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CHAPTER 11: Functional Organization of Nervous Tissue
11.1 Overview of the Nervous System
Definition: The nervous system is a communication system controlling body functions, composed of:
Central Nervous System (CNS): Brain and spinal cord
Peripheral Nervous System (PNS): Nerves, ganglia, and sensory receptors
Control Systems:
Nervous System: Uses electrical and chemical signals (neurons) to control functions.
Endocrine System: Utilizes hormones for control (refer to Chapters 17 and 18).
Study Breakdown:
Chapters focus on the physiology, anatomy of spinal cord, cranial nerves, special senses, and autonomic nervous system.
11.2 Functions of the Nervous System
LEARNING OUTCOMES: Understand the key functions of the nervous system:
Homeostasis Maintenance: Regulates bodily functions to maintain stable internal conditions.
Sensory Input: Receives external and internal stimuli through sensory receptors.
Examples of stimuli: Vision, hearing, taste, touch, pain, etc.
Integration of Information: Processes sensory information and initiates responses.
Muscle and Gland Control: Coordinates muscle contractions and glandular secretions.
Mental Activity: Supports consciousness, thought, memory, and emotions.
11.3 Structure of Nervous Tissue
Cell Types: Nervous tissue comprises:
Neurons: Electrically excitable cells; involved in signal transmission, primarily through action potentials.
Structure: Cell body, dendrites (receive signals), axon (transmits signals).
Glial Cells: Supportive cells that assist neurons; e.g. astrocytes, oligodendrocytes, microglia, and ependymal cells.
Neuron Classification
Function:
Sensory Neurons: Conduct signals towards CNS.
Motor Neurons: Conduct signals away from CNS.
Interneurons: Connect neurons within CNS.
Structure:
Multipolar Neurons: Multiple dendrites, one axon (e.g., motor neurons).
Bipolar Neurons: Single axon, single dendrite (e.g., in sensory organs).
Pseudo-Unipolar Neurons: Single, bifurcating axon (most sensory neurons).
Anaxonic Neurons: No true axon (found in brain and retina).
11.4 Gray Matter and White Matter
Gray Matter: Contains neuron cell bodies; darker in appearance due to minimal myelin. Found in the brain cortex and spinal ganglia.
White Matter: Comprises myelinated axons; lighter. Forms nerve tracts in CNS and nerves in PNS.
11.5 Electrical Properties of Neurons
Resting Membrane Potential: Difference in charge across the plasma membrane (~ -70 mV in neurons).
Factors Influencing Membrane Potential:
Ion Concentration Differences: Uneven distribution of ions (e.g. higher K+ inside cell, higher Na+ outside).
Ion Permeability: Permeability of the membrane varies for different ions, affecting resting potential:
More permeable to K+ due to more K+ leak channels.
Action Potentials: Rapid changes in membrane potential (depolarization and repolarization) generated by ion movement.
Ion Channels:
Leak Ion Channels: Always open, maintain resting potential.
Gated Ion Channels: Open or close in response to stimuli (ligand-gated, voltage-gated).
11.6 Neuron Communication
Phases of Communication:
Generation of Action Potentials at the axon hillock when graded potentials reach threshold.
Propagation of Action Potentials along the axon.
Communication at the Synapse through neurotransmitter release.
Graded Potentials vs Action Potentials
Graded Potentials: Small changes in membrane potential localized to specific areas, can summate to reach threshold.
Action Potentials: Full depolarization of the neuron that propagates along the axon in an all-or-none manner.
11.7 Synapses: Electrical vs Chemical
Electrical Synapses: Direct signal transmission through gap junctions; fast communication.
Chemical Synapses: Involve neurotransmitter release into the synaptic cleft; slower but versatile communication.
Neurotransmitter Release Mechanism: Action potentials open Ca2+ channels, leading to neurotransmitter exocytosis.
Removal of Neurotransmitters: Breakdown by enzymes, reuptake into presynaptic terminal, or diffusion away.
Postsynaptic Potentials
Excitatory Postsynaptic Potentials (EPSPs): Result in depolarization; increase likelihood of action potential generation.
Inhibitory Postsynaptic Potentials (IPSPs): Result in hyperpolarization; decrease likelihood of action potential generation.
Neuromodulators: Influence the effect of neurotransmitters, affecting the activity of postsynaptic cells.
