Neuroscience Chapter on Action Potentials

Nervous Tissue Overview

• Nervous tissue is responsible for communication throughout the body.
• The nervous system is organized into two main parts:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Cranial nerves, and spinal nerves. The PNS also includes enteric plexuses in the small intestine and sensory receptors in the skin.

Functions of the Nervous System

• Sensory: Detects changes through sensory receptors.
• Integrative: Analyzes sensory information, stores aspects, and makes decisions.
• Motor: Responds to stimuli.

Nervous Tissue Histology

• Neurons: Electrically excitable cells that transmit signals throughout the body. Key structures include:
  • Dendrites: Receive signals.
  • Cell body: Contains the nucleus.
  • Axon: Transmits signals.
  • Axon terminal: Passes signals to next cell.

Structural Classification of Neurons

• Multipolar: Many dendrites, one axon. Most common neuron type.
• Bipolar: One dendrite, one axon. Found in special sensory organs.
• Unipolar: One process, emerging from the cell body, that branches into peripheral and central processes. Most sensory neurons in PNS are unipolar.

Functional Classification of Neurons

• Sensory (afferent): Carry impulses toward the CNS.
• Motor (efferent): Carry impulses away from the CNS.
• Interneurons (association neurons): Located within the CNS; connect sensory and motor neurons.

Neuroglia

• Support cells in the nervous system.
• Not electrically excitable.
• Divided into 6 types:
  • 4 types of neuroglia found in the central nervous system (CNS)
    • Ependymal cells: line the central cavities of the brain; help produce and circulate cerebrospinal fluid (CSF)
    • Astrocytes: Most numerous neuroglia in CNS, guide axon regeneration, maintain the blood-brain barrier.
    • Oligodendrocytes: myelinate CNS axons (a type of glial cell).
    • Microglia: Smallest neuroglia in the CNS, act as phagocytes for the defense of the CNS
  • 2 types of neuroglia found in the peripheral nervous system (PNS)
    • Schwann cells: myelinate PNS axons; participate in axon regeneration.
    • Satellite cells: surround neuron cell bodies and regulate O2, CO2, neurotransmitters, and nutrients around the neurons.

Myelination of Neurons

• Myelin sheath: Formed by Schwann cells (PNS) and oligodendrocytes (CNS).
• Myelin insulates axons, speeding up signal conduction.
• Nodes of Ranvier: Gaps in the myelin sheath, allow for faster signal transmission (saltatory conduction).

Gray Matter vs. White Matter

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons
• White matter: Contains mostly myelinated axons

Electrical Signals in Neurons

• Excitable cells communicate via action potentials (AP) or graded potentials (GP).
• APs transmit signals over long distances, GPs over short distances.
• AP & GPs depend on the resting membrane potential (RMP).

Graded Potentials

• Small, localized changes in transmembrane potential.
• Can be depolarizing (positive changes) or hyperpolarizing (negative changes).
• Strength of the graded potential depends on the stimulus strength.
• May sum up to initiate an action potential.

Action Potentials

• Rapid, large changes in transmembrane potential.
• All-or-none principle (threshold potential must be reached to trigger an action potential).
• Consist of: depolarization, repolarization, and sometimes an after-hyperpolarization phase.

Propagation of Action Potentials

• Action potentials propagate along the axon.
• Propagation speed is influenced by axon diameter and myelination.
• Continuous conduction: along unmyelinated axons.
• Saltatory conduction: along myelinated axons (speedier)

Signal Transmission at Synapses

• Synapse: Junction between neurons or between a neuron and an effector.
  • Electrical synapses: fast, direct signal transmission.
  • Chemical synapses: one-way signal transmission using neurotransmitters.

Postsynaptic Potentials

• Postsynaptic neurons respond to neurotransmitters with either EPSPs (depolarizing) or IPSPs (hyperpolarizing) postsynaptic potentials. Summation of these potentials determines if an action potential is generated.

Neurotransmitters

• Chemical messengers that transmit signals across chemical synapses.
• Small molecule neurotransmitters: Acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide.
• Neuropeptides: More complex molecules.

Removal of Neurotransmitters

• Neurotransmitters are removed from the synapse to terminate the signal.
• Removal methods: diffusion, enzymatic degradation (e.g., acetylcholine), and uptake into cells.

Summation

• Spatial summation: Combining effects of multiple presynaptic neurons releasing neurotransmitter.
• Temporal summation: Combining effects of a single presynaptic neuron releasing multiple neurotransmitters.

Regeneration and Repair in Nervous Tissues

• The CNS shows limited ability to regenerate.
• The PNS shows a much greater potential for regeneration
  • Chromatolysis, Wallerian degeneration, and formation of a regeneration tube participate in PNS regrowth
  • Inhibitory factors from glial cells, specifically oligodendrocytes, may be responsible at least in part for the poor regeneration seen in the CNS.