Cellular Overview

Cellular Overview

• Neuroscience I, PT 8203, Rachel Wilcox, DPT, MS, Board-Certified Neurologic Clinical Specialist
• Course focuses on cellular components of the nervous system

Learning Objectives

• Describe the core components of a neuron.
• Categorize neuron and glial cell types based on function.
• Explain the role of myelin in signal transmission and its clinical applications.
• Detail action potential generation and transmission.
• Contrast different neurotransmitters.
• Explore the impacts of agonists and antagonists clinically.

Cellular Organization of Nervous System

• Two main cell types: neurons and neuroglia.
• Neurons are the basic structural and functional units, connecting with other neurons and body cells.
  • Receive input, process, and generate output (electrical).
• Neuroglia provide critical support to neurons.
  • Similar to support staff within the nervous system (utility poles, scaffolding).

Neuron Structure

• Neurons are designed for receiving, processing, and transmitting information.
• Key components include dendrites, soma, axon hillock, axon, myelin sheath, nodes of Ranvier, presynaptic terminal, and synapse.

Anatomy Review

• Dendrites: Branch-like extensions receiving input.
• Synapse: The point of communication between neurons.
• Soma: Cell body, contains organelles.
• Axon Hillock: Region where the axon emerges from the soma.
• Axon: Long extension carrying signals.
• Myelin Sheath: Fatty insulation around some axons.
• Nodes of Ranvier: Gaps in the myelin sheath aiding in signal transmission.
• Presynaptic Terminal: End of the axon at a synapse.
• Synaptic Cleft: The space between the presynaptic and postsynaptic neurons.

Classification of Neurons

• Morphology: Classification based on shape:

  • Multipolar: Multiple dendrites, one axon (most common).
  • Bipolar: One main dendrite and one axon.
  • Pseudounipolar: Has one axon root, with an extension acting as a dendrite

• Function: Classification based on the direction of signal flow:

  • Afferent (Sensory): Towards CNS
  • Efferent (Motor): Away from CNS
  • Interneurons: Relay signals within the CNS

Morphological Classification of Neurones

• Multipolar: Most frequent; several dendrites, one axon. Examples include lower motor neurons and Purkinje fibers.
• Bipolar: One axon and one dendrite. Includes retinal and olfactory neurons.
• Pseudounipolar: One axon root extending into two, acting as a dendrite and axon. Examples are somatosensory neurons.

Functional Classification of Neurons

• Afferent: Sensory neurons transporting information toward the CNS.
• Efferent: Motor neurons carrying information away from the CNS.
• Interneurons: Connect neurons within the CNS; important in relaying signals between neurons and forming circuits.

Classification of Neuroglia

• Neuroglia support and protect neurons.
• Factors to consider: primary function, location in the CNS or PNS (Central or Peripheral nervous system), and role in signaling/nourishing/cleaning and defending the CNS.

Myelinating Neuroglia

• Myelin, made of lipids and proteins, insulates axons to enhance signal transmission speed.
• Oligodendrocytes: Multiple axons myelinated in the CNS.
• Schwann cells: Single axon myelinated in the PNS.

Role of Myelin

• Prevents electrical charge loss, leading to faster nerve signal transmission.
• White matter in the brain and spinal cord is rich in myelinated axons.
• Gray matter predominates neuron cell bodies.

Demyelination Diseases

• Diseases disrupting myelin impair transmission speed of nerve signals.
• Multiple Sclerosis: Damage to oligodendrocytes affects CNS function.
• Guillain-Barré syndrome: Schwann cell damage disrupts PNS function.

Signaling/Nourishing/Cleaning Neuroglia

• Astrocytes: Crucial in the CNS for signaling and nourishing neurons, maintaining fluid balance, and forming parts of the blood-brain barrier.
• Satellite cells: Support neurons in the PNS, regulating their environment.

Defending Neuroglia

• Microglia: Function as immune cells within the CNS, clearing cellular debris, pruning nonfunctional neurons.

Clinical Correlate: Brain Tumors

• Many tumors of the CNS arise from glial cells.
• Types of gliomas are described and their origin and characteristics.

Knowledge Check

• Questions related to neuron types and oligodendrocyte (immune cell) functions, as well as the difference between UMN and LMN injuries. Further questions address effects of botulinum toxin on muscles, and types of brain tumors.

Transmission of Information

• Resting membrane potential, local potentials, action potentials, and synaptic events are described in detail in diagram form.

Resting Membrane Potential (RMP)

• Difference in electrical charge between the inside and outside of a neuron, typically around -70 mV.
• Maintained via passive ion diffusion through channels and active ion pumping.

Local Potentials

• Small, graded changes in membrane potential.
• Stimuli can either excite (+ depolarization) or inhibit (- hyperpolarization).
• Receptor potential: Sensory neurons detect stimuli; Ex. stretch, compressions or exposure.
• Synaptic potentials: Occur in the postsynaptic membrane due to neurotransmitter binding
• Their magnitude is proportional to the strength of the stimulus.

Summation

• Combining local potentials to initiate an action potential.
• Temporal summation: Adding potentials that occur in rapid succession.
• Spatial summation: Adding potentials that occur simultaneously from different locations on the neuron.

Action Potentials (AP)

• Large, rapid, and all-or-none changes in membrane potential.
• Result from rapid ion movement across the cell membrane.
• Depolarization (inside more positive), repolarization (inside becomes more negative). Key players: Na+ and K+
• Refractory period: Difficult to generate another AP due to ion channel inactivation.

AP Propagation

• Action potential movement along the axon.
• Unidirectional: Propagates in one direction.
• Saltatory conduction: Fast propagation in myelinated axons, "jumping" from node to node.

Synaptic Events

• Process of one neuron communicating with another at a synapse.
• Action potential triggers release of neurotransmitters at synapses.
• Neurotransmitter binding to the receptors on the dendrites of the postsynaptic neuron changes their membrane potential.

Neuromessengers

• Neurotransmitters: Act directly on postsynaptic receptors, influencing the membrane potential of the next neuron.
  • Examples include glutamate (excitatory), GABA (inhibitory).
• Neuromodulators: Affect synaptic transmission indirectly, without direct receptor binding.
  • Examples include cortisol influencing serotonin and dopamine receptors.

Postsynaptic Potentials

• Changes in membrane potential at the postsynaptic neuron due to neurotransmitter interaction.
  • EPSP: Excitatory postsynaptic potential (depolarizing).
  • IPSP: Inhibitory postsynaptic potential (hyperpolarizing).

Pharmacologic Agonists & Antagonists

• Agonists: Mimic neurotransmitters, stimulating postsynaptic receptors.
• Antagonists: Blocks neurotransmitters to prevent them from impacting postsynaptic receptors.

Clinical Correlates: Neurotransmitters

• The role of different neurotransmitters in various body functions.
• Clinical implications of agonists and antagonists (e.g. Botulinum toxin, nicotine).

Let's Practice!

• Practical application questions about nervous systems pathologies, diagnoses, and treatments.

Summary

• Key concepts of the human nervous system, including neuron and glial cell types, myelin's role, and neural transmissions, as well as the functions of neurotransmitters are summarized.

References

• Includes the course text and instructor(s).

Test your knowledge on the structure and function of neurons in this quiz. Explore the roles of different neuron parts, types, and properties like local potentials. Perfect for students studying neuroscience or biology.