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

Figure 12.11

Sensory neurons enter the spinal cord. Motor neurons leave the spinal cord. Interneurons connect the sensory and motor neurons.

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 ArcDorsal 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 three-neuron 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 1

2 2

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1. Dorsal (posterior) column2. Ventral (anterior) column3. Lateral column

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)

Major Nerves of the Upper

Extremity

Axillary

Musculocutaneus

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

Carpal Tunnel SyndromeApe Hand

Lumbo-Sacral Plexus

• Lumbar: – Femoral nerve

• Sacral: – Sciatic nerve

Nerves of the Lower

ExtremityFemoral

Obturator

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+

K+

K+

K+

K+

K+Na+

Na+ Na+

K+ channels are open most of the time, but Na+ channels are closed unless a nerve stimulates them.

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?

K+

Proteins

-

- -

-

-

---

--

K+K+K+

K+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

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.

Cell membranes contain K+ channels which are often open to allow K+ to flow into the cell and accumulate.

K+

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K+

Proteins

-

- -

-

-

---

--

K+K+K+

K+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

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.

That means that potassium will then flow out of cell until it is equally concentrated on both sides of the membrane.

K+

K+ K+ K+

K+

57

K+

Proteins

-

- -

-

-

---

--

K+K+K+

K+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Since there are now so many Na+

and K+ ions on the outside of the cell, and only a few negatively charged ions, such as Chloride (Cl-), the overall charge of the outside of the cell membrane is positive.

The inside of the cell is negative because there are a lot of proteins inside of a cell, and proteins are usually negatively charged. The negative charge is greater than the positive charges of the remaining K+

K+

K+ K+ K+

K+

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.

Cl-

Cl-

Cl-

Cl-

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.

K+

Proteins

-

- -

-

-

---

--

K+K+K+

K+

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

59

When a muscle cell or nerve cell is stimulated, Na+ channels in the cell membrane quickly open, and the sodium rushes into the cell.

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 overall negative charge.

K+

K+ K+ K+

K+

Cl-

Cl- Cl-

Cl-

Cl-

Cl-Cl-

Cl-

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)

Axon terminals (stimulate another cell)

Dendrites (receive signal)

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 junction, or synapse.

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.

Arteries of the Lower Extremity

External Iliac artery

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)