Neurology Term 5 Exam

surface anatomy: spinal cord

cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull

spinal cord characteristics

• occupies the upper two thirds of vertebral canal

• inferior margin ends at L1 or slightly beyond

• averages 1.8 cm thick and 45 cm long

• segment (the part supplied by each pair of nerves

spinal cord gives rise to how many pairs of spinal nerves

31 pairs

meninges of the spinal cord

• three fibrous membranes that enclose the brain and spinal cord

• they separate soft tissue of central nervous system from bones of cranium and vertebral canal

layers of the meninges from superficial to deep

• dura mater

• arachnoid mater

• pia mater

dura mater of the meninges of spinal cord

forms the dural sheath that surrounds the spinal cord and is separated from vertebrae by the epidural space. It’s in this space that the epidural anesthesia is made (common in childbirth

arachnoid mater of meninges

adheres to dura and is seprated from pia by fibers spanning the subarachous space that is filled with cerebrospinal fluid

lumbar puncture

• spinal tap

• takes sample of CSF

• taken below the medullary cone in that subarachnoid space

pia mater of meninges

delicate membrane that follows contours of the spinal cord and continues inferiorly as a fibrous terminal filum that fuses with dura to form coccygeal ligament

gray matter in spinal cord

• the spinal cord has a central core of gray matter that looks butterfly or H-shaped in cross section

• pair of posterior (dorsal) horns

• pair of thicker anterior (ventral) horns

pair of thicker anterior (ventral) horns

contains large cell bodies of motor neurons whihc connects with muscle fibers (anterior (ventral) root of spinal nerve carries only motor fibers)

pair posterior (dorsal) horns

receives sensory nerve fibers from the spinal nerves (which synapses with interneurons in the posterior horn (posterior (dorsal) root of spinal nerve carries only sensory fibers)

gray commissure connects right and left sides

has central canal lined with ependymal cells and filled with CSF

lateral horn: visible from T2 through L1

contains neurons of sympathetic nervous system

spinal tracts

• all fibers in a given tract have similar origin , destination and function

Spinal tracts: ascending tracts

carry sensory information up

spinal tracts: descending tracts

carry motor information down

spinal tracts: decussation

crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of body

spinal tract: contralateral

when the origin and destination of a tract are on opposite sides of the body

spinal tracts: ipsilateral

when the origin and destination of a tract are on the same side of the body; does not decussate

ascending tracts

Carry sensory signals up the spinal cord

sensory signals travel

• across three neurons from origin (receptors) to destinations in the sensory areas of the brain

  1. first order neurons

  2. second order neurons

  3. thirds order neurons

first order neurons

detect stimulus and transmit signal to spinal cord of brainstem

second order neurons

continues to the thalamis at the upper end of the brainstem

third order neruons

carries the signal the rest of the way to the sensory region of the cerebral cortex

descending tracts

Carry motor signals down the brainstem and spinal cord

descednding tracts involves two motor neurons

upper and lower motor neurons

upper motor neurons

originates in cerebral cortec or brainstem and terminates on a lower motor neuron

lower motor neurons

• is in brainstem or spinal cord

• axon of lower motor neuron leads to muscle or other target organ

nerve

a cord like organ composed of numerous nerve fibers (axons) bound together by connective tissue

mixed nerves

contains both afferent (sensory) and efferent (motor) fibers

nerve fibers of peripheral nercoud system are surrounded by

Schwann cells forming neurilemma and myelin sheath around the axon

endoneurium

loose connective tissue external to neurilemma

perineurium

lauyers of overlapping squamous cells that wrap fascicles bundles of nerve fibers

epineurium

• dense irregular connective tissue that wraps entire nerve

• blood vessels penetrate connective tissue coverings and provide plentiful blood supply

sensory (afferent; going up) nerves

carry signals from sensory receptors to the CNS

motor (efferent; going down) nerves

carry signals from CNS to muscles and glands

both sensory and motor fibers can also be described as

• somatic or visceral

• general or special

ganglion

• cluster of neurosomas outisde the CNS

• eveloped in and endoneurium continuous with that of the nerve

among neurosomas are

• bundles of nerve fibers leading into and out of the ganglion

• posterior root ganglion associated with spinal nerves

how many pairs of spinal nerves (mixed nerves) are there

31 pairs

how many cervical nerves

8 cervical (C1-C8)

