Central Nervous System - Peripheral Nervous System - see p. 470-489

The P.N.S. consists of 12 cranial nerves and 31 pairs of spinal nerves. These nerves
consist mostly of dendrites and axons, sheathed by collagenous fibrous tissue. Cell
bodies only occur in ganglia or in the Central N.S. Spinal nerves may have autonomic
function, but fibers also serve the Somatic Nervous System of muscles.

Meninges - three layers of connective tissues cover the brain and spinal cord,
serving a protection and aiding in the circulation of cerebrospinal fluid around and
within both structures. Meningitis may be bacterial (meningococcal, such as that
spreading through schools), or viral (transmitted by mosquitoes and rarer). See pg. 463,471.

1. Dura mater - the toughest and outermost layer.
2. Arachnoid - the inner surface of the arachnoid has collagen and elastic fibers.
   It appears "spidery."
3. Pia mater - the innermost layer immediately applied to the spinal cord.
   Between the pia and arachnoid is the subarachnoid space that contains
   cerebrospinal fluid.

 At vertebral level L1 and below, the spinal cord becomes a "horses tail" or cauda equina
of spinal nerves. The reason is that the development of the vertebral canal is faster than
the spinal cord, therefore, the spinal nerves must "chase" their distal openings caudally.
See page 462, look at L1-L5 and S1 to S5.

SPINAL CORD - See p. 471.

Label the skin innervation (dermatomes) of C6, T1, L5, and S2. See p. 516, 518

Gray Matter - spinal cord tissues include a central, butterfly-shaped gray matter.
The "wings" of the butterfly are called the  dorsal horn, lateral horn and ventral horn
of the gray matter, respectively. The "wings" are connected by a centralcommissure.
In the middle of the central commissure is the central or ependymal canal, a remnant
of the embryonic neural tube. See p. 472.
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 Spinal cord sections from cervical to sacral.

White Matter surrounds the gray matter. The white matter is organized into
ascending sensory tracts running toward the brain and descending motor tracts
proceeding away from the brain and down the spinal cord. See below, label the diagram.

Label as above
 

Label the meninges, roots of the spinal cord, the rami communicantes,
and the sympathetic trunk.

 The spinal canal.
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1. Fasciculus gracilis - sensory for the leg. Proprioception or "muscle position sense" and
discriminative touch sensations are carried to the higher brain.
2. Fasciculus cuneatus - sensory for the arm as above.
3. Spinothalamic tracts - ascend from the spinal cord to the thalamus just below the
cerebral hemispheres. The thalamus screens impulses, alerting the cerebral hemispheres to
relevant stimuli. Pain, temperature, crude touch and pressure are sensed.
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See the book and outline the f. gracilis and f. cuneatus, the ventral horn of the
gray matter, and the lateral and ventral (anterior) corticospinal tracts.

Descending-Motor  - see p. 479

1. Corticospinal - as parts of the Somatic N.S., they descend from the motor cortex of the
cerebral hemispheres where muscle movements are planned and ordered to the spinal cord
to spinalnerves to the effectors, the muscles. These pyramidal tracts descend in the pyramids
of the medulla from pyramidal cells in the cortex to skeletal muscles where precise,
voluntary movements take place.
2. Corticobulbar - pyramidal tract which is motor for the head and neck muscles.
3. Rubrospinal - descend from the red nucleus of the midbrain to muscles involved in posture.
The rubrospinal tracts and those below are extrapyramidal tracts.
4. Vestibulospinal - descend utimately from the inner ear to the medulla and on to the
spinal cord. The vestibule controls balance and equilibrium to muscles maintain such.
5. Reticulospinal - descend from the reticular formation of the medulla to muscles
that maintain muscle tone.
6. Tectospinal - descending from the midbrain, they coordinate movements with visual
and auditory stimuli.

The Simple Reflex Arc - See p. 521.

Reflexes are automatic responses to stimuli such as a pin prick or a patellar tap,
hey may be composed of two (monosynaptic) or three (polysynaptic) neurons,
which send the impulse to the spinal cord and back.

The path is as follows: A Sensory Neuron dendritic (receptor) will
pick up a stimulus and send a wave of depolarization via a dendron in a spinal
nerve to a cell body located in the dorsal root ganglion of the spinal cord. The
cell body will make an axon directed to the spinal cord. The spinal cord has an
outer cover of white matter (myelinated processes), and an inner butterfly shaped
gray matter (unmyelinated fibers, cell bodies) area. The axon synapses directly
with a motor neuron or with an Interneuron that then synapses with a
Motor Neuron. The motor neuron axon then begins a long journey to the
effector muscle, which contracts as a response.

The patellar reflex includes monosynaptic arcs. Reflex arcs can be much more
complicated and involve more interneurons; e.g., the tipping-over reflex of ones
arm flexing in the direction of the fall, while the opposite arm extends in an
attempt to catch your balance.

