book 8

• Nervous System


  • Receiving sensory input
  • Integrating information
  • Controlling muscles and glands
  • Maintaining homeostasis
  • Establishing and maintaining mental activity
• Main Divisions of Nervous System

  • Central nervous system (CNS) - brain and spinal cord
  • Peripheral nervous system (PNS) - All the nervous tissue outside the CNS
• Divisions of Peripheral Nervous System

  • Sensory division - Conducts action potentials from sensory receptors to the CNS
  • Motor division - Conducts action potentials to effector organs, such as muscles and glands
• Divisions of Motor Division

  • Somatic nervous system - Transmits action potentials from the CNS to skeletal muscles
  • Autonomic nervous system - Transmits action potentials from the CNS to cardiac muscle, smooth muscle, and glands
  • Enteric nervous system - A special nervous system found only in the digestive tract
• Neurons


  Receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs
• Glial cells

  Supportive cells of the CNS and PNS, meaning these cells do not conduct action potentials. Instead, glial cells carry out different functions that enhance neuron function and maintain normal conditions within nervous tissue.
• Parts of a Neuron

  • Cell body - which contains a single nucleus
  • Dendrite - which is a cytoplasmic extension from the cell body, that usually receives information from other neurons and transmits the information to the cell body
  • Axon - which is a single long cell process that leaves the cell body at the axon hillock and conducts sensory signals to the CNS and motor signals away from the CNS
• Structural Types of Neurons

  • Multipolar neurons - have many dendrites and a single axon
  • Bipolar neurons - have two processes: one dendrite and one axon
  • Pseudo-unipolar neurons - have a single process extending from the cell body, which divides into two processes as short distance from the cell body
• Types of Glial Cells

  • Astrocytes - serve as the major supporting cells in the CNS
  • Ependymal cells - line the cavities in the brain that contains cerebrospinal fluid
  • Microglial cells - act in an immune function in the CNS by removing bacteria and cell debris
  • Oligodendrocytes - provide myelin to axons of neurons in the CNS
  • Schwann cells - provide myelin to axons of neurons in the PNS
• Myelin Sheath


  • Specialized layers that wrap around the axons of some neurons, those neurons are termed, myelinated
  • Formed by oligodendrocytes in the CNS and Schwann cells in the PNS
  • An excellent insulator that prevents almost all ion movement across the cell membrane
• Nodes of Ranvier


  Gaps in the myelin sheath where ion movement can occur
• Myelination of an axon increases the speed and efficiency of action potential generation along the axon
• Multiple sclerosis is a disease of the myelin sheath that causes loss of muscle function
• Unmyelinated Axons


  Lack the myelin sheaths and rest in indentations of the oligodendrocytes in the CNS and the Schwann cells in the PNS
• Gray Matter


  Consists of groups of neuron cell bodies and their dendrites, where there is very little myelin
• White Matter


  Consists of bundles of parallel axons with their myelin sheaths, which are whitish in color
• Resting Membrane Potential


  The voltage that exists across the cell membrane when the cell is at rest
• Leak Channels


  Ion channels that are always open so ions can diffuse across the membrane, down their concentration gradient
• Gated Channels


  Ion channels that are closed until opened by specific signals
• Types of Gated Channels

  • Chemically gated channels - opened by chemicals such as neurotransmitters
  • Voltage-gated channels - opened by a change in the electrical property of the cell membrane
• Polarized


  When the inside of the membrane has a negative charge relative to the outside of the membrane
• Sodium-Potassium Pump


  Actively transports K+ into the cell and Na+ out of the cell to maintain resting membrane potential
• The sodium-potassium pump consumes 25% of all the ATP in a typical cell and 70% of the ATP in a neuron
• Excitable


  The resting membrane potential can change in response to a stimuli
• Action Potentials


  Changes in membrane potential that nerve cells use to communicate with other cells
• Depolarization


  When the inside of the cell membrane becomes positive due to the movement of Na+ into the cell
• Threshold


  The level of depolarization required to cause voltage-gated Na+ channels to open and generate an action potential
• Stimuli


  Cause action potentials
• Gated channels


  Closed until opened by specific signals
• Opening and closing of gated ion channels

  Changes the permeability of the membrane to ions and can therefore change the membrane potential
• Action potentials may result
• Action potentials


  Electrical signals conducted along the cell membrane similar to electricity travelling along an electrical wire
• Channels responsible for the action potential

  • Voltage-gated Na+ and K+ channels, which are closed during resting membrane potential
• Action potential generation


  1. Stimulus applied to nerve cell

  2. Na+ channels open briefly

  3. Na+ diffuses quickly into cell

  4. Causes depolarization
• Depolarization


  Inside of cell membrane becomes positive
• Threshold


  Minimum level of depolarization required to trigger an action potential
• Action potential propagation

  1. Voltage-gated Na+ channels open

  2. Massive increase in Na+ permeability

  3. Voltage-gated K+ channels open

  4. More Na+ enters cell, depolarization continues

  5. Charge reversal, Na+ channels close, K+ leaves cell, repolarization
• Hyperpolarization


  Charge on cell membrane briefly more negative than resting potential
• All-or-none


  Action potentials either occur fully or not at all
• Sodium-potassium pump


  Assists in restoring resting membrane potential