Cruz: Nervous System and Vision

Summary of "Autonomic Nervous System & Special Senses"

Nervous system divides into:

Central Nervous System (CNS): Brain and spinal cord

Peripheral Nervous System (PNS): 

Sensory input: 

• Exteroceptive (external environment, see, hear, touch, taste, and smell)

• Proprioceptive (limb position in space)

• Interoceptive (internal body sensations, such as hunger, thirst, heart rate, and breathing)

Motor divisions: 

• Somatic (voluntary control): 

• Somatic Nerves and Muscle Contraction: Somatic nerves always stimulate contraction in skeletal (somatic) muscles, meaning they activate movement rather than inhibit it.

• Monosynaptic Connection: Somatic efferent (motor) neurons connect directly to skeletal muscle fibers at a single synapse, which is called a monosynaptic connection. This direct pathway enables fast, precise control over voluntary movements.

• Autonomic (involuntary control): Maintains Homeostasis, regulates functions in visceral organs (like the heart and intestines) and glandular secretions to keep the body in balance without conscious effort.

• Sympathetic (thoracolumbar): fight-or-flight. 

• Sympathetic nerves in the body follow a specific pathway. The first group of sympathetic nerves, called preganglionic neurons, starts in the spinal cord from segments T1 to L3. In this region, these neurons are located in a section called the lateral horn.

• The axons (nerve fibers) of these preganglionic neurons leave the spinal cord and connect (or synapse) with a second group of neurons, called postganglionic neurons. This connection happens in two possible places:

1. Paravertebral ganglia: These are clusters of nerve cells positioned along each side of the spinal column. Together, they form the sympathetic chain, which runs parallel to the spine. To enter the sympathetic chain, the axons from the preganglionic neurons pass through a structure called the white ramus. Innervates thoracic structures, limbs and head

• After entering paravertebral ganglion, preganglionic sympathetic axons can: 

1. Synapse with the segmental paravertebral ganglion

2. Travel up/down the sympathetic chain within a neighboring paravertebral ganglion

3. Enter the greater or lesser splanchnic nerve to synapse within one of the ganglia of the prevertebral plexus (This plexus is connected to three main groups of nerve clusters: the celiac ganglion, and the cranial and caudal mesenteric ganglia. These ganglia are located near major blood vessels in the abdomen and send nerve signals to organs in this area)

2. Prevertebral ganglia: These are located in front of the spinal column, near the major arteries of the abdomen (around the abdominal aorta). These ganglia allow the nerves to connect and send signals to various abdominal and pelvic organs.

• Parasympathetic (craniosacral): rest-and-digest

• Cranial preganglionic fibers, part of the parasympathetic nervous system, leave the brainstem with cranial nerves III, VII, IX, and X to control various body functions. The preganglionic fibers connect to postganglionic neurons located in specific clusters:

• Ciliary ganglion (CG): Controls eye muscles.

• Pterygopalatine ganglion (PG): Affects the tear glands and nasal passages.

• Mandibular ganglion (MG): Influences salivary glands.

• Otic ganglion (OG): Also affects salivation.

• In the sacral spinal cord (specifically segments S2 and S3), there are preganglionic parasympathetic neurons that control organs in the lower body, like parts of the intestines, bladder, and reproductive organs.

  • These neurons are located in a similar area to where sympathetic neurons are found, but they don’t form a separate, clearly defined column.

  • Their axons (nerve fibers) exit the spinal cord through the ventral roots (front part of the spinal cord) and travel with pelvic splanchnic nerves.

  • The pelvic splanchnic nerves carry these signals to terminal ganglia (clusters of nerve cells) near the organs they control, such as the descending colon, rectum, bladder, and reproductive organs.

• The enteric nervous system (ENS) is a network of nerves that controls the digestive tract, including the pancreas and bile system.

  • Receives signals from both the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) parts of the autonomic nervous system (ANS).

• The ENS’s nerves are organized into two main plexuses (networks):

  • 1. The myenteric plexus controls motility, or how the muscles of the gut move food along.

    2. The submucosal plexus controls fluid and ion transport, which helps manage the gut's secretions and absorption.

• Both of these plexuses get signals from the vagus nerve (which mainly handles digestion) or the sacral nerves (for the lower parts of the gut, like the colon and rectum).

• The ENS can also function independently from the brain to control most gut activities, like:

• Starting peristalsis (the movement of food through the gut) when it senses the stomach stretching

• Managing fluid and nutrient absorption

• Triggering contractions in the biliary system to help with digestion