Spinal Nerves and Branches

• There are 31 pairs of spinal nerves that project from the spinal column.
• These nerves have sensory input and motor output.

Spinal Nerve Distribution:

• 8 Cervical nerves (C1-C8)
• 12 Thoracic nerves (T1-T12)
• 5 Lumbar nerves (L1-L5)
• 5 Sacral nerves (S1-S5)
• 1 Coccygeal nerve (Co1)

Somatic Branches:

• Includes anterior and posterior branches.
• These carry somatomotor output (motor control of skeletal muscles) and somatosensory input (sensory information from the body).
• Anterior branch goes to/from the front part of the body and appendages.
• Posterior branch goes to/from the back part of the body.

Autonomic Branch:

• Carries visceromotor (motor control of visceral organs) and viscerosensory (sensory information from visceral organs).

Meningeal Branch:

• Goes back and forth from the meninges (membranes surrounding the brain and spinal cord).

Cranial Nerves:

• Responsible for afferent (sensory) visceral sensations.
• CN X (Vagus nerve) contains osmoreceptors, chemoreceptors, and senses GI activity.
• Responsible for efferent function, promoting vegetative functions via the parasympathetic nervous system

Cervical Spinal Nerves:

• Have only somatic function (somatosensory/somatomotor).
• They lack an autonomic spinal branch.

Thoracic and Lumbar Spinal Nerves:

• Autonomic branch: efferent sympathetic nervous system for "flight, fight, fright" responses.
• Afferent function: visceral pain.
• T1-T4 spinal nerves carry viscerosensory information from the heart, with somatosensory input from the arm/shoulder.
• T1-T4 spinal nerves often carry sensory input.
• Visceral pain is referred to the soma (body wall).
• Heart attack pain refers to the arm, shoulder, and chest wall.
• Childbirth labor pain is felt as intense pain in the lower back.
• Pain is referred to a specific dermatome (called referred pain).
• Tissue destruction does NOT cause visceral pain.
• Smooth muscle contractions or ischemia cause visceral pain.

Sacral Nerves:

• Efferent function: vegetative functions in the lower body (via the parasympathetic nervous system).
• Afferent function: sensations from a full bladder, full rectum, and sexual sensations.

Dual Innervation:

• Refers to the opposing functions of the sympathetic and parasympathetic nervous systems at target organs.

Sympathetic Nervous System:

• Generally speeds up activity during emergencies (flight, fight, or fright).
• Inhibits vegetative functions.

Parasympathetic Nervous System:

• Generally increases vegetative functions.
• Inhibits emergency responses.

Opposing Systems:

• Sympathetic and Parasympathetic systems have opposing actions at organs or within the entire system.

Autonomic Nervous System Subdivisions:

• Sympathetic and Parasympathetic.
• Sympathetic system exits the spinal cord from the thoracolumbar region, causing general body-wide effects.
• Parasympathetic system exits the spinal cord from the craniosacral region, causing discrete effects.
• Sympathetic system described as a generalist
• Parasympathetic as discrete

Parasympathetic Cranial Nerves:

• CN III: functions of the eye.
• CN VII: salivation/tear glands.
• CN X: autonomic motor function and viscerosensory info.
• CN IX: salivation/tear glands.
• T1 to L3 involves L5 spinal nerves going further

Ganglia Lengths:

• Preganglionic neurons are long, and postganglionic neurons are short in the parasympathetic nervous system.
• Opposite in the sympathetic except the adrenal medulla
• Paravertebral, prevertebral, and chain ganglia are associated with the sympathetic nervous system.

Cholinergic Synapses:

• Neurons secrete acetylcholine (ACh).
• Choline is taken up by the cell, and ACh is synthesized from choline and acetyl Co A.
• ACh is released in response to action potentials and broken down by acetylcholinesterase (AChE).
• ACh binds to receptors associated with CR channels.
• Nicotinic and Muscarinic receptors

Nicotinic Receptors:

• N1 and N2 subtypes which are associated with CR Na+ channels.

Muscarinic Receptors:

• M1, M2, M3, M4, and M5 subtypes.

