Chapter 11: The Nervous System – Structure and Function Study Notes

Chapter 11: The Nervous System

General Functions of the Nervous System

The nervous system is responsible for receiving sensory input, integrating information, and producing motor output. These functions allow the body to respond to internal and external stimuli.

• Sensory Input: Detection of changes in the environment by sensory receptors.

• 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 parts: 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): Consists of nerves outside the CNS. Divided into:

  • 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 functional support to neurons.

• Astrocytes: Maintain the blood-brain barrier, provide nutrients, and regulate ion balance.

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

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

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

Comparison: Schwann cells myelinate axons in the PNS, while oligodendrocytes myelinate axons in the CNS.

The Neuron

Unique Features and Structure

Neurons are specialized cells for communication within the nervous system. They have unique structures and properties that enable rapid signal transmission.

• 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.

Functional Characteristics:

• Neurons transmit signals via graded potentials and action potentials.

• Neurotransmitters bind to receptors, opening ion channels and generating electrical changes.

• Axons may be myelinated, increasing conduction speed.

Myelin and Nodes of Ranvier

Myelin is a fatty substance that insulates axons, allowing for faster transmission of electrical signals.

• Myelinated Axons: Conduct impulses rapidly via saltatory conduction.

• Unmyelinated Axons: Conduct impulses more slowly.

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

White Matter vs. Gray Matter: White matter consists of myelinated axons; gray matter consists of neuron cell bodies and unmyelinated fibers.

Functional Types of Neurons

Neurons are classified based on their function:

• Sensory (Afferent) Neurons: Transmit information to the CNS.

• Motor (Efferent) Neurons: Transmit commands from the CNS to effectors.

• Interneurons: Integrate and process information within the CNS.

Membrane Potentials and Action Potentials

Resting Membrane Potential (RMP)

At rest, neurons maintain a voltage difference across their membrane, typically around -70 mV.

• Major Intracellular Cation: K+

• Major Extracellular Cation: Na+

• Na+/K+ Pump: Maintains ion gradients by pumping 3 Na+ out and 2 K+ in.

Equation:

$ext{RMP} = -70 ext{ mV}$

Ion Channels

Neurons have different types of ion channels that affect membrane potential:

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

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

Ligand-Gated vs. Voltage-Gated Channels: Ligand-gated channels open when neurotransmitters bind; voltage-gated channels open in response to changes in membrane potential.

Graded Potentials and Action Potentials

Changes in membrane potential can produce graded or action potentials.

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

• Action Potentials: Rapid, large changes in membrane potential that propagate along the axon.

Threshold: The minimum depolarization required to trigger an action potential.

Phases of Action Potential:

• Depolarization: Na+ channels open, Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell.

• Hyperpolarization: K+ channels remain open briefly.

Properties: Action potentials are all-or-none, self-propagating, and unidirectional.

Conduction Velocity

Two main factors influence the speed of nerve impulse conduction:

• Axon Diameter: Larger diameter = faster conduction.

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

Saltatory Conduction: Action potentials jump from node to node in myelinated axons.

Refractory Periods

After an action potential, neurons experience refractory periods:

• 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.

The Synapse

Structure and Function

A synapse is the junction between two neurons where information is transferred via neurotransmitters.

• Presynaptic Neuron: Releases neurotransmitters (NTS).

• Postsynaptic Neuron: Binds neurotransmitters and responds.

Neurotransmitters bind to receptors, opening ion channels and generating graded potentials in the postsynaptic neuron.

Types of Synaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing likelihood of action potential.

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.

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

• Peptides: Include substance P, endorphins, and enkephalins.

• Gases: Nitric oxide (NO) acts as a neurotransmitter.

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 changes in membrane potential.

• Indirect Effects: Longer-lasting changes via second messengers (e.g., cyclic AMP).

Second Messenger Systems: Involve intracellular signaling cascades that amplify and prolong the effects of neurotransmitters.

Table: Comparison of Glial Cells

Cell Type	Location	Main Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier
Microglia	CNS	Immune defense, phagocytosis
Ependymal Cells	CNS	Produce cerebrospinal fluid
Oligodendrocytes	CNS	Form myelin sheaths
Schwann Cells	PNS	Form myelin sheaths

Example: Action Potential Propagation

When a neuron is stimulated, voltage-gated Na+ channels open, causing depolarization. This triggers adjacent channels to open, propagating the action potential along the axon.

Additional info: Some context and definitions were expanded for clarity and completeness, including the table comparing glial cells and explanations of summation and second messenger systems.