Where is the first cervical nerve

exist between skull and atlas

where to the other cervical nerves exit

at intervertebral foramina

how many thoracic spinal nerves

12 thoracic (T1-T12)

how many lumbar spinal nerves

5 lumbar (L1-L5)

How many sacral spinal nerves

5 sacral (S1-S5)

how many coccygeal spinal nerves

1 coccygeal (Co1)

proximal branches

each spinal nerve is formed from two roots (proximal branches)

what are the two spinal roots

1. posterior (dorsal) root

2. anterior (ventral) root

posterior (dorsal) root

• sensory input to spinal cord

• posteriors (dorsal) root ganglion contains the neurosomas of sensory neurons carrying signals to the spinla cord

• six to eight rootlets enter posterior horn of cord

anterior (ventral) root

• motor output out of spinal cord

• six to eight rootlests leave spinal cord and converge to form anterior root

• cauda equina: formed from roots arising from L2 to Co1

nerve plexuses

anterior rami branch and anastomose repeatedly to form nerve plexuses

what are the five nerve plexuses

1. cervical plexus

2. brachial plexus

3. lumbar plexus

4. sacral plexus

5. coccygeal plexus

cervical plexus

• in the neck, C1 to C5

• supplies neck and phrenic nerve to the diaphragm

brachial plexus

• near the shoulder C5 to T1

• supplies upper limb and some of shoulder and neck

• median nerve - carpal tunnel syndrome

lumbar plexus

• in the lower back L1 to L4

• supplies abdominal wall anterior thigh and genitalia

sacral plexus

• in the pelvis, L4, L5, and S1 to S4

• supplies remainder of lower trunk and lower limb

coccygeal plexus

S4, S5 and Co1

pathway of somatic reflex arc

somatic receptors → afferent nerve fibers → integrating center → efferent nerve fibers → effectors

where are somatic receptors

in skin, muscle, or tendons

afferent nerve fibers

carry information from receptors to posterior horn of spinal cord or to the brainstem

intergrating center

• a point of synaptic contact between neurons in the gray matter of the cord or brainstem

• determines whether efferent neurons issue signals to muscles

efferent nerve fibers

carry motor impulses to muscles

effectors

the muscles that carry out the response

muscle spindle

stretch receptors embedded in skeletal muscles

proprioceptors

specialized sense organs to monitor position and movement of body parts

muscle spindles inform

the brain of muscle length and body movement

muscle spindles enable the brain to

send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture

stretch (myotatic) reflex

when a muscle is stretched, it ‘fights back’ and contracts

what does stretch reflex do

• helps maintain equilibrium and posture

  • head starts to tip forward as you fall asleep

  • muscles contract to raise the head

• stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions

• stretch reflex is mediated primarily by the brain its spinal component can be more pronounced if muscle is suddenly stretched by a tendon tap (knee jerk)

knee-jerk (patellar) reflex

• is a monosynaptic reflex arc

• one synapse between the afferent and efferent neurons

• testing somatic reflexes helps diagnise many diseases

reciprocal inhibition

reflecx phenomenon that prevents muscles from working against each other by inhibiting antagonist when agonist is excited

flexor (withdrawal) reflex

• the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus

• triggers contraction of the flexors and relaxation of the extensors in that limb

polysynaptic reflex arc

pathway is which signals travel over many synapses on their way to the muscle

the tendon reflex

• in response to excessive tension on the tendon

  • Inhibits muscle from contracting strongly

  • moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone

tendon organs

• proprioceptors in a tendon near its junction with a muscle

  • Golgi tendon organ: 1 mm long, nerve fibers entwined in collagen fibers of the tendon

