Download Spinal Cord and the Peripheral Nervous System

Peripheral Nerves and
Arteries
• CNS = Central Nervous System (Brain and spinal cord)
• PNS = Peripheral Nervous System (nerves to appendages)
• Neurons are grouped functionally according to the direction
the nerve impulse travels relative to the CNS.
• Sensoroy Neurons (afferent neurons) transmit impulses
toward the CNS. They originate in the PNS and terminate in
the CNS.
• Motor Neurons (efferent neurons) transmit impulses from
the CNS to effector organs (muscles and glands). They
originate in the CNS and terminate in the PNS.
• Interneurons (association neurons) connect sensory
neurons to motor neurons within the spinal cord and brain.
They originate and terminate in the CNS, and form complex
neuronal pathways. They make up 99.98% of the neurons in
the body, reflecting the vast amount of information
processed in the CNS.
Sensory Input and Motor Output
 Sensory (afferent) neurons are those that
pick up sensory signals from receptors in the
fingers, toes, etc

Bundles of the same kind of sensory neurons
travel together as NERVES, and go from the
PNS to the CNS
 Motor (efferent) neurons originate in the
brain, and the signals are carried away from the
CNS and go to the muscles and glands.
Information IN
Sensory or “afferent” neurons carry
information into the CNS from receptors
located throughout the body.
Information OUT
Motor or “efferent neurons” carry
electrical impulses away from the CNS
to innervate “effector organs,” like
muscles and glands.
Neurons Classified by Function:
Sensory vs. Motor Neurons
Sensory neurons enter the
spinal cord. Motor neurons
leave the spinal cord.
Interneurons connect the
sensory and motor neurons.
Figure 12.11
Sensory Receptors in Skin
•They detect
sensory input and
convert them into
electrical impulses
that will travel up
neurons along the
spinal cord.
•Sensory input
about touch, pain,
heat, cold, pressure.
The Reflex Arc
Dorsal root ganglia
contain cell bodies
of sensory neurons
Dorsal Root Ganglion
Simple Reflex Arc
 In the spinal cord, these three neurons together
(sensory, lower motor, and interneuron) form the
SIMPLE REFLEX ARC. They process information
without the brain. So if you touch a hot stove, the
sensory input comes into the spinal cord, the
association neurons send the information to the lower
motor neurons, the muscle contracts, and you take
your hand off the stove before your brain even knows
it. This is an example of a withdrawal reflex.
 Simple reflex behavior involves three neurons, and
no brain involvement. Reflexes are automatic events.
They involve both motor and sensory neurons, they
are rapid, involuntary, and they involve multiple
synapses.
 KNEE-JERK REFLEX is also an example of a threeneuron reflex.
Spinal Cord Reflexes
 Stretch Reflex (knee-jerk; patellar reflex)
Muscle contracts in response to a sudden stretch
force (with a reflex hammer).
 After a severe spinal cord injury, all spinal reflexes
are lost below the level of the injury for 2 weeks.
Then the patellar reflex returns but it is often
exaggerated (hyper-reflexic), indicating damage is
still present.
 Withdrawal Reflex



