AP1 Lab 5 – Meninges, Spinal Cord, Neurons, Rami, Nerve Plexuses, and Dermatomes

Project 1 - Meninges & Meningeal SpacesIn order from the most superficial to the deepest:

EPIDURAL SPACE The space between the dura mater and the periosteum of the adjacent vertebrae. Always exists around the spinal cord but usually not around the brain because the dura mater fuses

with the bony plates of the skull. The dura mater is therefore attached to the bony plates of the cranium but not to the vertebrae.

DURA MATER – Translates as: “TOUGH MOTHER” Fig. 12.22 & 12.23 the most superficial and toughest of the three meningeal layers is composed of 2 layers:

1) Periosteal layer – fuses to the inner surface of the bony plates of the cranium. This layer does not descend with the spinal cord and therefore is not fused to the vertebrae.2) Meningeal layer – is found around both the brain and spinal cordSeveral dural sinuses are formed where these two layers separate to form V-shaped channels filled with venous blood.

Venous blood collects in and travels through dural sinuses absorbing CSF (cerebrospinal fluid) by osmosis. The osmotic pressure of the venous blood (due to the presence of proteins) is greater than the OP of the CSF.

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SUBDURAL SPACE The space between the dura mater and the arachnoid mater. Is larger around the brain and contains blood vessels. Is much reduced around the spinal cord and is avascular at this level. What and where is a “subdural hematoma?”

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ARACHNOID MATER – MEANS: “SPIDERY” The 2nd of the 3 meningeal layers A thin delicate layer resembling cobwebs just under the dura mater and extending towards the pia

mater. Knoblike projections of the arachnoid mater called arachnoid villi (a.k.a arachnoid granulations)

extend through the dura mater into the superior sagittal sinus. CSF is absorbed through these into the venous blood by osmosis.

See Fig. 12.24

SUBARACHNOID SPACE The space between the arachnoid mater and pia mater. Always exists around both the brain and spinal cord. Filled with 140-150 ml of CSF (cerebrospinal fluid) Extends all the way to the level of S2. During a procedure called a “lumbar tap” or a “spinal tap” a sample of CSF is withdrawn from this

space in the lumbar region to test for meningitis

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PIA MATER – “GENTLE MOTHER” the deepest of the three meningeal layers It is bound tightly to the surface of the brain and cord. (It’s similar to a visceral serous

membrane… if you touch the surface of the brain you’re touching the pia mater.) See Fig. 12.29

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Project 2 - THE SPINAL CORD See figs. 12.29 through 12.33

FUNCTIONS of the Spinal Cord: 1. Serves as a 2-way communication path between the brain and the rest of the body. Sensory

(afferent) messages come in and travel up to the brain. Motor (efferent) messages travel down from the brain and then out to various target effectors such as muscle.

2. Serves as a reflex center for many reflexes involving skeletal muscle.

MENINGES & SPACES - Identify these (described earlier) on the spinal cord models.

EPIDURAL SPACE

DURA MATER

SUBDURAL SPACE

ARACHNOID MATER

SUBARACHNOID SPACE

PIA MATER

The spinal cord (nerve tissue) is shorter than the spinal column (bone tissue) and actually ends well before reaching the end of the spinal column. At the level of which vertebra does the cord end? ______

Define LUMBAR PUNCTURE (a.k.a. SPINAL TAP)

At the level of what vertebra is it performed and why at this location? ________________________

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ID the following on the models. Use Fig. 12.27 through 12.32. See also Fig. 13.7SPINAL NERVES - 31 pairs of spinal nerves emerge between the vertebrae and from the sacrum and coccyx.

In the cervical region each spinal nerve is numbered according to the vertebra immediately below it, so we have spinal nerves C1-C7.

The rule breaker is spinal nerve C8. It emerges below vertebra C7 and above vertebra T1. In the thoracic, lumbar, and sacral regions each spinal nerve is numbered according to the vertebra

immediately above it. Thus yielding spinal nerves T1-T12, L1-L5, S1- S5, and Co1 in the coccygeal region.

Each spinal nerve attaches to the spinal cord by a dorsal root and a ventral root.

VENTRAL (ANTERIOR) ROOT

Carries __________ (motor/sensory) nerve impulses away from the CNS via the axons of multipolar motor neurons. These impulses are also called _____________ (afferent/efferent) IMPULSES.

DORSAL (POSTERIOR) ROOT Carries incoming _ __________ (motor/sensory) nerve impulses to the CNS via the axons of

unipolar sensory neurons. These are also called __________________ (afferent/efferent) IMPULSES.

DORSAL (POSTERIOR) ROOT GANGLION (plural is GANGLIA) Is the slightly enlarged “swollen” portion of a dorsal or posterior root contains the ______ _____________ of unipolar sensory neurons… thus the enlarged size

CROSS SECTION OF SPINAL CORD Fig 12.29GRAY MATTER

consists mostly of unmyelinated neuron cell bodies and dendritesANTERIOR (VENTRAL) HORNS – appears more blunt and rounded

contains the cell bodies of multipolar motor neuronsPOSTERIOR (DORSAL) HORNS – appears more elongated or pointed

is where incoming sensory neurons form synapses with association neuronsWHITE COLUMNS

Consists mostly of _____________________ (myelinated/unmyelinated) axons grouped in bundles called ASCENDING and DESCENDING TRACTS (or PATHWAYS). These tracts are the pathways for messages up or down the spinal cord to or from the brain.

On spinal cord model #2Sensory impulses from sensory receptors will always travel in the ______________________ root and then travel up the spinal cord in ______________ (ascending/descending) tracts to the brain.

