The nervous system

• What must the nervous system be

  • quick to gather information and make decisions
  • Accurate
  • Reset quickly
• what are the two parts of the nervous system

  PNS-peripheral nervous system

  CNS- central nervous system
• what is nervous system tissue composed of?

  neurones which transmit nerve messages

  humans have about 100 billion in there brain alone
• what do all neurones have?

  • Dendrites
  • Cell body
  • axon
  • synaptic terminus
• dendrites 


  receive information from another cell and transmit the message to the body cell
• cell body 


  contains the nucleus, mitochondria and other organelles typical of eukaryotes
• axon 


  conducts messages away from the cell body
• synaptic terminus

  branches off of the axon

  responsible for releasing neurotransmitters

  the space between the synaptic terminus of one cell and the dendrites of another is called the synapse
• what does a neurone look like 


  Neurone structure

  
• what are some axons coated by?

  a phospholipid material called myelin, Schwann cells deposit myelin growing around the axon
• what is the gap between the myelin called

  nodes of ranvier
• membrane resting potential 


  -70mV

  the negative indicates that the inside of the cell os more negatively charged than the outside
• why does the membrane have an unequal concentration

  sodium ions are more concentrated outside the membrane (more + charge on outside)(3 x Na+ ions), potassium ions are more concentrated inside the membrane(2 x K+ ions)
• how is the unequal concentration maintained?

  sodium-potassium pump by actively transporting ions against their concentration gradients
• all or nothing principle

  action potential only happens if the stimulus reaches a threshold value, once it starts if travels to the synapse
• action potential 


  • a temporary reversal of the electrical along the membrane for a few milliseconds
  • the flip-flop of the charge in a region of the cell results in the propagation of the nerve impulse along the membrane
• steps in an action potential pt1.

  1. at rest the outside of the membrane is more positive than the inside. Na+/K+ Chanel’s are closed. the Na-K pump maintains at resting potential
  2. stimulus to a dendrite causes Na+ channels to open. stimulus may be from environment or a neurotransmitter. Na+ moves through facilitated diffusion into the cell
  3. The influx of Na+ causes the inside of the membrane to become more positively charged
• steps in an action potential pt2.

  4. if the charge reaches a threshold level(-50mV) then more Na+ channels will open causing an action potential

  5. If threshold is reached more Na+ channels open. more Na+ rushes in causing the cell to become positively rapidly positively charged.(depolarisation)

  6. Na+ channels close, K+ channels open, K+ flows out via facilitated diffusion. this restores the negative change inside the membrane(repolarisation)
• refractory period Is the time after depolarisation where no new action potential can start
• action potential is always the same size, the frequency can change
• Steps in an action potential Pt3.

  7. K+ continues to leave the membrane making it more negative than originally

  8. cell restores concentration by actively transporting sodium out and potassium in with the Na/K pump

  9. This is a refractory period. this prevents propagation of an action potential backwards along an axon
• Transmission of AP’s along an axon is the nerve impulse/electrical impulse
• what can you do to increase the speed on transmitting

  increase diameter

  increase diameter = faster transmission

  less resistance = less particles for the Na+ to collide with
• propagation 


  during depolarisation, Na drifts to neighbouring regions of the axon causing it to hit threshold and hence an action potential. the action potential can not go backwards because of the refractory period. Action potentials moving along an axon is an action potential
• what is myelin?


  a lipid, fatty substance that surrounds a neurone
• What does myelination do?

  improves the speed of electrical transmission via saltatory conduction
• unmyelinated neurones 


  action potentials propagate sequentially along the axon in a continuous wave of depolarisation
• myelinated neurones 


  the action potentials hop between the gaps on the myelin sheath (nodes of ranvier)
• Signals can be transported electrically

  Action potential simply jumps to the next cell

  Found in the CNS but is rare
• Signals can be transported chemically

  The action potential triggers the release of neurotransmitters from the synaptic terminus of one cell that can lead to an action potential in the next cells dendrite. A cholinergic synapse is a synapse for a neurone that produces acetylcholine which sends a message to another neurone to to skeletal muscle cells
• synapse process 


  1. arrival of the action potentials propagate sequentially causes voltage gated calcium channels to open. as a result calcium rushes into the cell
  2. the increase in calcium concentration causes vesicles containing neurotransmitters to move to the end of the axon and discharge their contents into the synapse via exocytosis
  3. Released neurotransmitters diffuse across the synapse and bind to the receptors on the other cells dendrite membrane causing chemically gated ion channels on that cel to open. Some neurotransmitters cause an action potential, others have an inhibitory effect
• synapses 


  Parts of a synapse drawing

  
• Neonicotinoids 


  def- a pesticide that targets the nervous system of animals, mainly insects

  Mode of action- acetylcholinesterase cannot break down neonicotinoids leading to paralysis due to them blocking ach receptors

  toxic to humans- no our metabolism can break it down

  Uses- farmers to rid of pests and get more crops

  Concerns- killing bees
• Pituitary gland

  Controls other glands/ hormones
• forebrain:

  cerebrum, thalamus, hypothalamus

  responsible for memory, thought, emotion and learning
• cerebral cortex

  responsible for memory, thought, emotion and learning
• spinal cord

  reflexes
• hindbrain

  PONS

  medulla oblongata

  cerebellum
• cerebellum

  responsible for movement and muscle memory
• the pacinian corpuscle

  -detects pressure

  -made of layers separated by gel

  Contains stretch mediated sodium ion channels which open when sufficient pressure is applied. Na+ enters the sensory neurons if threshold is reached and an AP occurs