Lecture Outline: The Central Nervous System

A. Anatomy:

1. Cell Body
2. Dendrites
3. Axon
4. Synapse

B. Bio-Electricity

1. Unequal distribution of charge across the cell membrane:

membrane potential: Na⁺, K⁺, Cl^-; protein^-.

2. Resting potential: Electro-Chemical Gradient; Na⁺/K⁺ PUMP
3. Ion Channels: making use of the membrane voltage (*** end lecture 1)
4. Electrical Currents - Ionic Currents (like water current)
5. Action Potential (AP): electrical impulse that travels down axon

C. Synapse: Junction between 2 cells

1. Electrical Synapse:
2. Chemical Synapse: Neurotransmitters / Receptors

D. Neurons can make decisions: "integration".

A. Three Basic Kinds of Neurons: Sensory Neurons; Motor Neurons; Interneurons.
B. Neurons are organized in circuits - connected one to the other via synapses
C. Evolution of a brain

1. Cephalization
2. behavioral selective pressures to elaborate brain?
3. Vertebrate: forebrain; midbrain; hindbrain; spinal cord

IV Human Brain / CNS

A.Two systems: somatic/autonomic nervous system
B. Brain

1. Hindbrain: medulla; pons; cerebellum
2. Midbrain: Reticular formation; inferior colliculi; superior colliculi
3. Forebrain - Cerebrum

a. Diencephalon:

Thalamus
Limbic System: Hypothalamus; Amygdala; Hippocampus; Olfactory bulb
b. Telencephalon

- Right and Left Cerebral hemispheres
- Basal Ganglia
- Cerebral Cortex:

Lecture Notes: The Central Nervous System

I. The Nervous system coordinates behavior.

II. The Neuron: The Fundamental Unit of the Nervous System

A. Anatomy

1. Cell Body - what you would expect in a cell

a. nucleus, site of protein synthesis, etc.
b. SPECIAL: ability to send proteins down axons OR dendrites

2. Dendrites - highly branched

a. INPUT - synaptic connections with "previous neuron" axons
c. Action Potentials? NO; not "electrically excitable"

3. Axon - long and slender, little branching

a. OUTPUT - make synaptic connection "downstream" neuron
b. Action Potential? YES; "electrically excitable

4. Synapse -

a. Junction: neuron-neuron; neuron-muscle; neuron-other cell.
b. 2 types

electrical - cells are essentially joined as one
chemical - neurotransmitter

- neurotransmitter (NT) released from axon / presynaptic side
- NT binds to Receptor in dendrite / postsynaptic side

B. Bio-Electricity

1. The source of bioelectricity is the unequal distribution of charge

across the cell membrane: membrane potential or membrane
voltage.
a. Na⁺, K⁺, Cl^-

- ions; atoms with either one more electron than proton (negative

electrical charge) or one more proton than electron (positive
electrical charge)

b. protein^-

- amino acids with OH groups will loose Hydrogen ion (H⁺) at

physiological pH, leaving Oxygen with one more electron than
proton: protein has an overall negative charge

c. Na⁺, K⁺, Cl^- can diffuse back and forth across cell membrane

but protein^- is trapped inside the cell.


2. Resting potential: charge difference across cell membrane (like battery)

a. High K⁺ IN / High Na⁺ OUT // Inside -70mV relative to outside
b. unequal distribution of charged ions.
c. Na⁺, K⁺, Cl^- can diffuse across membrane, protein^- cannot
d. Electro-Chemical Gradient: protein^- and Cl^-

- negative charges balance

protein^- IN drives Cl^- OUT
this is opposed by Cl^- concentration gradient pulling Cl^- back in

- result is a compromise,

Cl^- IN and OUT are not equal
charge IN and OUT are not equal

- this situation leaves Na⁺in=K⁺in and Na⁺out=K⁺out

unequal distribution of plus charge to compensate unequal neg. charge

e. Na⁺/K⁺ PUMP

- Na⁺ and K⁺ are sorted by a transmembrane protein "pump"

