nervous system

• Functions of the nervous system

  • Receiving sensory input
  • Integrating information
  • Controlling muscles and glands
  • Maintaining homeostasis
  • Establishing and maintaining mental activity
• We are aware of sensations from some stimuli, such as vision, hearing, taste, smell, touch, pain, body position, and temperature
• Other stimuli, such as blood pH, blood gasses, and blood pressure, are processed at a subconscious level
• Brain and spinal cord


  Major organs for processing sensory input and initiating responses
• Skeletal muscles


  Normally contract only when stimulated by the nervous system
• Nervous system


  Plays an important role in maintaining homeostasis
• Brain


  Center of mental activity, including consciousness, memory, and thinking
• Divisions of the nervous system

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


  All the nervous tissue outside the CNS (nerves and ganglia)
• The PNS functions to link the CNS with the various parts of the body
• The PNS carries information about the different tissues of the body to the CNS and carries commands from the CNS that alter body activities
• Possible results when the body feels a stimulus

  • Information will be ignored
  • It will trigger reflex
  • It will evaluate
• There are 12 pairs of Cranial Nerves and 31 pairs of Spinal Nerves, which are under the PNS
• Sensory division (afferent division) of the PNS

  Conducts action potentials from sensory receptors to the CNS
• Motor division (efferent division) of the PNS

  Conducts action potentials from the CNS to effector organs, such as muscles and glands
• Sensory neurons


  Transmit action potentials from the periphery to the CNS
• Motor neurons


  Transmit action potentials from the CNS toward the periphery
• Subdivisions of the motor division

  • Somatic (bodily) nervous system
  • Autonomic (self-governing) nervous system (ANS)
• Somatic nervous system


  Transmits action potentials from the CNS to skeletal muscles
• Autonomic nervous system (ANS)


  Transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands
• Divisions of the autonomic nervous system


  • Sympathetic
  • Parasympathetic
• Enteric nervous system (ENS)


  A unique subdivision of the peripheral nervous system, with both sensory and motor neurons contained wholly within the digestive tract
• The ENS can function without input from the CNS or other parts of the PNS, although it is normally integrated with the CNS by sensory neurons and ANS motor neurons
• Neurons (nerve cells)


  Receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs
• Neuron cell body


  Contains a single nucleus, which is the source of information for gene expression
• Dendrites


  Short, often highly branching cytoplasmic extensions that usually receive information from other neurons or from sensory receptors and transmit the information toward the neuron cell body
• Axon


  A single long cell process extending from the neuron cell body
• Axons can be surrounded by a highly specialized insulating layer of cells called the myelin sheath, which is formed by Schwann Cells in the PNS and Oligodendrocytes in the CNS
• Types of neurons based on structure

  • Multipolar
  • Bipolar
  • Pseudo-unipolar
• Multipolar neurons


  Have many dendrites and a single axon, most of the neurons within the CNS and nearly all motor neurons are multipolar
• Bipolar neurons


  Have two processes: one dendrite and one axon, located in some sensory organs
• Pseudo-unipolar neurons


  Have a single process extending from the cell body, which divides into two processes, one extending to the periphery and the other to the CNS, most other sensory neurons are pseudo-unipolar
• Glial cells (neuroglia)


  Primarily supportive cells of the CNS and PNS, do not conduct action potentials, more numerous than neurons
• Astrocytes


  Major supporting cells in the CNS, can stimulate or inhibit the signaling activity of nearby neurons, participate with the blood vessel endothelium to form the blood-brain barrier
• Ependymal cells


  Line the fluid-filled cavities within the CNS, some produce cerebrospinal fluid, others with cilia help move the cerebrospinal fluid
• Microglia


  Act as immune cells of the CNS, help protect the brain by removing bacteria and cell debris
• Oligodendrocytes and Schwann cells

  Provide an insulating material (myelin sheath) that surrounds axons in the CNS and PNS respectively
• Myelin sheath


  Specialized layers that wrap around the axons of some neurons, formed by the cell processes of oligodendrocytes in the CNS and Schwann cells in the PNS
• Myelin is an excellent insulator that prevents almost all ion movement across the cell membrane, gaps in the myelin sheath called nodes of Ranvier occur about every millimeter, ion movement can occur at the nodes of Ranvier</b>
• Myelination of an axon increases the speed and efficiency of action potential generation along the axon