Comprehensive Study Guide: Nervous System Organization and Function

Nervous System Organization

Major Functions of the Nervous System

The nervous system is responsible for sensation, integration, and response, enabling organisms to detect changes, process information, and react accordingly.

• Sensation: Detection of internal and external stimuli via sensory receptors.

• Integration: Processing and interpretation of sensory input; decision-making occurs here.

• Response: Activation of effectors (muscles or glands) to produce a reaction.

Central vs. Peripheral Nervous System

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neural structures outside the CNS (nerves, ganglia); connects CNS to receptors, muscles, and glands.

Sensory/Afferent and Motor/Efferent Divisions

• Sensory/Afferent Division: Carries information toward the CNS; involves sensory receptors detecting stimuli.

• Motor/Efferent Division: Carries information away from the CNS to effectors; subdivided into somatic and autonomic systems.

Somatic vs. Autonomic Nervous System

• Somatic Nervous System: Controls voluntary movements of skeletal muscle.

• Autonomic Nervous System: Regulates involuntary functions of cardiac muscle, smooth muscle, and glands.

• Sympathetic vs. Parasympathetic: Sympathetic prepares the body for 'fight or flight'; parasympathetic promotes 'rest and digest'.

Major Brain Structures

Cerebellum

Located posterior to the brainstem, the cerebellum coordinates movement and balance.

• Function: Fine-tunes motor activity, maintains posture and equilibrium.

• Damage: Results in ataxia, loss of coordination, and balance problems.

Brainstem

• Midbrain: Superior region; involved in visual and auditory reflexes.

• Pons: Middle region; bridges midbrain and medulla, regulates breathing.

• Medulla Oblongata: Inferior region; controls vital functions like heart rate and respiration.

Cerebrum and Cerebral Lobes

• Frontal Lobe: Motor functions, reasoning, problem-solving.

• Parietal Lobe: Sensory processing (touch, spatial awareness).

• Temporal Lobe: Auditory processing, memory.

• Occipital Lobe: Visual processing.

Broca's and Wernicke's Areas

• Broca's Area: Located in the frontal lobe; responsible for speech production. Damage causes expressive aphasia.

• Wernicke's Area: Located in the temporal lobe; responsible for language comprehension. Damage causes receptive aphasia.

Diencephalon

• Thalamus: Largest portion; relays sensory information.

• Hypothalamus: Regulates homeostasis, links nervous and endocrine systems; connected to pituitary gland via infundibulum.

• Epithalamus: Contains pineal gland; regulates circadian rhythms.

Hippocampus

• Function: Essential for learning and memory formation.

Nervous Tissue

Neurons

• Function: Specialized for communication; transmit electrical and chemical signals.

• Metabolic Requirements: High energy demand due to active transport and signaling.

Neuroglia

• Function: Support, protect, nourish, and maintain neurons.

CNS Neuroglia

• Astrocytes: Support neurons, maintain environment, regulate blood-brain barrier.

• Oligodendrocytes: Myelinate CNS axons, increase conduction velocity.

• Microglia: Immune defense, phagocytose debris.

• Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid.

PNS Neuroglia

• Schwann Cells: Myelinate PNS axons, aid in regeneration.

• Satellite Cells: Surround neuron cell bodies in ganglia, regulate environment.

Comparison: Oligodendrocyte vs. Schwann Cell

Cell Type	Location	Function
Oligodendrocyte	CNS	Myelination of multiple axons
Schwann Cell	PNS	Myelination of single axon

Neuron Anatomy

• Soma/Cell Body: Metabolic center; contains nucleus and organelles.

• Dendrites: Receive signals; highly branched for increased surface area.

• Axon: Conducts electrical impulses away from soma.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

• Axon Hillock: Initiates action potentials.

• Myelin Sheath: Insulates axon; produced by oligodendrocytes (CNS) and Schwann cells (PNS); increases conduction velocity.

• Nodes of Ranvier: Gaps in myelin; essential for saltatory conduction.

Neuron Classification

Structural Classification

Type	Processes	Location
Unipolar	One process	Sensory neurons
Bipolar	Two processes	Special senses (retina, olfactory)
Multipolar	Multiple processes	Most CNS neurons

Functional Classification

• Sensory/Afferent Neurons: Carry information toward CNS.

• Motor/Efferent Neurons: Carry information away from CNS to effectors.

• Interneurons: Located in CNS; process and relay information.

Resting Membrane Potential

The resting membrane potential is the voltage difference across the neuronal membrane when the cell is at rest, typically around -70 mV.

• Electrical Gradient: Results from ion concentration differences.

• Ion Concentration Gradients: Na+ higher outside, K+ higher inside.

• Membrane Permeability: More permeable to K+ than Na+.

• Sodium-Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in.

Equation:

$V_{m} = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Additional info: This is the simplified Nernst equation for potassium.

