Nervous system

Nervous system

Nervous system• 2 main parts

– 1) Central Nervous System (CNS)

– Brain: central processing center

– Spinal cord: extends down back below brain

Nervous system• 2 main parts

– 2) Peripheral Nervous System (PNS)

– Connects CNS to muscles, sensory cells, etc.

– 12 pairs of cranial nerves (attached to brain)

– _________ pairs of spinal nerves (attached to spinal cord)

Nervous system• Functions:

– 1) Monitors changes in external and internal environment

– Receptors: gather information

Nervous system• Functions:

– 1) Monitors changes in external and internal environment

– Receptors: gather information– 2) Sends messages in response to information

gathered– Effectors: cells that respond to message

(glands, muscles)

Nervous system• 3 types of nerve cells (neurons)

– Sensory neurons (act as receptors)• Found in skin, organs. Cell body of neuron outside of CNS.

Long dendrites, ____________ axons

Nervous system• 3 types of nerve cells (neurons)

– Motor neurons (carry message to effectors) • Cell bodies contained within CNS. Short dendrites, long axons

Nervous system• 3 types of nerve cells (neurons)

– Association neurons (often link sensory and motor neurons)

• Cell bodies in CNS. Connect motor and sensory neurons. Connect sensory neurons to ____________

The neuron• Functional unit of nervous system• Neuron: dendrite (incoming messages), cell

body, axon (outgoing message)

Neuroglia• Neuroglia (supporting cells)• Schwann cells (PNS) and oligodendrocytes (CNS)• Form myelin sheath, Nodes of Ranvier

Neuroglia• Myelinated axons are whitish: called white matter

in nerves/spinal cord/brain• Unmyelinated axons/dendrites and cell bodies are

grayish: called gray matter

Neuroglia• Secrete myelin (80% ___________)• Cells wrap axon in a whitish “jelly roll”:

myelin sheath

Neuroglia• Functions:

– 1) support and protect nerve cell from injury– 2) nourish nerve cell (lies between blood

vessels and neurons)– 3) electrical insulator: increases rate of impulse,

insulates against firing of other neurons– 4) aids in regeneration of sensory nerves after

______________

Neuroglia• Example of importance:

Multiple Sclerosis (MS)• 2 million people worldwide• Body’s immune system attacks

myelin sheaths• Result: poor nerve impulse

conduction. Lose ability to _____________ muscles

• Cause? Uncertain: genetics, viral infection

How a nerve works• Nerve impulse:

– electrochemical signal– great speed: up to _________ miles per hour– resting nerve has charge difference across membrane:

membrane “polarized”– impulse: wave of depolarization caused by sudden

influx of Na+ into axon– followed by repolarization: return to original

polarized state

How a nerve works• Resting neuron

– Lots Na+ outside membrane and K+ on inside– Charge is + outside and - inside due to Cl- and

proteins on inside of membrane– Maintained by Na-K pumps

How a nerve works• Resting neuron

– Neuron said to be polarized– Charge difference across membrane called _________

potential. About -70 millivolts (mV)

How a nerve works• Resting neuron movie

QuickTime™ and aCinepak decompressor

are needed to see this picture.

How a nerve works• Firing neuron

– stimulus arrives– if stimulus is enough, causes neuron to fire– firing is production of an action potential across

membrane: change in _____________ distribution– goes from -70 mV (resting potential) to +30 mV (at

peak of action potential)

How a nerve works• Firing neuron

– 1) membrane channels open to allow Na+ to rush in– 2) extra K+ channels open and lots of K+ flows out– This repolarizes membrane– 3) Refractory period: time during which original state

is regenerated by Na-K pumps. During this time, neuron __________ fire again.

How a nerve works• The firing neuron movie

QuickTime™ and aCinepak decompressor

are needed to see this picture.

