Neuroanatomy and Physiology: Brain Structure, Neurons, and Synaptic Function

Cerebrum

Largest part of the brain; responsible for voluntary actions, complex thought, and memory.

Cerebellum

The "little brain" at the back of the head; coordinates movement and balance.

Ventricles

Fluid-filled cavities within the brain that contain cerebrospinal fluid (CSF).

Gray matter

Brain tissue primarily composed of neuronal cell bodies.

White matter

Brain tissue primarily composed of myelinated axons.

Sulci

Grooves or crevices on the surface of the brain.

Fissures

Deep grooves in the brain (e.g., the longitudinal fissure separating the hemispheres).

Gyri

Bumps or ridges on the surface of the brain.

Nerves use electricity—how do we know? What experiments?

Galvani and du Bois-Reymond showed that applying electrical currents to nerve fibers caused frog muscles to twitch.

The brain has functional organization—how do we know? What experiments suggest cerebral localization?

Flourens used experimental ablation (destroying specific animal brain parts) to show different regions control different functions; Broca studied a patient with a left frontal lesion who lost the ability to speak.

The concept of shared ancestry allowed animal brains to provide two types of insights:

a) Animal models can be used to study generalized human brain function. b) Adaptations to specific environments can be studied across different species.

Neurons are the functional unit of the brain—how do we know? What experiments?

Golgi developed a stain showing individual neurons, and Cajal used it to prove the Neuron Doctrine (neurons are discrete cells, not a continuous web).

Nissl stain

Stains the rough endoplasmic reticulum in cell bodies; helps visualize the arrangement and density of neurons.

Golgi stain

Stains the entire neuron (soma, dendrites, and axon); reveals the total shape of the cell.

Astrocytes

Most numerous glial cells; regulate the extracellular chemical environment (like absorbing excess potassium).

Microglia

Immune cells of the brain; remove debris and dead cells via phagocytosis.

Ependymal cells

Cells that line the ventricles and direct the flow of cerebrospinal fluid (CSF).

Oligodendrocytes

Glial cells that provide myelin sheaths to multiple axons in the Central Nervous System (CNS).

Neuron parts (A-F from diagram)

A) Dendrite B) Soma/Cell Body C) Axon Hillock D) Myelin Sheath E) Node of Ranvier F) Axon Terminal

Classify by number of processes (3 options)

Unipolar, Bipolar, Multipolar.

Classify by direction of impulse (3 options)

Sensory (afferent), Motor (efferent), Interneurons.

Classify by shape (2 options)

Pyramidal (pyramid-shaped), Stellate (star-shaped).

4th way to classify neurons

By the neurotransmitter they release (e.g., cholinergic, dopaminergic).

Channel

Protein pore in the membrane that allows specific ions to pass through passively.

Pump

Protein that uses ATP energy to actively move ions against their concentration gradient.

Potential

The difference in electrical charge across the cell membrane.

Resting membrane potential value

Approximately -65 mV to -70 mV.

Selective permeability

The property of the membrane allowing certain ions to pass more easily than others (e.g., highly permeable to K+ at rest).

How does a steep K+ gradient influence potential?

It drives K+ to passively diffuse out of the cell, making the inside of the cell highly negative.

How does the neuron ensure equilibrium is not reached?

The Sodium-Potassium pump uses ATP to continuously push 3 Na+ out and pull 2 K+ in.

Why do ions gather at the membrane?

Opposite electrical charges attract each other across the thin membrane, creating a capacitor effect.

What matters more for resting potential—sodium or potassium? Why?

Potassium, because the resting membrane is much more permeable to K+ due to passive leak channels.

Depolarization

Membrane potential becomes more positive (moves toward zero).

Repolarization

Membrane potential returns to a negative state after the peak of an action potential.

Propagation

The continuous travel of the action potential down the axon.

Action potential diagram values

Top box (Peak) = ~+40mV. Middle box (Threshold) = ~-55mV. Bottom box (Resting) = ~-70mV. (Phases: 3=Rising/Depolarization, 4=Overshoot, 5=Falling/Repolarization, 6=Undershoot/Hyperpolarization, 7=Resting).

