The Nervous System, Part 1: Crash Course Anatomy & Physiology #8

Key Concepts

• Nervous system: Controls organs, physiological and psychological reactions, and the endocrine system.
• Sensory input: Detection of stimuli by sensory receptors.
• Integration: Processing sensory input and deciding on a response.
• Motor output: Activating parts of the body in response to processed information.
• Central Nervous System (CNS): Brain and spinal cord, the main control center.
• Peripheral Nervous System (PNS): Nerves branching from the brain and spine, facilitating communication between the CNS and the body.
• Afferent division: Sensory division of the PNS, carrying information to the brain.
• Efferent division: Motor division of the PNS, sending directions from the brain to muscles and glands.
• Somatic nervous system: Voluntary control of skeletal muscle movement.
• Autonomic nervous system: Involuntary control of heart rate, breathing, and digestion.
• Sympathetic division: Mobilizes the body into action.
• Parasympathetic division: Relaxes the body.
• Neurons: Nerve cells that respond to stimuli and transmit signals.
• Neuroglia (glial cells): Cells that support, protect, and insulate neurons.
• Astrocytes: Glial cells in the CNS that anchor neurons to blood supply and regulate material exchange.
• Microglial cells: Glial cells in the CNS that act as immune defense.
• Ependymal cells: Glial cells that line cavities in the brain and spinal cord, creating and circulating cerebrospinal fluid.
• Oligodendrocytes: Glial cells in the CNS that produce the myelin sheath.
• Satellite cells: Glial cells in the PNS that support neuron cell bodies.
• Schwann cells: Glial cells in the PNS that produce the myelin sheath.
• Soma: Neuron cell body containing the nucleus and organelles.
• Dendrites: Branch-like extensions of a neuron that receive messages from other cells.
• Axon: Long extension of a neuron that transmits electrical impulses away from the cell body.
• Multipolar neurons: Neurons with three or more processes extending from the soma.
• Bipolar neurons: Neurons with two processes extending from opposite sides of the cell body.
• Unipolar neurons: Neurons with one process extending from the cell body.
• Sensory (afferent) neurons: Neurons that transmit impulses from sensory receptors to the CNS.
• Motor (efferent) neurons: Neurons that transmit impulses from the CNS to muscles and glands.
• Interneurons (association neurons): Neurons that transmit impulses between sensory and motor neurons within the CNS.

Introduction to the Nervous System

The nervous system is crucial for controlling all bodily functions, including organs, physiological and psychological reactions, and even the endocrine system. It enables sensory input, integration, and motor output, which are fundamental to how we interact with the world.

Organization of the Nervous System

The nervous system is organized into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord, serving as the main control center.
• Peripheral Nervous System (PNS): Composed of nerves branching from the brain and spine, facilitating communication between the CNS and the rest of the body.

The PNS is further divided into:

• Sensory (Afferent) Division: Picks up sensory stimuli and transmits information to the brain.
• Motor (Efferent) Division: Sends directions from the brain to muscles and glands. This division includes:
  • Somatic Nervous System: Controls voluntary skeletal muscle movement.
  • Autonomic Nervous System: Controls involuntary functions like heart rate, breathing, and digestion. The autonomic system has two complementary forces:
    • Sympathetic Division: Mobilizes the body into action (e.g., "Gah! SPIDER!").
    • Parasympathetic Division: Relaxes the body (e.g., "it wasn’t a black widow or anything; you’re fine, breathe!").

Cells of the Nervous System: Neurons and Neuroglia

Nervous tissue is densely packed with cells, with less than 20% extracellular space. The primary cell types are:

• Neurons (Nerve Cells): Respond to stimuli and transmit signals. They are highly specialized and come in various shapes and sizes. Neurons share three key characteristics:

  1. Longevity: Some neurons can live as long as the individual.
  2. Irreplaceability: Most neurons are amitotic and cannot divide.
  3. High Metabolic Rate: Neurons require a constant supply of glucose and oxygen, consuming about 25% of daily calorie intake.

  Neurons consist of:

  • Soma (Cell Body): Contains the nucleus and other organelles.
  • Dendrites: Branch-like extensions that receive messages from other cells.
  • Axon: A long extension that transmits electrical impulses away from the cell body.

  Neurons are classified based on the number of processes extending from the cell body:

  • Multipolar Neurons: Have three or more processes (one axon and multiple dendrites); most common type.
  • Bipolar Neurons: Have two processes (one axon and one dendrite); found in special sensory areas like the retina.
  • Unipolar Neurons: Have one process; mostly found in sensory receptors.

  Neurons are also classified by function:

  • Sensory (Afferent) Neurons: Transmit impulses from sensory receptors to the CNS; mostly unipolar.
  • Motor (Efferent) Neurons: Transmit impulses from the CNS to muscles and glands; mostly multipolar.
  • Interneurons (Association Neurons): Transmit impulses between sensory and motor neurons within the CNS; mostly multipolar and the most abundant type.

• Neuroglia (Glial Cells): Support, protect, and insulate neurons. They outnumber neurons by about 10 to 1 and make up about half the mass of the brain. Types of glial cells include:

  • Central Nervous System (CNS):
    • Astrocytes: Anchor neurons to blood supply, regulate material exchange.
    • Microglial Cells: Immune defense against microorganisms.
    • Ependymal Cells: Create, secrete, and circulate cerebrospinal fluid.
    • Oligodendrocytes: Produce the myelin sheath.
  • Peripheral Nervous System (PNS):
    • Satellite Cells: Support neuron cell bodies.
    • Schwann Cells: Produce the myelin sheath.

Example: Spider on the Knee

The example of a spider crawling on a knee illustrates the integrated function of the nervous system:

1. Sensory Input: Unipolar sensory neurons in the skin detect the spider's legs.
2. Integration: The signal travels to the spinal cord and is passed to interneurons.
3. Motor Output:
   • Some interneurons send a signal to the quadriceps muscle, causing an involuntary kick.
   • Other interneurons send the signal to the brain, where it is interpreted, leading to a conscious decision to either scream or calmly remove the spider.

Conclusion

The nervous system, with its intricate organization and specialized cells, is essential for sensory input, integration, and motor output. Understanding the structure and function of neurons and glial cells, as well as the organization of the central and peripheral nervous systems, provides a foundation for comprehending how the body responds to stimuli and controls various functions. The next step is to understand how nerve cells use chemistry and electricity to communicate with each other.