State 6 actions of the sympathetic nervous system
Increases force of contraction
Constricts most blood vessels
Slows GI transit
Dilates pupil (mydriasis)
State 6 actions of the parasympathetic nervous system
Slows HR (no effect on force)
Relaxes blood vessels
Speeds up GI transit
Constricts pupil (miosis)
Describe the sympathetic supply to the head
Preganglionic fibres originate in hypothalamus
They descend to T1 & synapse with preganglionic neurons in the lateral horn and exit the spinal cord with T1 spinal nerve
Travel to sympathetic chain
Run up chain to cervical ganglia & synapse with postganglionic fibres
Enter head as plexus around internal carotid artery
Supply face via external carotid artery
Name the three sympathetic ganglia in the neck
Superior cervical: near the base of the skill (C1-C4)
Middle cervical (C5-C6)
Stellate: near apex of the lung (C7-T1)
Describe the structural organisation of the sympathetic nervous system
The cell bodies of sympathetic nerves lie in the lateral grey horn of the spinal cord from T1-L2.
Pre-ganglionic neurons have axons that travel through the ventral root (motor efferent) of their segmental spinal nerve and enter the sympathetic chain.
One chain is located on each side of the vertebral column (paravertebral) are are positioned in front of the ventral rami. The sympathetic chain extends from the base of the skull to coccyx where the two chains converge.
The sympathetic chain is made up of ganglia which is where preganglionic neurones synapse with postganglionic neurones.
Postganglionic fibres then leave the ganglia and are redistributed along the rami of spinal nerves via grey rami communicantes
How would the sympathetic nervous system be affected by damage to the spinal cord at T1?
What would the effect be?
T1 is the first branch of sympathetic neurons to enter the chain fron the spinal cord.
Fibres from T1-T4 enter the sympathetic chain and extend into the head and neck with
Damage to the spinal cord at T1 could remove or reduce sympathetic supply to the head and neck.
Could result in Horner's syndrome
How would injury at L1 vertebrae affect the autonomic nervous system?
The L1 vertebral level contains the roots of the sacral spinal nerves S1-S5. (L1-L5 spinal cord levels are at T11-T12).
Therefore damage to the L1 vertebrae will mainly affect the parasympathetic nervous system which has outflow from S2-S4. The sympathetic nervous system will not be affected as much or at all because outflow terminates at L1.
Describe the features of Horners syndrome
Loss of sympathetic supply to the head and neck
Pupil constriction (miosis)
Ptosis (drooping eyelids)
Lack of sweating (anhydrosis)
Name 3 common causes of Horner's syndrome
Pancoast tumour (apex of the lung)
Tumour at the base of the skull
Describe the structural organisation of the parasympathetic system
There are two clusters of preganglionic neurons at each end of the spinal cord - 4 cranial nerves (CNIII, CNVII, CNIX and CNX) and three spinal nerves (S2-S4).
Parasympathetic nuclei of the cranial nerves sit in the brainstem. Postganglionic fibres from the cranial nerves travel with branches of CN V (trigeminal nerve) except for the vagus nerve.
Sacral parasympathetic outflow comes from preganglionic neurons with cell bodies in S2-S4 of the spinal cord. Axons leave the spinal cord through the ventral root and leave the spinal nerves as separate pelvic nerves.
Name the four parasympathtic outflows from the crainal nerves
CN III - oculomotor nerve
CN VII - facial nerve
CN IX - glossopharyngeal
CN X - vagus nerve
What are the actions of the ANS on the heart?
Heart rate (chronotropic)
Force of heart contraction and stroke work (inotropic)
Speed of conduction through the AV node (dromotropic)
What are the actions of the ANS on blood vessels?
Contraction of smooth muscle in vessel wall
Dilation of coronary arteries
Dilation of arterioles – alter peripheral vascular resistance
Describe the autonomic innervation of the heart
Branches from both the parasympathetic and sympathetic systems contribute to the formation of the cardiac plexus.
This lies posterior to the aortic arch, to the bifurcation of the trachea.
Branches of the cardiac plexus affect nodal tissue which control the activity of the heart.
Vagus nerve: SA node and AV node
Sympathetic: SA node, AV node, Ventricular muscle
What is the prinipal transmitter released on the heart in response to sympathetic innervation
Sympathetic: Preganglionic - ACh, postganglionic - NA
What is the prinicpal transmitter released on the heart in response to parasympathetic innervation?
Parasympathetic: pre and postganglionic synapses release ACh
Preganglionic synapse has nicotinic receptor, postganglionic has muscarinic receptor
What receptor subtypes bring about sympathetic and parasympathetic effects in the heart?
