Nervous System Lecture Notes

Nervous System

• The nervous system controls all bodily functions.
• When the nervous system fails, overall function ceases.

Neural Tissue Composition

• Composed of neural tissues, including:
  • Neurons: Cells that receive and respond to signals.
  • Neuroglia (or glial cells): Support cells for the nervous system.
• Also includes:
  • Blood vessels, supplying glucose and oxygen.
  • Connective tissues (fatty and fibrous).

Neurons vs. Neuroglia

• Neurons:
  • Receive signals from external and internal environments.
  • Transmit signals to body parts for bodily functions.
• Neuroglia (Glial Cells):
  • Do not transmit signals.
  • Support the nervous system to function, not body function directly.

Neuron Structure

• Cell Body (Soma):
  • Contains the nucleus, cytoplasm, mitochondria, Golgi apparatus, etc.
  • Looks like a normal cell.
  • Soma is another term for cell body.
• Processes/Extensions:
  • Dendrites:
    • Shorter extensions that bring signals to the neuron.
    • A neuron can have multiple dendrites.
  • Axon:
    • Long extension that carries signals away from the neuron
    • Number of axons is fixed to one per neuron.
• Myelin:
  • Covers and insulates the axon.
  • Made of approximately 80% fat (lipid) and 20% protein.
  • Referred to as lipoprotein (mixture of lipid and protein).
  • Myelin color is white in real life.
• Axon Divisions:
  • Axons can branch out.
  • Specialized ends called synaptic knobs (telodendrium).
  • Synaptic knobs form connections (synapses) to other neurons or muscles.
• Synaptic Knobs (Telodendrium):
  • Contain neurotransmitters (chemical signals).
  • Release neurotransmitters to communicate with neurons, muscles, or glands.
• Schwann Cells:
  • Type of cell that makes myelin in the peripheral nervous system (PNS).
• Oligodendrocyte Cells:
  • Make myelin in the central nervous system (CNS).

Neuron Functions and Properties

• Neurons react to physical and chemical changes from internal and external environments.
  • Example: A needle prick (physical change) or changes in glucose levels (chemical change).
• Soma (cell body) contains cell membrane, cytoplasm, nucleus, and other cell components.
• Axons and dendrites are cell processes; neurons can have many dendrites but only one axon.

Signal Transmission

• Signals are transmitted within the neuron as bioelectrical signals.
• Connections between neurons are called synapses.
• Neurons communicate via neurotransmitters (chemical signals).
• Effectors are structures receiving signals from neurons (e.g., muscles).
• Depolarization occurs when a polarized cell receives a signal.
• Soma contains a single nucleus and cytoplasm.

Neurofibrils and Chromatophilic Substances

• Neurofibrils support the axon.
• Chromatophilic substance (Nissl bodies) is the rough endoplasmic reticulum (ER) of the cell.
• Cytoplasm contains glycogen (chains of glucose) and lipids (fat).
• Some neurons contain melanin (skin pigment).

Myelin Sheath

• Myelin sheath (lipoprotein) insulates axons.
• Myelinated axons transmit signals faster.
• Myelin is composed of 80% lipid and 20% protein.
• Myelin is made by two types of cells:
  • Oligodendrocyte cells(CNS)
  • Schwann cells(PNS).
• Neurolemma is found only when Schwann cells are involved in myelin formation (PNS).
• Nodes of Ranvier are tiny gaps in the myelin sheath.
• Axon collaterals are branches from the axon.
• Telodendria are specialized axon terminals.
• Dendrites are highly branched to provide receptive surfaces for communication with other neurons.

Neuron Classification

Based on Structure

• Multipolar: One axon and multiple dendrites (most common in CNS).
• Bipolar: One axon and one dendrite.
• Unipolar: Initially one process that splits into axon and dendrite.

Based on Function

• Sensory Neurons: Sense the environment and receive signals.
• Motor Neurons: Respond to signals.
• Association Neurons (Interneurons): Sit between sensory and motor neurons, passing signals between them.

Neural Pathways

• Sensory neuron collects a signal from a receptor and carries it to an interneuron.
• Interneuron carries the signal to a motor neuron.
• Motor neuron is connected to an effector (e.g., muscle) to produce a response.

Glial Cells

• Supporting cells in the nervous system (approximately half of the cells).
  • Astrocyte cells
  • Oligodendrocyte cells
  • Schwann cells
  • Ependymal cells
  • Microglial cells

Types of Glial Cells:

• Astrocyte Cells:
  • Star-shaped cells that connect blood capillaries with neurons, ensuring a constant supply of oxygen and nutrients.
• Oligodendrocyte Cells & Schwann Cells:
  • Oligodendrocytes (CNS):
    • Do not directly connect with the axon; they send processes to make myelin on multiple axons.
  • Schwann cells(PNS):
    • Physically connect to the axon and wrap around it to form myelin.
      *Note that neurolemma is only formed by the schwann cells.
• Ependymal Cells:
  • Ciliated cells that line the walls of brain ventricles.
  • Help circulate cerebrospinal fluid (CSF).
  • CSF functions:
    • Cushions the brain and spinal cord.
    • Supplies nutrients.
    • Maintains brain chemistry (pH balance).
    • Fights infections.
    • Removes waste.
• Microglial Cells:
  • Smallest glial cells that fight infections in the nervous system; they can become mobile during infection.

Cell Membrane Potential

• Resting Potential: Membrane is polarized (positive outside, negative inside) and does not function.
• Action Potential: Membrane is depolarized by importing positive charges from outside to inside, making it active.
• Depolarization: Occurs one part at a time; the previous part repolarizes after the signal moves on.
• Sodium ions (Na^+) move in to depolarize the membrane.
• Potassium ions (K^+) move out to repolarize the membrane.
• Sodium-potassium pump is essential for maintaining nervous system function.

Neurotransmitters

• Chemical messenger molecules that transmit signals to effectors.
• Examples include acetylcholine, serotonin, etc.
• At least 100 different types of neurotransmitters are found so far.