Chapter 12 Lecture: Week 2

Chapter 12: Nervous Tissue

12.1 Overview of the Nervous System

• Expected Learning Outcomes:

  • Describe the overall function of the nervous system.

  • Describe its major anatomical and functional subdivisions.

Overview of the Nervous System

• Coordination Systems:

  • Endocrine System:

    • Communicates via chemical messengers (hormones) into the bloodstream.

  • Nervous System:

    • Utilizes electrical and chemical means for cell-to-cell communication.

• Basic Steps in Nervous System Function:

  1. Sense organs collect information on changes in both internal and external environments. These changes are transmitted as coded messages to the central nervous system (CNS).

  2. The CNS processes this information, relates it to past experiences, and determines the appropriate response.

  3. The CNS sends commands to muscles and glands to implement the response.

Major Subdivisions of the Nervous System

• Central Nervous System (CNS):

  • Comprises the brain and spinal cord, protected by the cranium and vertebral column.

• Peripheral Nervous System (PNS):

  • Encompasses all nervous tissues excluding the brain and spinal cord, constituted by nerves and ganglia.

    • Nerve: Bundle of axons wrapped in connective tissue.

    • Ganglion: Swollen structure in a nerve where neuron cell bodies are concentrated.

PNS Divisions

• Sensory (Afferent) Division:

  • Carries signals from receptors to the CNS.

  • Subdivided into:

    • Somatic Sensory Division: Signals from skin, muscles, bones, and joints.

    • Visceral Sensory Division: Signals from the internal organs (heart, lungs, stomach).

• Motor (Efferent) Division:

  • Transmits signals from the CNS to effectors (muscles and glands).

  • Subdivided into:

    • Somatic Motor Division: Signals to skeletal muscles affecting voluntary movements and reflexes.

    • Visceral Motor Division (Autonomic Nervous System): Signals to cardiac and smooth muscles and glands which produce involuntary responses.

      • Sympathetic Division: Increases bodily activity (accelerates heart rate, respiration; inhibits digestive functions).

      • Parasympathetic Division: Decreases bodily activity (slows heart rate and breathing; stimulates digestion).

12.2 Properties of Neurons

• Expected Learning Outcomes:

  • Describe three functional properties of all neurons.

  • Define the three most basic functional categories of neurons.

  • Identify the parts of a neuron.

  • Explain how neurons transport materials.

Universal Properties of Neurons

• Excitability:

  • Ability to respond to stimuli.

• Conductivity:

  • Ability to produce electrical signals that travel along nerve fibers.

• Secretion:

  • Release of neurotransmitters at axon terminals influencing other cells.

Functional Classes of Neurons

• Sensory (Afferent) Neurons:

  • Transmit information about stimuli to the CNS.

• Interneurons:

  • Connect sensory and motor pathways, constituting about 90% of all neurons.

• Motor (Efferent) Neurons:

  • Send commands from the CNS to muscles and glands (effectors).

Structure of a Neuron

• Soma (Cell Body):

  • Houses nucleus and organelles (mitochondria, lysosomes, Golgi complex).

• Neuronal Processes:

  • Dendrites: Receive signals from other neurons.

  • Axon (Nerve Fiber): Specialized for rapid conduction of electrical signals originating from the axon hillock.

Axon Structure

• Components:

  • Axoplasm: Cytoplasm within axon.

  • Axolemma: Plasma membrane of the axon.

  • Myelin Sheath: Insulating layer around some axons, increasing the speed of signal conduction.

Structural Classification of Neurons

• Multipolar Neuron:

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

• Bipolar Neuron:

  • One axon, one dendrite (found in retina).

• Unipolar Neuron:

  • A single process leading away from the soma (sensory neurons).

• Anaxonic Neuron:

  • Many dendrites but no axon (found in brain).

12.3 Supportive Cells (Neuroglia)

• Expected Learning Outcomes:

  • Name and describe functions of six supporting cells in neurons.

  • Explain the importance of the myelin sheath.

  • How unmyelinated fibers relate to supportive cells.

  • Mechanisms of regeneration in damaged nerve fibers.

Supportive Cells

• Neuroglia, or glia, significantly outnumber neurons. They provide support, protect, and assist neurons.

• Types of Glia in CNS:

  • Oligodendrocytes: Form myelin sheaths.

  • Ependymal Cells: Line cavities of the brain; circulate cerebrospinal fluid.

  • Microglia: Macrophages that clean up debris.

  • Astrocytes (Most Abundant): Regulate the environment around neurons, form blood–brain barrier, and provide support.

Types of Glia in PNS

• Schwann Cells: Form myelin sheaths and assist in regeneration of damaged fibers.

• Satellite Cells: Provide electrical insulation and regulate the chemical environment around neuron cell bodies.

Myelin Sheath

• Composition:

  • Consists of glial cell plasma membranes, containing 20% protein and 80% lipids.

• Functionality: Speeds up nerve impulse conduction; nodes of Ranvier are gaps between myelin segments which play a crucial role in signal propagation.

12.4 Electrophysiology of Neurons

• Expected Learning Outcomes:

  • Explain voltage differences across membranes.

  • Describe local electrical responses and generation of nerve signals.

Electrical Potentials

• Membrane Potential:

  • Due to ion separation across the membrane, typically resting at -70 mV.

Action Potentials

• Characteristics:

  • All-or-none response, follows the nondecremental trend (does not weaken with distance).

• Phases:

  • Depolarization: Na+ influx.

  • Repolarization: K+ efflux.

  • Hyperpolarization: Briefly more negative than resting potential.

12.5 Synapses

• Expected Learning Outcomes:

  • Explain how messages are transmitted at synapses.

Synapse Structure

• Types:

  • Chemical Synapses: Involve neurotransmitter release (e.g., acetylcholine).

  • Electrical Synapses: Direct cell-to-cell electrical signal transmission through gap junctions.

Neurotransmitters

• Categories:

  • Acetylcholine, amino acids, monoamines, and neuropeptides.

Effects of Neurotransmitters

• Can be excitatory or inhibitory depending on the type of receptors on the postsynaptic neuron.

Memory and Synaptic Plasticity

• Memory types: Immediate, short-term, and long-term, linked to synaptic potentiation.

• Short-term Memory: Lasts seconds to hours, facilitated by circuit stimulation.

• Long-term Memory: Can last a lifetime, associated with permanent synaptic changes.

Diseases Related to the Nervous System

• Alzheimer's Disease: Characterized by memory loss, mood changes, and brain atrophy due to β-amyloid accumulation.

• Parkinson’s Disease: Involves the degeneration of dopamine-producing neurons leading to motor dysfunction.