Nervous System Overview and Neuron Structure

Nervous System Overview

• The nervous system comprises two main components:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Nerves transmitting info to and from the CNS.

Nervous Tissue Composition

• Primarily composed of two cell types:
  • Neurons: Excitable nerve cells transmitting electrical signals.
  • Neuroglia (glia): Supporting cells, meaning "nerve glue".

Neuron Structure and Function

• Dendrites: Receive signals from other neurons.
• Cell body: Organizes and maintains cell function.
• Nucleus: Controls entire neuron function.
• Axon hillock: Generates nerve impulses.
• Axon: Carries impulses away from the cell body.
• Axon terminals: Forms junctions with other cells, or organs.
• Schwann cell: Produces myelin sheath.
• Myelin sheath: Increases impulse speed.
• Node of Ranvier: Gaps in myelin, allowing impulse "jumps".

Neuron Classes

• Afferent neurons (sensory neurons): Transmit sensory signals to the CNS.
• Efferent neurons (motor neurons): Transmit signals from CNS to muscles and glands.
• Interneurons: Connect sensory and motor neurons, forming complex CNS pathways.

Neuroglia (CNS Glial Cells)

• Astrocytes: The most numerous, involved in metabolism and synapse formation.
• Microglia: Phagocytes, protecting the body by ingesting foreign particles.
• Ependymal cells: Line cavities of CNS and spinal cord, and assist in the formation of cerebrospinal fluid.
• Oligodendrocytes: Produce myelin sheaths in the CNS

Neuroglia (PNS Glial Cells)

• Satellite cells: Surround neuron cell bodies in ganglia.
• Schwann cells: Form myelin sheaths in the PNS.

Nerves

• Bundles of neurons.
• Some nerves contain few neurons others contain hundreds or thousands..
• Myelin sheath insulates axons.
• Nodes of Ranvier are gaps in the myelin sheath.
• Impulses "jump" across these gaps, increasing speed.

Nerve Impulse Route

• Signals travel from dendrite to cell body, to axon hillock, to axon, to axon terminals.

Nerve Impulse

• Electrical signal traveling along an axon.
• Electrical difference exists between the axon's inside and surroundings.

Action Potential

• Brief changes in electrical conditions at a neuron's membrane.
• Occurs when a neural signal arrives in the neuron and "triggers" the impulse to occur.

Polarized

• Cell membrane has a potential difference.
• When potential becomes more positive than resting potential the membrane becomes depolarized.
• When potential becomes more negative than resting potential the membrane becomes hyperpolarized.

Resting Membrane Potential (RMP)

• Electrical charge inside the cell is negative relative to the outside.
• Usually around -70 mV.

Formation of an Action Potential

• Step 1: Sensory cell or neuron stimulus causes depolarization toward threshold potential.
• Step 2: If excitation threshold is met, Na channels open.
• Step 3: At peak action potential, K channels open and K ions leave the cell.
• Step 4: Membrane becomes hyperpolarized as K+ ions continue to leave the cell.
• Step 5: Na+/K+ transporters restore the resting potential.

Synapses

• Junction between neurons.
• Information passed usually chemically.
• Presynaptic (towards synapse) and postsynaptic (away from synapse).
• Synaptic cleft is a small space between neurons.
• Neurotransmitters leave presynaptic, diffuse across cleft, bind to receptors, influencing postsynaptic activity.

Chemical Synapses

• Action potential triggers Ca2+ channels and releases neurotransmitters across synapse.

Electrical Synapses

• Direct physical connection between neurons; allowing ions to flow directly between cells.
• Faster and more reliable than chemical synapses.

Task

• (Diagram labels) These labels should accompany a diagram of a neuron for easier understanding and assignment.