Fundamentals and Organization of the Nervous System: Study Guide

Overview of the Nervous System

Major Divisions

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has distinct structures and functions essential for sensory processing, motor control, and integration.

• Central Nervous System (CNS):

  • Consists of the brain and spinal cord.

  • Responsible for integrating and processing information.

  • Coordinates voluntary and involuntary responses.

• Peripheral Nervous System (PNS):

  • Extends beyond the CNS to connect it with limbs and organs.

  • Contains two functional divisions:

    1. Sensory (Afferent) Division: Carries sensory input to the CNS from receptors.

    2. Motor (Efferent) Division: Carries motor output from the CNS to effectors.

       • Somatic Nervous System (SNS): Controls voluntary movements via skeletal muscles.

       • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion).

         • Sympathetic: Fight-or-flight response.

         • Parasympathetic: Rest-and-digest response.

Neuroglia (Glial Cells)

CNS Neuroglia

Neuroglia are supportive cells in the nervous system that maintain homeostasis, form myelin, and provide support and protection for neurons.

• Astrocytes: Regulate nutrient exchange, support neurons, maintain blood-brain barrier, and repair tissue after injury.

• Microglia: Act as macrophages, removing debris and pathogens.

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

• Oligodendrocytes: Form myelin sheaths around CNS axons, allowing faster signal transmission.

PNS Neuroglia

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

• Schwann Cells: Form myelin sheaths in the PNS, aid in regeneration of damaged fibers.

Neurons – Structure and Function

Special Characteristics

Neurons are highly specialized cells responsible for transmitting electrical signals throughout the nervous system.

• High metabolic rate: require constant oxygen and glucose.

• Extreme longevity.

• Amitotic: most do not divide after development.

Major Parts

• Dendrites: Receive signals and conduct graded potentials toward the soma.

• Cell Body (Soma): Contains nucleus and organelles; integrates incoming signals.

• Axon: Conducts action potentials away from the soma; ends in axon terminals that release neurotransmitters.

• Myelin Sheath: Insulates axons, increases conduction speed.

• Nodes of Ranvier: Gaps in myelin sheath; allow saltatory conduction (action potentials "jump" from node to node).

Classification of Neurons

Structural Classification

• Multipolar: Many dendrites, one axon (most common).

• Bipolar: One dendrite, one axon (rare; e.g., sensory organs).

• Unipolar (Pseudounipolar): Single process divides into peripheral and central branches (sensory neurons of PNS).

Functional Classification

• Sensory Neurons: Afferent; transmit impulses toward CNS.

• Motor Neurons: Efferent; transmit impulses away from CNS.

• Interneurons: Integration; most abundant type.

Principles of Electricity in Neurons

Key Electrical Concepts

• Voltage (V): Difference in charge between two points.

• Current (I): Flow of ions.

• Resistance (R): Hindrance to ion flow.

• Ohm's Law: $I = \frac{V}{R}$

Ion Channels

• Leakage Channels: Always open.

• Gated Channels: Open/close in response to stimuli (ligand, voltage, mechanical pressure).

Resting Membrane Potential (RMP)

Typical RMP

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically around -70 mV.

• Generated by ion concentration differences and membrane permeability.

• Na+/K+ pump maintains gradients:

  • 3 Na+ out / 2 K+ in

• Membrane more permeable to K+ than Na+.

Changes in Membrane Potential

• Depolarization: Inside becomes less negative.

• Hyperpolarization: Inside becomes more negative.

Graded Potentials vs. Action Potentials

Graded Potentials

• Short-distance signals.

• Magnitude varies with stimulus strength.

• Can be depolarizing or hyperpolarizing.

• Decrease with distance.

Action Potentials

• Long-distance signals.

• All-or-none response.

• Do not decrease with distance.

Phases of Action Potential

1. Resting State: Only leak channels open.

2. Depolarization: Voltage-gated Na+ channels open, Na+ influx.

3. Repolarization: Na+ channels close, K+ channels open, K+ efflux.

4. Hyperpolarization: K+ channels remain open briefly.

Propagation and Conduction Velocity

• Propagation: AP moves along axon.

• Conduction Velocity: Depends on myelination and axon diameter.

• Fiber Types:

  • Group A: Large, myelinated (fastest; somatic motor/sensory).

  • Group B: Medium, myelinated (ANS).

  • Group C: Unmyelinated (pain, temp.).

Synapses and Postsynaptic Potentials

Types of Synapses

• Electrical: Gap junctions, fast transmission.

• Chemical: Neurotransmitter release (most common).

Chemical Synapse Steps

1. AP arrives at axon terminal.

2. Voltage-gated Ca2+ channels open.

3. Neurotransmitter released into synaptic cleft.

4. NT binds to postsynaptic receptors.

5. EPSP or IPSP generated.

Postsynaptic Potentials

• EPSP (Excitatory): Na+ influx, graded depolarization.

• IPSP (Inhibitory): K+ efflux or Cl- influx, graded hyperpolarization.

Neurotransmitters and Receptors

Major Neurotransmitters

• Acetylcholine (ACh): Degraded by acetylcholinesterase (AChE).

• Amino acids: Glutamate, GABA, glycine.

• Biogenic amines: Dopamine, norepinephrine, epinephrine, serotonin, histamine.

• Peptides: Endorphins, substance P.

• Others: ATP, nitric oxide.

Receptors

• Direct (Ionotropic): Fast, ligand-gated ion channels.

• Indirect (Metabotropic): Slow, use second messengers (e.g., cAMP).

CNS Organization

Brain Regions

• Cerebral Hemispheres

• Diencephalon

• Brainstem

• Cerebellum

Gray vs. White Matter

• Gray: Cell bodies, dendrites, synapses.

• White: Myelinated axons.

Ventricles and CSF Flow

• Four ventricles: lateral, third, fourth.

• CSF circulates through ventricles and subarachnoid space.

Cerebral Cortex – Functional Areas

Motor Areas

• Primary motor cortex: Controls precise movements.

• Premotor cortex: Plans complex, patterned movements.

• Broca's area: Speech production.

Sensory Areas

• Primary somatosensory cortex: Receives sensory input.

• Visual, auditory, gustatory, visceral sensory areas.

Association Areas

• Integrate and interpret sensory information.

• Prefrontal cortex: cognition, personality, reasoning.

• Language areas: Broca's and Wernicke's areas.

White Matter

• Association fibers: Connect regions within a hemisphere.

• Commissural fibers: Connect hemispheres (e.g., corpus callosum).

• Projection fibers: Connect cortex to lower CNS.

Basal Ganglia

• Regulate motor activity, inhibit unwanted movement.

Diencephalon

• Thalamus: Relay station for sensory input.

• Hypothalamus: Homeostasis center (temperature, hunger, thirst, ANS).

• Epithalamus: Pineal gland secretes melatonin.

Brainstem

• Midbrain, pons, medulla oblongata: Control vital functions (breathing, heart rate, blood pressure).

• Cerebellum: Motor coordination, balance.

Functional Brain Systems

• Limbic System: Emotion, memory (amygdala, hippocampus).

• Reticular Formation: Alertness, arousal (RAS).

Brain Protection

• Meninges: Dura mater, arachnoid mater, pia mater.

• CSF: Cushions and nourishes brain.

• Blood-Brain Barrier: Selective permeability; limits passage of substances.

Brain Disorders

• Trauma: Concussion, contusion, hemorrhage.

• Degenerative Diseases: Alzheimer's, Parkinson's, Huntington's.

Additional info: These notes expand on the original outline by providing definitions, examples, and context for each major topic, suitable for exam preparation in Anatomy & Physiology.