ANS & Regulation of Cardiorespiratory Function

• Organisation of the nervous system

  • Nervous system -> somatic & autonomic `
  • Somatic nervous system (voluntary; controls skeletal muscle)
  • Autonomic nervous system (involuntary; controls smooth muscle, cardiac muscle & glands)
• Homeostasis


  Somatic vs autonomic (viseral) nervous system (afferent to CNS, efferent away from CNS)
• Somatic Nervous System

  
  

  effector : skeletal muscle

  controls : somatic reflexes + voluntary movements under conscious control

  efferent (motor) nerves : alpha motor neurons

  
  

  
  

  
  

  Autonomic Nervous System

  
  

  effector : cardiac + smooth muscle + some glands

  controls : autonomic reflexes + involuntary activity in visceral structures (heart, gut, salivary glands)

  efferent (motor) nerves : sympathetic and parasympathetic motor neurons
• Reflex : fast, involuntary, stereotypical motor response to a stimulus

  • key to coordinated movements and responses (may be under voluntary control)
  • involuntary -> suggests only involvement in autonomic homeostatic mechanism
• • Reflexes are mediated by synapses in the central nervous system, with degrees of higher control including voluntary control.
  • Both stimulatory and inhibitory control are present, with inhibition being a normal state
  • Parkinson's disease -> inhibitory problem
• Effector structures in ANS

  • cardiac muscle, smooth muscle (e.g. lining of gut, blood vessels) and glands (e.g. salivary, sweat)
  • autonomic reflexes : control of blood pressure, blushing, salivation
• Differences between Sympathetic and Parasympathetic divisions of the Autonomic Nervous System

  • Anatomical organisation, physiological effects, pharmacological properties
• Control/influence of/on the ANS

  Hypothalamus & parts of limbic system
• Anatomical properties of the somatic nervous system

  • Voluntary
  • Controls skeletal muscle
• Anatomical properties of the central nervous system

  • Brain
  • Spinal cord
• Arms of ANS

  • Sympathetic division (thoraco-lumbar) -> fight or flight
  • Parasympathetic division (cranio-sacral) -> rest & digest
• Autonomic Nervous Control


  Control of the respiratory system
• Autonomic Nervous Control of Breathing


  Controlled variables and effector mechanisms
• Functional similarities between the sympathetic and parasympathetic motor pathways

  • many structures receive input from sympathetic nervous system and parasympathetic nervous system
  • both divisions of the ANS show tonic (background) activity -> neurone release nerve impulse at rest
  • tonic activity can be increased or decreased from its resting level
  • structures within the CNS (*brainstem), co-ordinate activity in both divisions of the ANS to regulate functions
  • both can turn things ‘on’ and ‘off’
• Physiological differences between the sympathetic and parasympathetic motor pathways

  • When a target structure receives dual innervation from both the sympathetic and parasympathetic systems, their actions are usually reciprocal
  • Sympathetic activation of a target structure is usually consistent with a diffuse and widespread ‘fight or flight’ response
  • Parasympathetic activation is consistent with a ‘vegetative’ or ‘resting’ state
• Examples of autonomic function where arms have ‘opposing’ effects

  • pacemaker of heart -> SNS : increase heart rate PNS : reduce heart rate
  • cardiac ventricular muscle -> SNS : increase contraction force , PNS : reduce force of contraction
  • gastric motility -> SNS : reduces motility , PNS : increases motility
  • salivary glands -> SNS : thick, less secretion (dry mouth -> stressed + frightened), PNS : watery, profuse secretion
• Autonomic synaptic transmitters and post-synaptic receptor types


  • Different tissues have different receptors and effects from these receptors – drug targets
• How can one autonomic arm produce such diverse effects across tissues and organs?

  • same transmitter may interact with diff receptor types at diff synapses -> may evoke diff effects in diff cells + diff receptors signal via diff G-proteins and 2nd messenger proteins to give tissue specific effects
• Example- in sympathetic arm: Noradrenaline as a transmitter constricts many blood vessels via alpha (α) adrenergic receptors and speeds up heart rate via beta (β) adrenergic receptors - at the same time
• One arm of the autonomic system can produce diverse effects in the same tissue type=

  ?

  ?
• Sympathetic arm effects


  • Noradrenaline constricts many blood vessels via alpha (α) adrenergic receptors
  • Noradrenaline speeds up heart rate via beta (β) adrenergic receptors
• The ‘fight or flight’ response and its effect on small blood vessels (arterioles and sometimes venules)
• Increased sympathetic tone (drive) leads to

  1. Relaxation (or weak constriction) via β receptors in heart, muscle, lungs, brain

  2. Strong constriction via α receptors in skin, abdominal viscera, kidneys
• Adrenaline

  • released by sympathetic activation
  • effective for cardiac arrest, anaphylactic shock, and asthma emergency
• Adrenaline effects

  • acts as a powerful vasoconstrictor for bleeding and in local anaesthetic use, prolonging time in tissue -> acts on alpha receptors in 'peripheral' blood vessels (dentistry)
  • stressed animal -> adrenaline lvl already high
  • side effects -> anxiety, fast heart rate, high bp
• Autonomic Nervous Control of Blood Pressure

  controlled variables (bp) and effector mechanisms (heart rate)
• Autonomic control of blood pressure involves a homeostatic negative feedback system.

  fall in bp causes effectors to respond to increase bp (vice versa)
• Innate (unconditioned) vs acquired (conditioned) reflexes

  1. Innate reflexes

  • present from birth
  • stable, hard-wired neural circuitry=
  • predictable + reproducible +
  • e.g muscle stretch reflex

  2. Acquired reflexes

  • learned
  • require development of new functional connections between neurones -> strength of connection increased by reinforcement + response can be lost
  • unpredictable + varies
  • e.g flinching in response to neutral stimulus
• Key concepts to remember=

  • normal body function -> neither system operates in isolation
  • sympathetic and parasympathetic systems are active at all times and may be activated separately
  • effects of sympathetic activation -> more diffuse and widespread, involves neurocrine and endocrine pathways
  • The autonomic nervous system underlies key reflexes in homeostasis
• Somatic nervous system

  • CNS : brain & spinal cord
  • Peripheral nervous system : sensory nerves , sensory receptors and motor nerves)
• Autonomic control of heart

  • vagus nerve delivers parasympathetic signal to SAN and AVN
• Will heart stop beating if autonomic nervous input is removed?


  it will still beat as it has its own intrinsic cardiac nervous system