The Nervous System

Central Nervous System (CNS)Peripheral Nervous System (PNS)

Functions Sensory input: monitors internal and

external environments Integration: processes & interprets

sensory information Motor Output: Coordinates voluntary and

involuntary responses of effector organs 2 subdivisions:

CNS – brain and spinal cord (dorsal body cavity)▪ Integration, Intelligence, memory, emotion

PNS – all other neural tissue▪ Cranial nerves and Spinal nerves▪ sensory, motor

Give an example

Include: sensory input integration motor output

Receptors and Effectors Receptors – receive sensory info Afferent division – carries info from receptors

to the CNS (somatic & visceral) Efferent division – carries info from CNS to PNS

effectors (muscles, glands, adipose) Somatic Nervous System (SNS)▪ Controls skeletal muscles (voluntary)

Autonomic Nervous System (ANS)▪ Controls involuntary actions▪ Sympathetic Division (increase heart rate)▪ Parasympathetic Division (decreases heart rate)

The sensory part of the PNS is...

1 2 3 4

6%

47%

35%

12%

1. Somatic division2. Sympathetic

division3. Afferent division4. Efferent division

The fight or flight response is the...

1 2 3 4

6%

24%

0%

71%1. Somatic division2. Sympathetic

division3. Afferent division4. Efferent division

Neurons

Communicate w/other neurons Large Complex Cells:

Soma -cell body Dendrites -receive info Axon -sends signal to synaptic

terminals as nerve impulse Synapse – site of neural

communication (gap) Special characteristics:

Extreme longevity (100 years +) Amitotic – lose ability to divide

(G0) High metabolic rate – O2 & glucose

Cell Body AKA Soma Biosynthetic center Outgrowth of neuron

processes during embryonic development

Lacks centrioles Nissil bodies – Rough ER

stains darkly Nuclei - Clusters of cell

bodies in CNS Ganglia - Clusters of cell

bodies in PNS

Processes

Armlike processes - extend from cell body Tracts - Bundles of neuron processes in CNS Nerves - Bundles of neuron processes in CNS Dendrites

Convey graded potentials towards cell body Short and branching receptive regions Dendritic spines -bulbous ends that form synapses

Axon (single) Generates and transmits nerve impulse away from cell body Axon hillock – cone shaped area where axon extends from soma Nerve fiber – long axon (as long as 4 feet!) Axon collaterals – occasional 900 branch 1,000 - 10,000+ Terminal branches w/ Axon terminals (synaptic

knobs) Myelin Sheath

Protein-lipid electrical insulation on axons Increases speed of transmission Neurilemma – exposed plasma membrane of Schwann cell Nodes of Ranvier – gaps in the myelin sheath (widely spaced in CNS)

Structural Classification of Neurons

Multipolar multiple dendrites & single axon motor neurons most common in humans

Bipolar 2 processes: one dendrite and one

axon cell body between them Rare: special senses (retina &

olfactory) Unipolar

1 continuous dendrites & axon cell body lies to side sensory neurons (ganglia of PNS)

Functional Classification of Neurons

Sensory – afferent division info about surrounding environment position/movement skeletal muscles digestive, resp, cardiovasc, urinary, reprod, taste, and

pain Mostly unipolar (some bipolar in special senses)

Motor – efferent division (response) skeletal muscles cardiac and smooth muscle, glands, adipose tissue Mostly multipolar

Interneurons Integration Brain and spinal cord - memory, planning, and learning Mostly multipolar

Neuroglia of PNS Regulate environment around neurons, smaller &

outnumber neurons 2 Types in PNS: Satellite Cells

Surround neuron cell bodies of NS Function unknown

Schwann Cells Surround nerve fibers of PNS Secrete myelin sheath

Neuroglia AKA Glial cells

4 types inCNS: Astrocytes (most common in

CNS) Radiating processes connect to

capillaries Control chemical environment

Microglia Ovoid shape w thorny processes Moniter nueron health Can turn into macrophages

Ependymal Range shape from squamour to

columnar, usually ciliated Circulate CSF

Oligodendrocytes Wrap around nueron fibers &

produce myelin

The most common type of neuron is

1 2 3

100%

0%0%

1. multipolar2. bipolar3. unipolar

The part of the neuron that has receptor proteins on its surface is

1 2 3 4

94%

0%0%6%

1. Dendrites2. soma3. axon4. Myelin sheath

Action Potential & WKBK

Neurophysiology

Basic Electrical Principles Voltage

measure of electrical charge (mV = 1/1000 V)

potential difference measure between two points

Current – flow of electrical charge from one point to the next, used to do work

Membrane ion channels review

Membrane proteins that allow specific type of ion(s) to pass

Electrochemical gradient: ions move with concentration gradient and along electrical gradients (towards opposite charge)

