cell (including new neurons)
• The spot where axons and dendrites meet is called the synapse
  • There’s a gap between the two called a synaptic cleft
• Neurons have negative charge inside. When stimulated, gates open which makes the charge more positive. If the charge becomes positive enough, an action potential is triggered (the neuron fires).
  • After depolarization, Na+ (and K+) pumps restore negative charge
• Within neurons, communication uses electricity. Between neurons, communication uses chemicals (neurotransmitters).
• Interneurons carry messages between neurons (connectors)

Studying the Brain

• Lesion method: figuring out what parts of the brain do by removing them in animals and seeing what happens
• Transcranial Magnetic Stimulation (TMS): using electrical currents (a magnet) to stimulate or suppress certain neural circuits.
• Transcranial Direct Current Stimulation (tDCS): newer version of TMS.
• EEG: “listens” to brain activity by measuring fast electrical activity.
• ERP: variant of EEG for measuring waves of electrical activity in response to stimuli.
• PET scan: records biochemical changes in the brain (blood flow, glucose usage, receptors).
• MRI: gives a picture of the brain using magnetic fields or radio waves.
• fMRI: shows brain activity related to specific thoughts or behaviors by detecting blood oxygen in regions.

Parts of the Brain

• Brain stem: rises out of spinal cord
  • Medulla: unconscious bodily functions (breathing, heart rate)
  • Pons: sleeping, waking, dreaming
• Reticulating activating system (RAS): dense network of neurons that screens incoming information and arouses other centers
• Cerebellum: balance, muscle coordination, along with working memory, speech and language.

• Thalamus: sensory relay station.
  • The only sense that bypasses the thalamus is the olfactory bulb for smell.

• Hypothalamus: below thalamus (“hypo” = “under”), regulates homeostasis, body temperature, biological clock. Basic survival drives: feeding, fighting, fleeing, sex.
• Pituitary gland: “master gland”, secretes hormones. Instructions from hypothalamus → pituitary gland → other endocrine glands.

• Amygdala: evaluates sensory information and decides what to do with it.
  • “fear center”
  • Responds more strongly to fear than to happy/neutral
• Hippocampus: consolidates and forms memories, which may then be stored in the cerebral cortex
  • Damaging the hippocampus doesn’t damage your old memories, but it prevents you from forming new ones

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The upper part of the brain is the cerebrum, which contains two halves called cerebral hemispheres (connected by the corpus callosum).

Each cerebral hemisphere has four lobes:

• Occipital lobes: lower back, contain visual cortex
  • Damage can cause agnosia: person can see, but not recognize objects
    • prosopagnosia: difficulty identifying faces
• Parietal lobes: upper back, contain somatosensory cortex (pressure, pain, touch, temperature)
• Temporal lobes, behind the temples, audio processing
  • Houses Limbic System
    • Amygdala (“almond”): detection of threats
      • Forms emotional memories
    • Hippocampus (“seahorse”): acquisition of memories
• Frontal lobes, front, contain primary motor cortex to produce voluntary movement
  • Most forward part is prefrontal cortex
    • Large in humans compared to other animals
    • Reasoning, decision-making, planning
    • Ability to do tasks in sequence
  • Inhibits amygdala if possible threat is identified as harmless

Split Brains

Surgeons severed the connection between patients’ cerebral hemispheres, then showed a picture where the left half was a man and the right half was a boy.

When asked what they saw, the patients said a boy, because the right field of view is seen by the left half of the brain and the left half of the brain controls speech.

But when asked to point at who they saw with their left hand, which is controlled by the right half of the brain, they pointed at the man.

• People read faster if the words are flashed in their right field of view, because the left hemisphere is specialized for reading.
• Visually, the left hemisphere sees details and the right hemisphere sees how they fit together.
• Orally, the left hemisphere hears individual sounds and the right hemisphere hears intonation.

The Brain is Flexible

• The brain is a dynamic organ, that adapts to changes in the environment and its senses.

Neurotransmitters

Acetylcholine

• Controls muscles, vigilance, memory
• Curare blocks acetylcholine receptors, causing paralysis
• Botulinum toxin blocks the release of acetylcholine
  • This is botox! It prevents wrinkles by paralyzing face muscles

Dopamine

• Involved in movement, reward, addiction
• Parkinson’s Disease is decreased dopamine activity in the substantia nigra
• Cocaine inhibits the reuptake of dopamine
• Antipsychotic medications block excessive dopamine

Norepinephrine

• Fight or flight response
• Beta-blockers block post-synaptic norepinephrine receptors, decreasing blood pressure and heart rate
• Some antidepressants block reuptake

Serotonin

• Mood, appetite, sleep
• Some antidepressants block reuptake of serotonin, or prevent if from being degraded by enzymes