Chapter 11: Nervous System Structure and Function – Study Notes

Chapter 11: Nervous System Structure and Function

General Functions of the Nervous System

The nervous system is responsible for receiving sensory input, integrating information, and producing motor output. These functions are essential for maintaining homeostasis and responding to environmental changes.

• Sensory Input: Detection of stimuli from the internal and external environment.

• Integration: Processing and interpretation of sensory input in the central nervous system (CNS).

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Organization of the Nervous System

The nervous system is divided into two main anatomical and functional divisions: the central nervous system and the peripheral nervous system.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integration and command.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Transmits signals between the CNS and the rest of the body.

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Motor (Efferent) Division: Transmits motor commands from the CNS to effectors.

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

    • Autonomic Nervous System: Controls involuntary functions (smooth muscle, cardiac muscle, glands).

      • Parasympathetic Division: Rest and repose functions.

      • Sympathetic Division: Fight or flight responses.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system that provide structural and metabolic support to neurons.

• Astrocytes: Maintain the blood-brain barrier, regulate ion concentrations, and support neuronal metabolism.

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

• Ependymal Cells: Line ventricles of the brain and produce cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

Comparison: Oligodendrocytes myelinate multiple axons in the CNS, while Schwann cells myelinate a single axon in the PNS.

The Neuron

Neurons are the functional units of the nervous system, specialized for communication via electrical and chemical signals.

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body.

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

• Myelin Sheath: Insulates axons, increasing the speed of impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

Functional Characteristics:

• Neurons are excitable and can generate action potentials.

• They communicate via synapses using neurotransmitters.

• Neurons are classified as sensory, motor, or interneurons based on their function.

Gray Matter vs. White Matter

Gray matter consists mainly of neuronal cell bodies and unmyelinated axons, while white matter is composed of myelinated axons.

• Gray Matter: Site of synaptic integration and processing.

• White Matter: Responsible for transmission of signals over long distances.

Functional Characteristics of Neuron Types

Neurons are classified based on their roles in the nervous system.

• Sensory Neurons: Transmit information from sensory receptors to the CNS.

• Motor Neurons: Carry commands from the CNS to effectors.

• Interneurons: Integrate and process information within the CNS.

Membrane Potentials and Channels

Neurons maintain a resting membrane potential (RMP) due to differences in ion concentrations and membrane permeability.

• Resting Membrane Potential: Typically around -70 mV in neurons.

• Leakage Channels: Always open, allow passive movement of ions.

• Gated Channels: Open or close in response to stimuli (ligand-gated, voltage-gated).

Key Factors:

• Major intracellular cation: K+

• Major extracellular cation: Na+

• Membrane is more permeable to K+ than Na+

• Na+/K+ pump maintains ion gradients

Equation:

$V_m = V_{inside} - V_{outside}$

Graded Potentials and Action Potentials

Changes in membrane potential can produce graded or action potentials, which are essential for neuronal communication.

• Graded Potentials: Local changes in membrane potential; can be depolarizing or hyperpolarizing.

• Action Potentials: All-or-none electrical impulses that propagate along axons.

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

Phases of Action Potential:

• Depolarization

• Repolarization

• Hyperpolarization

Propagation: Action potentials travel unidirectionally along axons, with intensity encoded by frequency.

Refractory Periods

Refractory periods ensure unidirectional propagation and limit the frequency of action potentials.

• Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

• Absolute Refractory Period: No action potential can be initiated, regardless of stimulus strength.

Conduction Velocity

The speed of action potential conduction depends on axon diameter and degree of myelination.

• Axon Diameter: Larger diameter = faster conduction.

• Degree of Myelination: Myelinated axons conduct impulses faster via saltatory conduction.

Saltatory Conduction: Action potentials jump from node to node, increasing speed.

The Synapse

Synapses are specialized junctions where neurons communicate with other neurons or effectors via neurotransmitters.

• Presynaptic Neuron: Releases neurotransmitters (NTS).

• Postsynaptic Neuron: Binds neurotransmitters and responds.

Types of Synaptic Potentials:

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane.

Summation: Multiple EPSPs and IPSPs can combine to influence whether an action potential is generated.

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Signals from multiple synapses at the same time.

Neurotransmitters

Neurotransmitters are chemicals that transmit signals across synapses. They can be classified by structure and function.

• Biogenic Amines: Includes catecholamines (e.g., dopamine, norepinephrine, epinephrine).

• Peptides: Includes substance P, endorphins.

• Gases: Includes nitric oxide (NO).

Excitatory vs. Inhibitory: Some neurotransmitters (e.g., glutamate) are excitatory, while others (e.g., GABA) are inhibitory.

Neurotransmitter Effects and Second Messenger Systems

Neurotransmitters can act directly (opening ion channels) or indirectly (activating second messenger systems).

• Direct Effects: Fast, short-lived responses.

• Indirect Effects: Slower, longer-lasting responses via second messengers (e.g., cAMP).

Second Messenger System Example:

$\text{Receptor} \xrightarrow{\text{NT binding}} \text{G protein} \xrightarrow{\text{activation}} \text{Adenylate cyclase} \rightarrow cAMP$

HTML Table: Comparison of Glial Cells

Glial Cell Type	Location	Main Function
Astrocyte	CNS	Support neurons, maintain blood-brain barrier
Microglia	CNS	Immune defense, phagocytosis
Ependymal Cell	CNS	Produce and circulate CSF
Oligodendrocyte	CNS	Myelinate axons (multiple)
Schwann Cell	PNS	Myelinate axons (single)

HTML Table: Types of Neurons

Neuron Type	Function	Direction of Signal
Sensory (Afferent)	Transmit sensory info to CNS	Toward CNS
Motor (Efferent)	Send commands from CNS to effectors	Away from CNS
Interneuron	Integrate/process information	Within CNS

Example: Action Potential Generation

When a neuron receives sufficient excitatory input, the membrane depolarizes to threshold, triggering an action potential that propagates along the axon to the synaptic terminals.

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the details of neurotransmitter types and second messenger systems.

• Tables were constructed to summarize glial cell types and neuron classifications.