General Functions of the Nervous System

Overview of Nervous System Functions

The nervous system is responsible for controlling and coordinating bodily functions, enabling rapid communication between different parts of the body. It integrates sensory input, processes information, and initiates appropriate responses.

• Sensory Input: Detects changes in the internal and external environment via sensory receptors.

• Integration: Processes and interprets sensory input, deciding what action is needed.

• Motor Output: Activates effector organs (muscles and glands) to produce a response.

• Homeostasis: Maintains internal balance by regulating physiological processes.

• Mental Activity: Responsible for consciousness, memory, and learning.

Organization of the Nervous System

CNS vs. PNS: Structure and Function

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates and processes information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits information to and from the CNS.

Motor (Efferent) vs. Sensory (Afferent) Components

• Sensory (Afferent) Division: Carries sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits commands from the CNS to effector organs.

Nervous System as a Control System

• Sensory Receptors: Detect stimuli.

• Afferent Pathways: Carry information to the CNS.

• Control Center: CNS processes information.

• Efferent Pathways: Carry commands from CNS.

• Effector Organs: Muscles or glands that respond.

Somatic vs. Autonomic Motor Divisions

• Somatic Motor Division: Controls voluntary movements via skeletal muscles.

• Autonomic Motor Division: Regulates involuntary functions (e.g., heart rate, digestion) via smooth muscle, cardiac muscle, and glands.

General Anatomy of the Nervous System

Gray Matter vs. White Matter in CNS

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; found in the cortex and nuclei.

• White Matter: Composed of myelinated axons; forms tracts connecting different CNS regions.

Neuroglial Cells of the CNS

• Astrocytes: Support neurons, regulate environment, form blood-brain barrier.

• Oligodendrocytes: Myelinate CNS axons.

• Microglia: Immune defense in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

Structure of a Typical Nerve

• Neuronal Elements: Motor and sensory neurons.

• Neuroglial Elements: Support and protect neurons.

• Connective Tissue Wrappings: Endoneurium (surrounds each axon), perineurium (bundles axons into fascicles), epineurium (surrounds entire nerve).

Nucleus vs. Ganglion

• Nucleus: Cluster of neuron cell bodies in CNS.

• Ganglion: Cluster of neuron cell bodies in PNS.

Tract vs. Nerve

• Tract: Bundle of axons in CNS.

• Nerve: Bundle of axons in PNS.

Neurons

Major Components of a Typical Neuron

• Cell Body (Soma): Contains nucleus and organelles; metabolic center.

• Nucleus: Contains genetic material.

• Nucleolus: Produces ribosomes.

• Chromatophilic Substance (Nissl Bodies): Rough ER; protein synthesis.

• Axon Hillock: Initiates action potential.

• Dendrites: Receive input signals.

• Axon: Transmits output signals.

Structural Types of Neurons

Functional Types of Neurons

Neuroglial (Glial) Cells

Types of Neuroglial Cells

Myelination

• Definition: Formation of a myelin sheath around axons.

• Function: Increases conduction velocity, insulates axons.

• CNS: Oligodendrocytes myelinate multiple axons.

• PNS: Schwann cells myelinate single axons.

Neurophysiology

Major Ion Channels of Neurons

• Leak Channels: Always open; maintain resting membrane potential.

• Voltage-Gated Channels: Open in response to voltage changes; found in axons.

• Mechanically Gated Channels: Open in response to physical deformation; found in sensory receptors.

• Ligand-Gated Channels: Open in response to chemical signals; found in dendrites and soma.

Resting Membrane Potential (RMP)

• Definition: The voltage difference across the membrane at rest, typically -70 mV.

• Ion Channels: Leak channels for K+ and Na+.

• Ion Concentrations: High K+ inside, high Na+ outside.

• Electrochemical Gradient: Drives ion movement.

Sodium-Potassium ATPase Pump

• Function: Maintains RMP by pumping 3 Na+ out and 2 K+ in per ATP.

Depolarization, Repolarization, Hyperpolarization, Threshold

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

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

Graded Potentials vs. Action Potentials

Action Potential Diagram

• Depolarization: Na+ influx.

• Repolarization: K+ efflux.

• Hyperpolarization: Continued K+ efflux.

Conduction of Action Potential

• Propagation: Sequential opening of voltage-gated channels along axon.

• Importance: Ensures unidirectional transmission.

Axon Diameter and Myelination

• Larger Diameter: Faster conduction.

• Myelination: Enables saltatory conduction (jumps between nodes).

Saltatory vs. Continuous Conduction

• Saltatory: Myelinated axons; rapid, efficient.

• Continuous: Unmyelinated axons; slower.

Absolute vs. Relative Refractory Periods

• Absolute: No new action potential possible; Na+ channels inactivated.

• Relative: Action potential possible with stronger stimulus; some Na+ channels reset.

Neurotransmitters, Neuromodulators, and Synaptic Transmission

Synapse Types

• Electrical Synapse: Direct ion flow via gap junctions; rapid.

• Chemical Synapse: Neurotransmitter release; slower, more modifiable.

Neurotransmitter vs. Neuromodulator

• Neurotransmitter: Directly transmits signal across synapse.

• Neuromodulator: Modifies synaptic transmission, often indirectly.

Structures of a Chemical Synapse

• Axon Terminal (Synaptic Knob): Contains synaptic vesicles.

• Voltage-Gated Calcium Channels: Trigger neurotransmitter release.

• Synaptic Vesicles: Store neurotransmitters.

• Synaptic Cleft: Gap between neurons.

• Neurotransmitter Receptors: Located on postsynaptic cell.

Events of Synaptic Transmission

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open.

3. Ca2+ influx triggers neurotransmitter release.

4. Neurotransmitter diffuses across synaptic cleft.

5. Binds to receptors on postsynaptic cell.

6. Postsynaptic potential generated (EPSP or IPSP).

EPSP vs. IPSP

• EPSP (Excitatory): Depolarizes postsynaptic membrane; increases likelihood of action potential.

• IPSP (Inhibitory): Hyperpolarizes postsynaptic membrane; decreases likelihood of action potential.

Neurotransmitter Responses

• Same neurotransmitter can elicit different responses depending on receptor type.

Common Neurotransmitters

• Excitatory: Glutamate, acetylcholine.

• Inhibitory: GABA, glycine.

Temporal and Spatial Summation

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Multiple signals from different locations.

Termination of Neurotransmitter Activity

• Reuptake: Neurotransmitter reabsorbed by presynaptic cell.

• Enzymatic Breakdown: Enzymes degrade neurotransmitter.

• Diffusion: Neurotransmitter diffuses away from synapse.

Integration of Neural Information

Neural Circuits

• Definition: Networks of interconnected neurons.

• Converging Circuit: Multiple inputs, one output.

• Diverging Circuit: One input, multiple outputs.

Neural Plasticity

• Definition: Ability of neural circuits to change and adapt.

Application of Homeostatic Mechanisms

Nervous System and Homeostasis

• Role: Regulates body systems to maintain internal balance.

• Examples: Control of heart rate, breathing, digestion.

Disruption of Homeostasis

• Demyelinating Diseases: (e.g., Multiple Sclerosis) Slow conduction, cause motor and sensory deficits.

• Neuromuscular Junction Paralytics: Block neurotransmission, cause muscle weakness.

• Clinical Manifestations: Symptoms reflect underlying anatomical defects (e.g., muscle weakness in myasthenia gravis).

Additional info: Clinical case studies often illustrate the relationship between anatomical defects and functional outcomes, such as the impact of demyelination on nerve conduction velocity and resulting neurological symptoms.