PNS and Autonomic Nervous System

Question 1

Describe the peripheral nervous system:

Answer 1

The PNS consists of all nerves and ganglia outside the brain and spinal cord, connecting the central nervous system (CNS) to the rest of the body

Functions:
- sensory input from body to CNS
- motor input from CNS to muscle and glands
- controls involuntary physiological processes via ANS

Afferent (sensory) division:

• Transmits signals from sensory receptors to the CNS
  Types of sensory receptors:

Mechanoreceptors: Detect pressure and touch.
Thermoreceptors: Sense temperature changes.
Nociceptors: Respond to pain.
Proprioceptors: Provide feedback on body position and movement.

Efferent (motor) division:

Transmits commands from the CNS to muscles and glands.

Divided into:
Somatic Motor System: Controls voluntary muscle movements.
Autonomic Motor System: Regulates involuntary actions.

Divisions:

• Somatic nervous system controlling voluntary skeletal muscle movements
• ANS controlling autonomic functions

Question 2

Describe different receptors of the peripheral nervous system

Answer 2

Detect information from environment
Act as transducers, transforming physical or chemical stimuli into electrical impulses

Classification by source of stimulus:

Exteroreceptors - stimuli outside the body, respond to pain, temperature,
Proprioreceptors - relay information on body position and movement from muscles and tendons
Enteroreceptors - monitor events inside the body such as movement through the gastrointestinal system or blood pH

Classification by the mode of detection:

• Chemoreceptors – detect molecules that bind to the receptor
• Photoreceptors – detect light in the retina
• Thermoreceptors – detect temperature
• Mechanoreceptors – are stimulated by the mechanical opening of ion channels
• Nociceptors – detect signals that are associated with tissue damage

Based on Structure:

• Free Nerve Endings: Simple, unencapsulated receptors (e.g., pain, temperature).
• Encapsulated Nerve Endings: Complex structures surrounded by connective tissue (e.g., Meissner’s corpuscles for touch, Pacinian corpuscles for pressure).

Question 3

What is the autonomic nervous system ?

Answer 3

ANS is a subdivision of the PNS that controls involuntary physiological functions

Comprises both sensory and motor components, divided into sympathetic and parasympathetic divisions

Enteric nervous system, a third division, can act independently but is heavily influenced by these divisions

nfluences visceral organs, smooth muscle, cardiac muscle, glands

Question 4

Describe the parasympathetic nervous system of the ANS:

Answer 4

Promotes the “rest and digest” response

Functions:
- targets thoracic, abdominal and pelvic viscera
- decreases cardiac output + blood pressure
- speeds up peristalsis in the GI tract
- pupillary contraction + micturition

Leave in the cranial nerves III, VII, IX & X
Leave in sacral nerves S2, S3 & S4
1st order neurones brainstem nuclei & sacral spinal cord

Preganglionic parasympathetic axons exit the brainstem nuclei or sacral regions and travel to ganglia near the tissue they innervate

Question 5

Describe the sympathetic nervous system of the ANS:

Answer 5

Functions:
- “fight or flight response”
- increases cardiac output
- dilates airways facilitating oxygen uptake
- redirects blood flow to skeletal muscle
- slows gastrointestinal activity to conserve energy

Arises from the thoracolumbar region of the spinal cord T1–L2/L3 levels

Pre-ganglionic neurone pathway:
- originate in the lateral gray horns of the spinal cord
- exit the spinal cord via the ventral roots and enter sympathetic ganglia

Post-ganglionic neurone pathway:
- synapse with preganglionic neurones in ganglia
- extend to target organs or tissues to execute sympathetic effects

Question 6

Describe the enteric nervous system of the ANS:

Answer 6

Regulates gastrointestinal functions independently of CNS input

Neurones are arranged in clusters between the myenteric plexus and the submucosal plexus

The myenteric plexus controls gut motility, while the submucosal plexus controls fluid absorption

Both excitatory and inhibitory neurones end on smooth muscle and gland cells of the gut.

The GI system can act independently without CNS influence

Question 7

Describe the ganglia of the sympathetic nervous system:

Answer 7

Sympathetic side chains and Para-vertebral Ganglion:

• sympathetic side chain on each side of vertebral column
• each consists of a para-vertebral ganglion
• serve as relay points for pre- and post-ganglionic neurones
• para-vertebral ganglia exit the sympathetic chain via visceral branches, innervating different organs
• also exit sympathetic ganglia via gray rami, re-entering the spinal nerves to innervate tissues supplied by the nerve

Pre-vertebral Ganglia and Sympathetic Post-ganglionic Neurons:

• sympathetic post-ganglionic neurones located in the pre-vertebral ganglia, located anterior to the sympathetic chain on the abdominal aorta
• axons of pre-ganglionic neurones exit via anterior roots of corresponding spinal nerves, entering the sympathetic side chain via the white rami, and exiting the chain without synapsing