3 - Autonomic nervous system

• Afferent vs Efferent neurons:

  Afferent neurons: carry sensory information from the body to the CNS

  Efferent neurons: carry motor neurons from the CNS ti muscles or glands
• Divisions of the nervous system:

  
• The autonomic nervous system:

  • control motor neurons that regulate smooth and cardiac muscles, internal organs and skin
  • It adjusts body functions for optimal activity and operates subconsciously

  Homeostasis is a dynamic balance between the autonomic branches:

  • Parasympathetic activity dominates rest and digest
  • Sympathetic activity dominates fight or flight
• Efferent pathways in SNS vs ANS:

  SNS: somatic motor neurons have heavily myelinated axons that extend from the CNS to the effector

  ANS: Has a 2 neuron chain:

  • Preganglionic (first) neuron has a lightly myelinated axon
  • Ganglionic (second) neuron extends to the effector organ

  
• Neurotransmitters in SNS vs ANS:

  SNS: all motor neurons release ACh (Excitatory)

  ANS:

  • Preganglionic fibres release ACh
  • Postganglionic fibres release ACh or noradrenaline
  • Effect can be stimulatory or inhibitory depending on neurotransmitter and receptor type
• ANS division interaction:


  Most internal organs receive both sympathetic and parasympathetic input allowing opposing control E.G.

  • Sympathetic fibres: raises heart rate, slows digestion
  • Parasympathetic fibres: lowers heart rate, promotes digestion
• Parasympathetic division:

  • conserves energy “rest and digest”
  • Focuses on digestion, defacation and urination
  • Active during relaxation E.G. after eating:
  • low heart rate, blood pressure and respiration
  • High digestive activity
  • Constricts pupils

  Mainly effects internal organs, not the skin

  Neurotransmitter = acetylcholine at both ganglionic and effector organs
• Sympathetic division:


  • controls high energy “fight or flight” responses
  • Promotes adjustments during exercise: blood flow is reduced to organs and increased to muscles
  • Activity shown by someone threatened: increased heart rate, deep breathing, skin cold and sweaty, pupils dilate
  • Affects entire body
  • Neurotransmitters = Ach (ganglionic) and noradrenaline (effector organ)
• Sympathetic division : Adrenal gland

  • adrenal gland sits on top of kidneys
  • Major part of SNS
  • Activation causes systemic release of adrenaline
• Peripheral receptors at target tissue:

  1. Cholinergic receptors- mediate responses to acetylcholine - muscarinic
  2. Adrenergic receptors- mediate responses to adrenaline and noradrenaline - alpha and beta adrenergic