The body part is quickly removed from a painful stimulus.
Sensory neurons carry the information to the spinal cord,
and the muscles remove the limb immediately, before the
brain receives the pain information.
Three-Neuron Reflex
Figure 12.18a, b
Withdrawal Reflex
 When you touch a hot object, the sensory neuron
sends the impulse to the spinal cord where it
synapses on an interneuron.
 The interneuron synapses on a motor neuron
 The motor neuron tells your muscles to contract
to remove your hand.
 While you are taking your hand off the hot object,
a branch of the sensory neuron travels to the
brain to report the sense of pain. However, your
hand has already been withdrawn because of the
reflex command from the spinal cord.
Sensory Information goes to brain
 All sensory information goes to the brain for
interpretation.
 To get there, the axons have to travel up the
spinal cord.
 Sensory axons travel in the white matter of
the spinal cord to get to the brain.
White Matter
 White matter of the spinal cord forms conduction
pathways called NERVE TRACTS.
 The white matter in each half of the spinal cord is
organized into three columns:
 Dorsal (posterior) column
 Ventral (anterior) column
 Lateral column
 Each column has ascending tracts, which consist of
axons conducting impulses toward the brain and
descending tracts, which consist of axons
conducting impulses away from the brain.
Sensory Tracts are grouped into
columns
 Some tracts in the spinal cord are for neurons
transmitting sensory information about light
touch, pain, and temperature. They travel to the
thalamus in the dorsal column.
 Some tracts in the spinal cord are for neurons
transmitting sensory information about vibration.
They travel to the thalamus in the lateral column.
 Some tracts in the spinal cord are for neurons
transmitting sensory information about balance.
They travel to the cerebellum in the ventral
column.
1. Dorsal (posterior) column
2. Ventral (anterior) column
3. Lateral column
1
1
3
3
2
2
Sensory Tracts
 Therefore, when sensory information enters the spinal
cord, the signal goes to the brain via a TRACT.
 A tract is a collection of axons inside the central
nervous system.
 Sensory axons from neurons transmitting the sense of
touch, temperature, pain, or vibration send a branch to
the thalamus portion of the brain.
 SENSORY TOUCH  SPINAL NERVE 
POSTERIOR ROOT  TRACT (in the dorsal column)
 THALAMUS
Sensory Tracts
 Sensory axons from neurons called
proprioceptors transmit the sense of balance
and will travel to the cerebellum.
 PROPRIOCEPTORS SPINAL NERVE 
POSTERIOR ROOT  TRACT (in the ventral
column)  CEREBELLUM
Tracts to the Brain
 These tracts have various names, depending on
what types of neurons are traveling within them.
 For example, within the dorsal tract is a specific
region called the SPINOTHALAMIC TRACT
which transmits pain and temperature.
 Within the ventral tract is a specific region called
the SPINOCEREBELLAR tract which transmits
signals of balance and position to the
cerebellum.
 There are many other tracts as well. Some tracts
send sensory information to the brain, and some
tracts send motor commands from the brain to
the muscles.
PROPRIOCEPTION NEURONS
 These are sensors within the muscles that measure the
amount of force and movement (they are sensory).
 Proprioception neurons travel up the spinocerebellar
tract. The brain can then interpret whether you are off
balance, then send a command to the muscles to
contract and straighten yourself up so you don’t fall.
 Note that this sense of balance is NOT the same as the
sense of balance from equilibrium in the ears.
Proprioception neurons are located within the muscles.
 During a physical exam, a doctor will test the patient’s
proprioception ability by telling them to close their eyes
and place their finger on their nose. This may indicate a
lesion in the cerebellum. Who else may ask you to do
this test? Alcohol disrupts the cerebellum.
Proprioceptors
•Sensory receptors
that report on
internal events in
your muscles and
joints.
•They report on
muscle stretch and
joint position.
•They generate
electrical impulses
that will travel up
neurons to the CNS.
Proprioception Disorders
 Damage to proprioceptors can occur from
consuming excess vitamin B6 (pyridoxine).
 Patients cannot tell where their body parts
are unless they look at them.
 They have difficulty with all motor tasks
including walking, eating, dressing, etc.
 They must use their vision to watch each
body part to make it move in the right
direction.
Motor command come from the
brain
 All motor commands (the command to move
a skeletal muscle) originate in the brain.
 The cell body of these neurons (motor
neurons) are in the brain. Their axons travel
down the spinal cord.
 Motor neurons travel in the grey matter of the
spinal cord.
 There are always two motor neurons involved
in making skeletal muscles contract: an upper
motor neuron (UMN) and a lower motor
neuron (LMN).
Upper and Lower Motor Neurons
 The motor neuron whose cell body is in the
brain is called an upper motor neuron.
 It relays the signal to the motor neuron whose
cell body is in the spinal cord, called the lower
motor neuron.
 The lower motor neuron leaves the spinal
cord and synapses on a muscle, causing
contraction.
Lower Motor Neurons “Innervate” Muscle
Cells
Neuron “innervates” muscle
and triggers it to contract by
the release of a chemical
neurotransmitter.
Upper and Lower Motor Neuron
Diseases
 Some diseases only effect the UMN, and
some only effect the LMN.
 Lower motor neuron disorders:


Multiple Sclerosis
Polio
 Upper motor neuron disorder:
 Cerebral palsy
 Upper and Lower motor neuron disease

ALS
Autonomic Neuropathy
 Autonomic neuropathy is damage to
autonomic nerves.
 The autonomic nerves are those that supply
involuntary body functions, including heart
rate, blood pressure, perspiration and
digestion.
 A common symptom in autonomic
neuropathy is dizziness.
 Muscle twitches and lack of sensation are
NOT symptoms, since they are not supplied
by autonomic nerves.
Peripheral Nervous System
 Peripheral nerves are those that are outside
of the spinal cord.
 We will now discuss those nerves that supply
the arms and legs.
 As these nerves first exit the spinal cord, they
are called spinal nerves.
 They travel in groups; each group of spinal
nerves is called a plexus.
Spinal Nerve Plexi
 Interlacing network
 Each branch carries
fibers from several
spinal nerves
 Gives redundancy in
case of nerve damage
C1-C4- Cervical
plexus
C5-T1- Brachial
plexus
L1-L4- Lumbar
Plexus
L4-S4- Sacral
Plexus
Cervical Plexus
Nerves innervate skin of
neck, back of head and
upper shoulder.
The cervical plexus also
includes the phrenic
nerve (which innervates
the diaphragm, which
allows for breathing)
Brachial
Plexus
Brachial
Plexus
Brachial Plexus
 Damage to Brachial Plexus


Klumpke’s paralysis (brachial plexus damaged
during birth)
Acquired Brachial Plexus injuries
 Crutch paralysis (total upper extremity paralysis)
 Claw Hand / Ape hand (ulnar nerve damage)
 Hand of benediction (median nerve damage)
 Wrist Drop or “Waiter’s Hand” (radial nerve
damage)
Axillary
Musculocutaneus
Major Nerves
of the Upper
Extremity
Axillary Nerve
 Deltoid
 Teres minor
Musculocutaneus Nerve
 Supplies anterior muscles of the arm



Biceps brachii
Brachialis
Coracobrachialis
Median Nerve
 Supplies no muscles of the arm
 Supplies anterior forearm (except flexor carpi
ulnaris)
 Damage can cause



Carpal Tunnel Syndrome
Hand of benediction
Ape Hand
Ulnar Nerve
 Supplies flexor carpi ulnaris
 “Funny Bone”
 Damage can cause claw hand
Radial Nerve
 Supplies muscles on the posterior arm and
forearm



Triceps brachii
Extensor carpi radialis
Extensor digitorum communis
 Damage can cause wrist drop
Peripheral damage to the
Brachial Plexus
“Funny Bone”
damage
Ape Hand
Carpal Tunnel Syndrome
Lumbo-Sacral Plexus
• Lumbar:
– Femoral nerve
• Sacral:
– Sciatic nerve
Obturator
Femoral
Nerves of the
Lower
Extremity
Obturator Nerve
 Supplies adductor muscles
Femoral Nerve
 Anterior Thigh

Quadriceps femoris
Sciatic Nerve
 Supplies back of thigh



Biceps femoris
Semimembranosis
Semitendonosis
 Then it branches into the common peroneal and
tibial nerves to supply the leg and foot.
 The common peroneal branches into the
superficial and deep peroneal.
Tibial Nerve
 Posterior leg and foot



Gastrocnemius
Soleus
Tibialis Posterior
Common Peroneal Nerve
 Superficial branch


Lateral side of leg
Supplies peroneal muscles (lateral leg)

