Chapter 11: Nervous Tissue and the Nervous System – Study Guide

Chapter 11: Nervous Tissue and the 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 is divided into central and peripheral components, each with specialized functions.

• Main Functions: Sensory input, integration, and motor output.

• Organization: Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• Reflex Loop: A neural pathway that controls a reflex action.

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.

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

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, providing structural and metabolic support to neurons.

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

• Types in PNS: Schwann cells, satellite cells.

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

Neuron Structure and Function

Neurons have specialized structures for receiving, processing, and transmitting information.

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

• Dendrites: Receive incoming signals.

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

• Axon Hillock: Site where action potentials are initiated.

• Myelin Sheath: Insulates axons, increasing conduction speed.

Membrane Potential and Ion Channels

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane, primarily due to ion distribution.

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

• Key Ions: Sodium (Na+), potassium (K+), chloride (Cl-), and calcium (Ca2+).

• Ion Channels: Leak channels, voltage-gated channels, ligand-gated channels.

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

Equation:

$\text{Na}^+/\text{K}^+ \text{ ATPase:} \quad 3\ \text{Na}^+_{\text{in}} \rightarrow 3\ \text{Na}^+_{\text{out}};\ 2\ \text{K}^+_{\text{out}} \rightarrow 2\ \text{K}^+_{\text{in}}$

Action Potentials

Action potentials are rapid, transient changes in membrane potential that propagate along axons to transmit signals.

• Phases: Depolarization, repolarization, hyperpolarization.

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

• All-or-None Principle: Action potentials either occur fully or not at all.

• Propagation: Movement of the action potential along the axon.

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

Equation (Nernst Equation for Equilibrium Potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

Synaptic Transmission

Neurons communicate at synapses, which can be electrical or chemical. Chemical synapses use neurotransmitters to transmit signals across a synaptic cleft.

• Electrical Synapses: Direct passage of ions via gap junctions; rapid communication.

• Chemical Synapses: Neurotransmitter release from presynaptic neuron binds to receptors on postsynaptic neuron.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane.

• Neurotransmitter Examples: Acetylcholine (ACh), GABA, glutamate, dopamine, serotonin, endorphins.

Integration and Summation

Neurons integrate multiple synaptic inputs through summation, determining whether an action potential will be generated.

• Spatial Summation: Multiple synapses at different locations fire simultaneously.

• Temporal Summation: One synapse fires repeatedly in quick succession.

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

Neurotransmitters and Their Effects

Neurotransmitters are chemicals that transmit signals across synapses. They can be excitatory or inhibitory, and their effects depend on the receptors they bind to.

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; inhibitory in the heart.

• GABA: Main inhibitory neurotransmitter in the CNS.

• Glutamate: Main excitatory neurotransmitter in the CNS.

• Dopamine, Serotonin, Endorphins: Modulate mood, reward, pain, and other functions.

Table: Comparison of Electrical and Chemical Synapses

Feature	Electrical Synapse	Chemical Synapse
Speed	Very fast	Slower
Direction	Bidirectional	Unidirectional
Transmission	Direct ion flow	Neurotransmitter release
Modulation	Limited	Highly modifiable

Clinical Applications

• Neurotoxins: Substances that disrupt synaptic transmission (e.g., botulinum toxin blocks ACh release).

• Drugs: Many drugs act by enhancing or inhibiting neurotransmitter action (e.g., SSRIs increase serotonin levels).

Additional info: This guide expands on the study points by providing definitions, examples, and context for each concept, ensuring a comprehensive review for exam preparation.