Neurotransmitters and Structural Organization of the Brain: Cell Biology Study Notes

The Nervous System

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating sensory input, motor output, and higher cognitive functions. It is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves.

Neurotransmitters: Monoamines and Amino Acids

Monoamine Action and Inactivation

Monoamines are neurotransmitters derived from amino acids and include catecholamines such as dopamine and norepinephrine. Their action involves synthesis, release, receptor binding, and inactivation.

• Synthesis: Monoamines are produced and stored in synaptic vesicles.

• Release: Action potentials trigger vesicle fusion and neurotransmitter release into the synaptic cleft.

• Inactivation: Most monoamines are inactivated by monoamine oxidase (MAO).

Catecholamines

Catecholamines are a subgroup of monoamines and include dopamine, norepinephrine, and epinephrine. They do not use direct ion channels but act via second messenger systems, most commonly cyclic adenosine monophosphate (cAMP).

• Dopamine: Involved in motor control, emotional reward, and cognitive functions.

• Norepinephrine: Functions in both CNS and PNS, especially in the sympathetic nervous system for the 'fight-or-flight' response.

Dopamine as a Neurotransmitter

Dopaminergic neurons are concentrated in the midbrain and are involved in several key pathways:

• Nigrostriatal system: Motor control.

• Mesolimbic system: Emotional reward.

• Mesocortical system: Memory, cognition, and learning.

Norepinephrine as a Neurotransmitter

Norepinephrine is used by both CNS and PNS neurons. In the PNS, it is crucial for the sympathetic response:

• Increases muscle blood flow, dilates pupils, accelerates heart rate, and raises blood pressure.

• In the CNS, it is associated with arousal and alertness.

Amino Acids as Neurotransmitters

Amino acids such as glutamate and GABA serve as major neurotransmitters in the brain.

• Glutamate: The primary excitatory neurotransmitter, responsible for most excitatory postsynaptic potentials (EPSPs) in the cerebral cortex.

• GABA (Gamma-aminobutyric acid): The main inhibitory neurotransmitter, opening Cl- channels to hyperpolarize neurons.

Glutamate Receptors

Types of Glutamate Receptors

• AMPA receptors (iGluR): Ligand-gated ion channels, fast synaptic transmission.

• NMDA receptors (iGluR): Ligand and voltage-gated, allow Na+ and Ca2+ entry, involved in synaptic plasticity.

• Metabotropic receptors: G protein-coupled, slower, modulate synaptic activity via second messengers.

Glutamate Receptors: AMPA and NMDA

AMPA receptors mediate fast, transient depolarizations. NMDA receptors require both ligand binding and membrane depolarization to allow ion flow, and are blocked by Mg2+ at resting potential.

Synaptic Plasticity: Long-Term Potentiation (LTP)

Mechanism of LTP

LTP is a process where repeated stimulation of a synapse enhances its strength, crucial for memory formation.

• NMDA receptor activation allows Ca2+ influx, triggering signaling cascades.

• Insertion of AMPA receptors increases synaptic efficacy.

• LTP is prominent in the hippocampus, a key memory center.

Amino Acids as NTs: GABA

GABA Function and Receptors

• GABAA and GABAC: Ionotropic, ligand-gated Cl- channels.

• GABAB: Metabotropic, G protein-coupled.

GABAA Receptor Modulation

• Benzodiazepines: Enhance GABAA receptor activity, increasing Cl- permeability and neural inhibition.

• Ethanol, barbiturates, neurosteroids: Also potentiate GABAA receptor effects.

• Excessive inhibition can be dangerous, especially when combining drugs that act on GABAA receptors.

Structural Organization of the Brain

Central Nervous System

• Receives sensory input and directs motor output.

• Integrates information to maintain homeostasis.

• Divided into grey matter (cell bodies, dendrites) and white matter (axons).

The Adult Brain: General Info

• Contains 100 billion neurons.

• Weighs about 1.5 kg.

• Receives 15% of total blood flow per minute.

General Structure of the Brain

• Forebrain: Cerebrum, thalamus, hypothalamus.

• Midbrain

• Hindbrain: Pons, medulla oblongata, cerebellum.

Introduction to the Cerebrum

• Largest brain portion, divided into right and left hemispheres.

• Functions: touch, vision, hearing, speech, reasoning, emotions, learning, movement control.

• Hemispheres joined by corpus callosum.

• Each hemisphere controls the opposite side of the body.

Outer Layer of the Cerebrum: Cerebral Cortex

• Composed of 2-4 mm gray matter.

• Characterized by gyri (folds) and sulci (grooves).

• Divided into 5 lobes: Frontal, Parietal, Temporal, Occipital, Insula.

Frontal and Parietal Lobes

• Frontal lobe: Personality, emotions, judgment, planning, speech, motor control (precentral gyrus).

• Parietal lobe: Language, touch, pain, temperature, spatial perception (postcentral gyrus).

Maps of the Precentral and Postcentral Gyri

• Precentral gyrus: Primary motor cortex, controls voluntary movement.

• Postcentral gyrus: Somatosensory cortex, processes sensory input.

Temporal, Occipital, and Insula Lobes

• Temporal lobe: Auditory centers, memory, language.

• Occipital lobe: Vision, eye movement coordination.

• Insula: Memory encoding, sensory integration, visceral responses.

Brain Areas for Language

Broca's Area

• Located in left inferior frontal gyrus.

• Controls motor aspects of speech.

• Damage causes non-fluent (Broca's) aphasia: slow, poorly articulated speech.

Wernicke's Area

• Located in left superior temporal gyrus.

• Controls understanding of words.

• Damage causes fluent (Wernicke's) aphasia: rapid, meaningless speech ('word salad').

Angular Gyrus

• Located at the junction of parietal, occipital, and temporal lobes.

• Integrates multi-sensory information for language comprehension.

• Damage affects reading and writing abilities.

Cortical Network for Speech

• Broca's and Wernicke's areas connected by arcuate fasciculus.

• Damage causes conduction aphasia: impaired repetition and speech production.

Table: Comparison of Major Neurotransmitter Receptors

Receptor Type	Neurotransmitter	Channel Type	Main Function
AMPA	Glutamate	Ligand-gated ion channel	Fast excitatory transmission
NMDA	Glutamate	Ligand & voltage-gated ion channel	Synaptic plasticity, memory
GABAA	GABA	Ligand-gated Cl- channel	Inhibitory transmission
GABAB	GABA	G protein-coupled	Slow inhibitory transmission

Key Equations

• cAMP as a second messenger: $\text{ATP} \xrightarrow{\text{Adenylyl cyclase}} \text{cAMP} + \text{PP}_i$

• EPSP generation: $\text{EPSP} \propto \text{Na}^+ \text{influx}$

• NMDA receptor ion flow: $\text{NMDA channel open} \Rightarrow \text{Na}^+, \text{Ca}^{2+} \text{influx}$

Additional info: These notes expand on the original slides by providing definitions, context, and examples for neurotransmitter function, receptor types, and brain structure relevant to cell biology and neurobiology.