Histology of Nervous System Introduction

Organization: Anatomical

Brain
Spinal cord

- PNS

- - Cranial nerves = 12 pairs of nerves that are attached to the brain.
  - Spinal nerves = 31 pairs of nerves attached to the spinal cord

Neuron: Highly receptive branched dendrites taking info toward soma + unbranched axon with information away from soma. Axon ends with Terminal Bouton = allows contact with other neurons (via synapses) to allow chemical / electrical communication. Can be Axon / soma, axon / dendrite, axon / axon. When synapse on muscle = motor end plate.
Multipolar neuron: Neuron with a lot of processes – a typical neuron. Includes motor neurons. Muliple dendritic branches end on the soma, one axon leaves.
Bipolar neuron: One branched dendrite on one side, one axon on the other side.
Unipolar: soma with single process.
Pseudounipolar: have single process that then branches into two separate processes. Includes sensory neurons. One branch (called peripheral process) end has receptor the other branch (called central process) carries that received information to the CNS.
Nucleus: This term refers to a collection of soma that reside within the CNS.
Ganglion: Collection of soma that reside outside the CNS (i.e. not in brain / spinal cord).
Tract: a collection of nerve processes (esp. axons) bound in a fasicle in CNS.
Nerve: collection of nerve processes outside the CNS in a CT sheath.
Gray matter: Unmyelinated structures in the nervous system, normally the soma + dendrites.
White matter: structures that are myelinated, mostly the axons. Nerves will look white normally (not embalmed).

Spinal Cord:

Function: to connect the brain with the body.
Takes sensory (afferent) information from periphery (skin) and sends it to the brain.
Transmits (efferent) motor information from the brain out to the skeletal muscles. From brain to periphery.
Generates spinal reflexes

Structure of Apinal Cord:

16-18 inches in length cylinder.
Attached superior to brain (continuous with brain)
Inferiorly = it ends as a tapered cone (conus medullaris)

Has 2 areas of enlargement:

Cervical = between C4 and T1 à tasks of upper limb.
Lumbar = between L2 and S3 à tasks of lower limb
C = cervical, T = Thoracic, L = Lumbar, S = Sacral, Co = Coccyseal

Cauda Equina: continuation of dorsal and ventral roots (rootlets) inferior to L2 (second Lumbar) spinal nerve.

Protection for Spinal cord:

Bone = Vertebral column. External layer of protection. Forms a collar around the spinal cord. Opening in the middle of vertebra = Vertebral Foramen → oramina line up to form the vertebral canal which houses spinal cord.
3 layers of connective tissue = meninges.
- Dura Mater = “tough mother” outermost layer. Dense connective tissue forming tough protective long tubular sleeve. Continuous with dura mater over the brain. Extends below the length of the spinal cord and ends in the sacrum at S2 level (second sacral vertebra).
- Arachnoid Mater = “spider like” y→ thin layer of connective tissue pressed up against the internal aspect of Dura. Stretching from this membrane are web like filaments (trabeculae) that form second layer that stretch to the Pia Mater.
- Pia Mater = “delicate mother” = thin layer directly applied to the spinal cord. Cannot be removed from surface of spinal cord. Has extensions that act as anchors for spinal cord = 1. lateral extensions (denticulate ligaments) which run through arachnoid and attach to Dura Mater. 2. inferior extension (filum terminale) which is inferiorly anchored to dura mater.
Spaces
- Epidural space: Between bony vertebra and dura mater.
- Subdural space. Only a potential space, normally arachnoid held right up against dura mater. Between Dura Mater and Arachnoid Mater
- Subarachnoid space: Between arachnoid and pia / between trabeculae of arachnoid. Filled with cerebrospinal fluid = the last protection for spinal cord acting as a shock absorber to maintain structure.
Spinal Cord:

- Dorsal horn = Sensory
- Ventral horn = Motor
- Lateral horn = Autonomic

Nerve route: From skiny→ primary sensory neuron peripheral process (of pseudounipolar neuron)y→ to dorsal root ganglion which contains the somay→ central process continues through the dorsal root to the dorsal horn. Motor neuron soma in ventral horn y→ axon leaves through ventral root to muscle.

Spinal nerve = mixed nerve, carries both sensory and motor fibers. Always paired.

Dorsal root = Sensory only
Ventral root = Motor only

When two adjacent vertebrae articulate (touch) together, they create lateral opening = Intervertebral Foramen. Spinal nerves go through these spaces, allows attachment to spinal cord. Branches afterwards.

Dorsal Ramus: branch goes to skin and muscles of the back
Ventral Ramus: branch goes to skin and muscle of all other tissues. Much larger branch.

Spinal Nerves / Spinal Cord Segments:

31 pairs of Spinal nerves:

8 cervical spinal nerves = C1y→ C1 exits above first cervical vertebra. Does not use an intervertebral foramen, comes out between C1 and the skull.
12 Thoracic spinal nerves = T1y→ T12
5 Lumbar spinal nerves = L1y→ L5
5 Sacral spinal nerves = S1y→ S5
1 Coccygeal = Co1

Spinal cord segment: That portion of the spinal cord that contributes to the formation of a single spinal nerve. Each spinal nerve has many dorsal rootlets that come together from different levels to form dorsal root. Same happens for ventral root. Dura mater (as well as other sheaths) stops at interverterbral foramen, branches then continue in separate sheathes.

Vertebral level: Anatomical point / landmark associated with position of each vertebra.

Do not match with the spinal cord segment #’s. Spinal cord is not the same length as vertebral column. As embryo grew, bone grew more than the spinal cord. As it became disproportionate, dorsal / ventral roots had already been attached, so they stretch verticallyà end up in adult with spinal cord ending at bony vertebral level L2.

Conus Medullaris = At the bony L2 vertebral level.

Cauda Acuina = dorsal and ventral roots of spinal nerves below L2. There is no spinal cord at this portionà allows cerebral spinal fluid to be removed without fear of damaging spinal cord. This is where lumbar punctures occur (between L3 and L4) to sample fluid.

Dermatome: Segment of skin that is supplied by a single spinal nerve. Dermatomes may overlap, thus same portion of skin may be innervated by more than one spinal nerve. A dermatome map may allow diagnosis of nerve damage at particular levels.

Nerve Plexus: A network of intersecting nerves. Cardiac, brachial (shoulder)