Autonomic Nervous System (ANS)

• The lecture covers the structure and function of the Autonomic Nervous System (ANS), receptor subtypes, autonomic responses and reflexes, and autonomic disorders.

Somatic Nervous System (SNS)

• The SNS monitors the external environment.
• Sensory input includes somatic senses (tactile, thermal, pain) and special senses (vision, taste, hearing).
• Motor output is voluntary and goes to skeletal muscles.
• The effect of the SNS is always excitation, which causes muscle contraction.

Autonomic Nervous System (ANS)

• The ANS monitors conditions of the internal environment.
• Sensory input comes from autonomic sensory neurons associated with interoceptors in blood vessels, visceral organs, and muscles (chemo- and mechano-receptors).
• Motor output goes to cardiac muscle, smooth muscle, and glands.
• The involuntary response can either increase (excite) or decrease (inhibit) activity.
• It utilizes two motor neurons in series.
• These motor neurons release acetylcholine (ACh) or norepinephrine (NE).
• The preganglionic neuron has its cell body in the brain or spinal cord.
• The postganglionic neuron is located outside the CNS in the PNS.
• Synapses form in autonomic ganglia.

Divisions of the ANS

• Divisions of the ANS are the Sympathetic and Parasympathetic.
• The Sympathetic division generally causes excitation, known as the "fight or flight" response, increasing heart rate, blood pressure, dilating pupils, and releasing glucose from the liver.
• The Parasympathetic division generally causes inhibition, known as the "rest and digest" response, which conserves energy and replenishes nutrient stores.

Sympathetic Division Structure

• Cell bodies of sympathetic neurons are located in the 12 thoracic and the first 3 lumbar segments of the spinal cord.
• Sympathetic ganglia consist of sympathetic trunk ganglia (above the diaphragm) and prevertebral ganglia (below the diaphragm).

Parasympathetic Division Structure

• Cell bodies of parasympathetic neurons are located in brainstem nuclei and S2-S4 of the spinal cord.
• Parasympathetic ganglia are located close to or within the wall of a visceral organ.
• Means that preganglionic neurons are longer than those in the sympathetic division.

Cholinergic and Adrenergic Neurons

• Autonomic neurons are classified as either cholinergic or adrenergic based on the neurotransmitter they produce.
• Cholinergic neurons release acetylcholine (ACh).
• Adrenergic neurons release norepinephrine (NE), also known as noradrenaline.
• Receptors for neurotransmitters are integral membrane proteins located in the postsynaptic neuron or effector cell membrane.

Cholinergic Neurons and Receptors

• Cholinergic neurons include all sympathetic and parasympathetic preganglionic neurons.
• Sympathetic postganglionic neurons innervate most sweat glands.
• All parasympathetic postganglionic neurons are cholinergic.
• Cholinergic receptors
• Nicotinic receptors found on dendrites and cell bodies of both sympathetic and parasympathetic postganglionic neurons, chromaffin cells of the adrenal medulla, and the motor end plate of the neuromuscular junction (SNS).
• Muscarinic receptors found on all effectors innervated by parasympathetic postganglionic axons (smooth/cardiac muscle/glands).
• Nicotinic receptors activation by ACh causes depolarization of the postsynaptic cell.
• Muscarinic receptor activation by ACh can cause either contraction of circular muscles of the iris, or relaxation of smooth muscle sphincters of the GI tract.
• ACh is quickly inactivated (hydrolysed) by the enzyme.

Adrenergic Neurons and Receptors

• Adrenergic neurons release norepinephrine (NE), also known as noradrenaline.
• Most sympathetic postganglionic neurons are adrenergic.
• Adrenergic receptors bind both noradrenaline and adrenaline.
• Noradrenaline is released as a neurotransmitter.
• Adrenaline is released as a hormone by chromaffin cells.
• Alpha (α) receptors: α₁ and α₂ exist.
• Beta (β) receptors: β₁, β₂, and β₃ exist.
• Activation of α₁ and β₁ receptors generally produces excitation.
• α₁: vasoconstriction of smooth muscle in blood vessels, increased sweating
• β₁: increased rate/force of contraction in cardiac muscle, secretion of antidiuretic hormone from the posterior pituitary
• Activation of α₂ and β₂ receptors generally causes inhibition.
• α₂: vasodilation of smooth muscle in blood vessels, decreased insulin secretion from beta cells of the pancreas
• β₂: dilation of airways by relaxing smooth muscle