Absolute vs. Relative Refractory Period

Absolute: During the falling phase; a second stimulus absolutely cannot trigger a firing because Na+ channels are inactivated. Relative: During the undershoot; a second stimulus can trigger firing, but it requires a stronger-than-normal stimulus.

Faster/stronger signaling variables

Involves a WIDER diameter axon, a GREATER amount of myelination, and/or a HIGHER number of ion channels.

Why do Tetrodotoxin (antagonist) and Aconitine (agonist) both cause paralysis?

TTX blocks Na+ channels so no action potential can start. Aconitine forces them open so the neuron can't reset (repolarize). Both completely halt continuous nerve communication.

Connexons/Gap Junctions

Channel proteins that directly connect the cytoplasm of two cells in an electrical synapse.

Vesicles

Membrane-enclosed bubbles in the axon terminal containing neurotransmitters.

SNARE proteins

Proteins that physically bind vesicles to the presynaptic membrane to facilitate release.

Spatial summation

Adding together signals that arrive simultaneously from multiple different synapses on a dendrite.

Temporal summation

Adding together signals that arrive at the same synapse in rapid succession.

Integration

The process by which multiple synaptic potentials combine to dictate whether the postsynaptic neuron fires.

Modulation

Synaptic activation (usually via G-proteins) that modifies how effectively EPSPs generated by other synapses function.

Why are electrical synapses limited?

They are fast, but they cannot amplify signals, cannot easily be modulated, and usually only allow simple synchronous firing.

Neurotransmitter synthesis locations

a) Amino acids and amines are made in the axon terminal. b) Peptides are made in the cell body (Rough ER) and shipped down the axon.

Vesicle docking and release

a) SNARE proteins help dock. b) An influx of Calcium (Ca2+) ions signals the vesicles to release.

Receptor activation types

a) Fast/Simple: Ionotropic receptors (ligand-gated ion channels). b) Slow/Complex: Metabotropic receptors (G-protein coupled receptors).

Signal termination mechanisms (3 types)

a) Reuptake back into the presynaptic terminal. b) Enzymatic degradation in the cleft. c) Diffusion away from the synapse.

Dendrite signal strength

Dendrites that are NEAR TO the signal, have a WIDE diameter, and have a SHORT length will propagate the strongest signal.

Agonist

A chemical or drug that binds to a receptor and activates it, mimicking the natural neurotransmitter.

Antagonist

A chemical or drug that binds to a receptor and blocks the natural neurotransmitter from acting.

Ligand

Any molecule that specifically binds to a receptor site.

Receptor

A protein on the cell membrane that receives chemical signals.

Requirements for a neurotransmitter

1) Made and stored in the presynaptic neuron. 2) Released upon presynaptic stimulation. 3) When applied in vitro, produces the exact same response as natural release.

Finding where neurotransmitters are made

1) Immunocytochemistry (uses labeled antibodies). 2) In situ hybridization (uses labeled complementary mRNA).

Showing neurotransmitters are released

1) In vivo fluid collection mostly works for PNS neurons. 2) In vitro brain slice bathing is better for CNS neurons.

Excitatory vs Inhibitory changes

If membrane potential rises (depolarizes), the neurotransmitter is Excitatory. If membrane potential falls (hyperpolarizes), the neurotransmitter is Inhibitory.

Neurotransmitter Classes Table Data

Acetylcholine Precursor: Choline + Acetyl CoA. ACh Rate limiting step: Choline acetyltransferase (ChAT) availability. Serotonin Precursor: Tryptophan. Serotonin Rate limiting step: Tryptophan hydroxylase. Dopamine Precursor: Tyrosine. Dopamine Rate limiting step: Tyrosine hydroxylase.

Brain Lobes

Front (Purple) = Frontal Lobe. Top/Middle (Yellow) = Parietal Lobe. Back (Blue) = Occipital Lobe. Bottom/Side (Orange) = Temporal Lobe.

