Fundamentals of the Nervous System and Nervous Tissue: Study Notes

The Nervous System

Functions of the Nervous System

The nervous system is the main controlling and communicating system of the body. It is responsible for integrating sensory input, coordinating motor output, and maintaining homeostasis.

• 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 produce a response.

• Homeostasis: Sending signals to achieve balance in the body’s internal environment.

Divisions of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and command center.

• Peripheral Nervous System (PNS): Consists of nerves outside the CNS. It connects the CNS to limbs and organs.

Functional Divisions of the PNS

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

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

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

• Autonomic Nervous System: Controls involuntary responses via smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic and parasympathetic divisions.

Neurons

Characteristics of a Neuron

Neurons are excitable cells that transmit electrical signals. They have unique properties that allow them to respond to stimuli and conduct impulses.

• Excitability: Ability to generate an action potential in response to a stimulus.

• Conductivity: Ability to transmit the electrical signal along the plasma membrane.

• Longevity: Most neurons live as long as the organism.

• Amitotic: Most neurons do not divide after birth.

• High Metabolic Rate: Neurons require continuous supply of oxygen and glucose.

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Conducts impulses away from the cell body.

• Axon Hillock: Region where the axon originates from the cell body.

• Axon Terminals (Telodendria): Endings that form synapses with other cells.

Functional Segments of a Neuron

• Receptive Segment: Dendrites and cell body; receives stimuli.

• Initial Segment: Axon hillock; initiates action potential.

• Conductive Segment: Axon; propagates action potential.

• Transmissive Segment: Axon terminals; transmits signal to other neurons or effectors.

Neuroglial (Glial) Cells

Characteristics

Neuroglial cells support, protect, and insulate neurons. They do not conduct electrical impulses but are essential for neuron function.

• Physical Support: Provide scaffolding for neurons.

• Insulation: Form myelin sheaths around axons.

• Regulation: Maintain chemical environment and aid in repair.

Types of Neuroglial Cells

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

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

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

• Oligodendrocytes (CNS): Form myelin sheaths in the CNS.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia.

• Schwann Cells (PNS): Form myelin sheaths in the PNS.

Myelination

Myelination Process

Myelin is a fatty substance that insulates axons, increasing the speed of nerve transmission.

• PNS Myelination: Schwann cells wrap around axons, forming myelin sheaths. Each Schwann cell myelinates one segment of one axon.

• CNS Myelination: Oligodendrocytes myelinate multiple axons at once.

• Unmyelinated Axons: Still associated with Schwann cells, but no myelin sheath is formed.

Example: Myelinated axons conduct impulses faster than unmyelinated axons.

Neuron Electrophysiology

Resting Membrane Potential (RMP)

Neurons maintain a resting membrane potential, which is an electrical charge difference across the plasma membrane.

• Typical RMP Value: Usually around -70 mV.

• Establishment: Due to unequal distribution of ions (Na+, K+) across the membrane and selective permeability.

• Maintenance: Sodium-potassium pumps ($3 \text{Na}^+$ out, $2 \text{K}^+$ in) maintain the gradient.

Equation:

$\text{RMP} = \text{Potential inside cell} - \text{Potential outside cell}$

Channels and Pumps in Neuron Function

• Ion Channels: Allow ions to move down their concentration gradients. Types include leak channels, chemically gated channels, and voltage-gated channels.

• Pumps: Move ions against their concentration gradients (e.g., Na+/K+ pump).

Graded Potentials vs. Action Potentials

Graded Potentials

Graded potentials are changes in membrane potential due to the opening of chemically or mechanically gated channels. They are localized and decrease in strength with distance.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Summation: Multiple graded potentials can combine to reach threshold.

Action Potentials

Action potentials are rapid, large changes in membrane potential that propagate along the axon. They are initiated when the membrane potential reaches threshold (about -55 mV).

• Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell.

• Repolarization: Voltage-gated K+ channels open, K+ leaves the cell.

• Hyperpolarization: K+ channels remain open longer, membrane potential becomes more negative.

• Refractory Period: Time during which a new action potential cannot be generated.

Example: Action potentials allow rapid communication between neurons and muscle cells.

Equation:

$\text{Threshold} \approx -55 \text{ mV}$

Synaptic Transmission

Steps in Synaptic Transmission

• Action potential arrives at axon terminal.

• Voltage-gated Ca2+ channels open, Ca2+ enters the axon terminal.

• Ca2+ entry causes synaptic vesicles to release neurotransmitters.

• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.

• Binding of neurotransmitter opens ion channels, resulting in graded potentials.

• Neurotransmitter effects are terminated (by reuptake, degradation, or diffusion).

Table: Comparison of CNS and PNS Neuroglial Cells

Cell Type	Location	Function
Astrocytes	CNS	Support neurons, regulate blood-brain barrier
Microglia	CNS	Phagocytosis of debris and pathogens
Ependymal Cells	CNS	Produce and circulate cerebrospinal fluid
Oligodendrocytes	CNS	Form myelin sheaths around axons
Satellite Cells	PNS	Support neuron cell bodies in ganglia
Schwann Cells	PNS	Form myelin sheaths around axons

Additional info:

• These notes cover the core concepts of Chapter 11: Fundamentals of the Nervous System and Nervous Tissue, including neuron structure, function, neuroglia, myelination, membrane potentials, and synaptic transmission.

• Understanding these principles is essential for further study of the central and peripheral nervous systems, as well as neurophysiology and related clinical applications.