Chapter 11: Nervous Tissue and Nervous System – Study Guide

Chapter 11: Nervous Tissue and Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It consists of the central nervous system (CNS) and peripheral nervous system (PNS).

• Main Functions: Sensory input, integration, motor output, and homeostasis (including reflex loops).

• Organization: CNS (brain and spinal cord) and PNS (nerves and ganglia).

• Functional Divisions: Sensory (afferent), motor (efferent), and integrative functions.

• Reflex Arc: A neural pathway that controls a reflex action, typically involving a receptor, sensory neuron, integration center, motor neuron, and effector.

Structural and Functional Classification of Neurons

Neurons are classified based on their structure and function, which determines their role in the nervous system.

• Structural Types: Multipolar, bipolar, and unipolar neurons.

• Functional Types: Sensory (afferent), motor (efferent), and interneurons (association neurons).

• Location: Cell bodies of sensory neurons are found in ganglia outside the CNS; motor neurons are located in the CNS.

Neuroglia (Glial Cells)

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

• Types in CNS: Astrocytes, oligodendrocytes, microglia, and ependymal cells.

• Types in PNS: Schwann cells and satellite cells.

• Functions: Myelination, support, protection, and maintenance of the extracellular environment.

Membrane Potential and Action Potentials

Neurons communicate via electrical signals, primarily through changes in membrane potential and the generation of action potentials.

• Resting Membrane Potential (RMP): The voltage difference across the membrane at rest, typically around -70 mV.

• Ion Distribution: Na+ and K+ ions are distributed unevenly across the membrane, maintained by the sodium-potassium pump.

• Action Potential: A rapid change in membrane potential that propagates along the axon.

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), and hyperpolarization.

• Threshold: The minimum stimulus required to trigger an action potential.

• Propagation: Action potentials travel along axons without decreasing in strength.

Equation:

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

Where $V_m$ is the membrane potential.

Graded Potentials

Graded potentials are changes in membrane potential that vary in size and are localized to a specific region of the neuron.

• Types: Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

• Summation: Graded potentials can be summed spatially or temporally to reach threshold.

Synaptic Transmission

Neurons communicate with each other and with effector cells at synapses, which can be electrical or chemical.

• Electrical Synapses: Direct flow of ions through gap junctions; rapid transmission.

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

• Neurotransmitter Release: Triggered by Ca2+ influx following an action potential.

• Postsynaptic Response: Binding of neurotransmitter to receptors causes EPSPs or IPSPs.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses.

• Examples: Acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate, epinephrine, serotonin, and endorphins.

• Effects: Can be excitatory or inhibitory depending on the receptor type.

• Removal: By reuptake, enzymatic degradation, or diffusion away from the synapse.

Integration and Processing in Neurons

Neurons integrate multiple inputs to determine whether to generate an action potential.

• Summation: Spatial and temporal summation of EPSPs and IPSPs.

• Neural Circuits: Diverging, converging, reverberating, and parallel after-discharge circuits.

Table: Comparison of Electrical and Chemical Synapses

Feature	Electrical Synapse	Chemical Synapse
Transmission Speed	Very fast	Slower
Directionality	Usually bidirectional	Unidirectional
Mechanism	Ion flow through gap junctions	Neurotransmitter release and receptor binding
Example	Cardiac muscle cells	Neuromuscular junction

Additional info:

• Neurotransmitters can have different effects depending on the type of receptor present on the postsynaptic cell.

• Myelination by Schwann cells (PNS) and oligodendrocytes (CNS) increases the speed of action potential conduction via saltatory conduction.

• Diseases such as multiple sclerosis result from demyelination, affecting nervous system function.