Senescence

The Decline of Physiological Effectiveness

• Rate of cell division and number of cells reduce
  • All cells are capable to divide during embryological development
  • Cells lose ability to divide after birth or have a lower growth rate
  • Born with a fixed number of neurones → cannot divide/be replaced
• Decline in functional effectiveness of cells and organ systems
  • Deterioration in cells / slower responds to stimuli / slows homeostatic mechanism / increases change of dysfunction and death
• Ageing is controlled by genes but can be slowed down by
  • Regular (and adequate) sleep, (well balanced) meals, exercise
  • Refrain from smoking and alcohol
  • Keep body mass close to desirable mass for your height

Effect of Age on

• BMR
  • Number of cells decreases during ageing → lowers BMR
  • BMR decreases by ≈ 5% every 10 years above the age of 55
  • 10-20 years - rapid decrease associated with adolescent growth spurt
  • 20-35 - no change as body same size / same level of activity
  • 30-70 - slow decrease associated with loss of muscles / gain of fat / reduced activity
• CARDIAC OUTPUT = STROKE VOLUME  x  HEART RATE
  • Cardiac output decreases even though heart rate does not decline
  • Due to cardiac muscle fibres weaken (mainly left ventricle)
  • Decreases stroke volume of ventricles/volume of blood pumped per beat/cycle
• NERVE CONDUCTION VELOCITY
  • Cells in peripheral nervous system and brain get less
    • Neurones (nerve cells) are lost and cannot divide
    • Effect of cell loss depends on cells location
    • Brain loses ≈25% of cells that control muscular movement but hardly any that control speech → changes muscle coordination but not ability to speak
  • LOSS OF MYELIN: no saltatory conduction / impulses cannot jump from node to node / impulses must pass through greater amount of membrane
  • INCREASED WIDTH OF SYNAPSES: longer needed for diffusion/movement/greater distance to receptors/further to stimulate post-synaptic membrane/further diffusion distance of transmitter (across synapse)
  • SLOWER SYNAPTIC TRANSMISSION: presynaptic neurones produce less neurotransmitter
• Female reproductive capacity → MENOPAUSE (45-55 year old women)
  • Ovaries gradually become insensitive to FSH / secretion of oestrogen becomes less / ovulation becomes less / menstrual cycle becomes less / vagina walls become thinner / woman is infertile when oestrogen secretion stops
  • Levels of gonadotrophins (FSH, LH) rise to a peak after menopause
    • At menopause, oestrogen no longer secreted
    • FSH and LH no longer inhibited by negative feedback
  • SYMPTOMS: due to loss of oestrogen
    • Intense sweating / uncomfortable warmth / psychological problems
    • Increase risk of osteoporosis (loss of bone tissue) and heart diseases
  • TREATMENT: Hormone replacement therapy (HRT)
    • Postmenstrual woman take in small doses of oestrogen and progesterone
    • As tablets (orally) or apply implants beneath skin (skin patches)