11.8 Neural Pathways and Circuits
Convergent Pathways: Multiple presynaptic neurons synapse with fewer postsynaptic neurons (integration of signals).
Divergent Pathways: Few presynaptic neurons synapse with multiple postsynaptic neurons (amplifying effects).
Reverberating Circuits: Neurons repeatedly stimulate each other, prolonging responses.
Parallel After-Discharge Circuits: Non-linear pathways that lead to the production of multiple outputs for complex signal processing.
CHAPTER 11: Functional Organization of Nervous Tissue
11.1 Overview of the Nervous System
Definition: The nervous system is a complex communication network that governs body functions, composed of:
Central Nervous System (CNS): Encompasses the brain and spinal cord, acting as the control center for processing information and coordinating responses.
Peripheral Nervous System (PNS): Comprises nerves, ganglia (clusters of neuron cell bodies), and sensory receptors; responsible for transmitting signals between the CNS and the rest of the body.
Control Systems:
Nervous System: Utilizes rapid electrical impulses and neurotransmitter chemical signals via neurons to exert control over body functions almost instantaneously.
Endocrine System: Employs hormones, which are released into the bloodstream for slower, longer-lasting regulatory effects (see Chapters 17 and 18 for detailed insights).
Study Breakdown:The chapters focus on various aspects of the nervous system, including:
Physiology and anatomy of the spinal cord
Detailed structures and functions of cranial nerves
Mechanisms behind special senses such as vision and hearing
The role and functioning of the autonomic nervous system in regulating involuntary bodily functions.
11.2 Functions of the Nervous System
LEARNING OUTCOMES: Understand the key functions of the nervous system:
Homeostasis Maintenance: Actively regulates bodily functions including temperature, hydration, and nutrient levels to maintain stable internal conditions necessary for survival.
Sensory Input: Receives and processes a vast array of external and internal stimuli through specialized sensory receptors, which include sight (photoreceptors), hearing (auditory receptors), taste (gustatory receptors), touch (mechanoreceptors), and pain (nociceptors).
Integration of Information: Acts as a processing center where sensory information is interpreted, enabling appropriate responses to environmental changes.
Motor Control: Coordinates voluntary muscle contractions (skeletal muscles) and involuntary processes (smooth and cardiac muscles as well as glands) to facilitate movement and secretion.
Mental Activity: Encompasses the vast spectrum of cognitive processes including consciousness, thought, memory, emotions, and complex decision-making abilities.
11.3 Structure of Nervous Tissue
Cell Types: Nervous tissue primarily comprises two types of cells:
Neurons: The fundamental unit of the nervous system; these electrically excitable cells are responsible for signal transmission, primarily through action potentials.
Structure: Composed of three main parts:
Cell Body: Contains the nucleus and organelles; integrates incoming signals.
Dendrites: Branch-like structures that receive signals from other neurons or stimuli.
Axon: A long fiber that transmits signals away from the cell body to other neurons or effector cells.
Glial Cells (Neuroglia): Non-neuronal supportive cells that assist and protect neurons. Examples include:
Astrocytes: Provide structural support and regulate blood flow; involved in the formation of the blood-brain barrier.
Oligodendrocytes: Myelinate axons in the CNS, facilitating faster signal transmission.
Microglia: Act as the immune defense within the CNS, clearing debris and responding to injury.
Ependymal Cells: Line the ventricles of the brain and the central canal of the spinal cord, involved in the production and circulation of cerebrospinal fluid.
Neuron Classification
Function:
Sensory Neurons: Conduct signals towards the CNS, primarily from sensory receptors.
Motor Neurons: Conduct signals away from the CNS to muscles and glands to elicit responses.
Interneurons: Form connections between sensory and motor neurons within the CNS, playing a crucial role in reflexes and complex reflex arcs.
Structure:
Multipolar Neurons: Feature multiple dendrites and one axon (e.g., motor neurons found in the spinal cord).
Bipolar Neurons: Possess one axon and one dendrite (e.g., found in sensory organs like the retina).
Pseudo-Unipolar Neurons: Have a single, bifurcating axon, commonly found among sensory neurons of the PNS.
Anaxonic Neurons: Lack a true axon, mainly found in areas like the brain and retina, involved in local signal processing.