3 Na⁺ "pumped" OUT // 2 K⁺ is "pumped" in // 1 ATP consumed

- RESULT: HIGH Na⁺ out, HIGH K⁺ in

f. OVERALL RESULT: Inside is ~ 70 mvolts negative relative to outside
c. Potential Energy:

Na⁺ "wants to" move IN (would make inside positive)
Cl^- wants to move IN (would make inside more negative)
K⁺ "wants to" move OUT (would keep inside negative)


3. Ion Channels: making use of the membrane voltage

a. ion channels are transmembrane proteins

- penetrate the cell membrane, hole in center that can open and close

b. Two types:

- voltage sensitive: open if membrane voltage changes

restricted to axons

- ligand sensitive: receptors that open of NT binds to them

restricted to synapses (postsynaptic)

c. Voltage Sensitive Ion Channels

- electrical charges of amino acids are in balance with the

ionic charges on either side of the membrane

- the shape of the protein, open or closed, is sensitive to the

surrounding charge: reversing the membrane charge
changes the shape of the protein, opening (or closing) the
channel (hole)

d. membrane voltage (+)out/(-)in: channels closed
e. membrane voltage (-)out/(+)in: channels open
f. ion channels are selective:

- different channels specific for K⁺, Na⁺, Cl^-, Ca⁺⁺


4. Electrical Currents - Ionic Currents (like water current)

a. Na⁺ entering cell through ion channel creates a current loop
b. Na⁺ > ion channel > down cytoplasm > back across membrane (leakage)

> back along extracellular space > back to start (near channel).

c. back current across membrane is small per unit area, but over enough

membrane adds to the amount entering through the channel

d. because some current is constantly leaking across membrane, the

amount of current traveling down cytoplasm decreases with distance


5. Action Potential (AP): electrical impulse that travels down axon

a. FIRST: voltage sensitive Na⁺ channel opens

Na⁺ rushes IN >> membrane voltage reverses

inside changes from negative to positive

Na⁺ current is started down the axon cytoplasm
"Depolarization"

b. SECOND: voltage sensitive K⁺ channel opens, slightly delayed

K⁺ rushes OUT, reversing the effect of the Na⁺
inside changes back to negative (loss of positive charge)
"Repolarization"

c. Action Potential is ALL or NONE

The size (amplitude) of the AP is always the same

d. Action Potentials are self propagating

- the Na⁺ current entering one ion channel influences the membrane

voltage some distance away, opening neighboring ion channels

- RESULT: one ion channel opens the next, and so on down the axon

e. Action Potentials Travel Fast: 10-100 m/sec

- dependent of how far Na⁺ currents can "reach" down cytoplasm of Axon

-- dependent on relative resistance of membrane vs. cytoplasm
-- how "leaky" the membrane is for Na⁺ to move back across

- Increase diameter of axon: Faster APs

-- lowers cytoplasmic resistance relative to membrane resistance

- Increase Membrane resistance: Fastest APs

-- vertebrate neurons: special Schwann cells wrap around axons
-- Membrane wrap is called Myelin; extremely high resistance
-- Gaps in axon between Schwann cells: Nodes of Ranvier

--- voltage sensitive ion channels located at Nodes

-- Fastest APs, as APs "JUMP" from node to node

f. Action Potentials are property of: axons, skeletal muscle, heart


C. Synapse: Junction between 2 cells

1. Electrical Synapse:

a. two neurons physically joined with hole proteins:
b. contiguous cytoplasm; ionic current moves through holes, neuron to neuron


2. Chemical Synapse: Similar to Hormone System

a. Anatomy of Synapse: Presynaptic Cell <> Space <> Postsynaptic Cell
b. Neurotransmitters (NT): Chemical Signals / 4 Chemical Classes

- acetylcholine
- biogenic amines: norepinephrine, dopamine, serotonin (5HT)
- amino acids: GABA, glycine, glutamate, aspartate
- peptides (many)

c. Neurotransmitters are released from presynaptic neuron

- synthesized in presynaptic cell
- stored in membrane vesicles
- release: vesicle exocytosis
- release triggered by Calcium entry

e. NT release is regulated by Ca⁺⁺:

