Neurons, Glial Cells, and Central Nervous System: Structure and Function Study Guide

Neuron Anatomy

Structure and Function of Neuron Parts

Neurons are specialized cells responsible for transmitting electrical and chemical signals throughout the nervous system. Each part of a neuron has a distinct function:

Part	Function
Cell Body (Soma)	Contains the nucleus and other organelles; performs metabolic functions to keep the neuron alive.
Dendrites	Branched projections that receive chemical and electrical signals from other neurons and transmit them toward the cell body.
Axon	A long projection that carries electrical signals (action potentials) away from the cell body toward other neurons.
Axon Hillock	The region where the axon meets the cell body; the "trigger zone" where the action potential is generated if incoming signals are strong enough.
Axon Terminals	The end of the axon, forming a junction with another neuron or cell; releases neurotransmitters into the synapse.
Myelin Sheath	A fatty insulating layer covering many axons, made by glial cells; speeds up transmission of electrical impulses.
Nodes of Ranvier	Small gaps between sections of the myelin sheath; allow for ion exchange, crucial for signal transmission.

Membrane Potential and Action Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuron's membrane when it is not actively transmitting a signal. Typically, it is about -70 mV.

• K+ is the primary ion determining the resting membrane potential due to its high permeability.

• Na+ and Cl- also contribute but to a lesser extent.

Action Potential Phases

An action potential is a rapid change in membrane potential that travels along the axon. It consists of several phases:

1. Resting State: Neuron at -70 mV; voltage-gated channels are closed.

2. Depolarization: Voltage-gated Na+ channels open; Na+ influx makes the inside more positive.

3. Repolarization: Na+ channels inactivate; K+ channels open; K+ efflux restores negative potential.

4. Hyperpolarization: K+ channels close slowly, causing the membrane to become slightly more negative than resting.

5. Return to Resting Potential: Na+/K+ pump restores ion balance.

Equations for Membrane Potential

The Goldman-Hodgkin-Katz (GHK) equation calculates the membrane potential by considering the relative permeability and concentration gradients of multiple ions:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

The Nernst equation is used for a single ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Graded vs. Action Potentials

Feature	Graded Potential	Action Potential
Location	Dendrites and cell body	Axon (starts at axon hillock)
Strength	Varies with stimulus; can be summed	All-or-none; same size and strength
Distance	Signal loses strength over distance (decremental)	Signal does not decrease (non-decremental)
Cause	Ligand-gated or mechanically-gated channels	Voltage-gated Na+ and K+ channels

Refractory Periods

Feature	Absolute Refractory Period	Relative Refractory Period
Can another AP occur?	No, regardless of stimulus	Yes, but only with a strong stimulus
Na+ channel state	Open → inactivated	Closed, ready to open
Main cause	Na+ channel inactivation	K+ channel hyperpolarization

Glial Cells and Myelination

Types and Functions of Glial Cells

Glial cells support and protect neurons. They are classified by location and function:

Glial Cell	Location	Main Functions
Schwann Cells	PNS	Myelinate PNS axons; help regeneration
Satellite Cells	PNS	Support neuron cell bodies; regulate environment
Astrocytes	CNS	BBB, nutrient delivery, ion balance, repair
Oligodendrocytes	CNS	Myelinate CNS axons
Microglia	CNS	Immune defense; phagocytosis
Ependymal Cells	CNS	CSF production and circulation

Myelin and Saltatory Conduction

• Myelin is a fatty insulating layer that speeds up action potential conduction.

• Nodes of Ranvier are gaps in myelin where action potentials are regenerated.

• Saltatory conduction is the process where the action potential "jumps" from node to node, increasing conduction speed.

• Myelinated axons conduct impulses up to 50–100 times faster than unmyelinated axons.

Central Nervous System Structure

Gray Matter vs. White Matter

Feature	Gray Matter	White Matter
Contents	Cell bodies, dendrites, unmyelinated axons	Myelinated axons
Color	Gray (due to cell bodies)	White (due to myelin)
Function	Processing & integration	Fast communication
Brain location	Outer cortex	Inner regions
Spinal cord location	Inner "H" shape	Outer columns

Embryonic Development of the CNS

The CNS develops from the neural tube, which forms from the neural plate during embryogenesis. The neural tube gives rise to the brain and spinal cord.

