ANS final chapter review

The sympathetic nervous system (SNS) consists of a series of connected ganglia and neurons, leading to various physiological effects.

Chain ganglia are clusters of neurons located outside the central nervous system.
They serve as sites for synapse formation with postganglionic neurons.
Preganglionic neurons send axons to these ganglia where they synapse onto postganglionic neurons.
Postganglionic neurons extend axons to target organs to produce specific effects.

Preganglionic neurons originate in the CNS and have short axons.
Postganglionic fibers are longer and innervate target organs.
Examples:
- Sympathetic Chain Ganglia: Affect organs in the thoracic cavity, head, abdominal wall, and legs.
- Collateral Ganglia: Located in the abdominal cavity and involve different pathways.

The adrenal gland consists of a cortex and a medulla.
The medulla releases hormones like epinephrine into the systemic bloodstream, affecting various organs depending on receptor types.
Physiological effects include increased heart rate and blood flow diverted from digestion (e.g., increased circulation to the lungs and heart).

The parasympathetic nervous system (PNS) is crucial for 'rest and digest' functions.

Preganglionic neurons in the PNS are long and located in the brainstem or sacral spinal cord, sending axons to nearby ganglia.
Specific cranial nerves involved include cranial nerves III, VII, IX, and X, along with pelvic nerves.
Postganglionic neurons are typically located close to effectors, resulting in shorter axons.
Example: Cranial nerve III affects the intrinsic muscles of the eye, controlling pupil dilation and constriction, as well as focusing through ciliary muscles.

Functions include increasing digestion and controlling bodily secretions.
Vagus nerve (cranial nerve X) is a key player in controlling abdominal functions.
The PNS primarily uses acetylcholine as a neurotransmitter for both preganglionic and postganglionic neurons.

Both systems have opposing effects, with sympathetic activation preparing the body for 'fight or flight' responses and parasympathetic activation promoting 'rest and digest'.
Functional effects of the PNS are often localized and short in duration compared to the widespread effects of the SNS.

Preganglionic neurons release acetylcholine (ACh).
Most postganglionic neurons release norepinephrine (noradrenaline) but may release acetylcholine in certain cases (e.g. sweat glands).

Both preganglionic and postganglionic neurons use acetylcholine as the neurotransmitter.

Nicotinic Acetylcholine Receptors:
- Ligand-gated, found in skeletal muscle, prevalent in sympathetic and some parasympathetic synapses.
- Activates upon binding with acetylcholine or nicotine, allowing ion flow (mainly sodium).
Muscarinic Acetylcholine Receptors:
- Metabotropic (G-protein coupled) receptors responding to acetylcholine.
- Involved in various autonomic functions, both stimulatory and inhibitory.

Adrenergic receptors have two main classes—alpha and beta—with further subtypes (e.g., alpha-1, alpha-2, beta-1, etc.).
Effects vary based on the receptor type activated and the location within the body.

Activation of the sympathetic branch increases alertness, heart rate, blood pressure, and breathing rate.
It mobilizes energy reserves and increases muscle tone.
Conversely, parasympathetic activation decreases heart rate and increases digestive functions.

Autonomic responses can be reflexive, either short reflexes involving local ganglia or long reflexes involving the CNS.
The body collects information from both internal and external sources via receptors:
- Exteroceptors: External stimuli (e.g., touch, pain, temperature).
- Interoceptors: Internal stimuli (e.g., blood pressure, chemical balance, visceral stretch).

Baroreceptors: Detect changes in blood pressure.
Chemoreceptors: Monitor levels of CO2, O2, and pH, influencing breathing and cardiovascular responses as needed.

Reflexes help regulate both sympathetic and parasympathetic responses, often in response to physiological changes (e.g., increased food intake).
Appropriate responses involve activating or deactivating pathways via specific neural circuits.
Understanding these pathways is crucial for managing physiological responses effectively (e.g., heart rate regulation through changing autonomic influences).

A grasp of the autonomic nervous systems’ complexities is essential for comprehending physiological responses under varying conditions. By understanding the hierarchies of neuronal connections and their effects, one can appreciate the nuanced functionality of the body's involuntary control mechanisms.