Nervous system

• Nervous system


  One of the 11 body systems, with a mass of only 2 kg, about 3% of the total body weight, yet the most complex
• Main subdivisions of the nervous system

  • Central nervous system
  • Peripheral nervous system
• Central nervous system (CNS)

  Consists of the brain and spinal cord
• Brain


  • Part of the CNS located in the skull, contains about 100 billion (10^11) neurons
• Spinal cord


  • Connected to the brain through the foramen magnum of the occipital bone, encircled by the bones of the vertebral column, contains about 100 million neurons
• Central nervous system (CNS)


  • Works on many different kinds of incoming sensory information, source of thoughts, emotions, and memories, origin of most signals that stimulate muscles to contract and glands to secrete
• Peripheral nervous system (PNS)


  Consists of all nervous tissue outside the CNS
• Components of the PNS

  • Nerves
  • Ganglia
  • Enteric plexuses
  • 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
• Cranial nerves


  • 12 pairs emerging from the brain
• Spinal nerves


  • 31 pairs emerging from the spinal cord
• Ganglia


  Small masses of nervous tissue, consisting primarily of neuron cell bodies, located outside of the brain and spinal cord
• Enteric plexuses


  Extensive networks of neurons located in the walls of organs of the gastrointestinal tract, help regulate the digestive system
• 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 receptors in the nose
• Divisions of the PNS

  • Somatic nervous system (SNS)
  • Autonomic nervous system (ANS)
  • Enteric nervous system (ENS)
• Somatic nervous system (SNS)

  Consists of sensory neurons that convey information from somatic receptors to the CNS and motor neurons that conduct impulses from the CNS to skeletal muscles (voluntary)
• Autonomic nervous system (ANS)


  Consists of sensory neurons that convey information from autonomic sensory receptors to the CNS and motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands (involuntary)
• Divisions of the ANS


  • Sympathetic division
  • Parasympathetic division
• Sympathetic division


  Increases heart rate
• Parasympathetic division


  Slows down heart rate
• Enteric nervous system (ENS)


  The "brain of the gut", consists of over 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal tract, operates involuntarily
• Functions of the nervous system

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


  Sensory receptors detect internal or external stimuli
• Integrative function


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


  The nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves
• Nervous tissue cell types

  • Neurons
  • Neuroglia
• Neurons


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


  • Support, nourish, and protect neurons, maintain the interstitial fluid that bathes them, 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</b>
• Electrical excitability


  The ability of neurons 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


  An electrical signal that propagates (travels) along the surface of the membrane of a neuron, begins and travels due to the movement of ions between interstitial fluid and the inside of a neuron through specific ion channels
• Parts of a neuron

  • Cell body
  • Dendrites
  • Axon
• Cell body


  Also known as the perikaryon or soma, contains a nucleus surrounded by cytoplasm with typical cellular organelles, free ribosomes, and prominent Nissl bodies
• Cytoskeleton of neurons


  • Includes neurofibrils composed of bundles of intermediate filaments that provide shape and support, and microtubules that assist in moving materials between the cell body and axon
• Lipofuscin


  A pigment that occurs as clumps of yellowish brown granules in the cytoplasm of aging neurons, a product of neuronal lysosomes that accumulates as the neuron ages but does not harm it
• Dendrites


  The receiving or input portions of a neuron, contain numerous receptor sites for binding chemical messengers from other cells
• Axon


  A long, thin, cylindrical projection that propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell, arises at the junction of the axon hillock and the initial segment in the trigger zone
• Axoplasm


  The cytoplasm of an axon, surrounded by a plasma membrane called the axolemma