Brain Basics: Anatomy, Evolution, and Function

The Brain as a Nerve Center and Multitasking Hub

• The brain is the literal ‐nerve center‐ of the human body, containing billions of neurons.
• It functions by transmitting information from the body and the outside world, then programming responses that include conscious and unconscious movements, thoughts, emotions, and memories.
• A defining feature of the brain is its ability to perform multiple tasks simultaneously. Examples include:
  • Throwing a ball while talking to a friend.
  • Planning dinner while shopping.
  • Daydreaming about a balloon ride while driving to work.
• This multitasking is possible because the brain is split into many distinct regions specialized for specific tasks and abilities.

Major Landmarks of the Human Brain

• Cerebrum: The largest part of the human brain. It is divided into two large, separate hemispheres (left and right).
• Corpus Callosum: The largest bundle of nerve fibers that connects the two cerebral hemispheres, acting as a bridge to carry information between sides.
• Cerebral Cortex: The deeply folded surface layer of nerve tissue on the cerebrum.
  • The deep folds increase the surface area of the cortex, creating space for more neurons.
  • This increased neuron density enhances the brain's processing power.
• Lobes: Neuroscientists use the deepest divisions of the cerebrum to identify four distinct lobes in each hemisphere, each with characteristic functions.

The Four Principal Lobes and Their Functions

• Frontal Lobes: Located at the front of the brain, immediately above the eyes. Their functions include:
  • Coordinating voluntary movements and speech.
  • Managing memory and emotion.
  • Higher cognitive skills such as planning and problem-solving.
  • Defining many aspects of personality.
• Parietal Lobes: Located at the top of the brain, immediately behind the frontal lobes.
  • They integrate sensory signals from the skin.
  • They process taste.
  • They process certain types of visual information.
• Occipital Lobes: Located at the back of the brain.
  • They are responsible for processing visual information.
  • Specific tasks include recognizing colors and shapes and integrating them into complex visual understanding.
• Temporal Lobes: Located on the sides of the brain, at and below the level of the eyes.
  • They carry out some visual processing and interpret auditory information.
  • Hippocampus: Curved structures lying beneath the cerebral cortex within the temporal lobes; they encode new memories.
  • Amygdala: A deep structure within each temporal lobe that integrates memory and emotion.

Deep Brain Structures and the Forebrain

• Limbic System: A group of structures deep within the brain that help regulate emotion and motivation. It includes:
  • The Hippocampus.
  • The Amygdala.
  • The Thalamus: Functions to integrate sensory information and relay it to other brain regions.
  • The Hypothalamus: Sends hormonal signals to the body via the pituitary gland.
• Forebrain: Comprised of the limbic system structures and the cerebral cortex.

The Midbrain, Hindbrain, and Brainstem

• Midbrain: Located beneath the thalamus. It handles:
  • Coordination of eye movements such as blinking and focusing.
  • Triggering auditory reflexes (e.g., the startled jump caused by a loud noise).
  • Inhibition of unwanted body movements.
  • Coordination of sensory input and motor output for fine motor control (e.g., writing with a pen or playing an instrument).
• Basal Ganglia: A collection of structures formed by regions of the midbrain and parts of the forebrain; they help regulate complex body movements.
• Hindbrain: Plays roles in glucose regulation and sleep. It includes:
  • Cerebellum: Tucked under the occipital lobe at the very back. It is the second-largest part of the brain by volume and contains over half of the brain's neurons. It is deeply folded and divided into two hemispheres. It coordinates voluntary movements, helps learn new motor skills, and aids in spatial and temporal perception. Damage can lead to a jerky, arrhythmic gait or the inability to touch one's nose accurately.
  • Pons: Located below the cerebellum; it influences breathing and posture.
  • Medulla: Carries nerve pathways connecting the brain to the spinal cord. It contains neural networks for basic functions like swallowing, heart rate, and breathing.
• Brainstem: The collective term for the midbrain, pons, and medulla.

Brain Evolution

• The human brain evolved from a simple tube.
• Amphioxus (Lancelet): Modern lancelets have brains resembling early vertebrates—essentially a wide spot in a hollow nerve cord. Despite simplicity, they have specialized regions for light and chemical processing.
• Evolutionary Bulges: Early vertebrates developed three bulges at the ‐brain‐ end of the nerve cord:
  • Forebrain: Expanded for chemical detection (olfactory bulbs) and complex visual signals (with the evolution of eyes).
  • Midbrain: Expanded to process visual and auditory info to trigger escape, feeding, or mating.
  • Hindbrain: The cerebellum appeared here to expand control over escape movements and spatial orientation, crucial for active swimming.
• The forebrain eventually ‐ballooned out‐ to form cerebral hemispheres. In early mammals, cortical tissues in the cerebrum and cerebellum expanded further into layers and folds.