11.4 Gray Matter and White Matter
Gray Matter: Composed primarily of neuron cell bodies, dendrites, and unmyelinated axons; appears darker due to lower myelin content. It is found in regions such as the brain's cortex, spinal cord's interior, and spinal ganglia where processing occurs.
White Matter: Consists largely of myelinated axons which provide faster signal transmission; appears lighter and forms major nerve tracts within the CNS and peripheral nerves in the PNS, facilitating communication between brain regions and between brain and body.
11.5 Electrical Properties of Neurons
Resting Membrane Potential: Represents the voltage difference across the plasma membrane (~ -70 mV in neurons), indicative of the polarized state of the neuron.
Factors Influencing Membrane Potential:
Ion Concentration Differences: Variations in the distribution of ions such as potassium (K+) and sodium (Na+) create the potential; K+ is typically higher inside while Na+ is higher outside the cell.
Ion Permeability: The membrane's permeability is selectively adjusted, influencing the resting potential primarily due to K+ leakage through numerous channels.
Action Potentials: Refers to the rapid depolarization and subsequent repolarization of the neuron membrane, generated by the flow of ions moving through channels during signal propagation.
Ion Channels: Structure and function:
Leak Ion Channels: Always open, crucial for maintaining resting potential.
Gated Ion Channels: Open or close in response to specific stimuli, categorized as:
Ligand-gated: Activated by binding of neurotransmitters.
Voltage-gated: Activated by changes in membrane potential.
11.6 Neuron Communication
Phases of Communication:
Generation of Action Potentials: Initiated at the axon hillock when graded potentials surpass a threshold level, leading to the rapid change in membrane potential.
Propagation of Action Potentials: Action potentials travel down the axon via saltatory conduction in myelinated neurons, which greatly increases speed and efficiency of transmission.
Communication at the Synapse: Involves neurotransmitter release across synaptic clefts to convey signals to adjacent neurons.
Graded Potentials vs Action Potentials:
Graded Potentials: Represent small, localized changes in the membrane potential, which can summate to trigger an action potential when the threshold is reached.
Action Potentials: Characterized by a full depolarization of the neuron, propagating along the axon in an all-or-nothing response, crucial for long-distance communication.
11.7 Synapses: Electrical vs Chemical
Electrical Synapses: Facilitate direct transmission of signals through gap junctions, allowing for rapid, bidirectional communication between neurons. These are less common but extremely important in reflexes and synchronization.
Chemical Synapses: Involve the release of neurotransmitters into the synaptic cleft, translating electrical signals into chemical signals. This type of communication is slower but allows for greater versatility and complexity in signaling.
Neurotransmitter Release Mechanism: Upon the arrival of action potentials, voltage-gated calcium channels open, leading to an influx of Ca2+ ions, which trigger neurotransmitter exocytosis into the synapse.
Removal of Neurotransmitters: Critical for stopping signal transmission; occurs through:
Breakdown by enzymes in the synaptic cleft (e.g., acetylcholinesterase),
Reuptake into the presynaptic terminal for recycling,
Diffusion away from the synaptic cleft.
Postsynaptic Potentials:
Excitatory Postsynaptic Potentials (EPSPs): Result in depolarization of the postsynaptic membrane, increasing the likelihood of generating an action potential in the postsynaptic neuron.
Inhibitory Postsynaptic Potentials (IPSPs): Lead to hyperpolarization of the postsynaptic membrane, reducing the chance of an action potential occurring.
Neuromodulators: Substances that can influence the effectiveness of neurotransmitters, thereby modulating the overall activity of postsynaptic cells and contributing to the plasticity of synaptic connections.
11.8 Neural Pathways and Circuits
Convergent Pathways: Multiple presynaptic neurons synapse with fewer postsynaptic neurons, allowing for the integration of signals from various sources, essential for complex processing and decision-making.
Divergent Pathways: Fewer presynaptic neurons synapse with numerous postsynaptic neurons, amplifying signals and allowing for widespread information dissemination within the nervous system.
Reverberating Circuits: Neurons in the circuit stimulate each other in a loop, prolonging the response such as in breathing or rhythmic activities.
Parallel After-Discharge Circuits: These circuits often involve non-linear pathways to produce multiple outputs for intricate signal processing, frequently utilized in complex reflex actions and decision-making tasks.