Biology Unit 4.1.2- Nerves

Biology Unit 4.1.2- Nerves

1. Sensory receptors- specialised cells that can detect changes in the surroundings

   • Recpetors
     • rods and cones- light intensity and range of wavelength
     • olfactory cells- pressence of volatile chemicals- smell
     • taste buds, palate, epiglottis, oesophogus- soluble chemicals
     • pressure receptors (pacinan corpucles)- pressure on skin
     • cochlea- vibrations in air
     • muscle spindle (proprioceptors)- length of muscle fibres
   • Generating Nerve impulses
     • 1. changing membrane permeability
       • neurones
         • specific channel proteins- Na+ or K+
         • gated channel proteins- usually closed but can open
         • open- permeability increases, closed- permeability decreases
         • carrier proteins actively transport Na+ out and K+ in using sodium potassium pumps 3Na:2K
       • polarised- membrane that has a potential difference across it (Resting Potential)- inside= -vely charged
       • Depolarisation- loss of polarisation across membrane. increased permeability to NA+ down a concentration gradient into cell. inside= less -ve
     • 2.generator potential
       • generator potential- small depolarisation caused by sodium ions entering the cell. larger stimuli=more channels open
       • 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
       • sensory and motor neurones
         • sensory- carries action potential to CNS. long dendron carries AP from sensory to cell body
         • motor- carries action potential from CNS to effectors (muscles)- cell body in CNS and long axon carries action potential to effector
         • intermediate.relay- connect sensory and moto
         • 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

2. Resting and Action potential

   • Resting neurone
     • not transmitting an action potential
     • Interior of cell is more negative compared to exterior
     • Resting potential- potential difference/voltage across neurone at rest (-60mV)
   • Action Potential
     • At rest- sodium ion channels are closed
     • Sodium potassium ion pumps use ATP
     • Voltage gated channels- allows charged particles and ions. gates respond to change in potential difference.
     • 1. All or nothing
       • Depolarisation must be large enough to reach a threshold potential (-50mV) it will open nearby channels. depolarisation reaches =40mV Action Potential transmitted
     • 2. ionic movement
       • Action potential- depolarisation of membrane (inside more positive that outside) =40mV
   • Steps
     • 1. Resting state- polarised inside -60 mV compared to outside
     • 2. Na+ channels open, some diffuse in
     • 3. Depolarised- less negative inside and threshold reached (-50mV)
     • 4. Voltage gated sodium ion channels open (in more positive than out)
     • 5. potential difference- +40mV. in more positive than out
     • 6. Na+ channel close, K+ opens
     • 7. repolarisation- K+ diffuses out therefore more negative in than out
     • 8. hyperpolarised- potential difference overshoots slightly
     • 9. refractory period- cell recovery and potential difference restored

3. Transmission of Action potential

   • 1. local currents
     • 1. sodium ion channels open at particular point therefore, Na+ can diffuse across membrane from high concentration (outside) to low concentration (inside)
     • 2. movement upsets balance of ionic concentration created by pumps
     • 3. concentration of Na+ inside rises until more Na+ channels open.
     • 4. Na+ diffuses sideways away from increased concentration
     • 5. movement of charged particles is known as the local currents
   • 2. Voltage gated sodium ions
     • gated sodium ion channels are operated by a change in voltage across the membrane
     • movement of Na+ across neurones alters potential difference, when reduced, gates open
     • Na+ enters at point further across/ along membrane therefore, action potential moves across membranes
   • 3. myelin Sheath
     • insulating layer of fatty material
     • Na+ or K+ cannot diffuse through this fatty layer
     • Ionic exchange that causes an action potential therefore only occur at nodes of ranvier
     • Action potential jumps from one node to another - saltatory
   • 4. Advantages of Slatatory conditions
     • speeds up transmission of action potential
     • Myelinated neurones conduct action potential quicker (120ms-1) compared to non myelinated

4. Nerve junctions

   • cholinergic synapses
     • synanpse junction between 2+ neurones- communication and signals
     • synamptic cleft- gap between 2 neurones
     • presynaptic action potential causes release of chemicals that diffuse across the gap and generates action potential
     • neurotransmitter- chemical that diffuses across cleft of synapses to transmit signals to post synaptic neurone
     • cholinergic synapses use acetylcholine as a transmitter
   • 1. synaptic knob- swelling at the end of presynaptic neurone
     • mitochondria
     • smooth endoplasmic reticulum
     • vesicles of acetylcholine
     • voltage gates ca2+ channels in membranes
   • 2. postsynaptic membrane
     • Na2+ channels- 5 polypeptide molecules, two which have recpetor site to acetylcholine, when binds, channels open
   • Transmission across synapses
     • 1. action potential arrives at knob, Ca2+ opens and diffuses into knob
     • 2. ca2+ causes synaptic vesicles to make and fuse with presynaptic membrane
     • 3. acetylcholine released via excocytosis. diffuses across cleft and bind to receptor sites of Na+ channels in post synaptic membrane into neurone.
     • 4. generator potential created if threshold reached and new action potential created in post synapse
   • Acetylcholiinesterase- enzyme in synaptic cleft which breaks down acetylcholine to ethanoic acid and choline
     • stops transmission signals
     • re- enter synaptic knob (diffusion), recombine with ATP

5. signals and messages

   • Roles of synapses in the nervous systems
     • connects 2 neurones so a signal can be passed
     • several presynaptic converge to one postsynaptic. allows signals from different parts of nervous system
     • one presynaptic diverge to several postsynaptic. allows one signal to be transmitted to several parts of nervous system
     • synapses ensure signals transmitted in correct direction. only knob contains acetylcholine
     • synapses filter out unwanted low signals
     • low levels may be amplified via summation
       • generate several successive action potentials in presynaptic neurones
       • 1. temporal summation
         • 1 action potential in presynapticc does not produce in postsynaptic
         • series of action potentials are required in presynaptic
         • small excitatory postsynaptic potentials (EPSP) dont create an action potential in postsynaptic until they act together
       • 2. spatial summation
         • several presynaptic neurones may each contribute to producing an action potential in postsynaptic
       • acclimatisation- after repeated stimulation, vesicles with transmitter substances, therefore synapse fatigues and no longer responds to the stimuli- background noises
   • 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
   • myelinated and non myelinated neurones- non myelinated neurones: AP moves along the neurones in a wave rather than jumping between nodes
   • Advantages of myelinated...
     • speed: Myelinated: 100-120 ms-1, Non myelinated: 2-20ms-1
     • myelinated: sensory->CNS->effector (long), Non myelinated: co ordinates body finctions (shorter)