Physiology of the ANS

• Most organs receive input from both divisions of the ANS.
• Autonomic tone is a balance between sympathetic and parasympathetic activity, regulated by the hypothalamus.
• Turns up sympathetic tone at the same time as it turns down parasympathetic tone, and vice versa.
• Postganglionic neurons release different neurotransmitters, and effector organs contain different receptor subtypes, resulting in different effects.
• A few structures, like sweat glands, arrector muscles of the hair, the kidneys, the spleen, and most blood vessels, receive only sympathetic innervation.
• An increase in sympathetic tone has one effect, and a decrease in sympathetic tone produces the opposite effect.

Sympathetic and Parasympathetic Responses

• During physical or emotional stress, the sympathetic division dominates, resulting in "E situations"—exercise, emergency, excitement, and embarrassment. This is called the fight-or-flight response.
• Sympathetic responses include increased production of ATP, dilation of the pupils, increased heart rate and blood pressure & dilation of the airways.
• Parasympathetic responses conserve and restore body energy during times of rest, called the rest-and-digest response.
• Parasympathetic responses include increased digestive and urinary function and decreased body functions that support physical activity
• SLUDD pneumonic: Salivation, Lacrimation, Urination, Digestion, and Defecation are Parasympathetic Responses

Autonomic Reflexes

• Autonomic reflexes are responses that occur when nerve impulses pass through an autonomic reflex arc, differing from somatic reflex arcs which contract skeletal muscle.
• The autonomic reflex arc contains sensory receptor, sensory neuron, integrating center (CNS) and effector.

ANS Control by the Hypothalamus

• The hypothalamus is the major integrating center in the ANS.
• The hypothalamus receives sensory input from visceral functions and output influences autonomic centers in,
• The brainstem (cardiovascular, salivation, swallowing, vomiting)
• The spinal cord (urination, defecation).
• The hypothalamus connects to the ANS by axons of neurons with dendrites and cell bodies in various hypothalamic nuclei.
• Axons form tracts from the hypothalamus to parasympathetic and sympathetic nuclei in the brainstem and spinal cord.
• The posterior and lateral regions of the hypothalamus control sympathetic functions (increase HR, BP, body temperature, dilate pupils, inhibit GI tract).

Autonomic Dysreflexia

• Autonomic dysreflexia emerges after spinal cord injury and is an uncoordinated sympathetic response that may result in a potentially life-threatening hypertensive episode.
• Stimulus from a urological source (UTI or a distended bladder) results in mass stimulation of the sympathetic nerves inferior to the level of injury in 85% of cases.
• It can occur in susceptible individuals up to 40 times per day.
• Symptoms include severe vasoconstriction, increased blood pressure, headache, bradycardia, facial flushing, pallor, cold skin, and sweating in the lower part of the body.
• Spinal cord lesions (T6 or above) - complain of a severe headache and will need blood pressure checked.
• Untreated autonomic dysreflexia causes sustained, severe hypertension, which can result in a myocardial ischemia or infarction, renal failure, or pulmonary issues.

Raynaud Phenomenon

• Digits (fingers and toes) become ischemic (lack blood) after exposure to cold or with emotional stress.
• It involves excessive sympathetic stimulation of smooth muscle in the arterioles of the digits along with a heightened response to stimuli that cause vasoconstriction.
• With rewarming after cold exposure, the arterioles may dilate, causing the fingers and toes to look red.
• Patients may have low blood pressure or increased numbers of alpha adrenergic receptors.
• This disorder is common in young women, and occurs more often in cold climates.
• Treatment includes avoiding exposure to cold, wearing warm clothing, and medications like nifedipine or prazosin, which relax smooth muscle by blocking Cav channels and alpha receptors.