Part B: The Anatomy of a Neuron

Introduction to Neurons

• Neurons are the fundamental units of the nervous system and are essential for transmitting nerve impulses.

• There are three primary types of neurons, classified according to their function:

  • Sensory Neurons

  • Interneurons

  • Motor Neurons

Types of Neurons

• Sensory Neurons

  • Function: Carry nerve signals from sensory receptors to the Central Nervous System (CNS).

  • Definition: Sensory receptors are specialized structures of nerve tissue that detect environmental changes, whether internal or external.

• Interneurons

  • Location: Found entirely within the CNS.

  • Function: Receive input from sensory neurons and other interneurons. They sum up signals received and subsequently communicate with motor neurons.

• Motor Neurons

  • Function: Transmit nerve impulses away from the CNS to an effector, which can be a muscle fiber, organ, or gland.

  • Definition: Effectors execute the response to changes in the environment.

Anatomy of a Neuron

• A neuron comprises three main parts:

  • Dendrites

  • Structure: Small extensions off the cell body.

  • Function: Carry information toward the cell body, receiving signals from sensory receptors or other neurons.

  • Cell Body

  • Contains the nucleus and other organelles essential for the neuron's functionality.

  • Axon

  • Function: Conducts nerve impulses away from the cell body.

  • Myelination: Some axons are covered in myelin, a fatty substance that insulates the axon and speeds up signal transmission.

Function of Neurons

• Neurons specialize in conducting electric signals, referred to as nerve impulses, through their axons.

• Nerves: Bundles of axons that conduct signals in two directions:

  • From sensory receptors to the brain.

  • From the brain to muscles, glands, and other organs.

• Axon Terminals:

  • Connect to another neuron or tissue to facilitate the communication process.

  • Elicit responses in tissues (e.g., muscle contraction upon receiving a signal).

Structure of Different Neuron Types

• Sensory Neurons:

  • Structure: Possess sensory receptors at one end, a cell body, and a long axon covered by a myelin sheath.

  • Function: Transmit nerve impulses along the axon to the CNS, directing signals toward interneurons.

• Interneurons:

  • Structure: Located entirely within the CNS; typically exhibit short axons that do not have a myelin sheath.

  • Connection: Link between sensory neurons and motor neurons.

• Motor Neurons:

  • Structure: Long axon covered in a myelin sheath transmitting impulses from the CNS to effectors.

  • Function: Axon terminals connect to the effectors to induce a response.

Myelin Sheath

• In the Peripheral Nervous System (PNS), the myelin sheath is formed by Schwann Cells.

  • Schwann cells contain the lipid substance myelin within their plasma membranes.

  • These cells wrap around the axon multiple times, laying many layers of plasma membrane, sometimes up to 100 times.

  • Axons can be quite lengthy; thus, several hundred Schwann cells may be required for complete coverage.

• Nodes of Ranvier:

  • These are gaps (spaces) between myelin sheaths where the Schwann cells do not cover the axon.

  • Typically, long axons exhibit myelination while short axons generally do not.

Gray Matter vs. White Matter

• Gray Matter: Appears gray as it lacks myelinated axons.

• White Matter: Appears white due to the presence of myelinated axons.

• In the PNS, myelin contributes to the white, glistening appearance of nerve fibers and serves as an excellent insulator.

Clinical Relevance of Myelin

• In conditions such as Multiple Sclerosis (MS), the myelin sheath deteriorates, leading to difficulties in neuronal signal transmission.

• The myelin sheath plays a critical role in nerve regeneration, primarily within the PNS.

  • If an axon is severed, the remaining myelin sheath can help guide new fiber growth, aiding in the repair process.