Fundamentals of the Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and integrating all body activities. It detects changes, processes information, and initiates responses.

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to cause a response.

Structural and Functional Divisions of the Nervous System

• Structural Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

  • Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to the rest of the body.

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits impulses from receptors to the CNS.

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

    • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

    • Autonomic Nervous System (ANS): Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

      • Sympathetic Division: Mobilizes body systems during activity.

      • Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest.

Structural Components of a Neuron and Their Functional Roles

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

• Dendrites: Short, branched processes that receive signals from other neurons and convey them toward the cell body.

• Axon: Long process that transmits impulses away from the cell body to other neurons or effectors.

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

• Axon Terminals: Endings of the axon that form synapses with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse conduction.

Classification of Neurons by Structure and Function

• Structural Classification:

  • Multipolar: One axon, multiple dendrites (most common in CNS).

  • Bipolar: One axon, one dendrite (found in retina, olfactory mucosa).

  • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

  • Motor (Efferent) Neurons: Transmit impulses away from the CNS to effectors.

  • Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Functions of Neuroglia (Glial Cells)

• Astrocytes (CNS): Support neurons, regulate the blood-brain barrier, and maintain the extracellular environment.

• Microglia (CNS): Act as phagocytes, removing debris and pathogens.

• Ependymal Cells (CNS): Line ventricles, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes (CNS): Form myelin sheaths around CNS axons.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia, regulate the environment.

• Schwann Cells (PNS): Form myelin sheaths around PNS axons, aid in regeneration.

Structure and Function of the Myelin Sheath

• Myelin Sheath: A multilayered lipid and protein covering that insulates axons and increases the speed of nerve impulse conduction.

• CNS: Formed by oligodendrocytes; one cell can myelinate multiple axons.

• PNS: Formed by Schwann cells; each cell myelinates a single axon segment.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

Nucleus vs. Ganglion; Nerve vs. Tract

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Types of Membrane Ion Channels

• Leak Channels: Always open; allow ions to move down their concentration gradients.

• Ligand-Gated Channels: Open in response to binding of a chemical messenger (neurotransmitter).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential.

• Mechanically Gated Channels: Open in response to physical deformation of the membrane.

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the plasma membrane of a resting neuron, typically about -70 mV.

• Generated by differences in ion concentrations inside and outside the cell and selective permeability of the membrane.

• Maintained by the sodium-potassium pump and leak channels.

Key Equation:

(Nernst equation for potassium; actual resting potential is determined by all permeant ions)

Graded Potentials

• Short-lived, localized changes in membrane potential.

• Occur in dendrites and cell bodies; can be depolarizing or hyperpolarizing.

• Magnitude varies with stimulus strength; decay with distance.

• Examples: Postsynaptic potentials, receptor potentials.

Comparison: Graded Potentials vs. Action Potentials

Generation and Propagation of Action Potentials

• Action potentials are rapid, large changes in membrane potential that travel along axons.

• Generated when depolarization reaches threshold, opening voltage-gated Na+ channels.

• Sequence: Depolarization (Na+ influx) → Repolarization (K+ efflux) → Hyperpolarization.

• Propagated by sequential opening of voltage-gated channels along the axon.

Key Equation:

(Membrane current equation; relates to action potential propagation)

Absolute and Relative Refractory Periods

• Absolute Refractory Period: Time during which a second action potential cannot be initiated, regardless of stimulus strength (Na+ channels inactivated).

• Relative Refractory Period: Follows the absolute period; a stronger-than-normal stimulus can initiate another action potential (some Na+ channels reset, K+ channels still open).

Saltatory vs. Continuous Conduction

• Saltatory Conduction: Occurs in myelinated axons; action potentials "jump" from node to node (Nodes of Ranvier), increasing conduction speed.

• Continuous Conduction: Occurs in unmyelinated axons; action potentials propagate along every part of the membrane, slower than saltatory conduction.

Example: Motor neurons controlling skeletal muscles use saltatory conduction for rapid response.