Chemically (Ligand) gated channels Open when appropriate chemical

(neurotransmitter) binds Voltage gated channels

Open and close in response to changes in membrane potential

Mechanically gated channels Open in response to physical deformation

Non-gated (leakage) channels Always open

Resting Membrane Potential

-70mV (inside of cell is negatively charged in comparison to the outside of the cell)

Is said to be polarized due to difference of ionic concentrations of intracellular and extracellular fluids

Cytosol has low concentrations of Na+, and high conc of K+

K+ ions diffuse out of leak channels causing the cell to be neg inside (more than Na+ leak in)

Na+/K+ pumps stabilizes the resting membrane potential

Graded potentials

Incoming signals over short distance Decrease in magnitude with distance Magnitude dependent upon stimulus Stimulus causes gated channel to open

Receptor potential – heat, light, or other form of energy Post-synaptic potential – neurotransmitter Current carried by ions thru fluid in/out of cells

Positive ions move towards neg areas and vice versa K+ ions move away from depolarized area and accumulate in

neighboring membrane areas neutralizing neg ions Meanwhile positive ions move towards depolarized regions being

momentarily replaced by neg ions (Cl- or HCO3 -), then causing the neighboring membrane to depolarize

The plasma membrane is “leaky” and charge is quickly lost and dissipates quickly

Action potentials AKA nerve impulse Long distance signals of axons (do not decrease) Only cells w/excitable membranes (neurons & muscle) Transition from graded potential to action potential at

the axon hillock Brief reversal of membrane potential (-70mV

+30mV) Depolarization

reduction in membrane potential (less negative) Hyperpolarization

Increase in membrane potential (more negative)

Generation of Action Potential

Resting State – all voltage gated Na+ and K+ gated channels closed

Depolarizing phase – Na+ channels open (increasing + charge…opening more Na+ channels) Critical Threshold reached at -60 to -50mV and becomes

self-generating (+ feedback) Until all Na+ channels open and membrane potential

reaches +30mV Repolarizing phase – internal negativity restored

Na+ channels close, Na+ stops entering cell Potassium channels open, K+ leaves cell

w/electrochemical gradient Hyperpolarization

K+ channels remain open temporarily Na+ channels reset to their original position

Note: electrical conditions restores not ionic conditions, ionic distribution is restored by 1,000’s of Na+/K+ pumps in axon membrane

Propagation of Action Potential Action potential propagates (is

transmitted) away from its point of origin towards the axon terminals

Threshold – unstable equilibrium state Weak stimuli – generate subthreshold

depolarizations that do not generate AP AP is an ALL or NONE Phenomenon

Once AP is generated all alike

Action Potentialmyelinated vs. unmyelinated

Refractory period - When neuron membrane is generating AP and Na+ channels are open, neuron can NOT respond to any other stimulus

Conduction velocity – rate of propagation depend on Axon diameter – the bigger the faster Degree of myelination (insulation – preventing leakage)▪ Continues conduction - unmyelinated conduction is relatively

slow▪ Saltatory conduction – AP triggered only at nodes where Na+

channels are located (30x faster!) Nerve Fiber Classification

Group A – somatic sensory & motor (300mph) Group B & C – viscera sensory, ANS fibers to viscera,

and skin sensory (40mph – 2mph)

An excitatory neurotransmitter

1 2 3 4

25% 25%25%25%1. Increases electrical impulse

2. Causes the release of more neurotransmitters

3. Is released in a synaptic cleft

4. All of the above

The resting membrane potential inside a neuron is

1 2 3 4

25% 25%25%25%1. 0mV2. 30mV3. -60mV4. -70mV

After stimulus, the rush of sodium ions into the cell is called

1 2 3

33% 33%33%1. depolarization2. repolarization3. hyperpolarizatio

n

The action potential is propagated by

1 2 3 4

25% 25%25%25%1. More Na+ rushing into the cell

2. K+ leaving the cell

3. Neurotransmitters binding to dendrite

4. Vesicles release neurotransmitters

The cell’s charge at the peak depolarization is

1 2 3 4

25% 25%25%25%1. 0mV2. 30mV3. -60mV4. -70mV

During repolarization

1 2 3 4

25% 25%25%25%1. The resting potential is restored

2. K+ diffuse out of cell

3. The cell membrane becomes negatively charged again

4. All of the above

Once the action potential reaches the axon terminal, the signal will be carried to the next neuron by

1 2 3 4

25% 25%25%25%

1. Na+ ions2. Neurotransmitte

rs3. K+ ions4. All of the above

If an excitatory neurotransmitter binds to neuron number one, how will that affect the number of neurotransmitter released?

