NERVOUS SYSTEM

• Structures of the nervous system

  • Brain
  • Cranial nerves
  • Spinal cord
  • Spinal nerves
  • Ganglia
  • Enteric plexuses
  • Sensory receptors
• Basic functions of the nervous system

  • Sensory function
  • Integrative function
  • Motor function
• Sensory function


  Detect internal stimuli, carry sensory information into the brain and spinal cord through cranial and spinal nerves
• Integrative function


  Integrate (process) sensory information by analyzing and storing some of it and by making decisions for appropriate responses, perception is an important integrative function that occurs in the brain
• Motor function


  Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves
• Subdivisions of the nervous system

  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)
• Major structures of the nervous system

  • Brain
  • Nerves
  • Spinal cord
  • Ganglia
  • Enteric plexuses
  • Sensory receptors
• Brain


  Enclosed by the skull, contains about 100 billion neurons, 12 pairs of cranial nerves
• Nerves


  Bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord, each nerve follows a defined path and serves a specific region of the body
• Spinal cord


  Connects to the brain through the foramen magnum of the skull, encircled by the bones of the vertebral column, contains about 100 million neurons, 31 pairs of spinal nerves emerge from it
• Ganglia


  Small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside the brain and spinal cord, relay stations of the body's nervous system, play an essential role in connecting the parts of the peripheral and central nervous systems
• Enteric plexuses


  Extensive networks of neurons found in the walls of organs of the gastrointestinal tract, help regulate the digestive system
• Sensory receptor


  Dendrites of sensory neurons as well as separate, specialized cells that monitor changes in the internal or external environment, such as photoreceptors in the retina of the eye
• Parts of a neuron

  • Cell body
  • Nerve fiber (dendrites and axon)
• Cell body


  Perikaryon or soma, contains Nissl bodies (rough ER), cytoskeleton (neurofibrils and microtubules), lipofuscin
• Dendrites


  Receiving or input portions of a neuron, usually short, tapering, and highly branched
• Axon


  A long, thin, cylindrical projection that propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell, has a trigger zone (axon hillock and initial segment), contains axoplasm and axolemma
• Axon terminals (telodendria)


  Many fine processes of the ends of the axon and its collaterals, responsible for transmitting signals from one neuron to another across synapses
• Synapse


  Site of communication between two neurons or between a neuron and an effector cell, contains synaptic end bulbs and varicosities that release neurotransmitters
• Slow axonal transport


  Moves materials about 1–5 mm per day, conveys axoplasm in one direction only—from the cell body toward the axon terminals
• Fast axonal transport


  Moves materials a distance of 200–400 mm per day, uses proteins that function as "motors" to move materials in both directions—away from and toward the cell body—along the surfaces of microtubules
• Structural classification of neurons

  • Multipolar neurons
  • Bipolar neurons
  • Unipolar neurons
  • Purkinje cells
  • Pyramidal cells
• Functional classification of neurons

  • Sensory or Afferent neurons
  • Motor or Efferent neurons
  • Interneurons or association neurons
• Types of neuroglia in the CNS

  • Astrocytes
  • Oligodendrocytes
• Astrocytes


  Star-shaped cells with many processes, largest and most numerous of the neuroglia, play various roles in maintaining the structure, function, and homeostasis of the CNS
• Oligodendrocytes


  Responsible for forming and maintaining the myelin sheath around CNS axons
• Astrocytes


  • They facilitate the exchange of substances between the blood and the brain
  • They form tight junctions with endothelial cells of blood vessels, controlling the movement of substances into and out of the brain
  • They play a role in providing nutrients to neurons
  • They are involved in the uptake, storage, and release of energy substrates, such as glucose and lactate, which can be used by neurons as an energy source
  • They contribute to the repair and regeneration of damaged neural tissue
  • They can proliferate and form a scar at the site of injury, isolating and protecting the damaged area
  • They release factors that promote tissue repair and modulate the inflammatory response
  • They are involved in the regulation of synaptic function and plasticity
  • They can modulate synaptic transmission by releasing signaling molecules that influence the strength and efficacy of synaptic connections
• Oligodendrocytes


  • They resemble astrocytes, but are smaller and contain fewer processes
  • They are responsible for forming and maintaining the myelin sheath around CNS axons
• Myelin sheath


  A multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be myelinated.
• Microglia


  • They are small cells with slender processes that give off numerous spine like projections
  • They function as phagocytes
  • They remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue
• Ependymal Cells


  • They are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia
  • They line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord)
  • They produce, monitor, and assist in the circulation of cerebrospinal fluid
  • They form the blood-cerebrospinal fluid barrier
• Schwann Cells


  • They encircle PNS axons
  • They form the myelin sheath around axons
  • A single oligodendrocyte myelinates several axons, but each Schwann cell myelinates a single axon
  • They can also enclose as many as 20 or more unmyelinated axons (axons that lack a myelin sheath)
  • They participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS
• Satellite Cells


  • They are flat cells that surround the cell bodies of neurons of PNS ganglia
  • They provide structural support and regulate the exchanges of materials between neuronal cell bodies and interstitial fluid
• Myelination


  The process of forming a multilayered lipid and protein covering of the axon that electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction
• The medulla oblongata controls vital functions such as breathing, heart rate, blood pressure, swallowing, vomiting, sneezing, coughing, and digestion.
• The pons connects the cerebral cortex to the spinal cord and helps control sleep-wake cycles.
• The cerebellum is the part of the brain that coordinates movement.
• What is a Neurotransmitter?

  A chemical messenger that enables neurons to communicate with each other.