Peroneus longus, brevis, and tertius
 Deep branch


Supplies anterior leg muscles
Injury causes “Foot Drop”
Dermatomes
 You don’t need to memorize the dermatome
map; just know that a dermatome is the area
of skin innervated by a cutaneous branch of a
spinal nerve at a particular level.
Dermatome Map
Action Potentials
Cell membranes have gates embedded in them that allow only certain ions to go
through. These are called ion channels.
K+ channels are open most of the time, but Na+ channels
are closed unless a nerve stimulates them.
Na+
K+
K+
Na+
Na+
K+
K+
K+
K+
Therefore, K+ can go back and
forth from the cell, but Na+ is
blocked from entering the cell.
The outside of most cell membranes is positively
charged compared to the inside of the cell
membrane. How does this happen?
Na+
There are many potassium ions
(K+) on the inside of the cell
because the cell membrane is
more permeable to K+ then to
other ions.
Na+
Na+
Cell membranes contain K+
channels which are often open to
allow K+ to flow into the cell and
accumulate.
Na+
Na+
Na+
Na+
Na+
K+ K+
K+ K+ K+
Proteins - K+
However, that creates an area of
high concentration of potassium
on the inside of the cell, and all
molecules tend to diffuse from an
area of high concentration to an
area of low concentration.
Na+
Na+
That means that potassium will
then flow out of cell until it is
equally concentrated on both
Na+
sides of the membrane.
Na+
Na+
Na+
Na+
K+ K+
K+ K+ K+
Proteins - -
Na+
K+
K+
K+
K+
K+
56
There are also a lot of sodium ions (Na+) on the outside of the cell. The cell
membrane contains Na+ channels, but they are often closed, so this so they tend
to stay on the outside of the cell.
ClSince there are now so many Na+
+
Na
+
and K ions on the outside of the
cell, and only a few negatively
charged ions, such as Chloride
Na+
(Cl-), the overall charge of the
outside of the cell membrane is
positive.
Na+
Na+
Na+ Cl
Na+
Na+
K+ K+
K+ K+ K+
Proteins - -
The inside of the cell is negative
because there are a lot of proteins
inside of a cell, and proteins are
+
Na
usually negatively charged. The
negative charge is greater than ClK+
the positive charges of the
Cl
remaining K+
K+
K+
K+
K+
57
MEMBRANE POTENTIALS
• The charge difference on the outside and inside of the cell
membrane is called the resting membrane potential.
• Resting membrane potential is calculated at the point at
which potassium has reached equilibrium.
• The resting membrane potential is usually positive, since
sodium ions are also present when potassium is at its
equilibrium.
• The resting membrane potential will be changed if the
membrane permeability to one or more ions is
selectively altered.
When a muscle cell or nerve cell
is stimulated, Na+ channels in the
cell membrane quickly open, and
the sodium rushes into the cell.
ClCl-
That causes the inside of the cell
membrane to become positively
charged. The ions left on the
outside are Cl- which have a
negative charge, so the outside of
the cell membrane now has an
Cloverall negative charge.
Cl-
ClCl-
Cl-
Na+
K+ K+
+
K
K+ K+
Na+
+
+
Na
+
Na
+
Na
Na
Proteins - +
Na
- + Na
K+
Cl-
K+
K+
K+
K+
59
MEMBRANE POTENTIALS
• When the outside of a cell membrane goes from a positive
charge to a negative charge, this change is called
depolarization. It only lasts for a short time because the
Na+ channels quickly close, the sodium leaves the cell, and
the outside of the membrane becomes positively charged
again.
• The change back to resting potential is called
repolarization.
• The rapid depolarization and repolarization of the cell
membrane is called an action potential.
ACTION POTENTIAL
• The action potential begins when the charge reaches
a certain threshold.
• An action potential is like an electrical current being
conducted from cell to cell.
• That makes the outside of the cell next to it to also go
from positive to negative and back again.
• The action potential then continues from cell to cell.
When it reaches its target organ, such as a muscle,
the muscle will contract.
• In a muscle fiber, an action potential results in muscle
contraction.
Neuron (nerve cell)
Cell body
Axon (transmits signals)
Dendrites (receive
signal)
Axon terminals
(stimulate
another cell)
Two Neurons Communicate at a Synapse
A synapse refers to the junction between a nerve cell
and another nerve cell or a nerve cell and its target.
•
Presynaptic terminal  SYNAPTIC CLEFT  Postsynaptic terminal
Contains vesicles
filled with
neurotransmitter
Contains receptor
molecules to receive the
neurotransmitter
Direction of action potential
Motor neurons
• Skeletal muscle fibers do not contract
unless they are stimulated by motor
neurons.
• Motor neurons are nerve cells along which
