NEUROMUSCULAR JUNCTION II

1.   Thick myosin filament:
     -     made up of myosin molecule.
     -     has a long tail + double head (projects from filament’s surface).
     -      head - contains ATP binding site.

2.   Thin actin filament made up of:
     -     two strands of globular actin protein (twisted together)
     -     regulatory proteins (troponin + tropomyosin).
3.   When Ca²⁺ is not available:
     -     troponin + tropomyosin - block myosin binding sites (on actin filaments).

4.   Muscular junction:
     -     specialised form of synapse
     -     found between motor neuron + sleletal muscle.
     -     spans across one unit of sarcomere.
5.   Sarcolemma = postsynaptic membrane
6.   Acetylcholine = transmitter substance (in synaptic vesicles).
7.    Acetylecholine binding sites = located in the folds of the sacrcolemma
8.   Calcium ions:
     -     found in abundance
     -     in sarcoplasmic reticulum.
9.   T tubules = very closely associated with sarcoplasmic reticulum.

10. Nervous impulse:
     è arriving at the neuromuscular junction
     è generates action potential
     è which spreads along sarcolemma.

11. Action potential:
     è depolarises T tubule
     è causing sarcoplasmic reticulum
     è to release Ca²⁺ ions
     è into sarcoplasm.
     è allow actin + myosin filaments to interact.

12. ATP:
     -     provides energy needed
     -     to power the sliding + pulling motion
     -     between actin + myosin.

13.  If Ca²⁺ ions + ATP conc. is sufficiently high à sequence will be repeated.

NEUROMUSCULAR JUNCTION I

1.    Skeletal muscle:
     -     consists of a bundle of long fibres
     -      running the length of the muscle.
2.   Muscle fibre”
     -      long cylindrical cell
     -      with many nuclei
     -      each consists of smaller myofibrils
     -     arranged longitudinally.

3.   Content of muscle fibre:
     -     cytoplasm (sarcoplasm)
     -     mitochondria,
     -     endoplasmic reticulum (sarcoplasmic reticulum),
     -     plasma membrane (sarcolemma).

4.   Sarcolemma:
     -     folded inwards
     -     at intervals
     -     to form T tubules.
5.    T tubules - very closely associated with the sarcoplasmic reticulum.
6.   Myofibril = composed of thick + thin filaments.
     -     Thick filaments = composed of protein myosin.
     -     Thin filaments = composed of protein actin.
     -     organised into repeating units (sarcomeres).












7.   Sarcomeres:
     -     basic units of muscle contraction.
     -     shortening à causes whole muscle fibre to shorten.
     -     joined at their ends by 2 lines (Z lines).












8.   Arrangement of thick (myosin) + thin (actin) filaments:
     -     form sarcomere à hence the whole muscle contracts.
     -      form alternating light + dark bands in a skeletal muscle
     -     form striations pattern.
     -     banded pattern = Z lines, A band, I band + H zones.












9.   During muscle contraction:
     -     changes in banding pattern
     -     due to sliding motion
     -     between thick + thin filaments.

SYNAPSE

1.      Synapse = unique junction between 2 adjacent neurones.

2.      Presynaptic neurones (bounded by a presynaptic membrane)
       =.Neurones - which carry impulses to synapse
3.      Postsynaptic neurones ( bounded by a postsynaptic membrane)

       =.Neurones - which carry impulses away from synapses,

4.      Synaptic knob (axon terminal) = swollen distal end of a motor neurons axon

5.      Within the synaptic knob are:
      -     mitochondria
      -     vesicles (packed with neurotransmitter).

6.      A neurotransmitter:
      -     chemical messenger
      -     used by neurones
      -     to communicate
      -     with each other
      -    or with effectors.

7.      On the postsynaptic membrane:
      -    there are protein receptors
      -    for the attachment of neurotransmitters.

8.      When an action potential:
      à arrives at the synaptic knob
      à causes influx of Ca²⁺ ions
      è into synaptic knob
      è stimulates the fusion of vesicles + presynaptic membrane
      è release of neurotransmitter
      è into synaptic cleft
      è by exocytosis.

9.      The neurotransmitter:
      è diffuses across the synaptic cleft
      è binds to the protein receptors
      è on the postsynaptic membrane
      è causing depolarisation.
      è  it is removed from the synaptic cleft
      è by hydrolysis
      è re-uptake into the synaptic knob
      è to prevent further unwanted effect.

10.  Cocaine:
      è preventing the reuptake of dopamine
      è into the synaptic knob,
      è resulting in generation of repeated impulses,
      è producing feeling of euphoria.

