Nervous system  (part 1)
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Nervous system (part 1)






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Nervous system  (part 1) Nervous system (part 1) Presentation Transcript

  • The nervous tissue consists of: Neurones  transmit electrical impulses Glial cells  supporting cells - nonexcitable [Glial cells are NOT in syllabus]
  • Three types of neurones: Sensory or Afferent neurones Internuncial, Intermediate, Relay or Association neurones Motor or Efferent neurones
  • Basic plan of a nervous system change in the environment of the receptor stimulus muscle or gland relay neurones in receptor  sensory neurones  motor neurones  effector central nervous generates system response nerve impulses The functional unit of the nervous system is the neurone.
  • Characteristics of neurones Longevity:  can live and function for a lifetime Do not divide: foetal neurones lose their ability to undergo mitosis; neural stem cells are an exception High metabolic rate: require abundant oxygen and glucose
  • Each neurone possesses: Dendrites Dendrites Cell body [soma] Axon Synaptic terminals
  • Sensory Neurone AXON DENDRON Neurilemma: tough inelastic membrane surrounding the sheath
  • Motor neurone E F F E C T O R AXON O R G A N
  • Motor Neurone
  • Intermediate neurone
  • Cell body (Soma) contains most of the organelles 1. Action potential forms 2. Summation occurs
  • Nissl granules : stain darkly are groups of ribosomes:  form the neurotransmitter Transport of neurotransmitter
  • Schwann cells produce the myelin Schwann cells - wrap in concentric layers around the axon - cover the axon in a tightly packed coil of membranes
  • Theodor Schwann: (1810-1882) discovered the Schwann cell in the peripheral nervous system. Schwann cells are glial cells
  • Nodes of Ranvier: gaps along the axon  allow current exchange across axon membrane  occur at intervals of 1 to 2 m
  • Myelin Sheath: surrounds a nerve fibre • composed of the lipoprotein myelin Functions: 1. forms an insulating layer 2. increases the speed of impulse conduction Saltatory Conduction
  • Multiple Sclerosis (MS) What happens: - myelin sheath around some axons in the CNS becomes damaged
  • Multiple Sclerosis (MS) What is the result of this damage? - conduction along these axons slows down or stops completely Where can demyelination occur? - almost anywhere in the brain or spinal cord at various times
  • Two types of nerve fibres 1 Myelinated Each axon is wrapped by a Schwann cell
  • 2 Unmyelinated incompletely enclosed by a Schwann cell in autonomic nerves (control involuntary muscles) Up to 9 fibres are partially shrouded by one Schwann cell
  • Fig. 4 A nerve: is a BUNDLE of neurones Several axons in a bundle = a NERVE
  • Every cell has a voltage (difference in electrical charge) across its plasma membrane :  called a membrane potential -70mV  range is -50 to -90 mV [about -70 mV (millivolts)] in all species investigated  the inside of the cell is negative with respect to the outside
  • A voltmeter placed with one electrode inside the axon and the other outside the membrane reads the voltage
  • Resting potential is the :  potential difference across the axon plasma membrane  membrane potential of a neurone not sending signals (not firing) At the resting potential, the membrane is said to polarised
  • Differential distribution of positive & negative charges across a membrane [A] Membrane has no potential [B] Membrane has a potential
  • Differential distribution of positive & negative charges across a membrane
  • Table 1 The ionic composition of cytoplasm and extracellular fluid Ion Na+ K+ Cl- Cytoplasm (mmol/l) 15 150 7 Fixed anions:  proteins  nucleic acids Extracellular (mmol/l) 150 5 110 Equilibrium Potential (mV) +60 -90 -70
  • The gradients across the membrane are:  known as electrochemical gradients  maintained by sodium/potassium pumps
  • Resting Potential exists due to: 1 ions (Na+,K+, Cl-) being concentrated on different sides of the membrane
  • Membrane Potential is generated by: 2 differential membrane permeability to solutes  membrane: 20x more permeable to K+ What is the result if a lot of K+ are lost? A negative charge builds up within the axon.
  • Thus, the electrochemical gradient of K+ determines the value of the resting potential
  • Cells can be: excitable: sensory cells neurones muscle cells Resting potential changes with the activity of the cell non-excitable: all other cells - Membrane potential is constant
  • Membrane potential can shift: DEPOLARISATION : a shift in membrane potential in the positive direction e.g. -50 mV -70 mV HYPERPOLARISATION : a change in membrane potential that makes the inside of the membrane even more negative than it is at rest, e.g. -85 mV
  • Use the terms: polarised, depolarised and hyperpolarised to describe the membrane? Hyperpolarised - 85 mV Polarised -70 mV Depolarised + 40 mV
  • 3 Types of proteins in the axon membrane 1) Leak channels (passive channels)  always open 2) Sodiumpotassium pump Na+ K+ 3 Na+ 2 K+
  • 3) Voltage-gated:  Na+ channels  K+ channels  open & close in response to specific changes in the membrane potential  most gated channels are closed at the resting potential
  • [NOTE: Two types of gated channels] Chemically-gated ion channel Voltage-gated ion channel – along axon
  • Action Potential is:  a change in the potential across the membrane from a negative inside value of about -70 mV to a positive inside value of +40 mV + 40 mV  is an abrupt, short-lived reversal in the voltage difference across the plasma membrane
  • Inside of axon becomes positive as: Na+ voltage gated channels open
  • What causes an Action Potential ? A stimulus
  • How is an Action Potential generated? By a sudden brief increase in the permeability of the axon membrane to Na+ which enter the axon
  • Action Potential is propagated along axon
  • Action Potential is propagated along axon
  • Where is an Action Potential generated?
