Chapter 11: Introduction to the Nervous System and Nervous Tissue - Study Notes

Overview of the Nervous System

Functions of the Nervous System

The nervous system is essential for controlling perception, voluntary movement, consciousness, personality, learning, and memory. It also regulates homeostasis in conjunction with the endocrine system, affecting respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.

• Perception and Experience: The nervous system interprets sensory information from the environment.

• Voluntary Movement: Directs actions such as walking, running, and writing.

• Homeostasis: Maintains internal balance through rapid signaling.

Anatomical Divisions

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

• CNS: Consists of the brain and spinal cord, protected by the skull and vertebral column.

• PNS: Includes all nerves outside the CNS, such as cranial and spinal nerves.

Functional Divisions

Functionally, the nervous system is categorized into sensory, integrative, and motor divisions.

• Sensory (Afferent) Division: Gathers information from internal and external environments.

• Integrative Division: Analyzes and interprets sensory input to determine responses.

• Motor (Efferent) Division: Executes responses via effectors (muscles and glands).

Nervous Tissue

Neurons: Structure and Function

Neurons are excitable cells responsible for transmitting electrical signals. Each neuron consists of three main parts:

• Cell Body (Soma): Contains organelles for biosynthetic activity, including ribosomes, rough ER (Nissl bodies), Golgi apparatus, nucleoli, and mitochondria.

• Dendrites: Short, branched processes that receive input and transmit it toward the cell body.

• Axon: A single, long process that conducts action potentials away from the cell body. Key regions include the axon hillock, axon collaterals, telodendria, axon terminals, and axolemma.

Functional Regions of Neurons

• Receptive Region: Dendrites and cell body

• Conducting Region: Axon

• Secretory Region: Axon terminal

Neuron Classification

Neurons are classified by structure and function:

• Multipolar: One axon, multiple dendrites; most common.

• Bipolar: One axon, one dendrite; found in special sensory organs.

• Pseudounipolar: One fused axon that splits into two processes; sensory neurons.

Structural Class	Features	Location
Multipolar	One axon, many dendrites	Most neurons in CNS
Bipolar	One axon, one dendrite	Special sensory organs
Pseudounipolar	One fused axon, two processes	Sensory neurons in PNS

Neuron Groupings

• CNS: Nuclei (cell bodies), tracts (axons)

• PNS: Ganglia (cell bodies), nerves (axons)

Neuroglia

Types and Functions

Neuroglial cells provide structural support, protection, and environmental maintenance for neurons. They can divide and fill spaces left by dead neurons.

• CNS Neuroglia: Astrocytes, oligodendrocytes, microglia, ependymal cells

• PNS Neuroglia: Schwann cells, satellite cells

Astrocytes

• Anchor neurons and blood vessels

• Regulate extracellular environment

• Form blood-brain barrier

• Repair damaged tissue

Oligodendrocytes

• Myelinate axons in CNS

Microglia

• Act as phagocytes, ingesting debris and pathogens

Ependymal Cells

• Line CNS cavities, produce and circulate cerebrospinal fluid

PNS Neuroglia

• Schwann Cells: Myelinate axons in PNS

• Satellite Cells: Support cell bodies in PNS

The Myelin Sheath

Structure and Function

The myelin sheath is formed by layers of plasma membrane from Schwann cells (PNS) or oligodendrocytes (CNS). It insulates axons and increases the speed of action potential conduction.

• Myelinated Axons: Conduct action potentials 15–20 times faster than unmyelinated axons.

• Neurolemma: Present in PNS, absent in CNS.

• Internodes: Segments covered by myelin.

• Node of Ranvier: Gaps between myelinated segments.

Regeneration of Nervous Tissue

Repair Mechanisms

Regeneration is limited in the PNS and nearly nonexistent in the CNS. Neural tissue can regenerate only if the cell body remains intact.

• Wallerian degeneration: Axon and myelin sheath degenerate distal to injury.

• Growth processes form from proximal end.

• Schwann cells and basal lamina form regeneration tube.

• New axon reconnects to target cell.

Electrophysiology of Neurons

Resting Membrane Potential

Neurons maintain a resting membrane potential (RMP) of approximately −70 mV, with a thin layer of negative ions inside and positive ions outside the cell.

Ion Channels and Gradients

• Leak Channels: Always open, allow ions to flow down gradients.

• Gated Channels: Open in response to specific stimuli.

• Types: Ligand-gated, voltage-gated, mechanically-gated

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Returns to resting potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Local Potentials

• Small, local changes in membrane potential

• Can be depolarizing or hyperpolarizing

• Reversible and decremental

Action Potentials

Phases and Mechanisms

Action potentials are rapid, uniform changes in membrane potential, generated in trigger zones. They involve voltage-gated sodium and potassium channels.

• Depolarization: Membrane potential rises toward zero and becomes positive.

• Repolarization: Returns to negative value.

• Hyperpolarization: Temporarily more negative than resting.

Refractory Period

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

• Relative Refractory Period: Only a strong stimulus can trigger an action potential.

Local vs. Action Potentials

• Local potentials are graded, reversible, and decremental.

• Action potentials are all-or-none, irreversible, and nondecremental.

Propagation of Action Potentials

Mechanisms of Propagation

• Action potentials are self-propagating and travel in one direction.

• Conduction speed depends on axon diameter and myelination.

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

• Continuous Conduction: In unmyelinated axons, action potentials propagate along the entire axolemma.

Neuronal Synapses

Types of Synapses

• Electrical Synapses: Direct ion flow via gap junctions; bidirectional and nearly instantaneous.

• Chemical Synapses: Use neurotransmitters; unidirectional, allow variable signal intensities.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, moves closer to threshold.

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

Neural Integration

Summation

• Temporal Summation: Repeated release from a single neuron.

• Spatial Summation: Simultaneous release from multiple neurons.

Termination of Synaptic Transmission

• Neurotransmitter effects are terminated by diffusion, degradation, or reuptake.

Concept Boost: Channels and Pumps in Neurons

• Ligand-gated channels: Located on dendrites and cell body.

• Voltage-gated channels: Located on axon.

• Voltage-gated Ca2+ channels: Located on axon terminal.

• Leak channels and Na+/K+ pumps: Distributed throughout neuron membrane.

Neurotransmitters

Acetylcholine (ACh)

• Widely used in the nervous system; mostly excitatory.

• Quickly degraded by acetylcholinesterase (AChE).

Functional Groups of Neurons

Neuronal Pools and Circuits

• Neuronal Pools: Groups of interneurons processing specific information.

• Diverging Circuits: Single neuron communicates with multiple targets.

• Converging Circuits: Multiple neurons converge on a single target.

• Inhibitory Circuits: Provide negative feedback to stabilize activity.

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