Fundamentals of the Nervous System: Structure, Function, and Physiology (ch.11 part 1 _SG)

Fundamentals of the Nervous System

Basic Functions of the Nervous System

The nervous system is responsible for coordinating and regulating bodily activities through electrical and chemical signaling. It enables rapid communication between different parts of the body and the environment.

• Sensory Input: Detects changes in the internal and external environment via sensory receptors.

• Integration: Processes and interprets sensory input, making decisions about appropriate responses.

• Motor Output: Initiates responses by activating effector organs (muscles and glands).

• Homeostasis: Maintains stable internal conditions by regulating physiological processes.

• Mental Activity: Responsible for consciousness, memory, and thought.

• Example: Withdrawal reflex in response to a painful stimulus.

Structural and Functional Divisions of the Nervous System

The nervous system is divided into distinct anatomical and functional regions, each with specialized roles.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

• Functional Divisions:

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

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

• Example: The autonomic nervous system increases heart rate during exercise (sympathetic) and decreases it during rest (parasympathetic).

Types of Neuroglia and Their Locations

Neuroglia, or glial cells, support and protect neurons in both the CNS and PNS.

• Astrocytes: Found in the CNS; maintain the blood-brain barrier and provide structural support.

• Oligodendrocytes: CNS; form myelin sheaths around axons.

• Microglia: CNS; act as immune cells, removing debris and pathogens.

• Ependymal Cells: CNS; line ventricles and produce cerebrospinal fluid.

• Schwann Cells: PNS; form myelin sheaths around peripheral axons.

• Satellite Cells: PNS; support neuron cell bodies in ganglia.

• Example: Oligodendrocytes myelinate multiple axons in the CNS, while Schwann cells myelinate a single axon in the PNS.

Neuron Structure and Functional Roles

Neurons are the primary signaling cells of the nervous system, with specialized structures for communication.

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

• Dendrites: Receive incoming signals from other neurons.

• Axon: Transmits electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

• Functional Types:

  • Sensory (Afferent) Neurons: Carry information to the CNS.

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

  • Interneurons: Connect neurons within the CNS for integration.

• Example: Sensory neurons detect touch, interneurons process the information, and motor neurons cause muscle contraction.

Myelin Sheath: Importance and Formation

The myelin sheath is a fatty layer that insulates axons, increasing the speed and efficiency of electrical signal transmission.

• Importance: Enhances rapid conduction of action potentials via saltatory conduction.

• Formation in CNS: Oligodendrocytes wrap around axons to form myelin.

• Formation in PNS: Schwann cells wrap around axons, forming myelin sheaths.

• Example: Multiple sclerosis is a disease characterized by loss of CNS myelin, leading to impaired neural function.

Types of Membrane Ion Channels

Ion channels are proteins that allow specific ions to pass through the neuronal membrane, crucial for generating electrical signals.

• Leak Channels: Always open; maintain resting membrane potential.

• Voltage-Gated Channels: Open in response to changes in membrane potential; essential for action potentials.

• Ligand-Gated Channels: Open when a specific chemical (ligand) binds; important for synaptic transmission.

• Mechanically-Gated Channels: Open in response to mechanical deformation (e.g., touch receptors).

• Example: Voltage-gated sodium channels initiate the action potential in neurons.

Resting Membrane Potential and Electrochemical Basis

The resting membrane potential is the electrical charge difference across the neuronal membrane when the cell is not actively transmitting signals.

• Definition: Typically around -70 mV in neurons, with the inside of the cell negative relative to the outside.

• Electrochemical Basis: Established by differences in ion concentrations (mainly Na+ and K+) and selective permeability of the membrane.

• Key Equation: $V_m = E_K = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for potassium)

• Role of Na+/K+ Pump: Maintains ion gradients by actively transporting Na+ out and K+ in.

• Example: The resting membrane potential allows neurons to respond rapidly to stimuli by generating action potentials.