Chapter 11: Introduction to the Nervous System and Nervous Tissue

11.1 Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It is divided into anatomical and functional subdivisions.

• Anatomic Divisions:

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

  • PNS (Peripheral Nervous System): Includes all neural tissue outside the CNS, such as cranial and spinal nerves.

  • Number of Cranial and Spinal Nerves: 12 pairs of cranial nerves, 31 pairs of spinal nerves.

• Functional Divisions:

  • Somatic Nervous System: Controls voluntary movements (skeletal muscles).

  • Visceral (Autonomic) Nervous System: Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

• Main Functions: Sensory input, integration, motor output.

11.2 Neurons and Neuroglia

Neurons are the functional units of the nervous system, specialized for communication. Neuroglia (glial cells) support and protect neurons.

• Distinguishing Features of Neurons:

  • Excitability: Ability to respond to stimuli.

  • Conductivity: Ability to transmit electrical signals.

• Neuron Structure:

  • Dendrites: Receive signals.

  • Axon: Transmits signals away from the cell body.

  • Axon Hillock: Region where action potentials are initiated.

  • Axon Collaterals, Telodendria, Synaptic Knobs: Branches and terminals for communication with other cells.

• Axonal Transport:

  • Anterograde Transport: Movement from cell body to axon terminal.

  • Retrograde Transport: Movement from axon terminal to cell body.

• Classification of Neurons:

  • By structure: Multipolar, bipolar, unipolar.

  • By function: Sensory, motor, interneurons.

• Neuroglia (Glial Cells):

  • Six types: Four in CNS (astrocytes, oligodendrocytes, microglia, ependymal cells), two in PNS (Schwann cells, satellite cells).

  • Myelination: Oligodendrocytes (CNS) and Schwann cells (PNS) form myelin sheaths for faster signal conduction.

• Axon Regeneration: More likely in PNS than CNS due to presence of Schwann cells.

11.3 Membrane Potentials and Channels

Neurons communicate via electrical signals generated by changes in membrane potential, which depend on ion channels and pumps.

• Resting Membrane Potential (RMP):

  • Established by differences in ion concentrations and permeability.

  • Typical value: -70 mV (inside negative relative to outside).

  • Role of K+: Major contributor to RMP due to high permeability.

  • Role of Na+: Less permeable, but influx can depolarize the cell.

  • Na+/K+ Pumps: Maintain gradients by pumping 3 Na+ out and 2 K+ in per ATP.

• Plasma Membrane Characteristics:

  • Contains various channels: leak, voltage-gated, ligand-gated, mechanically-gated.

  • Selective permeability allows for transmission of electrical signals.

• Types of Channels:

  • Leak Channels: Always open, contribute to RMP.

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

  • Ligand-Gated Channels: Open/close in response to chemical signals.

  • Mechanically-Gated Channels: Respond to physical deformation.

11.4 Graded Potentials and Action Potentials

Neurons generate two main types of electrical signals: graded potentials and action potentials, each with distinct properties and roles in neural communication.

• Graded Potentials:

  • Local changes in membrane potential, vary in size.

  • Can be depolarizing or hyperpolarizing.

  • May lead to action potentials if threshold is reached.

• Action Potentials:

  • All-or-none electrical impulses that propagate along axons.

  • Steps:

    1. Threshold: Membrane potential reaches a critical level.

    2. Depolarization: Rapid influx of Na+ () ions.

    3. Repolarization: Efflux of K+ () ions restores negativity.

    4. Hyperpolarization: Membrane potential becomes more negative than RMP.

    5. Return to RMP: Na+/K+ pumps restore original ion distribution.

  • All-or-None Principle: Action potentials occur fully or not at all.

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be initiated.

  • Relative Refractory Period: Stronger stimulus required for initiation.

• Conduction:

  • Continuous Conduction: Occurs in unmyelinated axons; slower.

  • Saltatory Conduction: Occurs in myelinated axons; faster due to jumping between nodes of Ranvier.

Equation for Resting Membrane Potential:

Where is the membrane potential, and is the equilibrium potential for each ion.

11.5 Synapses and Neurotransmitters

Synapses are specialized junctions where neurons communicate with other neurons or effectors. Chemical synapses use neurotransmitters to transmit signals.

• Chemical Synapse Structure:

  • Presynaptic Neuron: Releases neurotransmitter.

  • Postsynaptic Neuron: Receives neurotransmitter.

  • Synaptic Cleft: Gap between neurons.

• Synaptic Transmission:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open.

  3. Neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic membrane.

  5. Postsynaptic potential generated (EPSP or IPSP).

• Synaptic Delay: Time required for neurotransmitter release and binding.

• Types of Synaptic Potentials:

  • EPSP (Excitatory Postsynaptic Potential): Depolarizes membrane, closer to threshold.

  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes membrane, further from threshold.

• Neural Integration: Summation of multiple synaptic inputs (temporal and spatial summation).

• Neurotransmitter Types:

  • Excitatory or inhibitory depending on receptor and location.

  • Major groups: Acetylcholine, biogenic amines, amino acids, neuropeptides.

• Neuromodulation: Regulation of synaptic activity by other chemicals.

11.6 Neuronal Pools and Circuits

Neuronal pools are groups of interconnected neurons that process specific types of information. Neural circuits determine patterns of signal flow.

• Neuronal Pools: Functional groups of neurons processing common information.

• Neural Circuits:

  • Diverging Circuits: One neuron stimulates many others.

  • Converging Circuits: Many neurons stimulate one neuron.

Table: Comparison of CNS and PNS Glial Cells

Additional info: Some explanations and definitions were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.