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

Chapter 11: The Nervous System

General Functions of the Nervous System

The nervous system is responsible for coordinating the body's response to internal and external stimuli. Its main functions include:

• Sensory Input: Gathering information from sensory receptors.

• Integration: Processing and interpreting sensory input.

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

Organization of the Nervous System

The nervous system is divided into two major parts:

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

• Peripheral Nervous System (PNS): Connects the CNS to limbs and organs. Subdivided into:

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

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

    • Somatic (Voluntary): Controls skeletal muscles.

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

      • Parasympathetic: Rest and repose functions.

      • Sympathetic: Fight or flight responses.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system. Their functions include:

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion and nutrient concentrations.

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

• Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid.

• 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.

• Unique Features: Excitability, conductivity, and ability to transmit electrical signals.

• Major Structures:

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

  • Dendrites: Receive incoming signals.

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

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

  • Myelin Sheath: Insulates axons, increasing conduction velocity.

• Types of Neurons:

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

  • Bipolar: One dendrite, one axon (sensory organs).

  • Unipolar: Single process (sensory neurons in PNS).

  • Interneurons: Integrate and process information within the CNS.

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the membrane of a resting neuron.

• Typical RMP value: $-70\text{mV}$

• Maintained by ion gradients and selective permeability of the membrane.

Ion Channels

Ion channels are proteins that allow ions to pass through the membrane.

• Leakage Channels: Always open; contribute to RMP.

• Gated Channels: Open or close in response to stimuli.

  • Voltage-Gated: Respond to changes in membrane potential.

  • Ligand-Gated: Respond to binding of specific molecules (e.g., neurotransmitters).

Factors Affecting Resting Membrane Potential

• Differences in ion concentrations inside and outside the cell.

• Selective permeability of the membrane to different ions.

• Role of the sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase):

$3\text{Na}^+$ ions are pumped out, $2\text{K}^+$ ions are pumped in, maintaining the negative charge inside the cell.

Graded and Action Potentials

Neurons communicate via changes in membrane potential:

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

• Action Potentials: Rapid, uniform depolarization and repolarization; all-or-none response.

Phases of Action Potential:

• Depolarization

• Repolarization

• Hyperpolarization

Key Equation:

$\text{Action Potential} = \text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: Action potential can be initiated with a stronger stimulus.

Conduction Velocity

• Influenced by axon diameter and myelination.

• Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

Multiple Sclerosis (MS)

MS is a disease characterized by loss of myelin in the CNS, leading to impaired nerve conduction.

• Symptoms: Muscle weakness, coordination problems.

• Compensation: Remyelination attempts, increased ion channel expression.

The Synapse

Synapses are junctions where neurons communicate with other cells.

• Presynaptic Neuron: Releases neurotransmitters (NTs).

• Postsynaptic Neuron: Receives NTs via receptors.

• Types of Postsynaptic Potentials:

  • EPSP (Excitatory): Depolarizes membrane, brings neuron closer to threshold.

  • IPSP (Inhibitory): Hyperpolarizes membrane, moves neuron away from threshold.

Summation

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Multiple signals from different locations.

Neurotransmitters

Neurotransmitters are chemicals that transmit signals across synapses.

• Categories: Amines, amino acids, peptides, gases.

• Examples: Acetylcholine, dopamine, serotonin, GABA, nitric oxide (NO).

• Functions: Excitatory or inhibitory effects, modulation of neural circuits.

Neurotransmitter Effects and Second Messengers

• Neurotransmitters can have direct (ion channel) or indirect (second messenger) effects.

• Second messenger systems (e.g., cAMP) amplify and prolong the signal.

Table: Comparison of Glial Cells

Cell Type	Location	Function
Astrocyte	CNS	Support, blood-brain barrier, regulate environment
Microglia	CNS	Phagocytosis, immune defense
Ependymal Cell	CNS	Produce and circulate CSF
Oligodendrocyte	CNS	Myelinate axons
Schwann Cell	PNS	Myelinate axons

Table: Types of Neurons

Type	Structure	Location	Function
Multipolar	Many dendrites, one axon	CNS	Motor and interneurons
Bipolar	One dendrite, one axon	Sensory organs	Sensory
Unipolar	Single process	PNS	Sensory

Additional info: Expanded explanations and tables were added for completeness and clarity, based on standard Anatomy & Physiology content for the nervous system.