Motor impulses from the brain will travel down the spinal cord in __________________ tracts and always travel out in the _______________________ (ventral/dorsal) root to muscles or other effectors.

**Take a spinal cord model to your instructor and confirm your identifications.

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RAMI fig. 13.8Contrary to many images, spinal nerves are surprisingly short, less than one inch. As soon as a spinal nerve emerges from the vertebral column it branches/splits and is no longer considered a spinal nerve. Instead these branches are initially called rami then eventually nerves.Like all spinal nerves, all rami are mixed nerves. What does this mean?

In the text images identify the shorter dorsal rami and the much longer ventral rami. NERVE Plexuses figs. 13.9 through13.12 Except for T2-T12 the ventral rami of all spinal nerves branch and merge with branches of other ventral rami to form nerve plexuses.See text images and identify the four plexuses: ____________________, ______________________, ___________________, and _________________________

These four plexuses primarily serve the limbs (arms & legs).

Significance: Within each plexus nerve fibers (neurons) from each ventral ramus are redistributed/recombined so that each branch of the plexus contains nerve fibers from several spinal nerves. As a result each muscle in a limb receives nerve fibers from several different spinal nerves.

Advantage: Injury to one spinal nerve does not result in complete loss of innervation of any limb muscle.

Disadvantage: Injury to the plexus results in more widespread loss of function.

DERMATOMES Fig. 13.13 A dermatome is an area of the skin innervated by the cutaneous sensory branches of a single spinal nerve.

By testing the various dermatomes for sensitivity (can your patient feel it when you prick the skin in this area?) you can determine which spinal nerve(s) is/are damaged. See dermatomal map in text.

OYO: All spinal nerves except ______ are associated with dermatomes.

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Project 3 - THE NEURON Use Fig 11.5

Your nervous system consists of millions and millions of cells called NEURONS that must continuously “talk” to each other through chemical synapses to create (or inhibit) ACTION POTENTIALS we will now call a NERVE IMPULSES.

Use the text to identify the following structures and their functions. Our neuron models are of a shape called MULTIPOLAR and would most likely function as MOTOR NEURONS or INTERNEURONS .

Define: Motor neuron____________________________________________________________

______________________________________________________________________________

Define: Interneuron ____________________________________________________________

______________________________________________________________________________

What are the other two possible “shapes” of neurons? _______________________________ and ______________________________

CELL BODY (a.k.a. SOMA or PERIKARYON) In the cell body identify: rough ER with ribosomes (a.k.a. nissl bodies), Golgi apparatus, vesicles, and

mitochondria. Most NTs are proteins synthesized by ribosomes, packaged into vesicles by the GA, and then

transported down the axon to the synaptic knob.

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Continue using Fig. 11.5. See also Fig. 11.6

DENDRITES appear on the models as numerous branches extending from the cell body. Be careful that you don’t

confuse these with the synaptic knobs that are the ends of axons of other neurons arriving at this neuron. What's the generic name for the chemical substances stored in and released from synaptic knobs? __________________________

Are the “receiving” components of a neuron. They receive signals via chemical synapses from other neurons.

When NTs (such as ACh) are released graded potentials, not action potentials, are created on dendrites and neuron cell bodies. These graded potentials may be:

1. EPSPs (excitatory postsynaptic potentials) - the RMP shifts closer to threshold2. IPSPs (inhibitory postsynaptic potentials) - the RMP shifts farther from threshold

These graded potentials on the dendrites and cell body travel toward the axon hillock and get “summed together” in the process of summation. Sometimes they add together and increase. Sometimes they add together and cancel each other out. If they achieve threshold potential at the axon hillock then you get an action potential that proceeds along the axon to the next cell.

AXON Is the single, long process arising from the cell body and extending to the target cell. Is the “conducting” portion of a neuron. It is the only part of the neuron that transmits a true action

potential (nerve impulse) to a target cell. Long axons are called nerve fibers. In motor neurons such as this one the axon transmits the impulse away from the cell body. Various materials (such as NTs) are transported along the interior of an axon from the cell body to

synaptic knobs. Unfortunately, materials such as viruses and bacterial toxins might also be transported from the

synaptic knobs to the cell bodies (which are usually in the brain or spinal cord) where they can do extensive damage. Several diseases enter the CNS (central nervous system) this way e.g. polio, rabies, herpes simplex viruses, and tetanus toxin

AXON HILLOCK Is the cone-shaped portion of the cell body where it narrows to form the axon This is where threshold is either reached or not reached as a result of summation of all the graded

potentials created on the dendrites and cell body. An action potential starts here or not at all. Therefore it is sometimes referred to as the “trigger zone.”

See note on summation above.

SCHWANN CELLS Are cells that wrap around the axons of neurons in the PNS (peripheral nervous system.) On the model they appear as sections of pale green and orange wrapped around the blue axon. Their presence creates the myelin sheath.

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MYELIN SHEATH – Fig. 11.6On our models it’s the pale green & orange material surrounding the axon. In a living person it is actually white in color.

3 Functions of the myelin sheath:

What makes the membrane of Schwann cells such a good insulator?

Explain why a patient with Multiple Sclerosis (MS) would have symptoms of muscle weakness, clumsiness, and paralysis.

Gaps between adjacent Schwann cells are called NODES OF RANVIER. There is no myelin sheath at these locations.

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NERVE TRACTS are bundles of axons in the CNS (Central Nervous System) (brain and spinal cord.)

NERVES are bundles of axons in the PNS (Peripheral Nervous System) (arms, legs, and torso.)

NUCLEI are clusters of neuron cell bodies in the CNS.

GANGLIA are clusters of neuron cell bodies in the PNS.

See fig 12.32

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