The Biology of Mind: Neural and Hormonal Systems

THE BIOLOGY OF MIND: Comprehensive Study Notes

Chapter 2 Overview: Key Questions in Human Biology and the Mind

• Why are psychologists concerned with human biology? This field helps understand the physiological basis of thoughts, emotions, and behaviors.

• How do biology and experience together enable neuroplasticity? The brain's ability to reorganize and form new connections is shaped by both its biological structure and lived experiences.

• What are neurons, and how do they transmit information? Neurons are the fundamental communication cells, transmitting electrical and chemical signals.

• How do nerve cells communicate with other nerve cells? They communicate electrically within a neuron and chemically across synapses.

• How do neurotransmitters influence behavior, and how do drugs and other chemicals affect neurotransmission? Neurotransmitters are chemical messengers that impact behavior, mood, and bodily functions, while drugs can agonize (boost) or antagonize (block) their actions.

• What are the functions of the nervous system's main divisions, and what are the three main types of neurons? The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS), each with specialized roles.

• How does the endocrine system transmit information and interact with the nervous system? The endocrine system uses hormones traveling through the bloodstream for slower, longer-lasting communication, often influenced by the nervous system.

• How do neuroscientists study the brain’s connections to the behavior and mind? Various imaging techniques allow researchers to observe brain structure and activity.

• What are the hindbrain, midbrain, and forebrain? These are major divisions of the brain, each containing structures with specific functions.

• What structures make up the brainstem, and what are the functions of the brainstem, thalamus, reticular formation, and cerebellum? These are ancient brain parts managing basic life functions, sensory relay, arousal, and motor control.

• What are the limbic system’s structures and functions? This system is crucial for emotions, memory, and drives.

• What four lobes make up the cerebral cortex, and what are the functions of the motor cortex, somatosensory cortex, and association areas? The cerebral cortex, divided into four lobes, is responsible for higher-level processing, including voluntary movement, sensory processing, and complex thought.

• Is it true that $90$ percent of our brain isn’t really used? This is a myth; virtually all of the brain is utilized.

• To what extent can a damaged brain reorganize itself, and what is neurogenesis? The brain has remarkable plasticity and can reorganize itself after damage, and neurogenesis is the birth of new neurons.

• What do split brains reveal about the functions of our two brain hemispheres? Split-brain studies show the specialized roles of the left and right hemispheres.

Neural and Hormonal Systems: Fundamental Components

Speed of Brain Processing

• Reaction time tests demonstrate how quickly the brain processes information and initiates a response.

Cells of the Nervous System

• Neurons - "Communication Cells"

  • Receive signals through dendrites.

  • Send signals through axons.

• Glial Cells - "Support Cells"

  • Provide the myelin sheath.

  • Nourish & protect neurons.

  • Perform housekeeping functions (waste removal, repair).

The Neuron: Structure

• Three main parts:

  1. Cell body (soma): The neuron's life support center, containing the nucleus.

  2. Dendrites: Branching extensions that receive messages from other neurons.

  3. Axon: A long, hair-like extension that sends messages away from the cell body to other neurons, muscles, or glands.

Myelin Sheath: Insulation and Speed

• A fatty coating around axons.

• Insulates electrical signals.

• Speeds up transmission of neural impulses.

• Boosts efficiency of neural communication.

• Without myelin: Signals would be slower, leading to clumsy or delayed responses.

• Multiple Sclerosis (MS):

  • Caused by the deterioration of the myelin sheath.

  • Slows or blocks neural communication.

  • Leads to movement and coordination problems.

  • Currently, there is no cure, but treatments help manage symptoms.

Action Potential: The Electrical Signal

• Action Potential: An electrical signal that travels down the axon.

  • Triggered when a neuron reaches a specific threshold.

  • It is an all-or-none process, meaning it either fires completely or not at all; there's no partial firing.

Resting Potential: The Neuron at Rest

• A neuron at rest is like a charged battery.

• Outside the neuron: Higher concentration of Sodium ($ext{Na}^+$) ions.

• Inside the neuron: Higher concentration of negatively charged Proteins ($ext{Proteins}^-$) and Potassium ($ext{K}^+$) ions.

• The membrane maintains a positive charge outside and a negative charge inside.

• This state is primarily maintained by the sodium-potassium pump, which actively transports $3$ Na$^+$ ions out for every $2$ K$^+$ ions pumped in.

• The inside of the neuron is approximately $-70 ext{ mV}$ compared to the outside.

• The neuron's membrane is selectively permeable.

Depolarization: The Firing Stage

• A stimulus opens voltage-gated sodium channels.

