Nervous System - Part 1: Structure & Function

Nervous System - Part 1: Basic Structure & Function

• The nervous system is a communicator and control system
• The central nervous system (CNS) includes the brain and spinal cord
• The peripheral nervous system (PNS) includes peripheral nerves connecting the CNS to other body parts

Functions of CNS and PNS

• Sensory function: Detects stimuli inside and outside the body via sensory receptors in the PNS; sensory impulses/signals are sent to the CNS to create sensations, memories, and thoughts
• Integration function: Processes and evaluates sensory information. The CNS takes sensory impulses & creates sensations, memories, and thoughts.
• Motor function: Responds to information by sending impulses from the CNS through the PNS to effectors (muscles and glands) to cause a response
• Sensory input: Sensory nerves detect stimuli and send information from receptors to the CNS
• Motor output: Motor nerves send information from the CNS to effectors

Neuron Structure

• Neurons are nerve cells that vary in shape and size but generally have the following components:
  • Soma (cell body): large nucleus with a prominent nucleolus
  • Neurofibrils: bundles of neurofilaments (proteins) that extend into axons and dendrites, providing tensile strength and forming the cytoskeleton
  • Nissl bodies: darkly staining membranous packets containing clusters of rough ER and free ribosomes; give gray matter its color
  • Dendrites: variable number of branches from the soma, function in receiving input and transferring it to the cell body; abundant surface area for receiving information.
  • Axon hillock: slightly elevated region off the soma that gives rise to the axon
  • Axons: one long, cylindrical process off the soma, conducts action potentials and releases neurotransmitters at synaptic knobs

Neuron Structure (continued)

• Axon terminals and synaptic knobs: many fine extensions at the end of an axon, each extension ending in a synaptic knob close to another cell; separated by a space/cleft.

Matching

• Soma- Main body of the neuron
• Neurofibrils – structure that provides strength to neuron
• Nissl bodies – Structures that give gray matter its color
• Dendrites-Finger-like extensions off soma
• Axon hillock-Connects axon to soma
• Axon – Long branch extending away from soma

Axons - Myelinated

• Fatty, lipid-rich sheaths around axons insulate them.
• Formed by Schwann cells (PNS, aka neurolemmocytes): elongated, flat cells that ensheath PNS axons with myelin; allows for faster action potential propagation
• Oligodendrocytes (CNS): Large cells with slender extensions; wrap around axons of neurons forming myelin sheath

Axons - Unmyelinated

• No myelin sheath; axons not insulated from ECF.
• Still protected by a Schwann cell, but not wrapped (neurilemma)

Axons - Neurilemma

• Every axon in the PNS is shielded from ECF by Schwann cells, whether myelinated or not
• Portions of the Schwann cell with the nucleus and most of the cytoplasm comprise the neurilemma sheath
• One Schwann cell can wrap around ~15 axons

Axons - Nodes of Ranvier

• Gaps in the myelin sheath between Schwann cells/oligodendrocytes
• Help speed up nerve signal transmission

Clinical Application - Multiple Sclerosis

• Autoimmune disease: Immune system attacks myelin in the CNS
• Scleroses (plaques) form in myelin
• Symptoms include impaired coordination