Nervous System: CNS and PNS

• The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• Includes the brain and spinal cord.
• Interprets sensory input and issues commands based on experience

Peripheral Nervous System (PNS)

• Includes cranial and spinal nerves.
• Consists of sensory neurons linking body surface receptors with processing from the CNS

PNS Divisions

• Somatic: Controls body movements.
• Autonomic: Manages involuntary internal organ functions.

Cranial Nerves (PNS)

• There are twelve pairs of cranial nerves located at the base of the brain.
• Olfactory and optic cranial nerves are part of the CNS, while 10 axons are PNS.
• A single cranial nerve can have multiple functions.
• They are essential for identifying neurological disorders.
• Cranial nerves have nuclei associated with the midbrain, pons, and medulla.

Brain Covering (Meninges)

• The CNS is encased in bone, which does not make direct contact.
• The brain is covered by a three-layered membrane called the meninges: dura mater, arachnoid membrane, and pia mater.

Dura Mater

• Outermost, tough covering of the meninges, described as tough and leather-like.

Arachnoid Membrane

• Located under the dura mater, it has a spider web-like consistency.
• There is no space between the dura and arachnoid layers; blood pools there in subdural hematomas.

Pia Mater

• Thin membrane near the surface of the brain with many blood vessels.
• The subarea contains cerebrospinal fluid (CSF).

Brainstem (Midbrain)

• Acts as information transport center between the spinal cord and forebrain Ascending and descending tracts meet in the midbrain.
• Red nucleus: Associated with motor control.
• Periaqueductal gray: Relays somatic pain sensations.
• Superior colliculus: Receives direct input from the eye and controls eye movement.
• Inferior colliculus: Receives sensory input from the ear and relays auditory information to thalamus and sound localization.
• The cerebral aqueduct is located in the core of the midbrain.
• Connects the third and fourth ventricles.

Brainstem (Medulla)

• Processes sensory information, controls voluntary movement, and regulates the autonomic system.
• Medullary pyramids: Bundles of axons descending to the spinal cord.
  • Found at the very bottom of the medulla.
  • Named for their shape.
  • Contain corticospinal tracts that control voluntary movement.
• Nuclear clusters: Associated with hearing, include dorsal and ventral cochlear nuclei, and superior olive.
• Inferior olive: Plays a role in motor control.
• Raphe nucleus: Modulates pain, mood, and wakefulness.

Medulla (Spinal Cord)

• Beginning of the spinal cord where the fourth ventricle disappears.
• Dorsal column nuclei: relays somatic sensory information.
• Axons from the dorsal column cross to the other side (decussate) to the thalamus.

Spinal Cord

• Located within the vertebral column
  • Neck: Cervical region (C1-C7).
  • Ribs: Thoracic region (T1-T12).
  • Lower back: Lumbar region (L1-L5).
  • Pelvic: Sacral region (S1-S5).
• Major conduit of information from the skin, joints, and muscles.

Transection of the Spinal Cord

• Results in anesthesia or lack of feeling in the skin.
• Results in paralysis of muscles caudal to the cut.
• Results in muscles that can't be controlled by the brain.
• Spinal nerves communicate with the cord through the somatic PNS.

Spinal Nerves

• Spinal nerves adopt the related vertebrae names.
• There are 8 cervical nerves for 7 cervical vertebrae.
• The spinal cord ends at L3.
• Does not grow after birth, the vertebral column continues.
• Cauda equina: Bundle of nerves extending down the lumbar and sacral spinal cord.

Ventral-Lateral Surface

• Spinal nerves with meninges attach to the spinal cord.
• Nerves split into two roots at the vertebral notch.
  • Dorsal root carries axons within the dorsal root ganglia.
  • Ventral root carries motor axons from the ventral gray matter.
• Butterfly-shaped core is gray matter, while the outside is white matter.
• Outside is highly myelinated.

Gray and White Matter Organization

• Gray matter is divided into 3 groups: dorsal, lateral, and ventral horns.
• White matter is divided into 3 columns: dorsal, lateral, and ventral columns.

Cross-Sectional Anatomy

• Ascending and descending pathways are part of the cross-sectional anatomy of the spinal cord.
  • Dorsal column consists of sensory axons to the brain which allow conscious touch appreciation.
  • Spinothalamic tract transmits painful stimuli and temperature sensations.

Descending Tracts for Control of Movement

• Tract names describe their origins and terminations.
• Dorsal column consists of sensory axons to the brain to allow conscious touch appreciation.
• There are two descending tracts:
  • Lateral: Controls muscle tone and antigravity.
  • Ventromedial: Maintains space and equilibrium.

Autonomic PNS

• Regulates internal organs, glands, and vasculature automatically.
• ANS divisions:
  • Sympathetic: Stimulates fight or flight response.
  • Parasympathetic: Stimulates rest-and-digest/feed and breed activities.

Sympathetic ANS

• Sympathetic Ganglia form a chain.
• Begins at the thoracic to lumbar regions (thoracolumbar outflow).
• Runs along the vertebral column with spinal nerves emerging from the column.
• Chemical signaling: Acetylcholine, nicotinic receptor, and norepinephrine.

Parasympathetic ANS

• Nerves originate from the CNS (craniosacral outflow).
• Cranial nerves have preganglionic parasympathetic nerves, including trigeminal branches: ophthalmic, maxillary, and mandibular nerve.
• Vagus nerve: Paired nerve typically referred to in the singular.
  • Supplies motor fibers to all organs from the neck down to the colon to regulate heart rate, sweating, and muscle movements.
• Pelvic splanchnic:
  • Innervates the hindgut.
  • Regulates bladder, sphincter, and sexual function.

Parasympathetic ANS Signaling

• Nicotinic receptors are channels, while muscarinic receptors are G-proteins.
• The main end-receptor is stimulated from postganglionic fibers.

Enteric Nervous System (ENS)

• A division of the ANS.
• Located in the lining of the esophagus, stomach, intestines, pancreas, and gallbladder.
• Controls the processes associated with the transport and digestion of food.
• 500 million neurons are part of the ENS.
• Employs 30 different neurotransmitters.
• Sequesters 90% of serotonin and 50% of domaine are used in the ENS. </existing_notes>

Explore the nervous system's central (CNS) and peripheral (PNS) divisions. The CNS includes the brain and spinal cord, interpreting sensory input. The PNS consists of cranial and spinal nerves, divided into somatic and autonomic systems.