How a nerve works• Nerve firing:

– All or none response. A nerve either fires or it doesn’t

– Very brief. Lasts about ________ millisecond– Transmitted along sections of axon like “the wave” in

a stadium

How a nerve works• Myelin sheaths speed impulse movement

– allow action potential to jump between nodes– action potential only produced at nodes, not in

between

How a nerve works• Myelin sheaths speed impulse movement

– like stadium with sections of empty seats. “Wave” jumps to next filled section

– called saltatory conduction

How a nerve works• Note speed of myelinated axons• Note also that axon diameter influences speed of

impulse. Larger axon, ___________ speed.

Neuron to neuron transmission• 2 neurons usually don’t touch• Share synapse: tiny intercellular space (synaptic

cleft)

Neuron to neuron transmission• Chemical transmission of impulse:

– 1)Action potential in firing axon stimulates release of synaptic vesicles into synapse

Neuron to neuron transmission• Chemical transmission of impulse:

– 2) Vesicles contain neurotransmitters– 3) Neurotransmitters diffuse across cleft to _________

proteins in postsynaptic membrane (dendrite of other neuron or muscle cell as shown here)

Neuron to neuron transmission• Chemical transmission of impulse:

– 4) Receptor proteins cause start of action potential in postsynaptic membrane

– 5) Enzymes ______________ neurotransmitters when transmission is completed. Prepares synapse for the next impulse.

Neurotransmitters• Vital substances in transfer of messages between

neurons

Neurotransmitters• 2 main types

– 1) excitatory transmitters: cause depolarization of postsynaptic membrane

– 2) inhibitory transmitters: stabilize membrane against depolarization

Neurotransmitters• Acetylcholine: Neurotransmitter between motor

nerve axon and skeletal muscle cell• Example of excitatory neurotransmitter• Causes depolarization of muscle cell membrane

and stimulation of ________________

Neurotransmitters• How stop stimulus? Acetylcholinesterase:

enzyme breaks down acetylcholine• What happens if acetylcholinesterase inhibited?

Muscle contraction uncontrolled.

• This is _________......• Some chemicals designed to kill by inhibiting

acetylcholinesterase

Neurotransmitters• Nerve gas: works by inhibiting acetylcholinesterase• Example, VX gas featured in “The Rock”

VX gas spheres in “The Rock”

Neurotransmitters• Nerve gas: works by inhibiting acetylcholinesterase• Example, sarin gas• Used in Tokyo subway attack by religious cult in

1995 (12 dead)

Neurotransmitters• Nerve gas: works by inhibiting acetylcholinesterase• Example, sarin gas• Used by Saddam Hussein and his pal “Chemical

Ali” in 1988 against Kurds at Halabjah (5,000 killed)

Neurotransmitters• Nerve gas: works by inhibiting

acetylcholinesterase• Example, sarin gas• US Army used incinerator at Anniston AL to

destroy some sarin gas (Sept. 2003)

Neurotransmitters• GABA: an inhibitory neurotransmitter• Allows ________ into neuron membrane,

making it even more negative inside.• Thus harder to develop action potential

Neurotransmitters• Some sedatives take advantage of GABA

mechanism• Ex, Valium: enhances binding of GABA to its

receptors. Harder for neurons to fire.

Neurotransmitters• Serotonin: Involved in sleep regulation and

emotional states• Insufficient serotonin production involved in

depression, Parkinson’s disease

Neurotransmitters• Prozac: blocks reabsorption of serotonin to make

up for _________ of production

Neurotransmitters• Serotonin• LSD: blocks serotonin receptors

in portion of brain to produce hallucinations or “trip”

Neurotransmitters• Cocaine: Affects neurons in brain’s “pleasure

pathways” (limbic system)

Neurotransmitters• Slows reabsorption of transmitters, so pleasure

messages intensified

Neurotransmitters• Addiction: body adjusts to drug. Example, cocaine• Decrease number of neurotransmitter receptors• When drug removed, synapse _________ sensitive

