Divisions of the Nervous System

Overview of Nervous System Organization

The nervous system is a complex network responsible for coordinating the body's activities. It is divided into central and peripheral components, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and is the main control center.

• Peripheral Nervous System (PNS): Composed of cranial nerves and spinal nerves that connect the CNS to the rest of the body.

• Sensory Input: The process of receiving information from sensory receptors and transmitting it to the CNS.

• Integration: The CNS interprets sensory input and determines the appropriate response.

• Motor Output: The CNS sends signals to effectors (muscles or glands) to elicit a response.

Functional Divisions

• Sensory (Afferent) Division: Transmits sensory information to the CNS.

• Somatic Sensory Division: Carries signals from skin, muscles, and joints.

• Visceral Sensory Division: Carries signals from organs.

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

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

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion); also called the visceral motor division.

Neurons

Structure and Function of Neurons

Neurons are the primary functional units of the nervous system, specialized for communication via electrical and chemical signals.

• Dendrite: Branch-like extensions that receive signals from other neurons.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Axon Hillock: The region where the axon originates; site of action potential initiation.

• Axolemma: The plasma membrane of the axon.

• Axoplasm: The cytoplasm within the axon.

• Axon: Long projection that transmits electrical impulses away from the cell body.

• Axon Collateral: Branches of the axon that allow communication with multiple targets.

• Myelin Sheath: Insulating layer around axons that increases signal speed.

• Telodendria: Fine branches at the end of the axon.

• Axon Terminal: The endpoint where neurotransmitters are released.

• Receptive Region: Includes dendrites and cell body; receives input.

• Conducting Region: The axon; transmits action potentials.

• Secretory Region: Axon terminals; release neurotransmitters.

Types of Neurons

• Multipolar Neuron: One axon, multiple dendrites; most common type in CNS.

• Bipolar Neuron: One axon, one dendrite; found in sensory organs (e.g., retina).

• Pseudounipolar Neuron: Single process splits into two branches; sensory neurons in PNS.

• Sensory (Afferent) Neuron: Transmits impulses toward the CNS.

• Interneuron: Connects neurons within the CNS; processes information.

• Motor (Efferent) Neuron: Transmits impulses away from the CNS to effectors.

Neuronal Groupings

• Nuclei: Clusters of neuron cell bodies in the CNS.

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Tracts: Bundles of axons in the CNS.

• Nerves: Bundles of axons in the PNS.

Neuroglia (Glia)

Types and Functions of Glial Cells

Neuroglia (or glial cells) support, protect, and nourish neurons. They are more numerous than neurons and have various specialized roles.

• Astrocyte: Star-shaped cells in the CNS; maintain the blood-brain barrier, regulate nutrients, and support synaptic function.

• Oligodendrocyte: Form myelin sheaths around CNS axons.

• Microglia: Act as immune cells in the CNS; remove debris and pathogens.

• Ependymal Cell: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CSF).

• Schwann Cell: Form myelin sheaths around PNS axons.

• Satellite Cell: Surround neuron cell bodies in PNS ganglia; regulate the environment.

Myelin Sheath

Structure and Function of Myelin

Myelin is a lipid-rich substance that insulates axons, increasing the speed of electrical signal transmission.

• Myelination: The process of forming a myelin sheath around an axon.

• Internode: The segment of a myelinated axon covered by myelin.

• Node of Ranvier: Gaps between myelin segments; sites of action potential regeneration.

• White Matter: Regions of the CNS with myelinated axons.

• Gray Matter: Regions with neuron cell bodies, dendrites, and unmyelinated axons.

Membrane Potential

Electrical Properties of Neurons

Neurons maintain a difference in electrical charge across their membranes, known as the membrane potential. This is essential for nerve signal transmission.

• Voltage: The difference in electrical potential between two points.

• Electrical Gradient: The difference in charge across the membrane.

• Resting Membrane Potential: The stable, negative charge of a neuron at rest (typically around -70 mV).

• Polarization (Polarized): The state of having a membrane potential.

• Ion Channel: Protein channels that allow specific ions to cross the membrane.

• Leak Channel: Always open; allow passive ion movement.

• Ligand-Gated Channel: Opens in response to a chemical signal (ligand).

• Voltage-Gated Channel: Opens in response to changes in membrane potential.