action potentials travel to skeletal muscle
fibers. Axons of these neurons enter
muscles and send out branches to several
muscle fibers.
• Each branch forms a junction with the
muscle fiber called a neuromuscular
If one neuron sends a signal, only its muscle fibers (the motor unit)
contract.
This allows for strength variations in lifting a chair vs. an eraser. For
full strength, all the motor units contract. For half strength, half of
the motor units contract.
Motor unit at a
neuromuscular
junction
• The axon terminal of the nerve
cell rests in indentations in the
cell membrane of the muscle
fiber.
• The enlarged knob of the axon is
called the presynaptic terminal
• The space between the
presynaptic terminal and the
muscle fiber membrane is the
synaptic cleft
• The muscle fiber membrane is
the postsynaptic membrane.
• The axon terminal contains
vesicles of acetylcholine, which
is a neurotransmitter that
stimulates the post synaptic cell.
• An action potential causes the release of Ach
(acetylcholine; the neurotransmitter at the
neuromuscular junction) into the synaptic cleft.
• Ach binds to receptor sites on the muscle fiber (muscle
cell) membrane. This opens up the Na+ channels so that
sodium rushes into the cell.
• When Na+ brings its positive charge to the inside of the
cell, it causes the outside of the cell to become negative
(depolarization). Then the Na+ channels close, the Na+
exit the cell, taking their positive charges with them, and
the membrane repolarizes. This event is called an action
potential, which travels along the length of the muscle
fiber and causes it to contract.
• The Ach that was released is rapidly broken down by an
enzyme, Ach-ase (acetylcholinesterase). This ensures
that the action potential will result in only one contraction
of each muscle fiber.
Acetylcholine Antagonists
• Some INSECTICIDES inhibit acetylcholinesterase, so Ach
accumulates in the synaptic cleft and acts as a
constant stimulus to the muscle fiber. The insects die
because their respiratory muscles contract and cannot
relax.
• Other poisons, such as CURARE, the poison used by
South American Indians in poison arrows, bind to the Ach
receptors on the muscle cell membrane and prevent Ach
from working. That prevents muscle contraction,
resulting in flaccid paralysis.
Myasthenia gravis
• Myasthenia gravis is an autoimmune disorder in which antibodies
attack and destroy some acetylcholine receptors.
• Acetylcholine is therefore less likely to stimulate muscle contraction,
resulting in muscle weakness and fatigue.
• Symptoms usually begin in the eyelid and facial muscles, and
manifests as drooping muscles on half or both sides of the face,
drooping eyelids, and slurred speech.
• Neostigmine is an anti-cholinesterase drug which reduces the
symptoms by inhibiting Ach-ase activity, preventing the breakdown
of Ach. Consequently, Ach levels in the synapse remain elevated, so
Ach is available to bind to those few functional Ach receptors that are
left.
Arteries of the
Upper Extremity
Arteries of the Upper Extremity
 Subclavian artery is so named because it goes under the
clavicle.
 The subclavian artery changes names when it enters the axilla.
It is now called the axillary artery.
 The axillary artery supplies triceps brachii
 The arm area is referred to as the brachium.
 The axillary artery changes names when it leaves the axilla and
enters the arm. It is now called the brachial artery (in the arm)
 The brachial artery supplies all the arm
muscles (biceps brachii, coracobrachialis,
brachialis).
 At the elbow, the brachial artery divides into the radial and
ulnar arteries to supply the forearm.
External
Iliac artery
Arteries of
the Lower
Extremity
Arteries of the Lower Extremity
 The abdominal aorta branches into the external iliac
arteries.
 When the external iliac artery leaves the abdomen at
the groin, it enters the thigh and is now called the
femoral artery.

Femoral artery supplies muscles of thigh
(including adductor muscles)
 The femoral artery changes names and becomes the
popliteal artery when it reaches the knee.
 The popliteal artery changes names to tibial artery in
leg. The tibial artery supplies the leg muscles.
Peripheral Vascular Disease (PVD)
 Refers to the obstruction of large arteries, frequently in
the lower extremity. Usually caused from
atherosclerosis (fatty plaques).
 Symptoms
 Claudication: pain, weakness, numbness, or
cramping in muscles due to decreased blood flow
 Sores, wounds, or ulcers that heal slowly or not at all
 Change in color (blueness or paleness) or
temperature (coolness) when compared to the other
limb
 Diminished hair and nail growth on affected limb and
digits (shiny, hairless skin)