Chapter 11: Nervous Tissue – Structure, Function, and Communication

Chapter 11: Nervous Tissue

Main Divisions of the Nervous System

The nervous system is divided into two primary divisions, each with distinct roles in body function and communication.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS. Subdivided into:

  • Somatic Nervous System (SNS): Controls voluntary movements and transmits sensory information.

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

General Components of CNS and PNS

The CNS and PNS contain specialized structures for processing and transmitting information.

• CNS: Brain (cerebrum, cerebellum, brainstem) and spinal cord.

• PNS: Nerves (bundles of axons), ganglia (clusters of neuron cell bodies), sensory receptors.

Main Parts of a Neuron and Their Functions

Neurons are the functional units of the nervous system, specialized for communication.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Receive signals from other neurons or sensory receptors.

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

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

Synapse and Neurotransmitter Function

A synapse is the junction between two neurons or between a neuron and another cell. Neurotransmitters are chemical messengers released from the presynaptic neuron, crossing the synaptic cleft to bind receptors on the postsynaptic cell, thereby transmitting the signal.

• Example: Acetylcholine is a neurotransmitter that stimulates muscle contraction.

Neuron Regeneration in the CNS

Most CNS neurons cannot divide or regenerate due to the absence of centrioles and the inhibitory environment created by glial cells and extracellular matrix.

• Additional info: Peripheral neurons have some capacity for regeneration, especially with Schwann cell support.

Classification of Neurons

Neurons are classified based on structure and function.

• Structural: Multipolar, bipolar, unipolar, and anaxonic neurons.

• Functional: Sensory (afferent), motor (efferent), and interneurons.

Connective Tissue Sheaths of Nerves

Nerves are protected and organized by three connective tissue layers:

• Endoneurium: Surrounds individual nerve fibers (axons).

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Glial Cells of the CNS

Glia support and protect neurons in the CNS. The four types are:

• Astrocytes: Maintain blood-brain barrier, provide structural support, regulate ion and nutrient concentrations.

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing conduction speed.

• Microglia: Act as immune cells, removing debris and pathogens.

• Ependymal Cells: Line ventricles and central canal, produce and circulate cerebrospinal fluid (CSF).

Myelinated vs. Unmyelinated Axons

Myelination affects the speed of action potential transmission.

• Myelinated Axons: Insulated by myelin, allowing rapid, saltatory conduction.

• Unmyelinated Axons: Lack myelin, resulting in slower, continuous conduction.

• Additional info: Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS.

Glial Cells of the PNS

The PNS contains two types of glial cells:

• Schwann Cells: Form myelin sheaths around PNS axons; aid in regeneration.

• Satellite Cells: Surround neuron cell bodies in ganglia; regulate environment.

Factors Determining Resting Potential

The resting membrane potential is determined by:

• Ion concentration gradients (especially K+ and Na+)

• Selective permeability of the membrane

• Activity of the sodium-potassium pump

Equation:

$V_{m} = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

(Nernst equation for potassium)

Chemical Gradient vs. Electrical Gradient

Gradients drive ion movement across membranes.

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

Types of Gated Channels

Three main types of gated channels regulate ion flow:

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

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

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

Graded Potentials and Transmembrane Potential

Graded potentials are local changes in membrane potential caused by the opening of gated channels. They can depolarize or hyperpolarize the membrane, influencing the likelihood of action potential generation.

Depolarization, Repolarization, and Hyperpolarization

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Repolarization: Return of membrane potential to resting value after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Steps in Action Potential Formation

Action potentials are rapid, all-or-none electrical signals. The steps include:

1. Resting state: All channels closed.

2. Depolarization: Voltage-gated Na+ channels open; Na+ influx.

3. Repolarization: Na+ channels inactivate; voltage-gated K+ channels open; K+ efflux.

4. Hyperpolarization: K+ channels remain open briefly.

5. Return to resting state.

Absolute and Relative Refractory Periods

• Absolute Refractory Period: No new action potential can be generated; Na+ channels are inactivated.

• Relative Refractory Period: Action potential possible with stronger stimulus; some Na+ channels reset.

Saltatory vs. Continuous Propagation

• Saltatory Propagation: Action potential jumps between nodes of Ranvier in myelinated axons; faster.

• Continuous Propagation: Action potential moves along entire membrane in unmyelinated axons; slower.

Information Transfer at Synapses

Information travels from a presynaptic cell to a postsynaptic cell via neurotransmitter release, receptor binding, and subsequent changes in postsynaptic membrane potential.

EPSP, IPSP, and Summation

• Excitatory Postsynaptic Potential (EPSP): Depolarizes postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreasing likelihood of action potential.

• Summation: Integration of multiple EPSPs and IPSPs determines whether threshold is reached.

Temporal vs. Spatial Summation

• Temporal Summation: Multiple signals from one synapse in rapid succession.

• Spatial Summation: Signals from multiple synapses at the same time.