• Primary brain vesicles: Prosencephalon, Mesencephalon, Rhombencephalon

• Secondary brain vesicles: Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myelencephalon

• Neural crest cells form peripheral ganglia and other structures.

Cranial Nerves

Names and Functions

Cranial Nerve	Function
I. Olfactory	Smell
II. Optic	Vision
III. Oculomotor	Eye movement, pupil, lens
IV. Trochlear	Eye movement (superior oblique)
V. Trigeminal	Facial sensation, chewing
VI. Abducens	Eye abduction
VII. Facial	Expression, taste (anterior 2/3 tongue)
VIII. Vestibulocochlear	Hearing & balance
IX. Glossopharyngeal	Taste (posterior 1/3 tongue), swallowing
X. Vagus	Parasympathetic to organs
XI. Accessory	Neck/shoulder movement
XII. Hypoglossal	Tongue movement

Mnemonic: Oh, Oh, Oh, To Touch And Feel Very Good Velvet, AH!

Major Divisions of the Brain

Brain Regions and Functions

• Cerebrum: Largest part; conscious thought, memory, sensory processing, voluntary movement.

• Diencephalon: Thalamus, hypothalamus, epithalamus; sensory relay, homeostasis, hormone regulation.

• Brainstem: Midbrain, pons, medulla; controls autonomic functions (breathing, heart rate, reflexes).

• Cerebellum: Coordinates movement, balance, and fine motor control.

Thalamus: Structure and Function

The thalamus is a relay station for sensory and motor signals to the cerebral cortex. It is composed of gray matter nuclei and is located deep in the brain, surrounding the third ventricle.

• Receives input from sensory systems and sends output to the cortex.

• Plays a role in consciousness, sleep, and alertness.

Brain Lobes

• Frontal lobe: Voluntary movement, problem solving, planning, speech production.

• Parietal lobe: Sensory information, spatial orientation, body position.

• Temporal lobe: Hearing, memory, language processing.

• Occipital lobe: Visual processing.

Diffuse Modulatory Systems

Neurotransmitter Systems

Diffuse modulatory systems are networks of neurons in the brainstem that project widely and regulate brain function. They are classified by neurotransmitter:

System	Neurotransmitter	Origin	Key Functions
Noradrenergic	Norepinephrine	Locus coeruleus	Attention, arousal, stress, mood
Serotonergic	Serotonin	Raphe nuclei	Mood, pain modulation, sleep
Dopaminergic	Dopamine	Substantia nigra, VTA	Movement, reward, motivation
Cholinergic	Acetylcholine	Basal forebrain, pons	Learning, memory, wakefulness

Memory Systems

Short-Term vs. Long-Term Memory

• Short-term memory: Holds information for seconds to minutes; limited capacity.

• Long-term memory: Stores information for days, years, or a lifetime; unlimited capacity.

Reflexive vs. Declarative Memory

Memory Type	Conscious?	Examples	Brain Regions
Reflexive	No	Motor skills, habits	Cerebellum, basal nuclei
Declarative	Yes	Facts, events, knowledge	Hippocampus, medial temporal lobe, cortex

Memory Consolidation

Memory consolidation is the process by which short-term memories are converted into stable long-term memories, making them more permanent and resistant to forgetting.

Language Centers and Aphasia

Broca's and Wernicke's Areas

• Broca's area: Speech production; damage results in expressive aphasia (slow, effortful speech).

• Wernicke's area: Language comprehension; damage results in receptive aphasia (fluent but nonsensical speech).

Example of Broca's aphasia: "I... want... water..."

Additional info:

• Notes include expanded explanations and tables for comparison, classification, and summary.

• Equations are provided in LaTeX format for academic clarity.

• Mnemonic for cranial nerves is included for memorization aid.