Sympathetic stimulation releases NA. Cardiac muscle has b1-adrenergic receptors. Increases cAMP to increase heart rate and force of contraction
Parasympathetic stimulation releases ACh which binds to M2 receptors on cardiac muscle. This decreases heart rate and force of contraction
What are the effects of NA on the heart?
NA mediates its effects via b1-adrenergic receptor
Increases rate of SA node
Reduces delay at AV node
Increases automacity in the ventricles
Increases contractility in the ventricles and atria
What are the effects of ACh on the heart?
ACh acts via the M2 receptor
Decreases rate of the SA node
Increases delay at the AV node
Decreases atrial contraction
No innervation to the ventricle, therefore no effect
Describe the parasympathetic outflow of the cranial nerves
CN III: Preganglionic fibre leaves the brainstem and synapses with the ciliary ganglion. Postsynaptic fibres travel with CN V1 (opthalmic division) to the eyes. Brings about pupil constriction and accomodation
CN VII: Preganglionic fibres send branches to the pterygopalatine ganglion and submandibular ganglion. From the pterygopalatine ganglion, branches of CNVII travel with CN V2 to the mucus membranes above the maxilla. From the submandibular gangion, fibres travel with CN V3 to the mucus membranes and glands of the mandible.
CN IX: Preganglionic fibres travel to the otic ganglion via the lesser petrosal nerve. Postganglionic fibres travel to the parotid gland with CN V3
CN X: the vagus nerve leaves the brainstem and travels to specific organs, contributes to thoracic plexuse and prevertebral plexus in the abdomen and pelvis
Describe the parasympathetic outflow of the sacral nerves
Parasympathetic preganglionic fibres form pelvic splanchnic neves (visceral nerves) which originate from the anterior rami of S2-S4 and enter the prevertebral plexus at the abdominal aorta.
Fibres are then redistributed to pelvic and abdominal viscera along the blood vessels.
In the GI tract, preganglionic fibres synapse on neurons in the ganglia of the enteric nervous system.
Describe two effects of disrupting parasympathetic innervation
Lesion in the oculomotor nerve causes loss of the pupillary light reflex, unopposed sympathetic action means the pupil remains dilated
Damage to parasympathetic components in the cauda equina causes loss of bladder control and sphincter function
Frey's syndrome: damage to the parotid gland or surgery may damage postganglionic fibres, which then regrow and synapse to the M3 receptors on the sweat glands of the skin. Produces sweating in response to meals
Describe the outflow of the sympathetic nervous system
From the lateral horn of the spinal cord:
Preganglionic fibres synapse with ganglia in the sympathetic chain, postganglionic fibres then leave to innervate thoracic or cervical viscera.
Preganglionic fibres may ascend or descend in the sympathetic chain before synapsing in ganglia. Postganglionic fibres combine with fibres from other levels to form visceral nerves (may join with parasympathetic fibres to form a plexus near the target organ).
Preganglionic fibres may pass through the sympathetic chain and ganglia without synapsing and join with other fibres to form splanchnic nerves which connect with sympathetic ganglia at the major branches of the abdominal aorta (coeliac, SMA, IMA)
Describe the innervation of the adrenal gland
Preganglionic sympathetic fibres from the thoracic spinal cord to the adrenal glands via the pelvic splanchnic nerves and synapse directly with chromaffin cells of the adrenal medulla.
This results in secretion of catecholamines (NA, adrenaline)
Describe the effects of B1-receptor activation on the heart
Stimulates adenylate cyclase
Increases the rate of the SA node
Reduces delay at the AV node
Increases contractility and automaticity of the heart
Increases contractility of the atria
What are the ionotrophic effects of NA?
Increases contractility of the atria and ventricle
What are the ionotropic effects of Ach on the heart
Decreases contractility of the atria
What effect does activation of M2 receptors have on the heart
Inhibits adenylate cyclase
Causes decrease rate at SA node
Increased delay at AV node
Decreases contractility of the atria
What is the action of a1 adrenergic receptors?
Smooth muscle contraction of blood vessels
Relaxation of GI smooth muscle by hyperpolarisation
Contraction of sphincter muscles
What is the action of a2 adrenergic receptors?
Limits transmitter release at the synapse
Reduces salivation and insulin release
What is the action of b1 adrenergic receptors?
Increases heart rate, increases the speed of AV node conduction and increases the force of contractions
Relaxes GI smooth muscle
What is the action of b2 adrenergic receptors?
What is the action of b3 receptors?
Lipolysis in adipose tissue
What is the action of M3 receptors on the circulatory system?
Present on vascular endothelial cells in the circulatory system.
Couples to PLC
Causes smooth muscle relaxation by promoting Ca2+ influx and NO release from the endothelial cells. NO causes a decrease in Ca2+ in vascular myocytes