Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for regulating and coordinating body activities by detecting changes, interpreting sensory information, and responding appropriately.

• 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 cause a response.

Structural and Functional Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to the rest of the body.

• Functional Divisions:

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

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

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

  • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., cardiac and smooth muscle, glands); subdivided into sympathetic and parasympathetic divisions.

Structure of a Neuron

Neurons are the functional units of the nervous system, specialized for transmitting electrical signals.

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

• Dendrites: Short, branched processes that receive signals from other neurons and convey them toward the cell body.

• Axon: Long process that transmits impulses away from the cell body to other neurons or effectors.

• Axon Terminals: Endings that release neurotransmitters to communicate with other cells.

Classification of Neurons

• By Structure:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in special senses).

  • Unipolar: Single process that splits into two branches (sensory neurons in PNS).

• By Function:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

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

  • Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neuroglia (Glial Cells)

Neuroglia are supporting cells that provide structural and functional support to neurons.

• Astrocytes: Maintain the blood-brain barrier, provide nutrients, and regulate the extracellular environment.

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

• Ependymal Cells: Line ventricles of the brain and spinal cord; produce and circulate cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Surround neuron cell bodies in the PNS; regulate the environment around neurons.

Myelin Sheath

The myelin sheath is a fatty layer that insulates axons, increasing the speed of nerve impulse conduction.

• CNS: Myelin is produced by oligodendrocytes.

• PNS: Myelin is produced by Schwann cells.

• Function: Myelination allows for faster transmission of electrical signals via saltatory conduction.

Nucleus vs. Ganglion; Nerve vs. Tract

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Membrane Ion Channels

• Leak Channels: Always open; allow ions to move across the membrane continuously.

• Gated Channels: Open or close in response to specific stimuli.

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

  • Chemically-Gated (Ligand-Gated): Open when a specific neurotransmitter binds.

  • Mechanically-Gated: Open in response to physical deformation of the membrane.

Resting Membrane Potential

The resting membrane potential is the voltage difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Electrochemical Basis: Maintained by the sodium-potassium pump and differential permeability of the membrane to Na+ and K+ ions.

$V_m = -70\ \mathrm{mV}$

$\text{Na}^+/\text{K}^+$ ATPase pumps 3 Na+ out and 2 K+ in, maintaining the gradient.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential that can be depolarizing or hyperpolarizing.

• Examples: Receptor potentials, postsynaptic potentials.

• Characteristics: Magnitude varies with stimulus strength; decay with distance.

Graded Potentials vs. Action Potentials

Generation and Propagation of Action Potentials

Action potentials are rapid, large changes in membrane potential that travel along axons.

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

2. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits the cell.

3. Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.

Propagation occurs as the depolarization of one segment of the axon triggers the next segment to depolarize.

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength (Na+ channels are inactivated).

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential (some Na+ channels have reset, K+ channels are still open).

Saltatory vs. Continuous Conduction

• Saltatory Conduction: Occurs in myelinated axons; action potentials "jump" from one node of Ranvier to the next, increasing conduction speed.

• Continuous Conduction: Occurs in unmyelinated axons; action potentials propagate along every part of the membrane, slower than saltatory conduction.

Example: Myelinated motor neurons conduct impulses much faster than unmyelinated sensory neurons.