neuroanatomy lecture 20

Lecture 17: Post-Lecture Quiz Answers

• Acetylcholine is the neurotransmitter used by a somatic efferent neuron.
• Sympathetic chain ganglia contain the cell bodies that contain norepinephrine.
• The craniosacral nervous system being activated means a state of relaxation.
• A post-ganglionic parasympathetic neuron has a cell body that resides distant from the Central Nervous System (CNS).

Lecture 20: Learning Objectives

• The external anatomy of the spinal cord and its associated structures.
• The internal anatomy of the spinal cord.
• How neural information is organized within the spinal cord and the direction of its flow.
• How neural information enters and exits the spinal cord.
• The spinal nerves and how neural information travels through them between the body and the CNS.
• The structure of a peripheral nerve.

Autonomic Nervous System Recap

• The craniosacral division is parasympathetic
• Fibers originate in the brain stem or sacral spinal cord.
• Preganglionic fibers are long, while postganglionic fibers are short.
• Ganglia are close to visceral effector organs.
• The thoracolumbar division is sympathetic
• Fibers originate in the thoracic and lumbar spinal cord.
• Preganglionic fibers are short, and postganglionic fibers are long.
• Ganglia are close to the spinal cord.

External Anatomy of the Spinal Cord

• It starts at the foramen magnum, which is the opening at the base of the skull.
• It ends at the inferior border of the first lumbar vertebra (L1).
• The spinal cord sits within a sac of meninges inside the spinal cavity.
• Within vertebrae, the spinal cavity extends to the coccygeal vertebrae.

Non-Neural Structures

• A meningeal sac extends around the spinal cord and is filled with cerebrospinal fluid (CSF).
• The conus medullaris is a tapered cone at the end of the spinal cord.
• It consists of non-neural tissue and serves as the attachment point for the filum terminale at the location of L1/L2.
• The filum terminale extends from the conus medullaris to the end of the spinal cavity.
• Anchors the spinal cord
• Is composed of fibrous, non-neural tissue.

Spinal Nerves

• There are 31 segments in the spinal cord.
• A pair of spinal nerves is associated with each segment.
• Includes 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair of spinal nerves.

Spinal Nerves: Exit Points

• Spinal nerves are named according to the vertebra they exit below.
• There is one exception: the first cervical spinal nerve exits between the skull and the first cervical vertebra.
• There are 8 cervical spinal nerves, even though there are only 7 cervical vertebrae.
• Spinal nerves exit the vertebral column at a level appropriate to their origin.
• For example, nerves originating in the lumbar spinal cord exit between the lumbar vertebrae.
• A large collection of long nerves inferior to the end of the spinal cord is called the cauda equina, or "horse's tail".

The Dermatome

• Areas of skin are innervated by spinal nerves.
• All spinal nerves except C1 innervate skin.

Internal Anatomy of the Spinal Cord

• The sulcus is Latin for furrow, and a fissure is a deep sulcus.
• White matter is composed of axons, while grey matter is composed of cell bodies.
• Dorsal refers to posterior and ventral refers to anterior.
• Key components include:
• Dorsal and ventral columns (axons)
• Lateral horn (cell bodies)
• Dorsal and ventral nerve roots (axons)
• Dorsal root ganglion (cell bodies)

Organization and Flow of Information

• Sensory (afferent) information enters the spinal cord from the Peripheral Nervous System (PNS) into the dorsal side.
• Motor (efferent) information exits from the spinal cord on the ventral side and goes into the PNS.

Flow of Efferent (Motor) Information

• Motor commands are sent to their effectors in the body via ventral roots.
• This includes autonomic neurons and somatic motor neurons.
• Damage to the ventral horn results in paralysis of muscles supplied by somatic motor neurons from that spinal cord segment, on the same side only.

Flow of Afferent (Sensory) Information

• Cell bodies of sensory neurons are located in the dorsal root ganglion.
• The input zone in the body associates with receptors for sensory stimuli.
• The output zone enters the spinal cord via the dorsal roots.
• Dorsal root ganglion contains cell bodies of sensory neurons.
• Damage leads to a loss of sensation from the region of the body supplied by sensory neurons from that spinal cord segment, on the same side only.

Spinal Nerves

• Spinal nerves carry both afferent and efferent information.
• Dorsal nerve roots carry only afferent information.
• Ventral nerve roots carry only efferent information.
• SAD DAVE is a mnemonic: Sensory = Afferent = Dorsal, Dorsal = Afferent, Ventral = Efferent.

What Happens when Leaving the Spinal Column

• Components include the dorsal and ventral nerve roots, the dorsal root ganglion, the dorsal and ventral ramus, rami communicans, and sympathetic ganglion.
  • The dorsal ramus (branch) carries efferent information to the back and afferent information from the back.
  • Rami communicans are sympathetic and only at T1-L2.
  • The ventral ramus (branch) carries efferent information to the ventral body and afferent information from the ventral body.

Structure of a Peripheral Nerve

• Bundles of axons in the Peripheral Nervous System (PNS) are called a nerve.
• Bundles of axons in the Central Nervous System (CNS) are called a tract.
• Individual axons may be myelinated or unmyelinated.
• Key Layers
• Endoneurium covers axons.
• Endoneurium-covered axons bundle together to form a fascicle.
• Perineurium covers fascicles.
• Fascicles bundle with each other and with blood vessels to form a nerve.
• Epineurium covers nerves.

Lecture 20: Post-Lecture Quiz

• The dorsal white columns are part of the sensory system.
• Spinal nerves contain all of the following except the filum terminale.
• The spinal cord contains spinal nerves.
• A peripheral nerve contains fascicles.