Fundamentals of the Nervous System: Structure, Function, and Signaling

I. Organization of the Nervous System

Major Functions of the Nervous System

• Sensation: The process of detecting changes in the internal or external environment through sensory receptors.

• Integration: The interpretation and processing of sensory input, leading to decision-making.

• Response: The activation of effectors (muscles or glands) to produce a reaction based on integrated information.

Central Nervous System (CNS)

• Structures: Brain and spinal cord.

• Role: Acts as the main control center for processing and integrating sensory data and issuing instructions.

Peripheral Nervous System (PNS)

• Structures: Cranial nerves, spinal nerves, and associated ganglia.

• Role: Connects the CNS to sensory receptors, muscles, and glands, facilitating communication between the body and CNS.

Sensory (Afferent) Division

• Direction: Carries information toward the CNS.

• Type of Information: Sensory input from receptors (e.g., touch, pain, temperature).

• Relationship: Sensory receptors detect stimuli and transmit signals to the CNS for processing.

Motor (Efferent) Division

• Direction: Carries commands away from the CNS to effectors.

• Subdivisions:

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

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

Somatic vs. Autonomic Nervous System

• Somatic: Voluntary control of skeletal muscles.

• Autonomic: Involuntary control of cardiac muscle, smooth muscle, and glands.

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

II. Major Brain Structures

Cerebellum

• Location: Inferior to the occipital lobes, posterior to the brainstem.

• Function: Coordinates voluntary movements, balance, and posture.

• Damage: Leads to ataxia (uncoordinated movements), tremors, and difficulty with balance.

Brainstem

• Regions: Midbrain, pons, medulla oblongata.

• Functions:

  • Midbrain: Visual and auditory reflexes, eye movement.

  • Pons: Relays signals between cerebrum and cerebellum; regulates breathing.

  • Medulla Oblongata: Controls vital autonomic functions (heart rate, blood pressure, respiration).

Cerebrum and Cerebral Lobes

• Lobes:

  • Frontal: Voluntary motor control, planning, reasoning, speech production.

  • Parietal: Somatosensory processing (touch, pressure, pain).

  • Temporal: Auditory processing, memory, language comprehension.

  • Occipital: Visual processing.

• Non-lobe structures: Insula (sometimes considered a fifth lobe), corpus callosum, etc.

Broca's Area

• Location: Left frontal lobe (usually).

• Function: Speech production.

• Damage: Broca's aphasia—difficulty producing speech, but comprehension is relatively preserved.

Wernicke's Area

• Function: Language comprehension.

• Damage: Wernicke's aphasia—fluent but nonsensical speech and impaired understanding.

Thalamus

• Part of: Diencephalon.

• Function: Relay station for sensory information to the cerebral cortex.

Hypothalamus

• Functions: Regulates homeostasis, controls the autonomic nervous system, links nervous and endocrine systems.

• Relationship: Connected to the pituitary gland via the infundibulum.

Diencephalon

• Major Structures: Thalamus (largest), hypothalamus, epithalamus.

Hippocampus

• Function: Essential for learning and formation of new memories.

III. Nervous Tissue

Neurons

• Function: Specialized for rapid communication via electrical and chemical signals.

• Specialization: High metabolic rate, longevity, inability to divide after maturity.

Neuroglia (Glial Cells)

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

• Types: Vary between CNS and PNS (see below).

IV. CNS Neuroglia

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

• Oligodendrocytes: Produce myelin sheaths in the CNS.

• Microglia: Act as immune cells, phagocytize debris and pathogens.

• Ependymal Cells: Line ventricles and central canal; produce and circulate cerebrospinal fluid (CSF).

V. PNS Neuroglia

• Schwann Cells: Form myelin sheaths around peripheral axons; aid in regeneration.

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

Oligodendrocyte vs. Schwann Cell

• Oligodendrocyte: CNS, myelinates multiple axons.

• Schwann Cell: PNS, myelinates a single axon segment.

VI. Neuron Anatomy

• Soma (Cell Body): Metabolic center; contains nucleus, organelles.

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

• Axon: Conducts action potentials away from soma.

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

• Axon Hillock: Site of action potential initiation.

• Myelin Sheath: Insulating layer; increases conduction velocity; produced by oligodendrocytes (CNS) or Schwann cells (PNS).

• Nodes of Ranvier: Gaps in myelin; sites of action potential regeneration during saltatory conduction.

VII. Neuron Classification

Structural Classification

• Unipolar: Single process splits into peripheral and central branches (sensory neurons).

• Bipolar: One axon, one dendrite (rare; e.g., retina, olfactory epithelium).

• Multipolar: One axon, multiple dendrites (most common; motor and interneurons).

Functional Classification

• Sensory (Afferent) Neurons: Transmit impulses toward CNS.

• Motor (Efferent) Neurons: Transmit impulses away from CNS to effectors.

• Interneurons: Located within CNS; process and integrate information.

