Fundamentals of the Nervous System and Nervous Tissue: Study Notes

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

11.1: The Nervous System Integrates and Responds to Information

The nervous system is responsible for integrating sensory input, processing information, and generating motor output. It works closely with the endocrine system to maintain homeostasis.

• Sensory Input: Gathering information from internal and external environments.

• Integration: Processing and interpreting sensory input.

• Motor Output: Activating effector organs (muscles and glands) to respond.

Divisions of the Nervous System:

• Central Nervous System (CNS): Brain and spinal cord; main control center.

• Peripheral Nervous System (PNS): Nerves outside the CNS; connects CNS to limbs and organs.

  • Somatic: Voluntary control (skeletal muscles).

  • Autonomic: Involuntary control (smooth muscle, cardiac muscle, glands).

Neuroglia: Supporting cells in nervous tissue.

11.2: Neuroglia Support and Maintain Neurons

Neuroglia are non-neuronal cells that provide support, protection, and nutrition to neurons. They are essential for the proper functioning of the nervous system.

• Types of Neuroglia in the CNS:

  • Astrocytes: Most abundant; support neurons, regulate environment, form blood-brain barrier.

  • Microglia: Immune defense; phagocytize debris and pathogens.

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

  • Oligodendrocytes: Form myelin sheaths in CNS.

• Types of Neuroglia in the PNS:

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

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

11.3: Neurons are the Structural Units of the Nervous System

Neurons are specialized cells that transmit electrical signals throughout the body. They have unique properties such as longevity, high metabolic rate, and inability to divide.

• Neuron Cell Body: Contains nucleus and organelles; metabolic center.

• Neuron Processes:

  • Dendrites: Receive signals; conduct impulses toward cell body.

  • Axon: Transmits impulses away from cell body; may be myelinated for faster conduction.

• Functional Characteristics:

  • Axonal Transport: Movement of materials along axon.

  • Myelin Sheath: Insulates axons; increases speed of impulse transmission.

Classification of Neurons

Neurons can be classified by structure and function.

• Structural Classification:

  • Multipolar: Many processes; most common type.

  • Bipolar: Two processes; found in special senses.

  • Unipolar: One process; mainly sensory neurons.

• Functional Classification:

  • Sensory (Afferent): Transmit impulses toward CNS.

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

  • Interneurons: Connect sensory and motor neurons; integration within CNS.

11.4: The Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest. It is essential for the generation and propagation of electrical signals in neurons.

• Basic Principles:

  • Voltage: Difference in electrical charge between two points.

  • Membrane Ion Channels: Allow movement of ions; contribute to RMP.

• Establishing the RMP:

  • Difference in ion composition inside and outside the cell.

  • Difference in plasma membrane permeability.

  • Key ions: Na+, K+, Cl-

  • Na+/K+ pump: Maintains concentration gradients.

Equation for RMP (Nernst Equation):

$E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

11.5: Graded Potentials and Action Potentials

Neurons communicate via electrical signals called graded potentials and action potentials. Graded potentials are short-lived and local, while action potentials are long-distance signals.

• Graded Potentials: Local changes in membrane potential; can summate to trigger action potential.

• Action Potentials: Rapid, large changes in membrane potential; follow the "all-or-none" principle.

• Phases of Action Potential:

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

  • Repolarization: K+ efflux; membrane potential returns to resting value.

  • Hyperpolarization: Membrane potential becomes more negative than resting.

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

Action Potential Equation:

$I = g(V - E)$

Where $I$ is the current, $g$ is the conductance, $V$ is the membrane potential, and $E$ is the equilibrium potential.

Conduction Velocity

The speed at which an action potential travels along an axon depends on axon diameter and degree of myelination.

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Saltatory Conduction: Occurs in myelinated axons; faster due to jumping between nodes of Ranvier.

11.6: Synapses Transmit Signals Between Neurons

Synapses are junctions where neurons communicate with other neurons or effectors. They can be electrical or chemical.

• Chemical Synapses: Use neurotransmitters to transmit signals across synaptic cleft.

• Events at a Chemical Synapse:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

  3. Neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic membrane.

  5. Ion channels open; postsynaptic potential generated.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes postsynaptic membrane; may trigger action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes postsynaptic membrane; inhibits action potential.

Integration at Synapses

Summation of EPSPs and IPSPs determines whether a neuron will fire an action potential.

• Temporal Summation: Multiple signals in rapid succession.

• Spatial Summation: Multiple signals from different locations.

11.8: The Effect of Neurotransmitters Depends on Receptors

Neurotransmitters are classified by chemical structure and function. Their effect depends on the type of receptor they bind to.

• Major Classes of Neurotransmitters:

  • Acetylcholine (ACh): Excitatory or inhibitory; found at neuromuscular junctions.

  • Biogenic Amines: Catecholamines (dopamine, norepinephrine, epinephrine), indolamines (serotonin, histamine).

  • Amino Acids: Glutamate, GABA, glycine.

  • Peptides: Endorphins, substance P.

  • Purines: ATP, adenosine.

  • Gases and Lipids: Nitric oxide (NO), carbon monoxide (CO), endocannabinoids.

• Classification by Function:

  • Excitatory vs. Inhibitory: Effect on postsynaptic membrane.

  • Direct vs. Indirect Action: Direct binding to receptor vs. second messenger systems.

11.10: Neurons Act Together

Neurons are organized into pools and circuits to process information efficiently.

• Neuron Pools: Groups of neurons working together.

• Types of Circuits:

  • Diverging: One input, many outputs.

  • Converging: Many inputs, one output.

  • Parallel: Inputs processed simultaneously in different pathways.

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate environment, form blood-brain barrier
Microglia	CNS	Immune defense, phagocytosis
Ependymal Cells	CNS	Produce and circulate CSF
Oligodendrocytes	CNS	Form myelin sheaths
Satellite Cells	PNS	Regulate environment around neuron cell bodies
Schwann Cells	PNS	Form myelin sheaths, aid in regeneration

*Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and tables have been added to support understanding of key concepts.*