neurons

• Nerve cells are made up of:

  • cell body - typical cell structure
  • dendrites - carries messages into the cell body
  • axon - carries nerve impulses away from the cell body
  • myelin sheath - fatty material
• Synapses:

  • the gap between neurons
  • neurotransmitters carries messages across the gap
  • neuromuscular junction - axon meets skeletal muscle cell
• Myelin sheath:

  • formed by Schwann cells
  • nodes of Ranvier - gaps between the sheath
  • neurilemma - helps repair injured fibres
  • produced by oligodendrocytes
• Functions of the myelin sheath:

  • insulates
  • protects axon
  • speeds up nerve impulses
• Nerve fibres:

  • axons and dendrites are nerve fibres
  • PNS - nerve fibres group up to form a nerve
• Dendrites receive information from other neurons or sensory receptors
• Axons carry electrical signals away from the cell body towards another neurone, muscle or gland
• Bipolar neurons have two extensions, one is an axon and the other is a dendrite
• Pseudounipolar neurons have one extension which divides into two at the end
• Multipolar neurons have many extensions from the cell body
• Bipolar neurons have two extensions from the cell body; one is an axon and the other is a dendrite
• Multipolar neurons have many extensions (dendrites) extending outwards from the cell body
• The myelin sheath insulates the axon so that impulses can travel faster along its length.
• Sensory neurones are unipolar and bipolar
• Nodes of Ranvier are gaps between sections of myelin sheath on the surface of the axon where there is no myelin sheath.
• pseudounipolar neurons have one extension that extends from the cell body and branches off to form both dendrites and an axon
• Sensory receptors are specialised cells or organs that detect changes in the environment and convert them into electrical signals
• Unipolar neurons have one extension from the cell body
• Motor neurons carry impulses to muscles and glands
• Sensory receptors detect stimuli such as light or sound waves
• Motor neurons carry impulses to effectors such as muscles and glands
• Axons transmit electrical signals away from the cell body to another nerve or muscle fibre, while dendrites receive electrical signals from other neurones.
• Sensory receptors detect changes in the internal or external environment.
• Neurons communicate with each other by transmitting electrical signals called action potentials down their long extensions called axons.
• Axons carry nerve impulses away from the cell body to synapses with target cells.
• Dendrites extend from the cell body and receive information from other cells.
• Dendrites receive signals from other neurons at synapses.
• The spinal cord contains sensory (afferent) and motor (efferent) nerve fibers.
• Axons carry electrical signals away from the cell body to other cells or organs
• Unipolar neurons have only one process which branches at the end into both an axon and a dendrite
• Axons transmit electrical impulses away from the cell body
• Interneurons connect sensory neurons with motor neurons, allowing information to be processed within the CNS
• A nerve impulse is an electrochemical change, it involves:

  • a change in electrical voltage
  • change is brought about by changes in chemicals
• Electrical charge:

  like charges will repel each other

  • the closer the charges get to each other, the stronger the force gets
  • positive + negative = release of energy
  • if separated by a barrier - they have potential (measured in volts) to release energy
• Potential difference across a cell membrane:

  • ions - electrically charged particles
  • extracellular fluid - high concentration of sodium chloride (positive sodium ions and negative chloride ions)
  • intracellular fluid - low concentration of sodium and chloride ions (positive potassium ions and various negative ions)
  • membrane potential - differences between inside and outside the cell membrane
• Resting membrane potential:

  an unstimulated neuron

  • -70mV maintained by sodium-potassium pump
  • K+ moves out of the cell along the concentration gradient
  • the sodium-potassium pump moves three sodium ions out of the cell for every two potassion ions entering (active transport)
• Depolarisation: (sudden increase in the membrane potential)

  occurs when stimulations about 15mV or the threshold

  • sodium channels are opened → intracellular fluid less negative → increase in potential difference
  • if -55mV → independent movement of sodium ions into cell (all-or-none response)
• Repolarisation:

  • sodium channels close → no influx of sodium ions
  • potassium channels open → K+ moves out of cell → repolarisation
  • potassium channels open too long → hyperpolarisation
• Refractory period:

  • sodium channels become quickly inactived (unresponsive)
  • lasts from -55mV to -70mV
• Nerve impulse transmission along unmyelinated fibres:

  • in refractory period → moves only in one direction
  • transmits messages at approx. 2m/s
  • stimulus in the middle → travels both directions