Autonomic Nervous System controls:
- vegetative functions (heart rate, blood pressure, body temp, digestion etc.)
- autonomic reflexes = visceral reflex arc (unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors
Autonomic Reflexes (Baroreceptor reflex = blood pressure)
- Blood pressure detected by arterial stretch receptors
- Afferent neuron carries signal to CNS
- Efferent signals travel to the heart
- Heart then slows, reducing blood pressure
(Negative feedback loop!)
Autonomic vs Conscious
- Purely autonomic: blood pressure, kidney function, digestion
- Linked to consciousness: temperature, hunger, thirst, defecation, urination
Autonomic reflexes with conscious control (two points of control)
– Afferents go to both conscious and subconscious centres
– Efferents
• ANS controls automatic functions (secretion, smooth
muscle)
• Somatic NS controls behavioral override (skeletal muscle)
– Spinal cord reflexes
• Defecation and micturition reflexes are integrated in spinal cord
• We control these functions because of our control over skeletal muscle sphincters; if the spinal cord is damaged, the smooth muscle of bowel and bladder is controlled by autonomic reflexes built into the spinal cord
Autonomic reflexes with conscious control (spinal cord reflexes in urination and defecation)
- Internal sphincter
• Smooth muscle - Autonomic control - External sphincter
• Skeletal muscle - Somatic motorneuron control
Neural Control of Defacation
SYMPATHETIC stimulation causes contraction, PARASYMPATHETIC stimulation causes relaxation
Neural control of Urination
Micturition reflex
- Afferents sense stretch (300-400mL)
- Detrusor and internal urethral sphincter muscle is innervated by the sympathetic (storage) and parasympathetic (to wee!) nerve fibres from the spinal cord.
- External urethral sphincter muscle is under voluntary control, and as such is innervated by the somatic nervous system
Control of Autonomic Function
ANS regulated by several levels of CNS
– hypothalamus (major visceral motor control centre)
• nuclei for primitive functions – hunger, thirst, sex
– midbrain, pons, and medulla
• nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes
– cerebral cortex has an influence (anger, fear, anxiety)
• Powerful emotions influence the ANS because of the connections between our limbic system and the hypothalamus
ANS centres
- Respiratory centre (control of respiration)
- Cardiac centres (cardiac control)
• Cardioacceleratory
• Cardiac slowing - Blood vessels (control of blood pressure)
• vasomotor
Sympathetic vs Parasympathetic activities
- Sympathetic activity: “fight or flight”
- Parasympathetic activity: “rest and digest”
Fight or Flight response (FLIGHT)
Get more oxygen and fuel to your muscles
- Bronchodilation, increased HR & CO, vasodilation at muscles, breakdown of stored glucose (liver, muscle) & fat (adipose tissue) increases plasma [glucose] & [fats], blood vessel constriction (increased BP drives blood to where you need it)
- Surely that’s a compromise? Shut down renal/hepatic/gastrointestinal tract blood flow (and function), peripheral & visceral vasoconstriction
Fight or Flight response (FIGHT)
– See better
• Dilate pupils
– Increased strength & skill
• Increased muscle tone, increased mental activity
– Lose less blood
• Peripheral vasoconstriction, increased clotting response
Rest and Digest response
• Slow down Heart Rate
• Digest things
– Increased glandular (secretion) and muscular (peristalsis) activity in the gut, increased blood flow to the gut to pick up all the goodies.
• Do other things
– Increased sexual arousal, eye ciliary muscle constriction
(all the better to see nearby faces)
• Go to sleep!
– Pupillary constriction
Dual Innervation
• Organs innervated by both parasympathetic and sympathetic divisions
– Antagonistic effect: oppose each other
• Autonomic tone—normal background firing rate
balance between the two divisions
– Parasympathetic tone
• Maintains smooth muscle tone in intestines
• Holds resting heart rate down to about 70 to 80 beats per minute
– Sympathetic tone
• Keeps most blood vessels partially constricted and maintains blood pressure
• Antagonistic effects—opposeeachother
– MAY BE….Exerted through dual innervation of same effector cells
• Heart rate decreases (parasympathetic)
• Heart rate increases (sympathetic)
OR…
– Exerted because each division innervates different cells
• Pupillary dilator muscle (sympathetic) dilates pupil
• Constrictor pupillae (parasympathetic) constricts pupilExceptions
- synergistic effects: two divisions act on different effectors to produce a unified overall effect
• PSNS increase salivary serous cell secretion
• SNS increase salivary mucous cell secretion - not all organs receive dual innervation
• sweat glands receive only sympathetic neurons
• blood vessels (arterioles and veins) receive only sympathetic nerve fibres, except penis & clitoris
• sympathetic tone (vasomotor tone)
- increase in firing frequency = vasoconstriction
- decrease in firing frequency = vasodilation
Kidney & Liver Bloodflow Control
- SNS stimulation causes potent vasoconstriction
• reduced renal and hepatic blood flow
• diverts blood away from kidneys and liver
• no vasodilator fibres
