intro to autonomic nervous system

• Neuroeffector junction

  The junction of a post-ganglionic axonal terminal with its effector cell
• Nerves transmit impulses across synapses and neuroeffector junctions using specific chemicals called neurohumoural transmitters or neurotransmitters
• The Autonomic Nervous System includes the Sympathetic Nervous System (Adrenergic Nervous System) and the Parasympathetic Nervous System (Cholinergic Nervous System)
• Efferent (Motor)

  Nerves that convey impulses from the brain and spinal cord (CNS) to muscles, glands and other effector organs
• The Autonomic Nervous System modulates the involuntary activity of secretory glands, smooth muscles, and visceral organs
• The Autonomic Nervous System sustains homeostatic conditions during periods of reduced physical and emotional activity and assists in internal bodily reactions to stressful circumstances
• Differences between Autonomic and Somatic Nervous System

  • Efferent nerves of A.N.S. supply all innervated structures of the body except skeletal muscles
  • The most distal synaptic junction in A.N.S. occurs in ganglia that are entirely outside the cerebrospinal axis
  • Many autonomic fibres form extensive peripheral plexuses
  • When cut, organ supplied generally show some level of spontaneous activity independent of intact innervations
  • Pre-ganglionic and post-ganglionic fibres are present
  • Post-ganglionic autonomic nerve fibres are generally non-myelinated (slow conduction of impulse), whereas pre-ganglionic fibres are myelinated (fast conduction of nerve impulse)
  • Somatic nerves supply skeletal muscles
  • Somatic nerves contain no peripheral ganglia, and synapses are located entirely within the cerebrospinal axis
  • No peripheral plexus is present in somatic nerve fibres
  • When cut, skeletal muscles supplied become paralyzed and undergo atrophy
  • Most fibres of skeletal muscles are myelinated
• Ganglion

  An aggregation of synapses
• Nerve plexus

  A network of nerve fibres
• Figure: 'The Autonomic Nervous System: Schematic representation of the autonomic nerves and effector organs based on chemical mediation of nerve impulses. Yellow, cholinergic; red, adrenergic; dotted blue, visceral afferent; solid lines, preganglionic; broken lines, postganglionic. The rectangle at right shows the finer details of the ramifications of adrenergic fibers at any one segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerve'
• Afferent (Sensory)

  Nerves that convey flow of impulse from peripheral to CNS
• Autonomic drugs exert their actions on smooth muscles, cardiac muscles, glands, and visceral organs by mimicking or modifying the action of neurotransmitters released by autonomic fibres
• Effector tissues

  • Heart
  • Sinoatrial (SA) node
  • Atria
  • Atrioventricular (AV) node
  • His-Purkinje system
  • Ventricles
• Table: Typical responses of effector tissues to sympathetic and parasympathetic nerve impulses
• It is not known whether the vasodilator fibers in the dorsal roots of the spinal nerves are motor or sensory or where their cell bodies are situated
• Parasympathetic Nervous System characteristics

  • Arises as a craniosacral outflow with specific cranial and sacral nerves
  • Ganglia are away from the C.N.S. and on or close to the organs
  • Distribution is much more limited
  • Neurotransmitter is acetylcholine in both ganglia and at neuroeffector junctions
  • Function involves modulating body functions towards inactivity and repair of energy deficits
  • Activity predominates during rest
  • Endowed with the medullary functions
  • If taken out, the function is usually normal but the animal will not survive long
• Parasympathetic-mediated responses

  • General inhibition
  • Decrease heart rate
  • Decrease contractile force
  • Decrease conduction velocity; AV block
  • Decrease contract
• Details of the ramifications of adrenergic fibers at any one segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves are shown in the rectangle
• Differences between the Sympathetic and Parasympathetic Nervous System:
• Sympathetic Nervous System characteristics

  • Arises as thoraco-lumbar outflow (T1 to L3)
  • Ganglia are nearer to the C.N.S.
  • Distributed to effector organs throughout the body
  • Neurotransmitters are acetylcholine (in ganglia) and norepinephrine (at neuroeffector junctions)
  • Function involves preparing the body for activity and energy expenditure
  • Activity increases in stress and emergency
  • Responsible for providing continuous stimulus to the organs and parts supplied
  • If nerve is cut, the animal will survive with some physiological change
• Sympathetic-mediated responses


  • General excitation
  • β1 – increase heart rate
  • β1 – increase contractile force, conduction velocity
  • β1 – increase automaticity, conduction velocity
  • β1 – increase automaticity, conduction velocity
  • β1 – increase contractile force, conduction velocity, irritability
• Effects of α1 stimulation on urinary bladder

  • Contraction
• Effects of β2 stimulation

  • Dilation
• Effects of α1 stimulation on piloerector muscles

  • Contraction
• Effects of α2 stimulation on adrenergic nerve terminals

  • Decrease release of norepinephrine
• Effects of β1 stimulation on urinary bladder

  • Relaxation
• Effects of β1 stimulation

  • Increase automaticity
  • Increase conduction velocity
  • Increase contractile force
• Effects of GI tract stimulation

  • Smooth muscle contraction
  • Sphincters contraction
  • Secretions
  • Gall bladder & ducts contraction
  • General inhibition
• Effects of General inhibition

  • Decrease heart rate
  • Decrease contractile force
  • Decrease conduction velocity
  • AV block
• Effects of α1 stimulation on salivary glands

  • Scant, viscous secretion
• Effects of α1 stimulation on uterus

  • Contraction
• Effects of β2 stimulation on adrenergic nerve terminals

  • Increase release of norepinephrine
• Effects of β stimulation on genitalia

  • Erection
• Effects of α2 stimulation

  • Dilation
• Effects of α2 stimulation on platelets

  • Aggregation
• Effects of β2 stimulation on bronchioles

  • Relaxation
• Effects of α1 stimulation

  • Constriction
• Effects of β2 stimulation on sweat glands

  • Secretion
• Effects of β2 stimulation on fat cells

  • Lipolysis
• Uterine responses vary depending on species and stage of estrous, pregnancy and menstrual cycle (when present)