Divisions of the Nervous System

Overview

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into structural and functional divisions, each with specific roles.

Structural Divisions

• Central Nervous System (CNS):

  • Consists of the brain and spinal cord.

  • Acts as the main control center for processing information and directing responses.

• Peripheral Nervous System (PNS):

  • Includes all nervous tissue outside the CNS.

  • Components:

    • Nerves: Bundles of axons that transmit signals to and from the CNS.

    • Nerve Plexuses: Networks of intersecting nerves.

    • Receptors: Specialized structures that monitor changes in the environment and relay information to the CNS.

    • Ganglia: Groups of neuron cell bodies located outside the CNS.

Functional Divisions

• Sensory (Afferent) Division:

  • Transmits sensory information from receptors to the CNS.

  • Examples: Touch, pain, temperature, and proprioception.

• Motor (Efferent) Division:

  • Transmits commands from the CNS to effectors (muscles and glands).

  • Subdivided into:

    • Somatic Nervous System: Controls voluntary movements by innervating skeletal muscles.

    • Autonomic Nervous System (ANS): Regulates involuntary functions by innervating cardiac muscle, smooth muscle, and glands.

      • Sympathetic Division: Prepares the body for 'fight or flight' responses.

      • Parasympathetic Division: Promotes 'rest and digest' activities.

Information Flow in the Nervous System

Pathways of Signal Transmission

Information in the nervous system flows through a series of steps involving sensory input, integration, and motor output.

• Sensory Input: Sensory receptors detect changes and send signals to the CNS.

• Integration: The CNS processes and interprets sensory input, making decisions about responses.

• Motor Output: The CNS sends commands via motor neurons to effectors (muscles or glands).

Example: Touching a hot surface activates sensory receptors in the skin, which send signals to the CNS. The CNS processes this information and sends a command to skeletal muscles to withdraw the hand.

Histology of Nervous Tissue

Cell Types

• Neurons: Specialized cells that generate and transmit electrical signals known as action potentials.

• Neuroglia (Glial Cells): Supportive cells that provide structural and metabolic support for neurons.

Neuron Structure

Major Components

• Cell Body (Soma):

  • Contains the nucleus and organelles.

  • Site of most metabolic activity and protein synthesis (chromatophilic substance).

  • Lacks centrioles, so neurons have limited ability to divide and repair.

• Dendrites:

  • Short, branched extensions that receive input from other neurons.

  • Transmit graded potentials toward the cell body.

• Axon:

  • Long projection that transmits action potentials away from the cell body.

  • Axon hillock: Region where action potentials are initiated.

  • Axon terminals: Branches at the end of the axon that form synapses with other cells.

Synapses

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Information is transmitted across the synaptic cleft via neurotransmitters.

Myelination

Myelin Sheath

• Myelin Sheath: A multilayered lipid-rich covering that insulates axons and increases the speed of action potential propagation.

• Formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated, allowing for saltatory conduction.

• Unmyelinated Axons: Lack a myelin sheath, resulting in slower impulse conduction.

Example: Myelinated axons in the spinal cord transmit signals much faster than unmyelinated axons in the autonomic nervous system.

Organization of Neurons and Nerves

Clusters and Bundles

• Nuclei: Clusters of neuron cell bodies in the CNS.

• Ganglia: Clusters of neuron cell bodies in the PNS (e.g., dorsal root ganglion).

• Tracts: Bundles of axons in the CNS (e.g., ascending nerve tracts).

• Nerves: Bundles of axons in the PNS (e.g., spinal nerves, cranial nerves).

Additional info: The rapid conduction of action potentials in myelinated axons is due to saltatory conduction, where the impulse jumps from one node of Ranvier to the next, greatly increasing speed and efficiency.