Neural Networks and Signal Processing

• Nerve Tracts: Distinct bundles of nerve fibers formed by neurons spanning different regions.
  • Example: Corpus callosum (left and right hemispheres).
  • Example: Anterior commissure (left and right temporal lobes).
• Neural Network: A group of nerve tracts connecting a series of brain regions. They route signals along linear pathways to analyze and organize information within fractions of a second.
• Visual Processing Network Example (Watching a Movie):
  1. Photoreceptors in the retina trigger electrical signals in response to light wavelengths.
  2. Signals travel via the optic nerve and optic tract to the thalamus.
  3. Thalamic neurons respond to shape, color, or movement and pass signals to the primary visual cortex (occipital lobe).
  4. Primary visual cortex neurons detect edges and integrate signals from both eyes for a 3D representation.
  5. Information splits into two parallel streams:
     • Temporal lobe: Recognizes and identifies objects.
     • Parietal lobe: Detects spatial location of objects.

Network Activity and Brain Waves (EEG)

• Thalamocortical Loop: A two-way circuit connecting the thalamus and cortex.
• Electroencephalograph (EEG): Detects rhythmic, oscillating electrical patterns known as brain waves.
• Alpha Waves: Frequency of $8-13\,Hz$. Originating in parietal and occipital lobes when relaxed with eyes closed.
• Beta Waves: Frequency of $14-30\,Hz$. Faster; typically from frontal and parietal regions during sensory processing or concentration.
• Theta Waves: Frequency of $4-7\,Hz$. Typical of sleep.
• Delta Waves: Frequency $< 3.5\,Hz$. Typical of deep sleep.
• Amplitude: Alpha and delta waves have higher amplitudes ($20-200\,\mu V$) compared to beta and theta waves ($5-10\,\mu V$).

Organization of Information through Loops and Tracts

• Spinal Tracts: Chains of neurons passing through the brainstem and spinal cord.
  • Upward signals: From sensory receptors (skin/muscles) to the thalamus and cortex.
  • Downward signals: From brain regions inducing movement through the medulla and spinal cord to muscles.
• Feedback Loops:
  • The basal ganglia loop takes info from cortical movement areas and feeds back to the cortex to excite or inhibit specific movements.
  • Cerebellum/brainstem loops influence move timing and strength, incorporating cortical tracts for environmental/emotional context.
  • Hippocampus loops analyze if signals are familiar or new.
  • Hippocampus/Thalamus/Hypothalamus loops allow memory to influence behavior and unconscious physiological responses.
• Reflex Loops: Circuits eliciting action before thought, controlled locally by the spinal cord or subcortical regions without reaching the cortex.

Neural Circuits and Cortical Organization

• Each brain region analyzes a specialized subset of information using neural circuits—interconnected neurons that turn input signals into output patterns.
• Cortical Organization:
  • Neurons are organized in a stack of distinct layers like shelves in a bookcase.
  • Circuits are arranged in columns. Signals enter and travel down the chain of neurons in the column.
  • Each step ‐feeds forward‐ and transforms the signal to encode complex information (e.g., recognizing a face).
• Dynamic Response: While each column has a specific task, neighbors influence them. Every neuron acts as a microprocessor, summing all received signals. Neighboring activity can shift a neuron's response, allowing flexible reactions.

Excitatory and Inhibitory Neurons

• Neurons are either excitatory or inhibitory.
• Excitatory Neurons ($80\%$): Push neighbors toward firing.
  • Pyramidal Cell: Most common excitatory neuron in the cortex, named for its cone-shaped soma. It has apical dendrites and shorter basal dendrites to collect signals from all layers, and a multi-branched axon to send signals to multiple destinations.
• Inhibitory Neurons ($20\%$): Suppress neighboring activity and regulate circuit activity. Often local interneurons that loop back to earlier segments.
• Interplay: The balance is crucial for learning and smoothing signals. Imbalances can cause seizure disorders like epilepsy.
• Input Architectures:
  • Feed-forward inhibition: Inhibitory interneurons in one column send signals to adjacent columns to reduce their activity.
  • Feedback inhibition: Neurons send signals to downstream neighbors AND to interneurons that reach back to inhibit preceding layers.
  • Recurrent Neural Networks: Circuits where neurons send feedback signals to one another.

