Nervous System PP’s

Nervous System Divisions

Central Nervous System (CNS)

• Brain

• Spinal Cord

Peripheral Nervous System (PNS)

• All neural tissue outside of the CNS (Commonly referred to as your nerves)

Central Nervous System CNS

Integrates, processes and coordinates sensory data and motor commands

Peripheral Nervous System PNS

Carries sensory info from periphery to CNS
Carries responsive Info from CNS to peripheral tissues + systems

PNS Subdivisions

Sensory

• Brings sensory information to CNS

Motor

• Brings motor commands from CNS

Afferent (SENSORY)

toward CNS from receptors

Somatic sensory receptors

Position, touch, pressure, pain and temperature sensations.

Special sensory receptors

Smell, taste, vision, balance and hearing

Part of somatic sensory division

Visceral sensory receptors

Monitors internal organs

Efferent (MOTOR)

away from CNS to effectors

Somatic NS (SNS)

Provides voluntary control over skeletal muscles

Autonomic NS (ANS)

Provides involuntary regulation of smooth muscles, cardiac muscle, glands and adipocytes

Nervous System Cells

NEURONS

Receive stimuli and transmit action potentials

Can’t divide

GLIAL

Support, nourish, and protect neurons

Can divide

Neuron Anatomy

Cell body (Soma)

Dendrites

input portion, receives

Axons

output portion, sends

Axoplasmic Transport

Movement of materials through axon along neurotubules

Bidirectional

Retrograde Flow

From synaptic terminal to cell body

Synapse

Site of communication between neuron and another cell

Neurons: Structural Classes

Anaxonic neuron

Bipolar neuron

Unipolar neuron

Multipolar neuron

Multipolar Neuron

2/+ dendrites

Single axon

Most common CNS neuron

PNS motor neurons

Bipolar Neuron

One dendritic process

W/ extensive distal branch

One axon

W/ several telodendria and synaptic terminals

Rare; special senses

Eyes, Nose, Ears

Unipolar Neuron

Dendrite Continuous with axon

Soma off to one side

Initial segment located where dendrites fuse

PNS sensory neurons

Anaxonic Neuron

Cannot differentiate between axons and dendrites (star like)

Brain, Special sense neurons

Function not yet understood

local, graded potentials

Which nervous system division would be responsible for receiving touch and pressure information from your fingers?

(Afferent) Somatic Nervous System

Subdivision of PNS

What type of neurons would you be likely to find within your nose?

Bipolar Neurons

(Olfactory Receptor Neurons)

If one's neurons within said nose were to be damaged or lost, can they be replaced? Why or why not?

Yes, Olfactory Sensory Neurons within nose can be replaced

Neurons: Functional Classes

Sensory Neurons

Interneurons

Motor Neurons

Sensory (Afferent) Neurons

PNS Afferent division

Carry info from sensory receptors to CNS

Unipolar

Cell bodies in sensory ganglia

Afferent fibers from sensory receptor to CNS

Afferent Neurons

Somatic Sensory Neurons

Monitor external conditions

Monitor body/limb position

Visceral Sensory Neurons

Monitor internal conditions

Monitor organ systems

Sensory Receptors

Process of specialized sensory neurons OR cells monitored by sensory neurons

Interoceptors

Proprioceptors

Exteroceptors

Interoceptors

Monitor digest, resp, cardiovasc, urinary, & reproduct systems

Provide Sensations of distention, deep pressure & pain

Exteroceptors

Carry info about external environment

Touch, Temp, Press, Sight, Smell, Taste, Hear, Balance

Proprioceptors

Monitor position and movement of skeletal muscles and joints

Motor (Efferent) Neurons

PNS efferent division

Carry instruct from CNS to peripheral effectors

Somatic motor neurons

Visceral motor neurons

Somatic Motor Neurons

Somatic subdivision of efferent PNS

Multipolar Neurons

Innervate skeletal muscles

Under voluntary control

Somatic Motor Neurons

Soma

In CNS

Axon

Extends to periphery to innervate skeletal muscles

Visceral Motor Neurons

Autonomic subdivision of efferent PNS

Innervate smooth muscle, cardiac muscle, glands, and adipose tissue

Under involuntary control

Visceral Motor Neurons (VMN)

Soma in CNS

Axon Innervate second set of VMN at autonomic ganglion

2nd VMN

Solar in autonomic ganglion

Axon Innervates peripheral effectors

Interneurons

Brain, Spinal cord, Autonomic ganglia

Interconnect other neurons

Receive/distribute sensory info

Coordinate motor commands

Higher brain functions

The more complex response, more Interneurons involved

Neuroglia of CNS

Ependymal cells

Astrocytes

Microglia

Oligodendrocytes

Ependymal Cells

Line brain ventricles and central canal of spinal cord

Produces CSF

Some may be ciliated

Astrocytes

Largest and most numerous

Create structural framework

Preform repairs

Guide neuron development

Control interstitial environment

Maintain homeostasis:

Secrete chem that make capillaries of CNS impermeable

Lipid soluble substances cross freely

Water-soluble substances must be actively transported

Microglia

Smallest and rarest

Mobile cells:

Migrate to areas of injured nerve tissue

Engulf invading microbes and dead cell debris

Oligodendrocytes

Have cytoplasmic extensions that wrap around axons

Form the myelin sheaths of neurons

Neuroglia of PNS

Satellite cells

Schwann Cells

Satellite Cells

Amphicytes

Surround neuron cell bodies in ganglia

Regulate interstitial environment

Schwann Cells

Neurilemmocytes

Surround axons

Form a sheath around the axon

Myelin Sheath (CNS & PNS)

Internodes

Areas of axons covered by myelin sheath

Nodes of Ranvier

Gaps between internodes

White Matter Vs Gray

White: Myelinated axons

Gray: Unmyelinated axons

Cell bodies and dendrites

Why do you think it can be important for nerves to have a sheath around them?- what happens to those whose myelin sheathes are damaged/destroyed?

crucial for Insulating nerves, increasing speed of signal, and protecting nerve fibers

-Slow/Block signals, MS Disease

Transmembrane Potential

Difference in electrical charge between inside and outside of cell membrane (voltage)

Outside +

Inside -

Banana in Ocean

Element Charges

K +

Na +

Ca +

Cl - Only negative

ECF VS ICF

OUT

High Na, Cl-, Ca

IN

High K, proteins-

Ion Channels

Protein tunnels for specific ions

3 Types

Leak, Gated, Active Transport proteins

Leak (PASSIVE) channels

Always open

More K leak than Na leak

50-100x more permeable to K than Na

More K leaks out than Na in

Gated Channels

Open and close in response to stimuli

Usually closed at resting potential

Types of Gated Channels

3: Chemically, Voltage, Mechanically

Chemically (LIGAND) Gated Channels

Open in response to specific channels

Locations: cell body and dendrites

Voltage Gated Channels

Open in response to changes to transmembrane potential

have an activation gate and inactivation gate

Found in excitable membranes

Axons (as well as skeletal muscles cells and cardiac muscle cells)

Mechanically Gated Channels

Open response to membrane distortion

Locations: sensory receptors

Touch, press, vibration

Gated (ACTIVE) Channels Summary

Chem Gated (Dendrites, Cell Bodies)

Mech Gated (Sensory receptors)

Voltage Gated (Axons, Axon Terminals)

Active Transport Proteins

Use ATP to push molecules against conc. gradient

ex: Na+/K+ pump

Transmembrane Potential

Passive forces acting across plasma membrane

Chemical Gradients

Diff conc. of specific ions (Na, K, Ca, Cl)

Chem gradient tries to equalize concentrations via diffusion

Electrical Gradients

Difference in positive and negative charges and outside the cell

Result in potential difference / resting potential

Neurons -70mv

Active forces acting across plasma membrane

Na/K ATPase/exchange pump

Establishing Transmembrane Potential

Established and maintained at -70mv by

Sodium potassium ATPase pump

Ion channels

Na+ / K+ Exchange Pump

Moves 3 Na ions out for every 2 K in

Inside gets less positive or more negative

Na/K pump

Na wants to move into cell

K wants to move out of cell

Must move through ion channels

TMP & electrochemical gradients

K ions

Chemical gradient stronger than electrical gradient

Wants to move out

Na ions

Chemical gradient combined with electrical gradient

Wants to move in

Equilibrium Potential

Neurons transmembrane potential at which there is no net movement of ions across a membrane

Resting Membrane Potential

Transmembrane potential of a resting (Unstimulated) neuron

Usually about -70 mV

Changes in Transmembrane Potential

Rises or falls in response to temporary changes in membrane permeability to Na and K

Open/Close Na and K gated Channels

Membrane Potential Changes

Depolarization

Repolarization

Hyperpolarization

Depolarization

Gated Na Channels open

Na enters cell

Membrane potential becomes more positive (less negative)

Repolarization

Gated Na channels close

Na/K pumps move Na back out of cell

Membrane potential returns to resting level -70 mV

Hyperpolarization

Gated K channels open

K exits cell

Membrane potential becomes more negative (-80 mV)

Return to resting potential

Gated K channels close

Na/K pumps move K back into cell

Membrane potential returns to resting level (-70 mV)

Electrical Signals

Graded potentials

Affect only small portion of cell membrane

Allows communication over short distances

Action potentials

Affect the entire surface of cell membrane

Allows communication over long distances

Electrical Signals Sections

Graded Potentials

Dendrites, Cell bodies, Synaptic terminals

Action Potentials

Axons

Graded Potential Phases

4:

Resting,

Stimulation,

Depolarization

Repolarization

OR

Hyperpolarization

Return to Resting

GP: Resting Cell

Transmembrane potential -70mV

Chem regulated Na Channels closed

GP: Stimulation

Membrane exposed to chemical

Chem regulated Na Channels open

Na ions begin to enter cell

GP: Depolarization

Movement of Na into cell depolarizes membrane (At stimulation site)

Potential moves from -70mV to towards 0 (more positive, less negative)

Movement of Na into cell also produces local current that depolarizes adjacent membrane segments

GP: Repolarization

Chemical (stimulus) removed

Chem regulated Na Channels close

Na/K pump removes Na from cytosol

Transmembrane potential -70mV

GP Stimulation Strength & Distance

The stronger stimulation:

more chem regulated Na channels open

more Na ions enter cell

greater degree of depolarization

larger area is depolarized

The greater distance from stimulation:

smaller effect on adjacent membrane segments

Only produce local effects

Electrical Signals

Graded Potentials

Affect only small portion of cell membrane

Allows communication over short distances

Action Potentials

Affect entire surface of cell membrane

Allows communication over long distances

Threshold

Transmembrane potential at which action potential begins

Happens around -55mV

Degree of depolarization that must occur by GP to start an action potential

Once threshold reached, action potential triggered

All or Nothing

Action Potentials

6 Phases:

Resting

Threshold

Depolarization/ Activate

Repolarization/ Inactivate

Hyperpolarization

Return to Rest

Resting Phase

Transmembrane potential around -70mV

Voltage gated Na channels are closed

Activation gate is closed

Inactivation gate is open (still no movement of Na ions)

Threshold Phase

Graded Potential (in AXON HILLOCK) depolarizes initial segment of axon to threshold (-55mV)

Depolarization/ Activation

Voltage gated Na channels open

Activation gate opens

Na ions rush through membrane, causing depolarization within axon

Repolarization/ Inactivation

When membrane potential reaches about +30mV

Voltage gated Na ion channels inactivate

(Inactivation gate closes)

Membrane potential reaching +30mV,

Voltage gated K ion channels open

K rushes out of cell, repolarizing membrane of axon

Repolarization Phase Membrane

As membrane potential reaches around -40mV,

Voltage gated Na channels reset:

Activation Gate closes

Inactivation Gate opens

Hyperpolarization

As membrane potential reaches around -90mV,

All voltage gated K ion channels close

membrane is still hyperpolarized

Return to Resting Phase Membrane

Na/K pumps returns transmembrane potential back to resting point (-70mV)

Pumps Na back out, pumps K back in

Refractory Period

Period of time during which another action potential cannot be generated

Begins with Depolarization

Ends when Repolarization is near completion

Absolute Refractory Period

Membrane cannot respond to another stimulus

Why? - Na Channels are already open / inactivated

Relative Refractory Period

Membrane potential almost normal

Na channels in ready state

K channels are not yet closed

Larger than normal stimulus can initiate action potential

Membrane is still hyperpolarized (-90mV)

Action Potential Propagation

Movement of action potential

From axon hillock, along axon, ends at synaptic terminals

Continuous or Saltatory

Action Potential Propagation Cont VS Salt

Continuous

Unmyelinated axons

Slower

Every Portion of Axon membrane must depolarize

Saltatory

Myelinated axons

Faster

Only Axon membrane at nodes of Ranvier must depolarize

Continuous Propagation EXP

Domino effect

Propagation can only occur in one direction. Why do you think thats the case? (Keep in mind steps of involved in action potential)

Due to refractory period of voltage gated Na

Saltatory Propagation EXP

‘Jumping’ to each Node of Ranvier only with sections covered in Myelin

Myelin prevents flow of ions across membrane

local current skips over internode and depolarizes node of Ranvier

Saltatory = Saltar- Leap

Multiple Sclerosis

Causes damage to myelins and supporting cells

Affects normal neural signaling and possible degradation of axon itself

Common clinical symptoms

Vision probs, Sensory disturb, Motor weak, Imbalance

Propagation Speed

The larger the axon diameter, the faster the action potential moves.

Type A (position, balance, vision, motor, commands)

Large; myelinated; over 300mph

Type B (sensory, other than vision)

Medium; myelinated; around 40mph

Type C (smooth muscle, gland secretion)

Small; Unmyelinated; about 2mph

If action potentials are all or nothing, how does sensation become more intense?

Through frequency coding and population coding

Stronger stimulus = neurons fire more frequently/S, recruits larger number of sensory neurons to fire simultaneously

Synapse

Site of communication between neuron and another cell

Synapse Sections

Presynaptic neuron

Synaptic cleft

Postsynaptic neuron

Types of Synapses

Electrical

Direct physical contact between cells

NS (rare) - eye, areas of brain, some ganglia

Chemical

Signal transmitted across a gap (synaptic cleft) by chemical neurotransmitters

Electrical Synapse

Action potential always produced in postsynaptic cell

Chemical Synapse

Most common - In all synapses between neurons and other cells

Cells not in direct contact → synaptic cleft