Fundamentals of the Nervous System and Nervous Tissue (Chapter 11) – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Introduction to the Nervous System

The nervous system is the body's master control and communication system, responsible for receiving sensory input, integrating information, and producing motor output. It enables rapid and precise regulation of body activities.

• Sensory Input: Detection of changes in the internal and external environment.

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

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

• Example: Seeing a glass of water (sensory input), deciding to pick it up (integration), and moving your arm to grasp it (motor output).

Organization of the Nervous System

Central and Peripheral Divisions

The nervous system is divided into two main anatomical divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integration and command center.

• Peripheral Nervous System (PNS): Composed of paired spinal and cranial nerves; carries messages to and from the CNS.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits sensory information from receptors(skins/muscle/organs) to the CNS.

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

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

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

Histology of Nerve Tissue

Principal Cell Types

Nerve tissue consists of two principal cell types:

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (Glial Cells): Supporting cells that surround, protect, and nourish neurons.

Functions of Neuroglia

• Provide scaffolding and support for neurons.

• Insulate neurons and segregate them (similar to insulation on wires).

• Guide young neurons to proper connections.

• Support health and growth of neurons.

Neuroglia in the CNS and PNS

Neuroglia of the CNS

• Astrocytes: Most abundant, versatile, and highly branched; support and brace neurons, anchor neurons to nutrient supplies, control the chemical environment (blood-brain barrier), and signal neurons.

• Microglia: Small, ovoid cells with spiny processes; act as phagocytes, traveling the CNS to remove pathogens.

• Ependymal Cells: Range from squamous to columnar; line central cavities of the brain and spinal cord, circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Branched cells that wrap CNS nerve fibers, forming the myelin sheath in the CNS.

Neuroglia of the PNS

• Schwann Cells (Neurolemmocytes): Surround axon fibers of the PNS, form the myelin sheath in the PNS.

• Satellite Cells: Surround neuron cell bodies within ganglia in the PNS.

Neurons (Nerve Cells)

Structure and Properties

Neurons are the structural units of the nervous system, specialized for rapid communication.

• Anatomy: Consist of a cell body, axon, and dendrites.

• Properties: Long-lived, amitotic (do not divide), and have a high metabolic rate.

Neuron Cell Body

• Major biosynthetic center; contains typical organelles except centrioles (neurons are amitotic).

• Rough endoplasmic reticulum is called Nissl bodies (contributes to gray matter).

• Contains a specialized region called the axon hillock, where axons arise.

Neuron Processes

• Dendrites: Short, tapering, diffusely branched; receive signals.

• Axons: Arm-like extensions; transmit signals away from the soma. In the CNS, axons are bundled into tracts; in the PNS, into nerves.

Axon Function

• Generate and transmit action potentials.

• Secrete neurotransmitters from axonal terminals.

• Movement along axons: Anterograde (toward terminal), Retrograde (away from terminal).

Myelin Sheath

• Whitish, fatty (protein-lipoid), segmented sheath around most long axons.

• Functions: Protects axon, electrically insulates fibers, increases speed of nerve impulse transmission.

Myelin Sheath Formation

• Formed by Schwann cells in the PNS and Oligodendrocytes in the CNS.

Nodes of Ranvier (Neurofibral Nodes)

• Gaps in the myelin sheath between adjacent Schwann cells.

• Sites where axon collaterals can emerge.

Regions of the Brain and Spinal Cord

• Gray matter: Mostly soma and unmyelinated fibers; Nissl bodies contribute to gray color.

• White matter: Dense collections of myelinated fibers; myelin sheath contributes to white color.

Neuron Classification

Structural Classification

Neurons are classified based on the number of processes extending from the cell body.

Neuron Type	Processes	Example
Multipolar	Three or more processes	Most CNS neurons
Bipolar	Two processes (axon and dendrite)	Retina of eye, olfactory mucosa
Unipolar (Pseudounipolar)	Single, short process	Most sensory neurons in PNS

Functional Classification

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

• Motor (Efferent): Carry impulses away from the CNS.

• Interneurons (Association Neurons): Shuttle signals through CNS pathways.

Neurophysiology: Signal Conduction

Electrochemical Gradient

Neuronal signaling depends on the movement of ions along gradients:

• Chemical Gradient: Ions move from areas of high to low concentration.

• Electrical Gradient: Ions move toward areas of opposite charge.

• Electrochemical Gradient: The combination of electrical and chemical gradients.

Role of Ion Channels

• Passive (Leakage) Channels: Always open.

• Chemically Gated Channels: Open with binding of a specific neurotransmitter.

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

• Mechanically Gated Channels: Open and close in response to physical deformation of receptors.

Resting Membrane Potential (Vr)

The resting membrane potential is the electrical potential difference (~ -70 mV) across the membrane of a resting neuron.

• Generated by different concentrations of Na+, K+, Cl-, and protein anions (A-).

• Maintained by differential permeability of the neurilemma to Na+ and K+ and the operation of the sodium-potassium pump.

Action Potentials (APs)

An action potential is a brief reversal of membrane potential, serving as the principal means of neural communication.

• Generated only by muscle cells and neurons.

• Do not decrease in strength over distance.

• All-or-none phenomenon: always the same regardless of stimulus strength.

• Phases: Depolarization, Repolarization, Hyperpolarization.

Changes in Membrane Potential

• Depolarization: Inside of the membrane becomes less negative.

• Repolarization: Membrane returns to its resting membrane potential.

• Hyperpolarization: Inside of the membrane becomes more negative than the resting potential.

Refractory Periods

• Absolute Refractory Period: Neuron cannot generate another action potential.

• Relative Refractory Period: A stronger stimulus could generate another action potential.

Summary Table: Structural and Functional Classes of Neurons

Type	Processes	Location	Function
Multipolar	Many dendrites, one axon	CNS	Motor, interneurons
Bipolar	One dendrite, one axon	Retina, olfactory mucosa	Sensory (special senses)
Unipolar	Single process splits into two branches	PNS (sensory neurons)	Sensory (general senses)

Key Equations

• Resting Membrane Potential: $V_r \approx -70\ \text{mV}$

• Sodium-Potassium Pump: $3\ \text{Na}^+ \text{out} : 2\ \text{K}^+ \text{in}$

Additional info:

• Neurotransmitters are chemical messengers used for neuronal communication; examples include acetylcholine, dopamine, serotonin, and GABA.

• Graded potentials are local changes in membrane potential that can initiate action potentials if strong enough.