Somatic Nervous System:

• Involves 1 motor neuron that is cholinergic.
• Skeletal muscle has CR Na+ channels, with nicotinic-N1 receptors that are excitatory.
• Involves 2 motor neurons that are both cholinergic.
• The preganglionic/postganglionic synapse has nicotinic-N2 receptors that are excitatory.
• The postganglionic/effector target e.g. smooth muscle, cardiac mucle, or gland has muscarinic receptors.

Muscarinic Subtypes:

• M1, M3, and M5 are excitatory.
• M2 and M4 are inhibitory.

Sympathetic Nervous System:

• Involves 2 motor neurons, with the preganglionic neuron being cholinergic.
• Preganglionic/postganglionic synapse; the receptors are nicotinic-N2 and excitatory.
• The postganglionic neuron is adrenergic and secretes norepinephrine (NE) or epinephrine (E).
• Postganglionic/effector targets are smooth muscle, cardiac muscle, or glands; they have alpha and beta receptors:
• Alpha 1, Alpha 2, Beta 1, Beta 2, Beta 3.
• Alpha/ Beta 1 and 3 are excitatory.
• Alpha/ Beta 2 are inhibitory.

Sympathetic Pattern Exception:

• Adrenal medulla instead of pre-ganglionic short and post-ganglionic it has preganglionic long and postganglionic absent = the adrenal medulla.
• The innervation to adrenal medulla is directly innervated, thus it is considered sympathetic
• 80% Epinephrine and 20% Nor-epinephrine is adrenergic (hormone)

Sympathetic Adrenergic Neurons:

• Usually have short preganglionic and long postganglionic neurons.
• Secrete norepinephrine (noradrenaline) and epinephrine (adrenaline).
• Adrenal glands inner layer (medulla which is nervous tissue).

Drug Effects:

• Adrenergic drugs cause 80% epinephrine, and 20% norepinephrine
• Released into the bloodstream, causing distant and system-wide effects.

NE/E Synthesis:

• L-tyrosine is converted to L-DOPA, then to dopamine, norepinephrine (NE), and epinephrine (E) through various enzymatic steps.
• The metabolites are taken into the neurons and vessels via Uptake 1.
• Monoamine oxidase (MAO) breaks down NE, E, DA, and 5-HT.
• Catecholamines need to be transported to vesicles to protect from MAO.
• Resperine blocks the uptake of NE into vesicles this decreases sympathetic activity
• MAO inhibitors are used for anti-depression as it blocks the break down of NE,E,DA and 5-HT leading to increase activity
• Uptake 2 is NE/E taken up by the effector/postsynaptic where it is broken down via Catechol-O-methyltransferase (COMT) .

Sympathetic Alpha Receptors:

• Alpha receptors used to be characterized by preferentially binding NE but can bind E; some say equal affinity.
• Alpha 1 are excitatory at the target.
• Alpha 2 are inhibitory at the target.

Sympathetic Beta Receptors:

• Beta receptors used to be characterized as preferentially binding E.
• B1, B3 Excitatory
• B2 Inhibitory

Location and Function of Alpha Receptors:

• Alpha 1 are predominantly located at smooth muscle cells associated with systemic arteries serving skin, mucosae, abdominal viscera, kidney and salivary glands; virtually all sympathetic target organs except the heart.
• Alpha 2 are associated to membranes of adrenergic axon terminals creating negative feedback to turn off NE secretion for inhibitation

Function Effects of Alpha locations:

• Alpha 1 constricts blood vessels via vasoconstriction and visceral organ sphincters, increasing blood pressure and inhibiting digestion.
• Alpha 2 inhibits smooth muscle walls of coronary artery , increasing blood FLOW.

Alpha receptor types:

• Gq receptor leads to vasoconstriction
• Gi receptor leads to decreased NE sympathetic neurons

Beta receptor types:

• Gs receptor leads to increased heart rate and contractility with decreased NE

Beta receptors Location and Function:

• Beta 1 mainly in the heart, functions to to increases heart rat and rate of contraction
• Beta 2 is generally inhibitory with lungs, functioning to bronchodilate
• Beta 3 is generally excitatory with fat cells functioning to Stimulates lipolysis

Beta receptor types:

• Yohimbine and prazosin as antagonists
• Epinephrine as an agonist