  • Action Potential in a neurone Cathode Ray Oscillopscope
  • Does any stimulation result in an action potential? NO!! - 55mV The membrane potential must reach the threshold of excitation.
  • Action potentials are all-or-none events Action potential
  • Events during an action potential
  • Course of the Action Potential • The action potential begins with a partial depolarisation (e.g. from firing of another neuron ) [A]. • When the excitation threshold is reached there is a sudden large depolarisation [B]. • This is followed rapidly by repolarisation [C] and a brief hyperpolarisation [D]. • There is a refractory period immediately after the action potential where no depolarisation can occur [E] +40 Membrane potential 0 (mV) [C] [B] [E] [A] [D] excitation threshold -70 0 1 2 3 Time (msec)
  • A nerve impulse is self-propagating. What does this mean? An impulse at any point on the membrane causes an impulse at the next point along the membrane.
  • Conduction of Acton Potentials Why does the membrane become quickly repolarised? To allow subsequent firing
  • Action potentials travel in only one direction: toward the synaptic terminals. Why? Inactivated Na+ channels behind the zone of depolarisation prevent the action potential from travelling backwards [see next slide]
  • Saltatory Conduction: Action Potential seems to JUMP • Neurones are excitable: they can change their membrane potential in response to stimulation • Myelinated regions of axon are electrically insulated • Electrical charge moves along the axon rather than across the membrane • Action potentials occur only at unmyelinated regions: nodes of Ranvier Myelin sheath Node of Ranvier
  • Voltage gates occur at these nodes.
  • Extracellular fluid Cytoplasm At rest, the inside of the neurone is slightly negative due to a higher concentration of positively charged sodium ions outside the neurone.
  • Na+ Na+ When stimulated past threshold (about –30mV in humans), voltage-gated sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. This is called depolarisation
  • K+ K+ Na+ Na+ The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential. This is called repolarisation.
  • K+ K+ Na+ Na+ This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon.
  • K+ K+ Na+ Na+ The sodium/potassium pump restores the resting concentrations of sodium and potassium ions.
  • Action Potentials in an Unmyelinated Axon Action potentials do not decrease in amplitude as they are conducted
  • The Refractory Period of an axon is related to the period of time required so that a neurone can generate another action potential
  • The Refractory Period : prevents the action potential from moving in both directions on axon sets an upper limit on frequency of action potentials
  • The Refractory Period is a period during which a nerve or muscle is incapable of responding to stimulation, especially following a previous stimulation 1 Divided into: 2
  • ABSOLUTE RELATIVE refractory period: refractory period: There is a total inability to respond. There is a response to very strong stimuli
  • Duration of the Refractory Period 1 ms 5-10 ms
  • During the Absolute Refractory Period the axon membrane cannot respond to a depolarisation, even if the stimulus is increased. Reason: Na+ voltage gates are closed and inactivated
  • During the Relative Refractory Period a highintensity stimulus may produce a depolarisation. Reason: Some Na+ voltage gates are available K+ voltage gates remain open – i.e. membrane is returning to resting potential
  • The maximum frequency with which action potentials can be triggered and conducted is fixed by the absolute refractory period What is the maximum frequency if the absolute refractory period is 2 ms? 1 s = 1000 ms 1 action potential can occur per 2 ms ? action potentials can occur per 1000 ms 1000/2 = 500 500 in 1 s  then the action potentials must have a time interval of at least (1/500) s;  the maximum frequency is 500 s-1 This possible maximum frequency is never used in vivo (normally is at most: 200 s-1 )
  • Stimulus intensity is in the frequency of action potentials
  • Stronger Stimuli do NOT change Amplitude of Action Potential (Frequency increases)
  • Speed of Conduction of Nerve Impulses
  • Transmission Speeds: • range from 0.5 m s-1 to over 100 m s-1 • affected by: 1. Axon diameter 2. Myelin sheath 3. Temperature
  • Impulse Transmission can be speeded up by: 1. increasing the diameter of the axon - electrical resistance is inversely proportional to cross-sectional area but this imposes constraints on packing a large number of large axons into a nerve 1 mm in squid
  • 2. having a myelin sheath;
  • • since the intervening parts of the axon membrane do not have to be successively depolarised it takes • less time for the action potentials to pass from node to node • this results in nerve impulse transmission that is much faster, • the consequence of which is that smaller myelinated nerves can transmit impulses much faster than larger non-myelinated ones • which alleviates the ‘packing problem’.
  • 3. Temperature - affects the: 1. rate of diffusion 2. rate of energy release by respiration for active transport (since it is controlled by enzymes)
  • In which animal is nerve impulse transmission faster? In the lion (endothermic animals) which maintain a high body temperature.