EXSS 155: The Nervous System--Overview and Divisions, Neurons and Synapses, The Brain, The Spinal Cord, Reflex Arcs, Cranial Nerves, and Neurophysiology and Action Potentials

Conus medularis

true end of the spinal cord

Cauda equina

"horse's tail", branching of nerve fibers below the spinal cord

Filum terminale

anchors spinal cord to coccyx

Map of Spinal Cord and Spinal Nerves

Gray matter in the Spinal Cord

processing

White matter in the Spinal Cord

the "two and from". While because of myelin.

Nerve Pairs: Cranial

8 pairs

Nerve Pairs: Thoracic

12 pairs

Nerve Pairs: Lumbar

5 pairs

Nerve Pairs: Sacral

5 pairs

Nerve Pairs: Coccygeal

1 pair

Cervical Nerve Pairs...

above vertebrae (except C8)

Thoracic and Lumbar Nerve Pairs...

below vertebrae

Cervical Plexus

C1-C5

Brachial plexus

C5-T1

Lumbar Plexus

L1-L4

Sacral Plexus

L4-S4

Cervical Plexus: Hypoglossal (CN XII)

1st branch; vertical

Cervical Plexus: Lesser occipital

2nd branch; vertical

Cervical Plexus: Greater Auricular

3rd branch; vertical

Cervical Plexus: Transverse Cervical

Straight across big loop

Cervical Plexus: Superior root of ansa cervicalis

Upper big loop

Cervical Plexus: Inferior root of ansa cervicalis

Lower big loop

Cervical Plexus: Supraclavicular

2nd lowest; pitchfork

Cervical Plexus: Phrenic

Lowest branch; going to diaphragm

Brachial Plexus: Musculocutaneous

Letter M; Upper

Brachial Plexus: Axillary

Letter V; Upper

Brachial Plexus: Median

Letter M; middle

Brachial Plexus: Radial

Letter V; lower

Brachial Plexus: Ulnar

Letter M; lower

Lumbar Plexus: Iliohypograstric

1st branch; fork

Lumbar Plexus: Ilioinguinal

2nd branch

Lumbar Plexus: Genitofemoral

Drapes over Lateral cutaneous nerve of thigh

Lumbar Plexus: Lateral cutaneous nerve of thigh

Genitofemoral crosses this nerve

Lumbar Plexus: Femoral

Lower; thick

Lumbar Plexus: Obturator

Most inferior

Sacral Plexus: Sciatic

Huge!
Common Fibular (posterior): lateral side

Tibial (anterior): medial side

Sacral Plexus: Superior gluteal

Most superior branching off sciatic

Sacral Plexus: Inferior gluteal

Middle branching off sciatic

Sacral Plexus: Nerve to piriformis

Most inferior branching off sciatic

Reflex: Definition

fast, predictable, automatic response to changes in the environment that helps to maintain homeostasis
- Simplest type of pathway
- Level of brain involvement varies
- How ANS operates
- Some inborn (touch hot stove)
- Some learned (driving)

Gray vs. White Matter in Brain

Gray: integration center for reflexes
White: highways for nerve impulse propagation

Reflexes: Necessary Components

1. Receptor
2. Sensory Neuron (posterior)
3. Integrating Center (brain or spinal cord)
4. Motor neuron (movement; anterior)
5. Effector (muscles or glands)

Sensory Receptors in Muscle: Muscle Spindle

1. Senses muscle length (how much stretch)
2. Muscles attached to spindle stretch
3. Neurons "report" stretch to CNS
4. Muscles contract (reflex) to resist further stretching

Sensory Receptors in Muscle: Golgi Tendon Organs (GTO)

1. Sense changes in muscle tension
2. Located close to tendon and muscle attachment
3. Inhibit agonist muscle contraction and excite antagonist muscles to prevent injury

Mechanoreceptors:

respond to mechanical force such as touch, pressure, vibration, and stretch

Theromoreceptors:

respond to changes in temperature

Photoreceptors:

respond to light to allow vision

Chemoreceptors:

respond to chemical stimuli from foods, odors, and changes in blood concentrations

Osmoreceptors:

respond to changes in the osmotic pressure of body fluids

Cranial Nerves: Map and Basic Information

Sensory, motor, and mixed nerves
Name and roman numerals to identify

Cranial Nerves: How many pairs?

12

Cranial Nerve 1

Olfactory Nerve

Olfactory (I) Nerve

Sense of smell
Sensory nerve
Olfactory cells converge to become nerve

Cranial Nerve 2

Optic

Optic (II) Nerve

Vision
Sensory Nerve
Ganglion cells in retina of each eye join to form

Cranial Nerve 3

Oculomotor

Oculomotor (III) Nerve

Supply extrinsic eye muscles to control adduction and downward movement of eyeball, movement of upper eyelid, and reaction to light
Motor nerve
Originates in Midbrain