Does your brain command this movement? Does your brain command this
movement? If not, how does the brain know of its occurrence? Can reflexes
occur in paraplegics, below the point of the spinal cord transection (after recovery
from spinal shock, a period of areflexia occurring for a month after the accident) ?
 
 

Polio virus attacks motor neurons in the ventral horn of the gray matter of the spinal cord.
Can polio patients feel their muscles? Can they move them?

Involuntary autonomic impulses to smooth muscle pass through motor neurons in the lateral horns.

The Stretch Reflex - See p. 523.

If a muscle is stretched, it has a receptor called the muscle spindle which detects the stretch
and fires off an impulse to the spinal cord to a motor neuron and back to the stretched muscle
or agonist which reflexively contracts. However, synapses are made with Inhibitory Association
Neurons which send impulses to the antagonist muscle in the opposite muscle compartment,
causing it to relax. The stretch reflex attempts to control muscle length.

The Tendon Reflex - See p. 525.

In the tendons of muscles are receptors called golgi or tendon organs. They detect increased
tension in muscles. If a muscle is developing so much force that the tendon is about to tear, an
impulse is sent to the spinal cord where Inhibitory Association Neurons synapse with motor
neurons which send messages back to the agonist, causing it to relax. Stimulatory
Association Neurons send messages to the antagonist, causing it to contract.

Multi- or Intersegmental Reflex Arcs

Reflex arcs can be much more complicated and involve more than one set of spinal nerves
on one side of the body (ipsilateral). The sensory messages sent to the spinal cord in a
withdrawal reflex (foot withdrawn from a tack) will synapse with association neurons that
will carry the stimulus to segments above and below the spinal nerve that the impulse came
in on, e.g., one comes in on T3 and its spread upward to T2 and downward to T4,
thereby involving more muscles in the reflex.

Contralateral or Crossed Reflexes - See page 526.

The tipping-over reflex will result in one's arm flexing in the direction of the fall, while
the opposite arm extends in an attempt to catch your balance. Association neurons are
relaying directions to the opposite side of the body. When the biceps on the falling side
contracts and its antagonist triceps relaxes, the opposite side biceps relaxes and
triceps contracts. The falling side arm flexes and the opposite arm extends.

Explain why this occurs.
 

DIAGNOSIS WITH REFLEXES
 

 

Reflex tests check the health of the spinal cord and nerves.
See reflex tests from the U. of Fla. at http://www.medinfo.ufl.edu/year1/bcs/clist/neuro.html#AA29

When reflexes are tested, the physician is primarily looking for their absence or exaggeration as
symptoms of diseases. Absence of reflexes occurs in spinal shock, a period of areflexia
of the spinal cord which characterized by swelling and inflammation of the cord after it has
been damaged. After recovery from spinal shock, exaggerated reflex activity will return
to those areas even below the point of a complete transection (reflex arcs will be lost at
the exact point of transection). Cutting of the highways to and from the brain will, of course,
result in loss of voluntary muscle control, sensation and the ability of the brain to influence
the elaboration of the reflex. Why then is this reflexive activity exaggerated?

Exaggerated reflex activity is also seen in spinal cord compression. Christopher Reeve,
the Superman actor, fell in a hose-riding accident and crushed C1 and C2.
What are his symptoms? See pg. 467//483.

Spinal cord injuries are now treated with antiinflammatory drugs like cortisone.
Prognosis is greatly improved if the inflammation and scar tissue forming responses
are weakened.

Diseases like sugar diabetes which causes damage to the circulatory system's ability
to distribute nutrients like glucose and oxygen to neurons, and neurosyphilis can damage
motor tracts so that normal reflexes can not occur.

Upper Motor Neuron Syndrome results when motor neurons are damaged in the
brain or when descending tract of the spinal cord are disrupted. The symptom associated
with this is spastic paralysis such as occurs in compressions, transections or hemisections
(partial cuts) of the spinal cord (see above).

Lower Motor Neuron Syndrome occurs when the ventral horn neurons are damaged
as in polio. The resulting symptom is flaccid (limp) paralysis. See pg. 481.

Spina bifida - a neural tube defect in the spinal cord/column. Spina bifida cystica//myelomeningocole  is
shown in Fig.12.36, p. 483.
 

Common Reflex Tests

1. Patellar reflex (knee jerk) - when the patella is tapped, the quadriceps muscles
contract, causing the leg to extend. See pg. 523.

2. Achilles reflex (ankle jerk) - when the Achilles tendon is tapped, the gastrocnemius
muscle contracts to cause a plantar flexion (big toe and sole down, foot extends).
The Achilles and patellar stretch reflexes are stretch reflexes.

3. Babinski - the outer margin of the sole of the foot is stroked lightly. In children older
than 2 and adults, the normal reflexive response is a plantar flexion, toes curling under,
also called the negative Babinski. In an abnormal positive Babinski, the toes extend or
fan upward.

Absence of these reflexes are seen in chronic sugar diabetics, chronic syphilis infections,
and sometimes in multiple sclerosis and other demyelinating diseases.