- intracellular [Ca⁺⁺] kept very low, extracellular [Ca⁺⁺] relatively high
- Ca⁺⁺ causes release of vesicles/NT across presynaptic membrane
- AP arrives and synapse >> opens "voltage sensitive Ca⁺⁺ channels"

>> Ca⁺⁺ enters >>> NT is released

- AP stops, Ca⁺⁺ is pumped out of cell, intracellular [Ca⁺⁺] drops


d. Neurotransmitters are detected by Receptors (R):

- Receptors are located in postsynaptic membrane
- Two kinds of Receptors:

-- Type 1: "ligand gated ion channel" (not voltage sensitive)

--- an ion channel that opens if NT binds to it,

ion enters - ionic current electrically excites postsynaptic cell

--- many different types: specific to NT and ion

---- all type of NT
---- Na⁺, Cl^-, Ca⁺⁺

-- Acetylcholine receptors in skeletal muscle are of this type
-- VERY FAST

-- Type 2: "G-protein coupled Receptors", (second messenger)

--- like hormone receptors - second messenger
--- NT-R > G-protein activation > second messenger >>>

ion channel opens > ionic current

--- "SLOWER", Can modify/modulate multiple processes

- NT+R >> excite (Na⁺ or Ca⁺⁺) or inhibit (Cl^-) postsynaptic cell


D. Neurons can make decisions: "integration".

1. Two types of decision:

a. ON or OFF: "fire" an action potential? Yes or No
b. STRONG or WEAK: "fire" one or many APs? Yes or No

- AP is "all or nothing" in size (amplitude)
- Strength is encoded in frequency

-- Multiple APs can release more NT than a single AP

2. AP initiates at axon ONLY IF dendrite is SUFFICIENTLY stimulated
3. Dendrites contain many synapses: effects are SUMMED

a. may be thousands, from several hundred different neurons
b. single activated synapse initiates a small ionic current

ONLY short distance influence

c. multiple synapses contribute many small currents which add together
d. if enough synapses are activated, axon is influenced to fire AP

4. Inhibition / Excitation:

a. Some receptors / synapses are excitatory: create Na⁺ currents
b. Some receptors / synapses are inhibitory: create Cl^- currents
c. The overall currents are added, resulting in a "decision" whether or not

the axon will initiate an AP and at what frequency.

A. Three Basic Kinds of Neurons

1. Sensory Neurons - peripheral neurons conveying sensory info inward
2. Motor Neurons - neurons synapsing onto muscles to simulate contraction
3. Interneurons (Association Neurons) - all the rest


B. Neurons are organized in circuits - connected one to the other via synapses

1. humans - 10¹² neurons (nearly all interneurons)
2. a neuron (e.g. hypocampus) may connect to several hundred other neurons

and itself receive thousands of inputs (synapses)

3. a simple circuit - skeletal muscle reflex

a. the spinal cord has a left and right side
b. motor neuron (MN) cell bodies are located in the ventral region of each half
c. MN axons leave spinal cord, project to skeletal muscle where they synapse
d. sensory neurons in muscle are stimulated during muscle stretch

"stretch receptors" - activated when you hit tendon just below knee

e. sensory neuron sends axon into spinal cord, making synapse on local

interneuron in dorsal region of same side half.

f. interneuron synapses onto motorneuron, as well as other local neurons
g. muscle stretch

stretch receptor fires

interneuron stimulated

interneuron stimulates motor neuron

motor neuron fires > stimulates muscle to contract

h. local interneuron signals activity to brain via other interneurons
i. brain communicates command decision re. muscle movement via axons

down spinal cord - synapsing onto local interneurons in spinal cord which
activate motor neurons.