Cranial Nerve 4

Trochlear

Trochlear (IV) Nerve

Downward movement of eyeball
Motor nerve
Originates in Midbrain

Smallest of cranial nerves

Cranial Nerve 5

Trigeminal

Trigeminal (V) Nerve

Touch, pain, temperature. Chewing (mastication) muscles.
Mixed Nerve

Branches: Opthalmic, maxillary, mandibular
Largest Nerve

Cranial Nerve 6

Abducens

Abducens (VI) Nerve

Abduction of eyeball
Motor nerve
Originates from pons

Cranial Nerve 7

Facial

Facial (VII) Nerve

Sensory: Taste
Motor: Facial Expression
Mixed Nerve

Cranial Nerve 8

Vestibulocochlear

Vestibulocochlear (VIII) Nerve

Vestibular branch: equilibrium
Cochlear branch: hearing
Sensory Nerve

Originates in inner ear

Cranial Nerve 9

Glossopharyngeal

Glossopharyngeal (IX) Nerve

Sensory: Taste
Motor: Saliva
Mixed Nerve

Cranial Nerve 10

Vagus

Vagus (X) Nerve

Sensory: proprioception (where we are in space), stretching
Motor: swallowing, vocalization
Mixed Nerve

Main nerve of Parasympathetic Nervous System (rest/digest)

Cranial Nerve 11

Spinal Accessory Nerve

Spinal Accessory (XI) Nerve

Supplies Sternocleidomastoid and Trapezius
Motor Nerve

Cranial Nerve 12

Hypoglossal

Hypglossal (XII) Nerve

Speech and swallowing
Motor Nerve

Cranial Nerve Map

Resting Membrane Potential (RMP)

- Accumulation of negative ions inside the cell (cytosol), and positive ions outside the cell (extracellular fluid)
- RMP: Negative charge difference inside compared to outside: -70mV (maintained by Na/K pump
- Changes in membrane potential occur when ion gates in the membrane open, permitting ions to move from one side to the other
If membrane potential becomes less negative (by 15-20 mV) then it reaches the threshold and it results in an action potential
*Threshold value is -55. If we become more positive (-55), it is considered a meaningful change = action potential

Graded Potentials

small, localized deviations from the RMP of -70mV

Characteristics:
- vary in amplitude
- localized
- depend upon the strength of the stimulus
- occur most often in the dendrites and cell body of a neuron

Graded Potentials: the amplitude of a graded potential depends on the...

stimulus strength

Graded Potentials: Shifts in Membrane Potential

Polarization/
Hyperpolarization: membrane potential becoming more negative
Depolarization: becoming less negative (more positive)

EPSP: Excitatory Postsynaptic Potential

- excitatory
- depolarizing (Na influx)
- one doesn't usually initiate a nerve impulse (aka graded potential, takes more than 1)

IPSP: Inhibitory Postsynaptic Potential

- inhibitory
- hyperpolarizing (K outflux)
- makes harder to generate action potential

Summation

effects of presynaptic neurons may be combined to generate a nerve impulse

Integrator

postsynaptic neuron that recieves and integrates signals, then responds

Summation may be:

1. Spatial
2. Temporal

Spatial Summation

neurotransmitters released from several presynaptic end bulbs onto 1 postsynaptic neuron
"space" issue

Temporal Summation

neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a neuron
"time" issue

What is an Action Potential?

1. "Sequence of rapidly occuring events that starts with depolarization (rushing in of Na) and then restore it to the resting state (repolarization, with outflux of K)"
2. Rapid, transient change in started with depolarization
3. Starts as graded potential in response to stimuli
4. Requires depolarization of the membrane
5. Once threshold is met, the all or none principle applies

Events during AP:

1. Resting state (RMP)
2. Depolarization
3. Propagation of an action potential
4. Repolarization
5. Return to the resting state with help from the Na/K pump

Depolarizing Phase

- Upstroke
- Stimulus causes a graded potential to reach at least 55mV or threshold
- Voltage-gated Na+ channels open & Na+ rushes into cell
- Gate closes again in few ten-thousandths of second
- Positive feedback process

Action Potential Propagation (Spreading)

- Spreads over cell surface without dying
- Na+ flows in during depolarization, the voltage of adjacent areas is affected and their voltage-gated Na+ channels open
- Self-propagates along the membrane
- Traveling action potential is called a nerve impulse

Action Potential Conduction Velocity: Depends on...

Diameter of nerve fiber (larger=faster)
Myelination of fibers
Nerves (motor=fast, sensory=slow)
Temperature (higher=faster)
*Not related to stimulus strength

Continuous Conduction

slow conduction that occurs in nonmyelinated axons

Saltatory Conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.

Repolarizing Phase

- Downstroke
- When threshold is reached, voltage-gated K+ channels open
- K+ channels open much slower than Na+ channels
- K+ outflow returns membrane potential back to -70mV
- If enough K+ leaves cell it reaches a -90mV membrane potential (hyperpolarization)

All-or-none principle

if a stimulus reaches threshold, the action potential is always the same
*A stronger stimulus will not cause a larger impulse

Amplitude

change from RMP

Overshoot

change from zero

Absolute refractory period

- even a very strong stimulus cannot initiate a second action potential
- inactivated Na+ channels cannot open
"Absolutely no way to have another AP"