major portions of the brain

• cerebrum

• cerebellum

• brainstem

cerebrum

• 83% of brain’s volume

• pair of cerebral hemispheres marker by gyri and sulci

• longitudinal fissure separates L and R hemispheres

• connected by a thick bundle of nerves: corpus callosum

cerebellum occupation percentage

• 10% of volume but contains 50% of brain neurons

• occupies posterior cranial fossa, inferior to cerebrum

brain stem

• contains the midbrain, pons and medulla oblogata

• ends at the foramen magnum (CNS continues after this point)

gray matter in brain

• formed from cell bodies, dendrites and synapses

• Form the cortex over the cerebrum and cerebellum

• forms the nuclei depps in the white matter

white matter of brain

• bundles of axons

• composed of tracts that connect one part of the brain to another and to the spinal cord

• deep to the cortec

• bright peraly color due to myelination

gray and white matter in the brain is what to the spinal cord

opposite placement in spinal cord than in the brain

embryonic development

• The nervous system develops from the ectoderm

  • outermost tissue layer of an embryo

• by the third week of development, a neural plate forms

  • Gives rise to most neurons and glial cells

• neural plate sinks and thickens to form the neural groove with raised neural folds

  • Neural folds fuse along the midline, like closing a zipper

embryo → fully developed

• forebrain / prosencephlaon

  • telencephalon → cerebrum

  • diencephalon → thalamus, hypothalamus

• mid brain/ mesencephalon

  • mesencephalon → midbrain

• hindbrain/rhombencephalon

  • metencephalon → pons, cerebellum

  • myelencephalon → medulla oblongata

meninges in the brain

• three connective tissue membranes envelop the brain (between the nervous tissue and bone)

• Meninges protect the brain and provide a structural framework for its arteries and veins

meninges in the brain from superficial to deep

• dura mater (2 layers)

• arachnoid mater

• pia mater

ventricles and CSF

• on the floor and wall of each ventricle is a spongy mass of blood capillaries called a choroid plexus

• Ventricles, canals and choroid plexuses are lined with ependyma (ependymal cells)

  • these cells produce cerebrospinal fluid (CSF)

cerebrospinal fluid

• A small amount of CSF fills the central canal of the spinal cord

• most escapes through the 3 openings or aperatures in the 4th ventricle

• apertures lead to the subarachnoid space in the brain surface

• from the subarachnoid space, CSF is reabsorbed by arachnoid granulation (or villi)

  • granulations (or villi) are protrusions through the dura mater into the superior sagittal sinus

brain barrier system

• Strictly regulates what substances can get into the brain tissue fluid

• the blood is also a source of antibodies, macrophages, bacterial toxins, pathogens and other potentially harmful substances

two areas that need to be guarded by the brain barrier system

1. Blood capillaries in the brain tissue = blood-brain barrier (BBB)

2. choroid plexuses = blood-CSF barrier

the medulla oblongata

• The myelencephalon in the embryo becomes one adult structure the medulla oblongata

• It begins at the foramen magnum and extends about 3 cm rostrally

the anterior surface of the medulla oblongata features

• a pair of ridges called the pyramids which contain the corticospinal tracts (motor)

• ridges separated by the anterior median fissure

the olive

a bulge found lateral to each pyramid on the medulla oblongata

the gracile and cuneate fasciculi (sensory) are found

two ridges posteriorly on the medulla oblongata

the descending fibers of the medulla oblongata

they are the corticospinal tracts that fill the pyramids

the corticospinal tracts

• carry motor signals from the cerebral cortex down the spinal cord to the skeletal muscles below the neck

• Most fobers decussate (cross-over)

• Muscles are controlled by the contralateral side of the brain

• tectospinal tract passes through the medulla to control muscles of the neck

signals for functions to enter and leave the medulla not only through the spinal cord, but also by four pairs of cranial nerves, what are they

1. glossopharyngeal nerve (CN IX)

2. Vagus nerve (CN X)

3. Accessory nerve (CN XI)

4. Hypoglossal nerve (CNXII)
   

reticular formation of medulla oblongata

Comprised of many nuclei that extend throughout the brainstem