11.  Curare :
      è binding to the protein receptor for acetylcholine
      è on postsynaptic membrane
      è prevents the transmission of the impulse
      è causing paralysis.

NERVE IMPULSE

1.   Membrane potential:
      -     difference in electrical charge
      -     across the plasma membrane.











2.   Resting potential:
     -     neurone in its resting state
     -     usually has a membrane potential of -70mV
     -     maintained by the sodium-potassium pumps
     -     which actively transport sodium ions ß out of  the cell
     -     potassium ions à into the cell.
     -     require ATP from respiration.

3.   In resting neurone:
     -     membrane is more permeable to K⁺ than to Na⁺.
     -     potassium ions move out
     -      sodium ions move in
     -     thru passive ion channels.

4.   When the axon is stimulated:
     -      Na⁺ ions diffuse in
     -     polarity of the membrane in reversed
     -     from -70mV inside value
     -     to +40mV  inside.
     -     axon is depolarised.

5.   Polarity change = action potential = nerve impulse.

6.   Stimulus:
     -     opens up Na⁺ channels,
     -     which allow Na⁺ to flood in by diffusion
     -     thus depolarising the axon
     -     sodium-potassium pumps stop.

7.   In response to depolarisation:
     -     K⁺ ions diffuse out along a concentration gradient
     -     starting off repolarisation.
     -     inside membrane becomes more -ve than resting potential
     -     thus creating a refractory period.
     -     sodium-potassium pumps then start again.
     -     re-establishes the resting potential

8.      Axon can also be excited by direct application of:

     -     mechanical stimuli
     -     osmotic stimuli
     -     chemical stimuli
     -     thermal stimuli
     -     electrical stimuli.

9.      Action potential:

     -     can be generated if stimulus reaches certain threshold level.
     -     more intense stimulus:
         à will not give greater action potential.
     -     once threshold level is reached:
         à size of an impulse
         à is independent of the intensity of the stimulus
         = all-or-nothing law.

10.  During refractory period:

      -     neurone is insensitive to depolarisation.

11.  In absolute refractory period à cannot transmit another action potential à no matter how great a stimulus is applied.

12.  In relative refractory period à axon can transmit impulses à but stimulus must be stronger than usual.

13.  2 factors determine the speed of impulse transmission:

      -     axon diameter
      -     myelin sheath.

14.  An action potential:

      -    self-propagation (regenerating repeatedly along the axon)
      -     when wave of depolarization travels down the axon.

15.  In non-myelinated neurones:

      à speed of impulse transmission

      à proportional to diameter of axon.

16.  In myelinated neurones:

      à action potential jumps from node of Ranvier to the next

      à speed up impulse transmission = saltatory conduction.

THE NERVOUS SYSTEM

1.     Nervous system:
      -    ensures the whole body responds appropriately
      -     to external conditions.

2.     Human nervous system can be divided into 2 parts:
      -     central nervous system (CNS)
      -     peripheral nervous system(PNS).
3.      CNS is:
      -     divided into somatic + autonomic nervous system.
      -     made up of brain + spinal cord,
4.      PNS is:
      -     made up of paired cranian + spinal nerves.












5.      Brain is:
      -     made up of grey matter (located in the outer cortex) + white matter (inside brain)
      -     divided into 3 regions: forebrain, midbrain, hindbrain.

6.      PNS has 2 sets of neurons: sensory + motor neurones.

7.      Motor neurones are divided into: somatic + autonomic nervous system.
8.     Divisions of the autonomic nervous system are distinguished by where the  nerves originate.
9.    Nerves of the sympathetic nervous system emerge from:
      -     upper + central spinal cord regions
      -     thoracic regions
      -     lumbar regions.
10.  Nerves of the parasympathetic nerves system originate from:
      -      cranial nerves
      -     sacral region of the spinal cord.
11.  Autonomic nervous system:
      -     controls the involuntary activities
      -     of the internal organs.












12.  Parasympathetic nervous system:
      -     maintains the normal functioning of the body
      -     in a non-threatening situation
      -     helps keep the body in a relaxed + unstressed condition.
13.  Sympathetic nervous system:
      -     brings about physiological responses
      -     to threatening situations.










14.  A neurone:
      -      nerve cell:
      -     which conveys information
      -     in the form of nerve impulses
      -     basic functional unit of the nervous system.
15.  There are 3 types of neurones:
      -      motor,
      -     sensory
      -     interneurones.

16.  Motor neurones - carry impulses - from the CNSà to effectors.
17.  Sensory neurones - carry impulses - from sensory receptors à to the CNS.
18.  Interneurones - relay impulses - between sensory + motor neurones.