• Sodium ($ext{Na}^+$) ions rush into the neuron due to electrochemical gradients.

• The inside of the neuron becomes positive (e.g., from $-70 ext{ mV}$ to $+40 ext{ mV}$).

• An electrical change within a neuron from a negative charge to a positive charge.

Repolarization: Resetting the Charge

• Shortly after depolarization, potassium channels open.

• Potassium ($ext{K}^+$) ions exit the neuron.

• The inside of the neuron becomes negative again (e.g., from $+30 ext{ mV}$ to $-80 ext{ mV}$).

• An electrical change within a neuron from a positive charge to a negative charge.

Hyperpolarization and Refractory Period

• Sometimes, too much potassium leaves the neuron, causing the inside to become overly negative (hyperpolarization).

• Refractory period: A brief period after an action potential during which a neuron cannot fire again immediately; it needs to reset.

Threshold & All-or-None Response

• A neuron must reach a threshold of excitation (approximately $-55 ext{ mV}$) to trigger an action potential.

• If the stimulus is too weak, the threshold is not met, and no action potential occurs.

• The all-or-none principle means the action potential fires with either full strength or not at all, regardless of the stimulus intensity once the threshold is met.

Action Potential Summary Steps

1. Stimulus triggers a voltage change, potentially reaching the threshold.

2. A. Depolarization (rising phase): Sodium ($ext{Na}^+$) gates open, $Na+$ rushes in, depolarizing the neuron, making the inside positive.

3. B. Repolarization (falling phase): Slower Potassium ($ext{K}^+$) gates open, $K+$ ions leave the cell, repolarizing the neuron, making the inside negative again.

4. C. Hyperpolarization: Occurs as the inside becomes temporarily too negative.

5. The $ext{Na}^+/ ext{K}^+$ pump then works to restore the resting ion balance, bringing the neuron back to its resting state.

Interneural Communication: The Synapse

The Synapse: Bridge Between Neurons

• A tiny junction (ranging from $20$ to $40 ext{ nanometers}$) between neurons.

• At the synapse, the electrical signal (action potential) is converted into a chemical signal.

• Neurotransmitters are the chemical messengers that carry the message across the synapse.

• The synapse connects the pre-synaptic ("sending") cell to the post-synaptic ("receiving") cell.

Steps of Synaptic Transmission

1. An action potential reaches the axon terminal of the pre-synaptic neuron.

2. This stimulates the release of neurotransmitters (NTs) from specialized sacs called vesicles into the synaptic gap.

3. NTs travel across the synaptic gap and bind to specific receptors on the dendrites or cell body of the post-synaptic neuron, causing either excitation or inhibition of the receiving neuron.

4. Excess neurotransmitters in the synaptic gap are then removed by one of three mechanisms:

   • Reabsorption (reuptake) back into the pre-synaptic neuron.

   • Diffusion away from the synapse.

   • Broken down by enzymes.

Neurotransmitters: Chemical Messengers of the Brain

• Glutamate: Primary excitatory neurotransmitter, involved in learning and memory.

• GABA (gamma-aminobutyric acid): Primary inhibitory neurotransmitter, calms the nervous system.

• Acetylcholine (ACh): Involved in movement, learning, and memory; a deficit is linked to Alzheimer's disease.

• Dopamine: Important for movement, learning, attention, and emotion; associated with reward and pleasure.

• Serotonin: Influences mood, hunger, and sleep.

• Norepinephrine: Contributes to alertness and arousal.

• Endorphins: Natural painkillers produced by the body, related to pleasure and stress reduction.

Neurotransmitter Analogy: Keys and Locks

• Neurotransmitters = keys

• Receptors on dendrites = locks

• Synapse = the small gap between the door and the keyhole

How Drugs Affect Neurotransmission

• Agonist: A chemical that boosts or mimics the action of a neurotransmitter (like a fake key that opens the lock). Example: Nicotine is an ACh agonist.

• Antagonist: A chemical that blocks the action of a neurotransmitter (like a blocker that jams the lock). Example: Curare is an ACh antagonist.

Hormones and the Endocrine System

Hormones vs. Neurotransmitters

• Neurotransmitters: Act locally at synapses, effects are rapid (milliseconds), and typically short-acting.

• Hormones: Travel through the bloodstream, effects are slower to onset, and longer-lasting.

• Both hormones and neurotransmitters significantly influence behavior, mood, and physiology.

Endocrine System Glands (Overview)

• Adrenal glands: Release adrenaline (epinephrine) and cortisol, crucial for stress response.

• Pancreas: Produces insulin to regulate blood sugar levels.

• Thyroid: Secretes hormones vital for metabolism.

