nervous tissue

• Nervous system


  Regulates body activities by responding rapidly using nerve impulses
• Endocrine system


  Responds by releasing hormones
• Nervous system and endocrine system

  Have the same objective: to keep controlled conditions within limits that maintain life
• Nervous system


  • Responsible for our perceptions, behaviors, and memories
  • Initiates all voluntary movements
• The nervous system is quite complex, so it is discussed in several chapters

  
• This chapter focuses on the organization of the nervous system and the properties of neurons and neuroglia

  
• The autonomic nervous system operates without voluntary control
• The somatic senses and their sensory and motor pathways will be discussed in Chapter 16
• The special senses will be discussed in Chapter 17
• Nervous tissue


  Its excitable characteristic allows for the generation of nerve impulses that provide communication with and regulation of most body organs

  
• Central nervous system (CNS)

  • Consists of the brain and spinal cord
  • Processes many different kinds of incoming sensory information
  • Is the source of thoughts, emotions, and memories
  • Most signals that stimulate muscles and glands originate in the CNS

  
• Peripheral nervous system (PNS)


  • Consists of all nervous tissue outside the CNS
  • Includes nerves and sensory receptors
• Components of the PNS

  • Nerves
  • Sensory receptors

  
• Nerve


  A bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord

  
• Nerves that emerge from the CNS

  • 12 pairs of cranial nerves
  • 31 pairs of spinal nerves
• Sensory receptor


  A structure of the nervous system that monitors changes in the external or internal environment
• Examples of sensory receptors

  • Touch receptors in the skin
  • Photoreceptors in the eye
  • Olfactory (smell) receptors in the nose
• Divisions of the PNS

  • Sensory (afferent) division
  • Motor (efferent) division
• Sensory (afferent) division


  Conveys input into the CNS from sensory receptors in the body

  
• Motor (efferent) division


  Conveys output from the CNS to effectors (muscles and glands)
• Subdivisions of the motor (efferent) division

  • Somatic nervous system
  • Autonomic nervous system
• Somatic nervous system (SNS)


  • Conveys output from the CNS to skeletal muscles only
  • Its motor responses can be consciously controlled, so the action is voluntary

  
• Autonomic nervous system (ANS)


  • Conveys output from the CNS to smooth muscle, cardiac muscle, and glands
  • Its motor responses are not normally under conscious control, so the action is involuntary
• Branches of the ANS


  • Sympathetic nervous system
  • Parasympathetic nervous system
  • Enteric nervous system

  
• Sympathetic nervous system


  Helps support exercise or emergency actions - the "fight-or-flight" responses
• Parasympathetic nervous system


  Takes care of "rest-and-digest" activities
• Enteric nervous system (ENS)


  • An extensive network of over 100 million neurons confined to the wall of the gastrointestinal (GI) tract
  • Helps regulate the activity of the smooth muscle and glands of the GI tract
• The ENS can function independently, but it communicates with and is regulated by the other branches of the ANS
• Basic functions of the nervous system

  • Sensory (input)
  • Integrative (process)
  • Motor (output)
• Sensory function


  Sensory receptors detect internal and external stimuli, and this information is carried into the brain and spinal cord

  
• Integrative function


  The nervous system processes sensory information by analyzing it and making decisions for appropriate responses

  
• Motor function


  Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands)

  
• Neurons


  • Provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions
  • Most neurons have lost the ability to undergo mitotic divisions due to their specialization
• Neuroglia


  • Smaller cells that greatly outnumber neurons, perhaps by as much as 25 times
  • Support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them
  • Unlike neurons, neuroglia continue to divide throughout an individual's lifetime
• Both neurons and neuroglia differ structurally depending on whether they are located in the central nervous system or the peripheral nervous system

  
• Electrical excitability


  The ability to respond to a stimulus and convert it into an action potential
• Stimulus


  Any change in the environment that is strong enough to initiate an action potential
• Action potential (nerve impulse)


  An electrical signal that propagates (travels) along the surface of the membrane of a neuron

  
• Basic parts of a neuron

  • Dendrites
  • Cell body
  • Axon

  
• Axonal transport


  1. Slow axonal transport: Conveys axoplasm in one direction only - from the cell body toward the axon terminals

  2. Fast axonal transport: Moves materials in both directions - away from and toward the cell body