Nervous System

• Nervous System


  The master controlling and communicating system of the body
• Structural Classification of Nervous System

  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)
• Central Nervous System (CNS)

  • Consists of the brain and spinal cord
  • Occupies the dorsal body cavity
  • Acts as the integrating and command centers of the nervous system
  • Interprets incoming sensory information and issues instructions based on past experience and current conditions
• Peripheral Nervous System (PNS)


  • Includes all parts of the nervous system outside the CNS
  • Consists mainly of the nerves that extend from the spinal cord and brain
  • Spinal nerves carry impulses to and from the spinal cord
  • Cranial nerves carry impulses to and from the brain
  • Serves as communication lines
  • Links all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles
• Functional Classification of Nervous System

  • Sensory division or Afferent division
  • Motor division or Efferent division
• Sensory division or Afferent division


  • Consists of nerves that convey impulses to the central nervous system from sensory receptors located in various parts of the body
  • Somatic sensory fibers deliver impulses from the skin, skeletal muscles, and joints
  • Visceral sensory fibers transmit impulses from the visceral organs
• Motor division or Efferent division


  • Carries impulses from the CNS to effector organs, the muscles and glands
  • Activates muscles and glands
  • Somatic nervous system allows us to consciously, or voluntarily, control our skeletal muscles
  • Autonomic nervous system regulates events that are automatic, or involuntary, such as the activity of smooth muscle, and glands
  • Sympathetic "fight-or-flight" system
  • Parasympathetic "rest-and-digest" system
• Supporting Cells in the CNS (Neuroglia)

  • Astrocytes
  • Microglia
  • Ependymal cells
  • Oligodendrocytes
• Astrocytes


  • Abundant star-shaped cells
  • Form a living barrier between capillaries and neurons
  • Help determine capillary permeability
  • Play a role in making exchanges between the two
  • Help protect the neurons from harmful substances in the blood
  • Help control the chemical environment in the brain
• Microglia


  • Spiderlike phagocytes
  • Monitor the health of nearby neurons
  • Dispose of debris, such as dead brain cells and bacteria
• Ependymal cells


  • Line the central cavities of the brain and spinal cord
  • The beating of their cilia helps to circulate the cerebrospinal fluid
• Oligodendrocytes


  • Wrap their flat extensions tightly around the nerve fibers, producing fatty insulating coverings called myelin sheaths
• Gliomas


  Brain tumors formed by neuroglia because one of the ability of neuroglia is that they never lose their ability to divide unlike neurons
• Supporting Cells in the PNS

  • Schwann cells form the myelin sheaths around nerve fibers
  • Satellite cells act as protective, cushioning cells for peripheral neuron cell bodies
• Neurons


  Highly specialized to transmit messages (nerve impulses) from one part of the body to another
• Neuron Structure


  • Cell body contains the nucleus and is the metabolic center of the cell
  • One or more slender processes extending from the cell body—fibers
• Myelin sheath


  Protects and insulates the fibers and increases the transmission rate of nerve impulses
• The importance of nerve insulation is best illustrated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, the electrical current is short-circuited and may "jump" to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The affected person may have visual and speech disturbances, lose the ability to control his or her muscles and become increasingly disabled. Multiple sclerosis is an autoimmune disease in which the person's own immune system attacks a protein component of the sheath.
• Clusters of neuron cell bodies and collection of nerve fibers in the CNS vs PNS

  • CNS: Nuclei, Tracts, White matter, Gray matter
  • PNS: Ganglia, nerves
• Functional Classification of Neurons

  • Sensory or Afferent
  • Motor or Efferent
  • Association neurons or Interneurons
• Sensory or Afferent Neurons

  • Carry impulses from sensory receptors (in the internal organs or skin) to the CNS
  • Cell bodies are always found in a ganglion outside the CNS
  • Keep us informed about what is happening both inside and outside the body
  • Dendrite endings are usually associated with specialized receptors: cutaneous sense organs and proprioceptors
• Motor or Efferent Neurons


  • Carry impulses from the CNS to the viscera and/or muscles and glands
  • Cell bodies are always located in the CNS
• Association neurons or Interneurons

  • Connect the motor and sensory neurons in neural pathways
  • Cell bodies are located in the CNS
• Structural Classification of Neurons

  • Based on the number of processes extending from the cell body
• Irritability


  The ability of neurons to respond to a stimulus and convert it into a nerve impulse
• Conductivity


  The ability of neurons to transmit the impulse to other neurons, muscles or glands
• Resting neuron's membrane


  Polarized, with fewer positive ions on the inner face than the outer face
• Action Potential Generation


  1. Stimulus excites neuron

  2. Sodium channels open, sodium rushes in, depolarization occurs

  3. Sodium channels close, potassium channels open, potassium rushes out, repolarization occurs
• Factors that can impair conduction of nerve impulses include sedatives, anesthetics, cold, and continuous pressure
• Transmission of Signal at Synapses

  1. Action potential reaches axon terminal

  2. Calcium channels open, calcium ions enter

  3. Neurotransmitter-containing vesicles fuse with membrane, releasing neurotransmitter

  4. Neurotransmitter diffuses across synaptic cleft and binds to receptors on next neuron

  5. Electrical changes in receiving neuron generate graded potential and possibly action potential
• Types of Reflexes


  • Somatic reflexes
  • Autonomic reflexes
• Somatic Reflexes


  • Stimulate the skeletal muscles
  • Involuntary
• Autonomic Reflexes


  • Regulate the activity of smooth muscles, the heart, and glands
  • Examples: salivary reflex, pupillary reflex
• Elements of a Reflex Arc

  • Receptor
  • Effector
  • Sensory neuron
  • Motor neuron
  • CNS integration center
• The simple patellar, or knee-jerk reflex, is an example of a two-neuron reflex arc, the simplest type in humans
• Reflex testing is an important tool in evaluating the condition of the nervous system. Reflexes that are exaggerated, distorted, or absent indicate damage or disease in the nervous system.
• Major Regions of the Central Nervous System

  • Cerebral hemispheres
  • Diencephalon
  • Brain stem
  • Cerebellum
• Cerebral Hemispheres


  • Exhibit elevated ridges of tissue called gyri and shallow grooves called sulci
  • Separated by a deep fissure called the longitudinal fissure
  • Consist of a superficial cortex of gray matter, an internal area of white matter, and the basal nuclei
• Functions of the Cerebral Cortex

  • Speech
  • Memory
  • Logical and emotional responses
  • Consciousness
  • Interpretation of sensation
  • Primary somatic sensory area in parietal lobe
  • Visual area in occipital lobe
  • Auditory area in temporal lobe
  • Olfactory area in temporal lobe
  • Primary motor area in frontal lobe
  • Broca's area for speech in frontal lobe
  • Higher intellectual reasoning in anterior frontal lobes
• Cerebral hemispheres


  • Divided by a single deep fissure—longitudinal fissure
  • Divided into lobes by other fissures or sulci