MMED2931 wk1 and wk2 l4 p2

Phasic Activation of Sympathetic Nervous System

• During the fight or flight response:
    - Precondympanic neurons are activated.
    - They selectively innervate important glands; specifically, the adrenal medulla.
    - The adrenal medulla is located at the center of the adrenal gland, which is situated atop the kidneys.
    - Cells in the adrenal medulla resemble sympathetic ganglion cells but lack postganglionic axons.
    - They contain nicotinic receptors similar to those found on ganglion cells.
• Activation of sympathetic nerves leads to the release of acetylcholine that acts on:
    - Nicotinic receptors on the chromaffin cells of the adrenal medulla.
    - Chromaffin cells release epinephrine (adrenaline) into the bloodstream as neurohormones, affecting various body cells.

Parasympathetic Nervous System Overview

• Parasympathetic Preganglionic Neurons:
    - Origin in two main regions:
      - Brainstem
      - Sacral region of the spinal cord
    - Preganglionic axons activate visceral organs via cranial nerves.
• Cranial Nerves Involved:
    - Total of 12 pairs of cranial nerves, mostly mixed nerves (both sensory and motor).
    - Specific nerves include:
      - Cranial Nerve III: Oculomotor nerve
      - Cranial Nerve VII: Facial nerve
      - Cranial Nerve IX: Glossopharyngeal nerve
      - Cranial Nerve X: Vagus nerve (longest cranial nerve, innervates thoracic and abdominal organs).
• Anatomical Location of Parasympathetic Preganglionic Neurons:
    - Found between dorsal and ventral horns; located in the intermediate gray zone.
• Parasympathetic Ganglia:
    - Small, often intramural (within organ).
    - Contrast with sympathetic ganglia, which are typically close to the spinal cord.

Functions of the Parasympathetic Nervous System

• Tonic Activity Examples:
    - Decreased cardiac output:
      - Achieved by acetylcholine from preganglionic neurons activating M2 muscarinic receptors, resulting in decreased heart rate.
    - Pupil diameter regulation:
      - Preganglionic neurons in cranial nerve III synapse at ciliary ganglia near the eye.
      - Postganglionic fibers activate circular muscle in the iris, leading to pupil constriction via acetylcholine.
    - Tear production for ocular lubrication is routinely driven by the parasympathetic system.
• Phasic Activity Examples:
    - Gut motility increases upon eating:
      - Involves acetylcholine acting on nicotinic receptors in ganglia of the enteric nervous system.
    - Urination:
      - Full bladder activates parasympathetic nerves to release acetylcholine, resulting in contraction of the bladder smooth muscle through muscarinic receptors, facilitating urination.

Anatomical and Functional Differences: Sympathetic vs. Parasympathetic

• Anatomical Origin:
    - Sympathetic: Preganglionic neurons mainly in thoracic and upper lumbar spinal cord.
    - Parasympathetic: Preganglionic neurons in brainstem and sacral spinal cord.
• Peripheral Autonomic Ganglia Location:
    - Sympathetic ganglia: Close to spinal cord (sympathetic chain and prevertebral ganglia).
    - Parasympathetic ganglia: Near or within target organs (intramural ganglia).
• Neurotransmitter Substances:
    - Sympathetic: Majority use noradrenaline (norepinephrine).
    - Parasympathetic: Primarily use acetylcholine acting on muscarinic receptors.
• Dual Innervation of Organs:
    - Many organs receive both sympathetic and parasympathetic innervation with opposing effects:
      - Heart:
        - Sympathetic increases heart rate.
        - Parasympathetic decreases heart rate.
      - Gut:
        - Sympathetic inhibits motility.
        - Parasympathetic activates motility.

Enteric Nervous System Overview

• Definition:
    - The enteric nervous system consists of neurons within the gastrointestinal tract, controlling motility from the esophagus to the rectum.
    - Input received from both sympathetic and parasympathetic systems.
• Neuron Count:
    - Enteric nervous system contains over 500 million neurons, surpassing the combined number of sympathetic and parasympathetic neurons (approximately 100,000).
• Sensory Neurons in Enteric Nervous System:
    - Named intrinsic efferent neurons, operate with local circuitry without sending information to the higher brain regions, hence left unnoticed unless discomfort occurs.
• Structure and Function:
    - Enteric system includes interneurons and two types of motor neurons:
      - Excitatory motor neurons (contract smooth muscle).
      - Inhibitory motor neurons (relax smooth muscle).
• Plexus Composition:
    - Subdivided into two main plexuses:
      - Myenteric plexus: Located between longitudinal and circular muscle layers, controls gut motility.
      - Submucous plexus: Located between gut lining and circular muscle, regulates secretion and blood flow.

Enteric Reflexes

• Reflex Example - "Lof of Intestine":
    - Discovered in the 19th century; involves cutting a segment of gut and observing contraction and relaxation responses to balloon distension.
    - Oral contraction called standing excitatory reflex; anal relaxation called distending inhibitory reflex.
• Propulsive/Peristaltic Movements:
    - Represent motor patterns achieved via enteric neural circuits independent of central nervous system guidance.
    - Video documentation captures peristaltic movement within isolated gut segments, moving from oral to anal ends without CNS involvement.