Action Potentials

Sequence of Events

1. Resting membrane potential

2. Stimulus

3. Threshold

4. Depolarization

5. Repolarization

6. Hyperpolarization

7. Return to resting conditions

• Depolarization: Na+ enters neuron; membrane potential becomes less negative.

• Repolarization: K+ exits neuron; membrane potential returns toward resting.

• Hyperpolarization: Membrane becomes more negative than resting due to continued K+ efflux.

Threshold and All-or-None Principle

• Threshold: Minimum depolarization required to trigger an action potential.

• All-or-None Principle: Action potential either occurs fully or not at all; amplitude does not vary with stimulus strength.

• Frequency Coding: Stronger stimuli increase action potential frequency, not size.

Refractory Periods

• Absolute Refractory Period: No new action potential possible; Na+ channels inactivated.

• Relative Refractory Period: Action potential possible with stronger stimulus; K+ channels open, membrane repolarizing.

Action-Potential Conduction

• Axon Diameter: Larger diameter = faster conduction due to lower resistance.

• Myelination: Myelinated axons conduct impulses more rapidly via saltatory conduction.

Continuous vs. Saltatory Conduction

Type	Location	Speed	Mechanism
Continuous	Nonmyelinated axons	Slow	AP regenerated along entire axon
Saltatory	Myelinated axons	Fast	AP jumps between nodes of Ranvier

Clinical Applications

• Multiple Sclerosis: Autoimmune attack on CNS myelin; causes demyelination and impaired conduction.

• Local Anesthetics: Block Na+ channels; prevent pain signal transmission.

• Nerve Compression: Reduced blood supply and oxygen impairs neuronal function; causes numbness.

Synapses

• Presynaptic Cell: Sends signal.

• Postsynaptic Cell: Receives signal.

Chemical Synaptic Transmission Sequence

1. Action potential reaches axon terminal

2. Calcium enters terminal

3. Neurotransmitter released

4. Neurotransmitter crosses synaptic cleft

5. Neurotransmitter binds postsynaptic receptors

Chemical vs. Electrical Synapses

Type	Mechanism	Speed
Chemical	Neurotransmitter release	Slower
Electrical	Gap junctions, ion flow	Faster

Neurotransmitters

• Excitatory: Depolarize membrane; increase likelihood of action potential (e.g., Glutamate, Acetylcholine).

• Inhibitory: Hyperpolarize membrane; decrease likelihood of action potential (e.g., GABA, Glycine).

• Other Examples: Dopamine, Substance P, Norepinephrine, Serotonin, Adenosine, Endocannabinoids, Nitric oxide.

Neurotransmitter Effects Depend on Receptors

• Same neurotransmitter can have different effects depending on receptor type.

• Example: Acetylcholine excites skeletal muscle but inhibits cardiac muscle.

Direct vs. Indirect Neurotransmitter Actions

• Direct: Neurotransmitter directly opens ion channel; rapid effect.

• Indirect: Neurotransmitter activates second messenger pathways (often via G proteins); slower, longer-lasting effects.

Ionotropic vs. Metabotropic Receptors

Type	Mechanism	Speed
Ionotropic	Direct ion channel	Rapid
Metabotropic	G protein, second messenger	Slower

EPSPs and IPSPs

• EPSP (Excitatory Postsynaptic Potential): Local depolarization; increases action potential likelihood.

• IPSP (Inhibitory Postsynaptic Potential): Local hyperpolarization; decreases action potential likelihood.

• Both are graded potentials, not all-or-none.

Summation

• Temporal Summation: Multiple signals from one neuron in rapid succession.

• Spatial Summation: Signals from multiple neurons arrive simultaneously.

Presynaptic Inhibition

• One neuron can reduce neurotransmitter release from another via axoaxonal synapse.

Termination of Neurotransmitter Effects

• Reuptake: Neurotransmitter taken back into presynaptic cell.

• Enzymatic Degradation: Neurotransmitter broken down by enzymes.

• Diffusion: Neurotransmitter diffuses away from synapse.

Neural Integration

• Neuronal Pools: Groups of neurons processing information together.

• Allows complex integration and output to other areas.

Neural Circuits

Circuit Type	Organization	Function
Diverging	One input, many outputs	Amplifies signal
Converging	Many inputs, one output	Integrates information
Reverberating	Loop	Prolongs activity
Parallel After-Discharge	Multiple pathways, common output	Complex processing

Reflexes

A reflex arc is the basic functional unit of the nervous system, enabling rapid, automatic responses.

1. Receptor

2. Sensory neuron

3. CNS integration center

4. Motor neuron

5. Effector

Example: The patellar (knee-jerk) reflex follows this sequence.

Additional info: This guide expands on brief points with academic context, definitions, and examples for self-contained exam preparation.