C. Evolution of a brain

1. Cephalization

a. Bilateral Symmetry with "brain" observed in flat worms

acoelomate animals - very "primitive"

b. concentration of sensory systems in "head"
c. brain coordinates sensory input with MEANINGFUL motor output
d. increasing anatomical complexity with increasing behavioral complexity

2. behavioral selective pressures to elaborate brain?

a. avoiding being eaten
b. eating
c. sex

3. Vertebrate

a. Basic structures / parts

forebrain

- complex sensory input - information processing - memory - emotions

midbrain - relays
hindbrain - much motor coordination
spinal cord - local motor control

b. Fish vs. Human - primitive to you

fish have the parts, however, great elaboration of cerebral cortex is seen

IV Human Brain / CNS

A.Two systems:

1. somatic nervous system: responses to external environment
2. autonomic nervous system: coordinates internal organs

sympathetic system: cell bodies in spinal cord
parasympathetic division: cell bodies in medulla:

STRUCTURE		PARASYMPATHETIC			SYMPATHETIC
pupil			constricts			dilates
salivation		stimulates			inhibits
heart			slows				accelerates
bronchi (lungs)		constricts			relaxes
stomach			stimulates			inhibits
			stimulates gall bladder 	stimulates liver

B. Brain

1. Hindbrain:

a. medulla -

enlarged extension of spinal cord
neuron cell bodies at center
autonomic functions: breathing, heart rate, blood pressure, swallowing

b. pons

influence transitions between sleep and wakefulness, stages of sleep
rate and pattern of breathing

c. cerebellum - .

coordinates body movement
compares:

sensory info from position detectors in joints and muscles
sensory info from visual and auditory systems
info from cerebrum re. commands to move

coordinates info into smooth output; balance, coordination

e.g. hand - eye coordination


2. Midbrain

a. Reticular formation

relay between medulla and forebrain
sleep, arousal, emotion , muscle tone, certain movements and reflexes
filters sensory input before it reaches conscious levels
decides which stimuli require attention

e.g. mother sleeping through traffic noise but waking to baby cry

b. inferior colliculi - part of auditory system

all fibers involved in hearing ass through or terminate here

c. superior colliculi - part of visual system

in non-mammalian vertebrates: optic lobes, maybe only visual centers
in mammals, vision integrated in forebrain, leaving SC to coordinate visual
reflexes and perform limited perceptual functions.


3. Forebrain - Cerebrum

a. Diencephalon (lower subdivision)

- Thalamus

-- major relay of sensory info to limbic system and cerebrum
-- many nuclei, each dedicated to specific sensory types
-- incoming information from all the senses (NOT OLFACTORY) is

sorted out in the thalamus and sent onto the appropriate higher
brain centers

-- receives input from brain centers regulating emotion and arousal
-- little processing, mostly channeling

- Limbic System

- functional group of nuclei / axon tracts in arc between thalamus

and cerebrum

- linked to prefrontal cortex (complex learning, reasoning, personality)
- most basic and primitive emotions, drives, behaviors
- fear, rage, tranquillity, hunger, thirst, pleasure, sexual responses
-- Hypothalamus

--- neurosceretory - hormones
--- homeostasis - pituitary connections
--- Suprachiasmatic Nucleus -

---daily (circadian) rhythms - biological clock

-- Amygdala

--- behavioral responses to environmental stimuli
--- input from auditory and visual areas of cortex
--- produce pleasure, punishment, sexual arousal
--- reduce/enhance aggressive behavior

-- Hippocampus

--- long-term memory, learning
--- rage, sexual arousal

-- Olfactory bulb (olfactory system) - smell emotions

--- part of the limbic system / projects "outside" of brain
---(taste, hearing, vision enter limbic system AFTER passing

through cerebral cortex)


b. Telencephalon

- Right and Left Cerebral hemispheres

-- outer layer of gray matter - cerebral cortex (cell bodies)
-- inner layer of white matter - (axons)
-- cluster of nuclei deep in white matter: basal ganglia

- Basal Ganglia

-- motor coordination

- Cerebral Cortex:

-- highly folded in mammals
-- 80% mammalian brain, 0.5 m2 surface area
-- corpus callosum - nerve tracts joining hemispheres
-- motor cortex

-- sends commands to skeletal muscles, responses to sensory stimuli

-- somatosensory cortex

-- represents a sensory map of the body (pain, touch, pressure,

heat, cold)

-- other specific sensory regions: olfactory, vision, hearing, taste, etc.

Vocabulary