Biology Unit 4.1.2- Nerves

Sensory receptors- specialised cells that can detect changes in the surroundings
1. Recpetors
  1. rods and cones- light intensity and range of wavelength
  2. olfactory cells- pressence of volatile chemicals- smell
  3. taste buds, palate, epiglottis, oesophogus- soluble chemicals
  4. pressure receptors (pacinan corpucles)- pressure on skin
  5. cochlea- vibrations in air
  6. muscle spindle (proprioceptors)- length of muscle fibres
2. Generating Nerve impulses
  1. 1. changing membrane permeability
    1. neurones
      1. specific channel proteins- Na+ or K+
      2. gated channel proteins- usually closed but can open
      3. open- permeability increases, closed- permeability decreases
      4. carrier proteins actively transport Na+ out and K+ in using sodium potassium pumps 3Na:2K
    2. polarised- membrane that has a potential difference across it (Resting Potential)- inside= -vely charged
    3. Depolarisation- loss of polarisation across membrane. increased permeability to NA+ down a concentration gradient into cell. inside= less -ve
  2. 2.generator potential
    1. generator potential- small depolarisation caused by sodium ions entering the cell. larger stimuli=more channels open
    2. action potential- when membrane is depolarised to =40mV. All or Nothing response. leading up to action potential- membrane depolarises and reached threshold- lots of NA+ enters axon and action potential is reached
    3. sensory and motor neurones
      1. sensory- carries action potential to CNS. long dendron carries AP from sensory to cell body
      2. motor- carries action potential from CNS to effectors (muscles)- cell body in CNS and long axon carries action potential to effector
      3. intermediate.relay- connect sensory and moto
      4. similarities in structure...
        long and transmits over long distances
        gated sodium potassium ion pumps
        sodium potassium pumps
        maintain potential difference
        fatty sheath- schwann cells and nodes of ranvier
        numerous dendrites connected to other neurones
Resting and Action potential
1. Resting neurone
  1. not transmitting an action potential
  2. Interior of cell is more negative compared to exterior
  3. Resting potential- potential difference/voltage across neurone at rest (-60mV)
2. Action Potential
  1. At rest- sodium ion channels are closed
  2. Sodium potassium ion pumps use ATP
  3. Voltage gated channels- allows charged particles and ions. gates respond to change in potential difference.
  4. 1. All or nothing
    1. Depolarisation must be large enough to reach a threshold potential (-50mV) it will open nearby channels. depolarisation reaches =40mV Action Potential transmitted
  5. 2. ionic movement
    1. Action potential- depolarisation of membrane (inside more positive that outside) =40mV
3. Steps
  1. 1. Resting state- polarised inside -60 mV compared to outside
  2. 2. Na+ channels open, some diffuse in
  3. 3. Depolarised- less negative inside and threshold reached (-50mV)
  4. 4. Voltage gated sodium ion channels open (in more positive than out)
  5. 5. potential difference- +40mV. in more positive than out
  6. 6. Na+ channel close, K+ opens
  7. 7. repolarisation- K+ diffuses out therefore more negative in than out
  8. 8. hyperpolarised- potential difference overshoots slightly
  9. 9. refractory period- cell recovery and potential difference restored
Transmission of Action potential
1. 1. local currents
  1. 1. sodium ion channels open at particular point therefore, Na+ can diffuse across membrane from high concentration (outside) to low concentration (inside)
  2. 2. movement upsets balance of ionic concentration created by pumps
  3. 3. concentration of Na+ inside rises until more Na+ channels open.
  4. 4. Na+ diffuses sideways away from increased concentration
  5. 5. movement of charged particles is known as the local currents
2. 2. Voltage gated sodium ions
  1. gated sodium ion channels are operated by a change in voltage across the membrane
  2. movement of Na+ across neurones alters potential difference, when reduced, gates open
  3. Na+ enters at point further across/ along membrane therefore, action potential moves across membranes
3. 3. myelin Sheath
  1. insulating layer of fatty material
  2. Na+ or K+ cannot diffuse through this fatty layer
  3. Ionic exchange that causes an action potential therefore only occur at nodes of ranvier
  4. Action potential jumps from one node to another - saltatory
4. 4. Advantages of Slatatory conditions
  1. speeds up transmission of action potential
  2. Myelinated neurones conduct action potential quicker (120ms-1) compared to non myelinated
Nerve junctions
1. cholinergic synapses
  1. synanpse junction between 2+ neurones- communication and signals
  2. synamptic cleft- gap between 2 neurones
  3. presynaptic action potential causes release of chemicals that diffuse across the gap and generates action potential
  4. neurotransmitter- chemical that diffuses across cleft of synapses to transmit signals to post synaptic neurone
  5. cholinergic synapses use acetylcholine as a transmitter
2. 1. synaptic knob- swelling at the end of presynaptic neurone
  1. mitochondria
  2. smooth endoplasmic reticulum
  3. vesicles of acetylcholine
  4. voltage gates ca2+ channels in membranes
3. 2. postsynaptic membrane
  1. Na2+ channels- 5 polypeptide molecules, two which have recpetor site to acetylcholine, when binds, channels open
4. Transmission across synapses
  1. 1. action potential arrives at knob, Ca2+ opens and diffuses into knob
  2. 2. ca2+ causes synaptic vesicles to make and fuse with presynaptic membrane
  3. 3. acetylcholine released via excocytosis. diffuses across cleft and bind to receptor sites of Na+ channels in post synaptic membrane into neurone.
  4. 4. generator potential created if threshold reached and new action potential created in post synapse
5. Acetylcholiinesterase- enzyme in synaptic cleft which breaks down acetylcholine to ethanoic acid and choline
  1. stops transmission signals
  2. re- enter synaptic knob (diffusion), recombine with ATP
signals and messages
1. Roles of synapses in the nervous systems
  1. connects 2 neurones so a signal can be passed
  2. several presynaptic converge to one postsynaptic. allows signals from different parts of nervous system
  3. one presynaptic diverge to several postsynaptic. allows one signal to be transmitted to several parts of nervous system
  4. synapses ensure signals transmitted in correct direction. only knob contains acetylcholine
  5. synapses filter out unwanted low signals
  6. low levels may be amplified via summation
    1. generate several successive action potentials in presynaptic neurones
    2. 1. temporal summation
      1. 1 action potential in presynapticc does not produce in postsynaptic
      2. series of action potentials are required in presynaptic
      3. small excitatory postsynaptic potentials (EPSP) dont create an action potential in postsynaptic until they act together
    3. 2. spatial summation
      1. several presynaptic neurones may each contribute to producing an action potential in postsynaptic
    4. acclimatisation- after repeated stimulation, vesicles with transmitter substances, therefore synapse fatigues and no longer responds to the stimuli- background noises
2. frequency of transmission- when stimuli is at higher intensity, the sensory receptor will produce more generator potentials, therefore more vesicles released. higher frequency means more intense stimuli
3. myelinated and non myelinated neurones- non myelinated neurones: AP moves along the neurones in a wave rather than jumping between nodes
4. Advantages of myelinated...
  1. speed: Myelinated: 100-120 ms-1, Non myelinated: 2-20ms-1
  2. myelinated: sensory->CNS->effector (long), Non myelinated: co ordinates body finctions (shorter)

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Biology Unit 4.1.2- Nerves

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	Created by Sarah Pirbhai almost 11 years ago