Anterior/Posterior

Front / Back.

Dorsal/Ventral

Top / Bottom.

Medial/Lateral

Toward the middle / Toward the side.

Sagittal section

Slicing the brain to separate the left and right halves.

Frontal section

Slicing the brain to separate the front and back halves (coronal).

Ventral section

Slicing the brain to separate top and bottom halves (horizontal).

Parts of the CNS

Brain (Cerebrum, Cerebellum), Brainstem (Midbrain, Pons, Medulla), Spinal Cord.

3 layers of meninges

a) Dura mater (closest to skull, tough, thick leather-like). b) Arachnoid mater (middle, web-like). c) Pia mater (closest to brain, thin, delicate membrane).

Cerebral cortex differences in humans

1) Highly folded (more sulci and gyri) maximizing surface area. 2) Massive expansion of the frontal lobe for complex associative functions.

Sympathetic nervous system

"Fight or flight" system; prepares the body for high-stress, active situations.

Parasympathetic nervous system

"Rest and digest" system; conserves energy and manages baseline organ functions.

Ganglion

A cluster of nerve cell bodies in the peripheral nervous system.

Hypothalamus

Master control center of the brain; regulates homeostasis and autonomic responses.

Anterior Pituitary

Gland that synthesizes and secretes its own hormones, dictated by the hypothalamus.

Posterior Pituitary

Gland that stores and releases hormones directly made by the hypothalamus (oxytocin, vasopressin).

Label A and B (PNS Pathway)

A = Parasympathetic pathway (long preganglionic, short postganglionic). B = Sympathetic pathway (short preganglionic, long postganglionic).

Serotonin (5-HT) main roles

Regulates mood, emotional behavior, and sleep cycles.

Dopamine (DA) main roles

Facilitates motor control, reward processing, and reinforcement learning.

Prandial state

The state immediately after eating where the blood is filled with absorbed nutrients.

Post-absorptive state

The fasting condition between meals where stored energy is mobilized.

Leptin Figure Analysis

Leptin ACTIVATES (arrow) POMC/CART neurons (promoting satiety). Leptin INHIBITS (blocked arrow) AgRP/NPY neurons (suppressing hunger).

Anabolism

Building up complex macromolecules to store energy.

Catabolism

Breaking down complex macromolecules to release and use energy.

Humoral response

Stimulating or inhibiting the release of hormones into the bloodstream.

Visceromotor response

Adjusting the balance of sympathetic and parasympathetic nervous system outputs.

Somatic response

Eliciting voluntary motor behaviors (like actively seeking out food).

Job of the lateral hypothalamus

Initiates hunger and coordinates the behavioral drive to seek and eat food.

Cephalic phase

Activated by the hormone Ghrelin. Released when stomach is empty and blood glucose is low. Activates the Homeostatic circuit (need) and Hedonic/Mesolimbic circuit (reward).

Gastric phase

The hormone Gastrin/CCK is produced. Enhances production of digestive enzymes and stomach churning. Mechanoreceptors communicate fullness to the brain via the Vagus nerve.

Intestinal phase

Hormone peptide CCK/PYY is produced in proportion to fat content.

Pacemaker

A cluster of cells (like the SCN) that dictates the rhythm for the rest of the brain.

Collective behavior

When many neurons fire synchronously together.

Orexin

Neuropeptide in the hypothalamus that powerfully promotes wakefulness.

ARAS (Ascending Reticular Activating System)

Network in the brainstem that sends widespread excitatory signals to wake the cortex.

VLPO (Ventrolateral preoptic nucleus)

Brain region that induces sleep by inhibiting wake-promoting networks.

Adenosine

Chemical that builds up during waking hours due to ATP breakdown, increasing the drive for sleep.

Melatonin

Hormone released by the pineal gland during dark cycles that promotes sleepiness.

Circadian proteins

Proteins (like Clock, BMAL1, Per, Cry) that operate on a ~24-hour negative feedback loop dictating the biological clock.