• Mechanically Gated Channel: Opens in response to mechanical deformation.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Example: The sodium-potassium pump ( out, in) helps maintain the resting membrane potential.

Action Potentials

Generation and Phases of Action Potentials

An action potential is a rapid, temporary change in membrane potential that travels along the axon, enabling neural communication.

• Local (Graded) Potential: Small, localized changes in membrane potential; can trigger action potentials if threshold is reached.

• Trigger Zone: The axon hillock; site where action potentials are initiated.

• Voltage-Gated Potassium Channel: Opens during repolarization to allow outflow.

• Voltage-Gated Sodium Channel: Opens during depolarization to allow influx.

• Activation Gate: Part of the sodium channel that opens rapidly with depolarization.

• Inactivation Gate: Part of the sodium channel that closes to stop entry.

• Threshold: The membrane potential at which an action potential is triggered (typically around -55 mV).

• Depolarization: Rapid influx of makes the inside of the cell positive.

• Repolarization: Outflow of returns the membrane to a negative potential.

• Hyperpolarization: Membrane potential temporarily becomes more negative than resting.

• Sodium-Potassium Pump: Restores ion gradients after an action potential.

• Refractory Period: Time during which a neuron cannot fire another action potential.

• Absolute Refractory Period: No action potential can be generated, regardless of stimulus strength.

• Relative Refractory Period: A stronger-than-normal stimulus can trigger an action potential.

Equation:

Where is the membrane potential, is the potential inside the cell, and is the potential outside the cell.

Action Potential Propagation

How Action Potentials Travel

• Conduction (Propagation): The movement of the action potential along the axon.

• Self-Propagating: Once initiated, the action potential continues without decrement.

• Saltatory Conduction: In myelinated axons, action potentials jump from node to node, increasing speed.

• Continuous Conduction: In unmyelinated axons, action potentials travel along every part of the membrane.

Example: Saltatory conduction allows rapid signal transmission in motor neurons controlling muscle movement.

Synapses

Types and Functions of Synapses

A synapse is the junction where a neuron communicates with another cell. Synapses can be electrical or chemical.

• Neuronal Synapse: Synapse between two neurons.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Synaptic Transmission: The process of transferring a signal across a synapse.

• Axosomatic Synapse: Axon to cell body.

• Axodendritic Synapse: Axon to dendrite.

• Axoaxonic Synapse: Axon to axon.

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

• Chemical Synapse: Uses neurotransmitters to transmit signals across a synaptic cleft.

• Synaptic Vesicle: Stores neurotransmitters in the axon terminal.

• Neurotransmitter: Chemical messenger released into the synaptic cleft.

• Neurotransmitter Receptor: Protein on the postsynaptic membrane that binds neurotransmitter.

• Postsynaptic Potential: Change in membrane potential of the postsynaptic cell.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the chance of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the chance of action potential.

Neural Integration

Processing of Synaptic Inputs

• Neural Integration: The process by which a neuron combines multiple synaptic inputs.

• Summation: The additive effect of multiple postsynaptic potentials.

• Temporal Summation: Multiple signals from one presynaptic neuron in rapid succession.

• Spatial Summation: Simultaneous signals from multiple presynaptic neurons.

Termination of Synaptic Transmission

How Neurotransmitter Action Ends

• Diffusion: Neurotransmitter drifts away from the synaptic cleft.

• Degradation: Enzymes break down neurotransmitter molecules.

• Reuptake: Neurotransmitter is reabsorbed by the presynaptic neuron.

Neurotransmitter Receptors

Types and Mechanisms

• Ionotropic Receptor: Ligand-gated ion channel; direct, fast response.

• Metabotropic Receptor: Indirectly linked to ion channels via G-proteins and second messenger systems; slower, longer-lasting effects.

Neurotransmitters

Major Classes and Examples

• Acetylcholine: Involved in muscle contraction and autonomic functions.

• Biogenic Amines: Include serotonin (mood), histamine (arousal), and catecholamines (e.g., norepinephrine, epinephrine, dopamine).

• Amino Acid Neurotransmitters: Glutamate (excitatory), GABA (inhibitory), glycine (inhibitory).

Example: Dopamine is involved in reward pathways and motor control; GABA is the main inhibitory neurotransmitter in the CNS.

Additional info: This guide expands on the vocabulary list by providing definitions, examples, and context for each term, suitable for exam preparation in a college-level Anatomy & Physiology course.