11.1 – Organization of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has specialized roles in processing and transmitting information throughout the body.

• CNS: Composed of the brain and spinal cord; responsible for integrating and processing information.

• PNS: Consists of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

• Functional Divisions of PNS:

  • Sensory (afferent) division: Transmits sensory information to the CNS.

  • Motor (efferent) division: Carries motor commands from the CNS to effectors.

  • Autonomic nervous system: Controls involuntary functions; subdivided into sympathetic and parasympathetic branches.

11.2 – Neuroglia: Types and Roles

Neuroglial Cells in CNS and PNS

Neuroglia are supporting cells in the nervous system that provide structural and functional support to neurons. They play crucial roles in homeostasis, protection, and repair.

• CNS Neuroglia:

  • Astrocytes: Maintain blood-brain barrier, regulate nutrients, and repair tissue.

  • Microglia: Act as immune cells, removing debris and pathogens.

  • Ependymal cells: Line ventricles, produce cerebrospinal fluid.

  • Oligodendrocytes: Form myelin sheaths around CNS axons.

• PNS Neuroglia:

  • Satellite cells: Support neuron cell bodies in ganglia.

  • Schwann cells: Form myelin sheaths around PNS axons.

11.3 – Neuron Structure and Function

Features of a Neuron

Neurons are the primary signaling cells of the nervous system, specialized for rapid communication via electrical and chemical signals.

• Characteristics:

  • Conduct impulses

  • Extreme longevity

  • Amitotic (do not divide after maturity)

  • High metabolic rate (require oxygen and glucose)

• Cell Body (Soma): Contains nucleus, organelles, and biosynthetic machinery.

• Dendrites: Receive signals from other neurons.

• Axon: Conducts action potentials away from the cell body; includes axon hillock and terminals.

• Myelin Sheath: Insulates axons, increases speed of impulse conduction.

11.4 – Neurophysiology: Electrical Signaling

Membrane Potentials and Action Potentials

Neurons communicate via changes in membrane potential, primarily through action potentials.

• Resting Membrane Potential: The voltage difference across the membrane at rest, typically around -70 mV.

• Action Potential: Rapid, transient change in membrane potential that propagates along the axon.

Key Equations:

• Nernst Equation: Calculates equilibrium potential for an ion:

• Ohm's Law: Relates current, voltage, and resistance:

Ion Channels and Signal Transmission

• Types of Ion Channels:

  • Leakage channels: Always open

  • Gated channels: Open/close in response to stimuli (chemical, voltage, mechanical)

• Phases of Action Potential:

  1. Resting state: Na+ and K+ channels closed

  2. Depolarization: Na+ channels open, Na+ influx

  3. Repolarization: K+ channels open, K+ efflux

  4. Hyperpolarization: K+ channels remain open briefly

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

11.5 – Graded Potentials

Short-Distance Electrical Signals

Graded potentials are changes in membrane potential that vary in size and decay with distance. They are important for initiating action potentials.

• Occur in dendrites and cell bodies

• Can be depolarizing or hyperpolarizing

• Summation of graded potentials can trigger an action potential at the axon hillock

11.6 – Speed of Conduction

Factors Affecting Conduction Velocity

The speed at which an action potential travels along an axon depends on several factors.

• Axon Diameter: Larger diameter = faster conduction

• Myelination: Myelinated axons conduct impulses faster via saltatory conduction

• Fiber Classification:

  • Group A: Fastest, large diameter, myelinated

  • Group B: Intermediate

  • Group C: Slowest, small diameter, unmyelinated

11.7 – The Synapse

Neuronal Communication

A synapse is the junction between two neurons where information is transmitted chemically or electrically.

• Presynaptic neuron: Sends the signal

• Postsynaptic neuron: Receives the signal

• Types of synapses: Axodendritic, axosomatic, axoaxonic

Electrical vs. Chemical Synapses

• Electrical synapses: Direct flow of ions via gap junctions

• Chemical synapses: Use neurotransmitters to transmit signals across the synaptic cleft

11.8 – Postsynaptic Potentials

Excitatory and Inhibitory Signals

Postsynaptic potentials are graded responses in the postsynaptic neuron that can be excitatory (EPSP) or inhibitory (IPSP).

• EPSP: Depolarizes the postsynaptic membrane, increasing likelihood of action potential

• IPSP: Hyperpolarizes the membrane, decreasing likelihood of action potential

• Summation: Multiple EPSPs/IPSPs can combine temporally or spatially to influence action potential generation

11.9 – Neurotransmitters & Receptors

Types and Functions of Neurotransmitters

Neurotransmitters are chemical messengers released by neurons to transmit signals across synapses. Their effects depend on the type of receptor they bind to.

• Classes of Neurotransmitters:

  • Biogenic amines: Dopamine, norepinephrine, epinephrine, serotonin

  • Amino acids: Glutamate, GABA

  • Peptides: Substance P, endorphins

  • Others: Acetylcholine, ATP

• Receptor Types:

  • Ionotropic receptors: Ligand-gated ion channels, fast response

  • Metabotropic receptors: G-protein coupled, slower but longer-lasting effects

11.10 – Neuronal Circuits

Integration and Processing

Neurons are organized into circuits that process and integrate information. These circuits determine the flow and complexity of neural signals.

• Types of Circuits:

  • Diverging

  • Converging

  • Reverberating

  • Parallel after-discharge

Table: Comparison of CNS and PNS Neuroglia

Additional info:

• Some content inferred for completeness, such as definitions and examples of neurotransmitter classes and circuit types.

• Equations and table structure expanded for clarity and academic context.