How is it that the infant does a positive Babinski rather than a negative Babinski?

Shingles is an outbreak with pain and blistering caused by herpes zoster, the chicken
pox virus. The virus lives in the dorsal root ganglion. Occasionally, for reasons largely
unknown (changes in diet and sunlight exposure are implicated) it migrates down the
dendrons to the surface of the skin.
 

SELECTED SPINAL NERVES AND FUNCTIONS - See p. 508-518.
 
 

Spinal Nerve	Function	Neuritis/Compression or Transection Signs
Cervical Plexus
Phrenic	controls diaphragm,  breathing	“hiccups"/inability to breath
Brachial Plexus
Musculocutaneous	biceps	/can’t flex forearm
Axillary/circumflex ((Watch deltoid injection location!!!)	deltoid	/ can’t raise arm
Radial	triceps, bends thumb,  extends hand	/"wrist drop"
Median	flexes hand	carpal tunnel syndrome - can't flex hand
Ulnar	abducts and adducts fingers innervates little finger	/can't abduct/adduct fingers
Lumbar Plexus
Genitofemoral	scrotum and labia	/loss of scrotal elevation
Femoral	flexors of thigh, extensors of leg	/can't flex thigh, extend leg
Obturator	adductors of thigh	/can't adduct thigh
Sacral Plexus
Gluteal	gluteus muscles	/can’t abduct, adduct or rotate thigh
Sciatic	hamstrings extend thigh, flex leg	/"foot drop," can’t extend thigh
Branches of Sciatic
Tibial	plantar flexes foot, gastrocnemius, post. leg compart	/foot can’t plantar flex, can't stand on toes
Common peroneal	everts foot, peroneus muscles,  tibialis anterior , dorsiflex foot	/can’t evert or dorsiflex foot.  Footdrop and equinovaurus (inversion).
Pudendal	penis, clitoris	/can’t erect either

Brachial plexus

 

Lumbar and Sacral Plexi

Label as above.

Watch where you give gluteal injections!!!  Slipped disks that compress the sciatic nerve can
cause sciatica, a pain that passes down the back of the thigh. Under which muscle does the
sciatic nerve pass?

CRANIAL NERVES - See p. 500-508.
 
 

Number	Function	Clinical application
I-Olfactory	smell	“anosmia” if olfactory nerves are broken by fractured olfactory foramina in the cribriform plate
II-Optic	vision	loss of vision
III-Oculomotor	eye movements of sup., med.&  inf. rectus, inf.obliq	loss causes external strabismus - squinting and eye moves down and  out, diplopia - double vision
IV-Trochlear	eye movement of superior oblique	eye turns down and medially, squinting and diplopia
V - Trigeminal branches:	see below	"Tic douloureux" - stabbing pains in face `
a. Ophthalmic	sensory to upper eyelid
b. Maxillary	sensory for upper teeth
c. Mandibular	sensory for lower teeth,  motor for chewing	paralysis of chewing muscles
VI-Abducens	lateral rectus of eye	eye moves medially, squinting and diplopia
VII-Facial  branches:	facial expressions, taste ant. 2/3 tongue, tears	"Bell's palsy", cannot close  eyes, face sags; sweet, salt, sour taste diminished
a. Temporal	Frontalis muscle
b. Zygomatic	orbicularis oculi
c. Buccal	orbicularis oris
d. Mandibular	depressor anguli
e. Cervical	platysma
VIII-Auditory (Vestibulocochlear)  branches:	hearing and equilibrium
a.Vestibular	equilibrium	“vertigo"
b. Cochlear	hearing	"nerve deafness"
IX-Glossopharyngeal	taste post.1/3 tongue	loss of acute bitter taste, salivation
X-Vagus	digestion, peristalsis,swallowing, slows heart	decreased motility digestive tract, decreased stomach HCl; bradycardia if oversecreting Ach on the pacemaker cells
XI-Accessory	motor to sternocleidomastoids  and clavotrapezius	"wry neck,” cant shrug shoulders
XII-Hypoglossal	motor to tongue	cannot speak, swallow,  tongue points to affected side

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Label the cranial nerves.
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Identify cranial nerves I, VII, and IX and the organs innervated.

Study Questions

1. Explain why spinal taps are done between L3-L5. See pg. 463//470.
2. Compare the causes and symptoms of upper and lower motor syndromes.
3. What would happen if you have paralysis causes by trauma to the median,
radial, ulnar, phrenic, femoral and sciatic nerves?
4. Explain what would happen if C1 and C2 were crushed into the spinal cord,
providing one survived (like Christopher Reeve).
5. Describe and compare the stretch, tendon and Babinski reflexes. What can the physician
determine by their observation?
6. Draw and label a simple polysynsaptic reflex arc.
7. Define and explain, spinal shock, spastic paralysis and flaccid paralysis.
 

jaliff @ gpc.edu