1 2 3

33% 33%33%

1. more2. less3. No effect at all

If previous neuron releases GABA, an inhibitory neurotransmitter, how will that affect neuron #2

1 2 3 4 5 6

17% 17% 17%17%17%17%

1. Increase electrical stimulus

2. Decrease electrical stimulus

3. Increase neurotransmitters released

4. decreased neurotransmitters released

5. 1&36. 2&4

0 of 25

Do Now: You spray your house with insecticide.

Shortly afterwards, you observe roaches lying on the ground with legs and wings twitching uncontrollably. What might the insecticide have done to the bug’s nervous system to cause this reaction?

Multiple Sclerosis is a disease in which the nerve fibers in the CNS lose their myelin. Why would this affect the person’s ability to control their skeletal muscles?

Insecticides

Most insecticides affect the nervous system by disrupting the Acetylcholine Esterase enzyme that regulates the neurotransmitter acetylcholine

ACh accumulates in the synapse repetitively stimulating receptors

Organophosphate pesticides were also used in World War II as nerve agents due to similar effects on humans

What is Multiple Sclerosis? Symptoms: visual disturbance,

weakness, clumsiness, paralysis, speech disturbance

Autoimmune disease Myelin sheaths in CNS gradually

destroyed leaving lesions (scleroses) Causes “short circuiting”, AP slows

until ceases Axons not damaged and more Na+

channels can appear

Synapses Synapse – junction that mediates info transfer from neuron

to neuron (or effector) Presynaptic neuron – conducts impulse towards synapse Postsynaptic neuron-conducts impulse away from synapse

Electrical synapse (uncommon) Gap junctions between adjacent cells that allow for direct

flow of ions and small molecules Rapid transmission for synchronized activity (eye

movements, hippocampus, and embryonic nervous tissue) Chemical synapse – release/receive neurotransmitters

Axon terminal of presynaptic neuron w/synaptic vesicles filled w/thousands of neurotransmitters

Synaptic cleft – fluid filled space in between Neurotransmitter receptor on dendrite membrane

Information transfer across Chemical Synapse

1. Ca2+ channels open in presynaptic axon terminal When nerve impulse reaches axon terminal Ca2+

gated channels also open w/Na+ channels, Ca2+

rushes in causing2. Neurotransmitters are released

Synaptic vesicles fuse w/membrane Ca2+pumped out, or taken in by mitochondria

3. Neurotransmitter binds to postsynaptic receptor4. Ion channels open in the postsynaptic membrane

Receptor changes shape, causing ion channels to open generating graded potential

5. Neurotransmitter effects are terminated Degradation by enzymes Reuptake by astrocytes or presynaptic terminal Diffusion away from synapse

*Note: Synaptic delay – rate determining step b/s slower than AP

Neurotransmitters 50+ have been

indentified Most neurons make 2 or

more Chemical

Classifications Ach Amines Purines Amino Acids Peptides Dissolved Gasses

Functional Classifications Effects

▪ Excitatory – cause depolarization

▪ Inhibitory – cause hyperpolarization

▪ Both – dependent on receptor type

Action Mechanism▪ Direct - bind to ion channels▪ Indirect – long lasting▪ Intracellular 20 messenger

Neural Integration Neurons function in groups Neuronal pools – integrate incoming info in CNS Circuits – patterns of neuronal pools

Diverging circuits▪ Amplify (1 triggers many, which each trigger many more)▪ Sensory & motor

Converging circuits▪ Funnel or concentrating effect▪ Different sensory can have same effect

Oscillating (reverberating) circuits▪ Chain of neurons w/colateral synapses (+) feedback▪ Sleep-wake cycle, breathing, arm swing w/walk

Reflexes Reflex – involuntary response

to stimulus w/o requiring the brain

Particular stimulus always causes the same response

Reflex arc- receptor sensory neuron Interneuron motor neuron effector

Ex. Knee jerk reflex Babinski reflex (infants only)

Stroke sole of foot toes fan out

Plantar reflex (adults only) Stroke sole of foot toes curl

Signals sent to brain by interneurons allow for control Ex. Toilet training, gag, blink