• Pineal gland: Produces melatonin, which regulates sleep-wake cycles.

• Gonads: Produce estrogen and testosterone, which are sex hormones involved in reproduction and secondary sex characteristics.

Serotonin in the Gut

• Approximately $90$ percent of the body's serotonin is found in the gut.

• Plays a significant role in regulating bowel function.

• Helps protect the digestive system.

Problems with Low Serotonin

• Psychological: Depression, anxiety, suicidal behavior, Obsessive-Compulsive Disorder (OCD), panic disorders, phobias, schizophrenia.

• Physiological: Sleep problems, digestive problems.

• Causes: The body may not produce enough serotonin, or it may not use serotonin effectively.

The Nervous System: Divisions and Functions

Central Nervous System (CNS)

• Comprises the Brain and Spinal Cord.

• Receives sensory information.

• Processes and coordinates information.

• Sends commands to the skeletal and muscular systems.

Peripheral Nervous System (PNS)

• Includes everything else in the nervous system outside of the CNS.

• Connects the CNS to the body's organs and muscles.

Divisions of the Peripheral Nervous System

• Somatic Nervous System:

  • Responsible for voluntary control of skeletal muscles.

  • Enables us to perceive, think, and coordinate our behaviors consciously.

• Autonomic Nervous System (ANS):

  • Responsible for involuntary control of glands, organs, and blood vessels.

  • Operates largely outside of conscious control.

Divisions of the Autonomic Nervous System

• Sympathetic Nervous System:

  • Arouses the body, preparing it for "fight or flight" responses.

  • Effects: Dilates pupils, increases heartbeat, dilates bronchial tubules (for more air), stimulates sweat glands, increases the rate of glycogen conversion to glucose (energy), decreases digestive system activity, stimulates adrenaline production from adrenal glands, causes vaginal contraction, relaxes the bladder, constricts blood vessels.

• Parasympathetic Nervous System:

  • Calms the body, promoting "rest and digest" functions.

  • Effects: Constricts pupils, slows heartbeat, constricts bronchial tubules, stimulates bile release, stimulates digestive system activity, causes relaxation of the uterus, increases urinary output.

Brain Imaging Techniques: Peeking Inside the Brain

• Electroencephalogram (EEG):

  • How it works: Electrodes placed on the scalp measure electrical activity in neurons.

  • Sample Finding: Symptoms of depression and anxiety correlate with increased activity in the right frontal lobe, an area associated with behavioral withdrawal and negative emotion.

• Magnetoencephalography (MEG):

  • How it works: A head coil records magnetic fields produced by the brain's natural electrical currents.

  • Sample Finding: Soldiers with PTSD show stronger magnetic fields in the visual cortex when viewing trauma-related images compared to those without PTSD.

• Positron Emission Tomography (PET):

  • How it works: Tracks where a temporarily radioactive form of glucose goes in the brain as a person performs a task, indicating metabolic activity.

  • Sample Finding: Monkeys with an anxious temperament have brains that use more glucose in regions related to fear, memory, and expectations of reward and punishment.

• Magnetic Resonance Imaging (MRI):

  • How it works: Uses magnetic fields and radio waves to provide a detailed, static map of brain structure (organs, soft tissues, bones, blood vessels).

  • Sample Finding: People with a history of violence tend to have smaller frontal lobes, particularly in regions aiding moral judgment and self-control.

• Functional MRI (fMRI):

  • How it works: Measures changes in blood flow to brain regions by comparing continuous MRI scans, thus indicating brain activity.

  • Sample Finding: Years after surviving a near plane crash, passengers viewing trauma-related material showed greater activation in the brain's fear, memory, and visual centers than when watching footage of the $9/11$ terrorist attacks.

Brain Structures: The Foundation of Mind

The Brainstem and Thalamus

• Brainstem:

  • The oldest, innermost brain region.

  • Responsible for automatic survival functions.

  • Medulla: Controls heartbeat and breathing (essential life support functions).

  • Pons: Involved in movement, balance, and coordination, and helps control sleep.

• Thalamus:

  • Acts as the brain's sensory relay station or "traffic controller."

  • Directs sensory messages (except smell) to the appropriate areas of the cerebral cortex.

  • Sends replies from the cortex to the cerebellum and medulla.

• Reticular Formation:

  • A nerve network extending through the brainstem and thalamus.

  • Crucial for controlling arousal and alertness.

The Cerebellum:

little brain’ at the back of brainstem
• Coordinates voluntary movement
• Helps with balance, fineLittle brain’ at the back of brainstem
• Coordinates voluntary movement
• Helps with balance, fine motor control
• Affected by alcohol

the limbic system