VIII. Resting Membrane Potential

• Definition: The voltage difference across the neuron's membrane at rest (typically -70 mV).

• Electrical Gradient: Difference in charge across the membrane.

• Ion Gradients: High K+ inside, high Na+ outside.

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

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

Equation:

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

Additional info: This is a simplified version of the Goldman-Hodgkin-Katz equation for membrane potential.

IX. Action Potentials

• Sequence:

  1. Resting membrane potential

  2. Stimulus

  3. Threshold reached

  4. Depolarization (Na+ influx)

  5. Repolarization (K+ efflux)

  6. Hyperpolarization (membrane more negative than rest)

  7. Return to resting potential

• Depolarization: Na+ channels open, Na+ enters, membrane potential becomes less negative.

• Repolarization: K+ channels open, K+ exits, membrane returns toward negative.

• Hyperpolarization: K+ channels remain open briefly, membrane potential dips below resting value.

X. Threshold and the All-or-None Principle

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

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

• Stimulus Intensity: Encoded by frequency of action potentials, not size.

XI. Refractory Periods

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

• Relative Refractory Period: Possible to trigger another action potential, but requires stronger stimulus; K+ channels still open.

XII. Action-Potential Conduction

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

• Myelination: Myelinated axons conduct impulses much faster than unmyelinated ones.

XIII. Continuous vs. Saltatory Conduction

• Continuous Conduction: Occurs in unmyelinated axons; action potential regenerates along entire length; slower.

• Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier; much faster.

XIV. Clinical Applications

• Multiple Sclerosis (MS): Autoimmune attack on CNS myelin; causes slowed or blocked nerve conduction, leading to neurological symptoms.

• Local Anesthetics: Block voltage-gated Na+ channels, preventing action potential propagation and pain sensation.

• Nerve Compression: Prolonged pressure reduces blood flow and oxygen, impairing neuronal function and causing numbness.

XV. Synapses

• Presynaptic Cell: Sends signal toward synapse.

• Postsynaptic Cell: Receives signal.

• Chemical Synaptic Transmission Sequence:

  1. Action potential reaches axon terminal

  2. Voltage-gated Ca2+ channels open

  3. Neurotransmitter released into synaptic cleft

  4. Neurotransmitter binds to postsynaptic receptors

  5. Postsynaptic response generated

XVI. Chemical vs. Electrical Synapses

• Chemical Synapses: Use neurotransmitters; involve synaptic vesicles and cleft; unidirectional; slower.

• Electrical Synapses: Use gap junctions; ions pass directly; bidirectional; faster; less common in adults.

XVII. Neurotransmitters

• Excitatory: Cause depolarization (e.g., glutamate, acetylcholine at neuromuscular junction).

• Inhibitory: Cause hyperpolarization (e.g., GABA, glycine).

• Other Examples: Dopamine (modulatory), norepinephrine (excitatory or inhibitory), serotonin, substance P, adenosine, endocannabinoids, nitric oxide.

XVIII. Neurotransmitter Effects Depend on Receptors

• The same neurotransmitter can have different effects depending on the receptor type (e.g., acetylcholine excites skeletal muscle but inhibits cardiac muscle).

XIX. Direct vs. Indirect Neurotransmitter Actions

• Direct: Neurotransmitter binds and opens ion channel directly; rapid response.

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

XX. Ionotropic vs. Metabotropic Receptors

• Ionotropic: Ligand-gated ion channels; neurotransmitter binding opens channel directly; fast response.

• Metabotropic: G protein-coupled receptors; activate intracellular signaling cascades; slower, modulatory effects.

XXI. EPSPs and IPSPs

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

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

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

XXII. Summation

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

• Spatial Summation: Signals from multiple presynaptic neurons arrive simultaneously.

XXIII. Presynaptic Inhibition

• One neuron can inhibit another's neurotransmitter release via axoaxonal synapse, reducing postsynaptic effect.

XXIV. Termination of Neurotransmitter Effects

• Reuptake: Neurotransmitter taken back into presynaptic cell.

• Enzymatic Degradation: Neurotransmitter broken down by enzymes (e.g., acetylcholinesterase).

• Diffusion: Neurotransmitter diffuses away from synaptic cleft.

XXV. Neural Integration

• Neuronal Pools: Groups of interconnected neurons that process specific types of information.

• Allow for complex processing and integration of signals.

XXVI. Neural Circuits

Circuit Type	Organization	Function
Diverging	One input, many outputs	Amplifies signal; e.g., motor pathways
Converging	Many inputs, one output	Integration of information; e.g., sensory pathways
Reverberating	Signal loops back	Rhythmic activities; e.g., breathing
Parallel After-Discharge	Input diverges, then converges	Complex processing; e.g., problem-solving

XXVII. Reflexes

• Reflex Arc Components (in order):

  1. Receptor

  2. Sensory neuron

  3. CNS integration center

  4. Motor neuron

  5. Effector

• Example: Knee-jerk reflex involves all five components in a rapid, involuntary response.