Module 3 - Lecture 2

Organisation of the Nervous System

• Information about the internal and external environment travels into the central nervous system (CNS) via afferent pathways.
• The CNS integrates and processes this information.
• The efferent division transmits the processing results, leading to actions (movement) or physiological responses.

Functions of the Nervous System

1. Sensory Input
   • Sensory receptors gather information about internal and external changes.
   • Somatic sensory input comes from the skin and muscles.
   • Visceral sensory input comes from organs.
2. Integration
   • The nervous system processes and interprets sensory input.
3. Motor Output
   • Activation of effector organs (muscles and glands) produces a response.

Protection of the Brain

1. Meninges and Bone
2. Cerebral Spinal Fluid
3. The Blood-Brain Barrier (discussed in lecture 3)

Meninges and Bone

• The brain is encased in the bony skull, or cranium.
• Meninges lie between the bone and tissues to stabilize neural tissue and protect from bruising.
  • Dura mater
  • Arachnoid membrane
  • Pia mater

Dura Mater

• Superficial layer
• Double-layered:
  • Outer layer: Periosteal layer, fused to the bone.
  • Inner layer: Meningeal layer
• Dural venous sinuses exist between the two layers (collect venous blood).
• Dural folds (septa), e.g., Falx cerebri

Arachnoid Mater

• Middle layer
• Trabeculae attach to the pia mater.
• The subarachnoid space contains cerebral spinal fluid (CSF) and blood vessels.

Pia Mater

• Deep layer
• Follows the contours of gyri and sulci.
• Rich with small blood vessels.

Cerebral Spinal Fluid (CSF)

• The brain floats in CSF.
• Salty solution, similar to plasma.
• Created by the choroid plexus in each ventricle.
• Functions in physical and chemical protection.
• Carries oxygen, glucose, and other important substances from the blood to nervous tissue cells.

Ventricles

• Lateral ventricles
• 3rd ventricle (in diencephalon)
• 4th ventricle (between pons, medulla, and cerebellum)

The Spinal Cord

• Starts at the medulla
• Enclosed in the vertebral column
• Ends at L1 or L2 at the conus medullaris
• Filum terminale: anchors the spinal cord to the coccyx
• Cauda equina: extends to the base of the vertebrae (tail of nerves)

Cervical and Lumbar Enlargements

• House nerves that service the upper and lower limbs.
• Provides two-way communication to and from the brain and body.
• Major reflex centre: reflexes are initiated and completed at the spinal cord (see lecture 8).

Spinal Cord Cross Section

• Grey matter on the inside
• White matter on the outside
• Ascending and descending pathways connect the brain and body.
• The central canal runs the length of the cord, filled with CSF.
  • Dorsal horn = sensory
  • Lateral horn (thoracic and lumbar) = autonomic
  • Ventral horn = motor

Spinal Meninges

• Dura mater (superficial): one layer thick, not attached to vertebrae
• Arachnoid mater (middle)
• Pia mater (deep)
• Epidural space: fat and network of veins between vertebrae and dura mater

Spinal Injections

• Epidural injections: injection into the epidural space between the dura and vertebrae at L3; used for pain relief
• Lumbar puncture: penetrates dura to access CSF; used for pathological testing or drug administration

The Peripheral Nervous System (PNS)

• Central nervous system (CNS): Brain and spinal cord
• Peripheral nervous system (PNS): Comprised of cranial nerves and spinal nerves
• Afferent division: sensory; carries information towards the CNS
• Efferent division: motor; carries information out of the CNS
  • Autonomic (see lecture 7)
  • Enteric nervous system: involved in digestion
• Ganglion (singular), Ganglia (pl): Collections of cell bodies in the PNS
• Plexus: Branching network of nerves

Spinal Roots

• Part of the PNS
• Ventral root: motor (efferent) output from the spinal cord
• Dorsal root: sensory (afferent) input to the spinal cord
• Dorsal root ganglia: cell bodies of sensory neurons

Spinal Nerves

• Spinal nerves = formed by the fusion of dorsal and ventral roots
• 31 pairs of spinal nerves
• Named according to where they emerge from the spinal cord:
  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal

Structure of Nerves

• Bundles of axons surrounded by connective tissue:
  • Endoneurium: wraps around axons
  • Perineurium: wraps around fascicles
  • Epineurium: wraps around the nerve
  • Blood vessels
• Most nerves carry both afferent and efferent axons (i.e., are both sensory and motor).

Dorsal and Ventral Rami

• Dorsal rami: innervation of the skin & back muscles (smaller)
• Ventral rami: innervation of the rest of the trunk and limbs (larger)

Nerve Plexuses

• Ventral rami branch and join to form nerve plexuses.
• Plexuses contain fibres of different spinal nerves.
• Fibres from each rami project to the periphery via many pathways.
• Overlapping territories mean that each part of the skin/muscle receives input from more than 1 spinal nerve (important when spinal nerves are damaged).
• 4 main plexuses:
  • Cervical
  • Brachial
  • Lumbar
  • Sacral

Plexuses Locations and Innervations

• Cervical (C1-C5): located under the sternocleidomastoid, innervates neck muscles and the skin
• Brachial (C5-T1): located in the neck and shoulder, innervates the upper limb
• Lumbar (L1-L4): located within the psoas muscle, innervates the abdominal wall, anterior and medial thigh
• Sacral (L4-S4): located posterior to the lumbar plexus, innervates buttocks, lower limb, pelvis
• Sciatic Nerve: largest branch, serves most of the thigh, leg, and foot muscles

Cranial Nerves

• Originate from the brain
• 12 pairs (CNI-CNXII)
• Sensory, motor, or both (mixed)
• Arranged along the longitudinal axis of the brain
• CNI attaches to the cerebrum
• CNII attaches to the diencephalon
• CNIII-XII attach to the brainstem