Neurotransmitters• Addiction example, nicotine

Neurotransmitters• Nicotine binds to brain receptors that normally

bind acetylcholine• Influences a number of other neurotransmitters

and their receptors• Result is stimulating

Neurotransmitters• Problem: brain cells adjust numbers and

sensitivities of many brain receptors • If nicotine removed (stop smoking), system

unbalanced. Effects unpleasant

Neurotransmitters• Example, nicotine• Takes time to recover

Synaptic Integration• Recall that there are two types of

neurotransmitters: excitatory and inhibitory• So, synapses can be either excitatory or

inhibitory, depending on the neurotransmitter produced

• CNS neurons often receive input from many other neurons

Synaptic Integration• Example, 1 spinal cord motor neuron might have

50,000 synapses!• Action of that neuron depends on the total effect

of both excitatory (red) and inhibitory (blue) inputs

Parts of the Nervous System• Central nervous system: brain and spinal cord

Parts of the Nervous System• Brain: Hindbrain

– 1) Medulla oblongata: continuation of spinal cord. Pathway to higher brain centers (traffic controller)

– Contains reflex centers that control: breathing, heart rate, blood pressure, swallowing

– Damage to this area is __________!

Parts of the Nervous System• Brain: Hindbrain

– 2) Pons: carries impulses from one side of ______________ to other. Regulates chewing, salivation, facial expressions, eye movements

Parts of the Nervous System• Brain: Hindbrain

– 3) Cerebellum: receives information from sense organs, muscles, cerebrum

– Involved in equilibrium, coordination of muscles

Parts of the Nervous System• Brain: Midbrain

– 4) Small area in humans: midbrain connects hindbrain to forebrain

5) Brain: Forebrain– Cerebrum: Main processing center. Regulates vision,

speech, hearing, olfaction, memory, logic, consciousness, etc.

5) Brain: Forebrain– Thalamus: analyzes sensory information and relays it

to cerebrum– Hypothalamus: regulates sleep, metabolism, body

temperature, water balance, appetite, thirst, pleasure– Note connection of hypothalamus to _____________

gland

Brain comparisons• Humans with highly developed cerebrum: largest

portion of brain

Parts of the Nervous System• Spinal cord: cable of neurons running down

back. Information highway!• Inner zone: gray matter. Interneurons and cell

bodies of motor neurons• Outer zone: white matter. Axons and dendrites

of neurons.

Parts of the Nervous System• Spinal cord: also contains reflex arcs• Very fast responses because information doesn’t

travel to _______________ for processing

Parts of the Nervous System• 2 neuron arc: has only sensory and motor neuron

involved. Example, knee jerk reflex

Parts of the Nervous System• 3 neuron arc: has sensory and motor neuron, but

________________ present also. Example, cutaneous spinal reflex

Parts of the Nervous System• Brain to spinal cord mass ratio• Gives rough measure of intelligence• Fish/amphibians 1:1• Cat 4:1• Tailed monkey 15:1• Human 55:1• ____________ 61:1• Who’s more intelligent??

Parts of the Nervous System• Peripheral Nervous System

– Afferent system: carry impulse from sensory neuron to CNS. Sensory pathways.

– Efferent system: carry impulses from CNS to muscles (effectors). Motor pathways.

Parts of the Nervous System• Peripheral Nervous System

– Efferent system: carry impulses from CNS to muscles (effectors). Motor pathways.

• Somatic: CNS to skeletal muscles (_________________)• Autonomic: CNS to smooth muscles (involuntary)

Parts of the Nervous System• Peripheral Nervous System

– Autonomic: CNS to smooth muscles (involuntary)– Made of two antagonistic subdivisions (have

__________ effects)• 1) Sympathetic• 2) Parasympathetic

Parts of the Nervous System• Autonomic

– 1) Sympathetic (on edge). Speed up body functions and activities to prepare for defense

– Increase heart rate, glucose release, decrease blood to digestive system, more blood to muscles

Parts of the Nervous System• Autonomic

– 2) Parasympathetic (laid back)– _________________ of sympathetic system