Physiology Chapter Review

Basis of Understanding Physiology

  • Next week will cover a challenging topic in physiology that requires a solid understanding of relevant terms, cells, and tissues.

Divisions of the Nervous System

  • Anatomically divided into:

    • Central Nervous System (CNS)

      • Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS)

      • Encompasses all cranial and spinal nerves that branch from the brain and spinal cord.

  • Functionally divided into:

    • Sensory Division (Afferent)

      • Responsible for bringing information towards the central nervous system.

      • Examples:

      • Eyes bringing visual information.

      • Sensory input when touching a hot stove.

    • Motor Division (Efferent)

      • Commands sent from the central nervous system out to effectors in the periphery.

      • Example: Moving your hand away after touching a hot surface.

Detailed Functional Divisions of CNS

  • Sensory portions (input towards CNS) include:

    • Somatic Sensory

      • Consciously perceived information from receptors like the eyes, ears, and skin.

    • Visceral Sensory

      • Sensory input not consciously perceived, e.g., blood pressure monitored by baroreceptors.

  • Motor portions (output from CNS) may be divided into:

    • Somatic Motor System

      • Voluntary control over skeletal muscles.

      • Example: Intentionally picking up an object.

    • Autonomic Motor System

      • Involuntary control over muscles like the heart and smooth muscles in blood vessels.

Subdivisions of the Motor Nervous System

  • Somatic Motor System

    • Involves voluntary actions and conscious control.

  • Autonomic Motor System

    • Further divided into:

      • Sympathetic Branch

      • Parasympathetic Branch

    • Concerns involuntary control; further discussion will follow in later chapters.

Overview of Nervous System Structure

  • Levels of Organization

    • System (Nervous System) → Organs (Brain, Spinal Cord, Nerves) → Tissues → Cells.

  • Specialized Cells in the Nervous System:

    • Neurons

      • Cells that transmit nerve impulses.

    • Glial Cells (Neuroglia)

      • Non-neuronal cells that provide support and maintain homeostasis in nervous tissue.

Characteristics of Neurons

  • Functions:

    • Conduct nerve impulses and send electrical signals.

  • Longevity:

    • Neurons have extreme longevity; they are amitotic and do not divide.

  • Metabolic Rate:

    • High metabolic rate to support functions and sustain ATP production.

  • Structure:

    • Components of a neuron include:

      • Cell Body

      • Contains organelles, including nucleus and ribosomes (Nissle bodies are specifically rough endoplasmic reticulum).

      • Axon

      • Conductive component that transmits impulses away from the cell body.

      • Dendrites

      • Receive incoming signals toward the cell body.

Distinguishing Neuron Components

  • Dendrites

    • Short, tapering branches that carry information to the cell body.

  • Axon

    • Long structure that transmits information away from cell body.

  • Axon Hillock

    • The region where the axon begins; serves as the trigger zone for impulses.

  • Teledendria/Terminal Branches

    • Final divisions of the axon leading to synaptic end bulbs/terminals.

  • Axon Collaterals

    • Branches that come off the main axon before the terminal segments.

Myelin Sheath

  • Insulating layer surrounding the axon:

    • Formed by glial cells – Schwann Cells (PNS) and Oligodendrocytes (CNS).

  • Function:

    • Increases impulse conduction speed.

  • Nodes of Ranvier

    • Gaps in the myelin sheath where the axon membrane is exposed, crucial for rapid signal transmission.

Types of Neurons

  • Multipolar Neuron

    • One cell body, multiple dendrites; common in the CNS.

  • Bipolar Neuron

    • One dendrite and one axon; found in sensory environments like the retina.

  • Unipolar Neuron

    • One projection that splits into a dendrite and an axon; often sensory neurons in the PNS.

Functional Classification of Neurons

  • Sensory Neurons

    • Carry information towards the central nervous system.

    • Generally unipolar in structure.

  • Motor Neurons

    • Carry commands away from the CNS.

    • Typically multipolar.

  • Interneurons (Association Neurons)

    • Connect sensory and motor neurons, located primarily in the CNS.

Neuroglial Cells in CNS

  • Ependymal Cells

    • Line ventricles and central canal; produce and circulate cerebrospinal fluid (CSF).

  • Astrocytes

    • Control exchanges between blood vessels and neurons; maintain chemical environment.

  • Microglial Cells

    • Act as immune defense cells; remove pathogens and dying neurons.

  • Oligodendrocytes

    • Form the myelin sheath in the CNS.

Neuroglial Cells in PNS

  • Schwann Cells

    • Form the myelin sheath around axons in the peripheral nervous system.

  • Satellite Cells

    • Surround neuron cell bodies in ganglia; provide structural support and regulate nutrient exchange.

Plasma Membrane and Membrane Potential

  • Structure:

    • Composed of a phospholipid bilayer with embedded proteins.

  • Resting Membrane Potential:

    • Negative 70 millivolts (mV) indicates the inside of a neuron is negative relative to the outside.

    • Maintained by selective permeability and the sodium-potassium pump.

    • Concentration gradient:

      • More sodium and chloride ions outside the cell.

      • More potassium ions inside the cell.

      • Large negative proteins present in the cytoplasm.

Ion Channels

  • Passive Channels:

    • Always open, allowing specific ions to diffuse along their concentration gradients.

  • Gated Ion Channels:

    • Open in response to specific stimuli:

      • Ligand-Gated Channels:

      • Open when a chemical (neurotransmitter) binds to a receptor on the channel.

      • Voltage-Gated Channels:

      • Open in response to changes in membrane potential.

      • Mechanically-Gated Channels:

      • Open in response to mechanical stimuli (e.g., pressure or sound).

Conclusion

  • The study of nervous system physiology encompasses understanding structure, function, and interaction of neurons and glial cells, their roles in signal transmission, and the mechanisms that maintain homeostasis and communication within the nervous system.

  • Next week will focus on action potentials and characteristics of nerve impulses.