Cellular Components: Neurons and Glia

• Neuron: The functional unit of circuits. Components include:
  • Soma (Cell Body): Contains the nucleus, cytoplasm, and machinery for protein building.
  • Dendrites: Branched projections that collect incoming signals.
  • Axon: An extension that transmits electrical signals (can be $< 1\,cm$ to $> 1\,m$ long). Ends in axon terminals.
  • Synapse: The junction where signals pass to other cells.
• Glia: Support cells. While previously thought to be a $10:1$ ratio to neurons, recent primate studies suggest a $1:1$ ratio in some regions. Types in the CNS:
  1. Astrocytes: Regulate ion concentrations, provide nutrients, and regulate new connections.
  2. Microglia: Immune cells/phagocytes protecting from infection and damage; they also regulate new connections.
  3. Ependymal Cells: Produce cerebrospinal fluid.
  4. Oligodendrocytes: Wrap axons in a fatty myelin sheath to improve function.

Ion Channels and Action Potentials

• Ion Channels: Tunnel-like protein gates allowing charged atoms (ions) to enter or leave the cell.
• Membrane Potential: Voltage difference across the membrane.
  • Resting Potential: Approximately $-70\,mV$ (more negative inside).
  • Depolarization: Membrane potential becomes less negative.
  • Hyperpolarization: Membrane potential becomes more negative.
• Action Potential: If the sum of signals reaches a threshold voltage, voltage-sensitive ion channels open, triggering an electrical impulse that moves down the axon.

Synapses and Neurotransmission

• Synaptic Cleft: The space between the axon terminal and the next neuron's dendrite (verified in the 1950s via electron microscopy).
• Neurotransmitters: Chemical signals that cross the cleft when electrical signals cannot.
• Mechanism of Release:
  1. Action potential arrives at the axon terminal.
  2. Voltage change opens ion channels, allowing calcium ions to flow in.
  3. Calcium binds to synaptic vesicles (packages of neurotransmitters).
  4. Vesicles fuse with the membrane and empty contents into the cleft.
  5. Membrane pieces cycle back to the soma to refilled as new vesicles.

Neurotransmitter Synthesis and Transport

• Small non-peptides (e.g., dopamine, acetylcholine) are synthesized in the axon terminal.
• Peptide-based neurotransmitters are built in the ribosomes of the cell body.
• Transport: Vesicles bud from the Golgi apparatus, bind to kinesin proteins, and move down the axon along microtubules (cellular skeleton filaments).
• Postsynaptic Density: A region on the dendrite with a high concentration of receptors. Receptors and neurotransmitters fit like a ‐key and lock.‐
• Clearing the Cleft: Astrocytes mop up excess neurotransmitters, or they are broken down/reabsorbed via reuptake to prevent continuous activation.

Receptors: Ionotropic vs. Metabotropic

• Ionotropic Receptors: The neurotransmitter binds directly to an ion channel protein, changing its shape to widen the tunnel for ions. Fast response.
• Metabotropic Receptors: The receptor and ion channel are different proteins. Binding triggers a biochemical cascade (second messengers). Slower response; can open distant channels or activate intracellular molecules.

Major Neurotransmitters: Glutamate and GABA

• Glutamate: Most common excitatory neurotransmitter, used by $\\approx 50\%$ of excitatory synapses. It binds to:
  • AMPA receptors: Fast and brief action.
  • NMDA receptors: Slower activation, responding to waves of action potentials. Interactions are vital for learning and memory.
• GABA (gamma-aminobutyric acid): Most important inhibitory neurotransmitter.
  • Ionotropic GABA receptors: Allow chloride ions (negative) to enter.
  • Metabotropic GABA receptors: Release potassium ions (positive) out of the cell.
  • Both push membrane potential downward to inhibit firing.

Molecular Signaling and Gene Expression

• Neuromodulators: Example: Endocannabinoids (suppress neurotransmitter release).
• Prostaglandins: Small lipids that increase pain sensitivity during inflammation.
• Hormones:
  • Surface binding: Triggers signal transduction pathways changing ion balance or enzyme activity.
  • Diffusion: Steroids like estradiol or cortisol diffuse into the soma, bind to internal receptors, and act as transcription factors to change gene activity in the nucleus.
• Gene Expression: All neurons have the same DNA, but differences arise from which genes are ‐expressed.‐
• Chromatin: DNA/protein complex. Open/unfolded chromatin allows protein building; tightly packed chromatin shuts down genes. This is reversible based on environmental/hormonal cues.
• Gene Variants (Alleles): Variations in nucleotide sequences can lead to structural differences in proteins.
  • Example: Tay-Sachs disease is caused by mutations in the gene for beta-hexosaminidase A. This variant fails to break down fats, which then build up and become toxic to neurons.