CHAPTER 48 & 49: NERVOUS SYSTEMSAP Biology 2013

OVERVIEW• Human brain consists of an estimated

100 billion neurons. Each neuron may communicate with thousands of other neurons that function in specialized circuits dedicated to different tasks

• All animals except sponges have some type of nervous system

• Cone snail kills prey with venom that disables neurons (nerve cells that transfer information)

• Neurons use two types of signals: electrical (long-distance), chemical (short distance)

• Processing information takes place in simple clusters of neurons called ganglia or a more complex structure called a brain

Fig. 48.1

INFORMATION PROCESSING• Three stages: sensory input, integration,

and motor output

• Sensory neurons - transmit information from sensors that detect external stimuli and internal conditions

• Interneurons -part of the CNS that receives and integrates sensory information

• Motor neurons - send motor output signals to the effector cells

• Central nervous system (CNS) - brain and nerve cord where integration takes place

• Peripheral nervous system (PNS) - carries information into and out of CNS

Fig. 48.3

Sensor

Effector

Sensory input

Motor output

Integration

Peripheral nervous system (PNS)

Central nervous system (CNS)

1

2

3

NEURON STRUCTURE• Most organelles are located in the cell body

• Dendrites - highly branched extensions that receive signals from other neurons

• Axon - longer extension that transmits signals to other cells at synapses

• May be covered in a myelin sheath

Fig. 48.4

Nucleus

Dendrites Stimulus

Axon hillock

Cell body

Presynaptic cell

Signal direction

Axon

Synapse

Neurotransmitter

Synaptic terminals

Postsynaptic cell

Synaptic terminals

NEURON SHAPE• Shape of a neuron reflects its input and output interactions

Figs. 48.5 & 48.6

Dendrites

Axon

Cell body

Portion of axon

Sensory neuron Interneurons Motor neuron

Glia

80 µm

Cell bodies of neurons

ION CHANNELS• Across the membrane,

every cell has a voltage (called membrane potential)

• Inside of the cell is negative to the outside

• Resting potential is the potential of a neuron that is not transmitting

• Depends on the ionic gradients Fig. 48.7

Key Na+ K+

Sodium- potassium pump

Potassium channel

Sodium channel

OUTSIDE OF CELL

INSIDE OF CELL

4

5

6

ION CHANNELS • Concentration of Na+ is higher in the extracellular fluid than in the cytosol, while the opposite is true for K+.

• A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane.

• The diffusion of K+ and Na+ through these channels leads to separation of charges across the membrane, producing resting potential.

• Gated ion channels open or close in response to a change in membrane potential.Fig. 48.8

Inner chamber

-90 mV Outer chamber

140 mM KCl

5 mM KCl

Potassium channel

Artificial membrane

K+ Cl-

(a) Membrane selectively permeable to K+

EK = 62 mV = -90 mV

+62 mV Inner chamber

Outer chamber

150 mM NaCl

15 mM NaCl

Sodium channel

Na+

Cl-

(b) Membrane selectively permeable to Na+

ENa = 62 mV = +62 mV

ACTION POTENTIALS• If a cell has gated ion channels, its membrane potential may change in response to stimuli

that open or close the channels.

• Stimuli either trigger hyperpolarization (increase in magnitude of the membrane potential) or depolarization (reduction of the magnitude of the membrane potential)

• A signal strong enough to produce a depolarization that reaches the threshold triggers a stronger response (action potential)

• Action potential - brief all-or-nothing depolarization of a neuron’s plasma membrane; type of signal that carries information along axons

Fig. 48.10

Stimulus

Threshold

Resting potential

Hyperpolarizations

Time (msec)

+50

0

-50

-100 1 0 2 3 4 5

+50

0

-50

-100

+50

0

-50

-100

Time (msec) 1 0 2 3 4 5

Time (msec) 1 0 2 3 4 5 6

Threshold

Resting potential

Threshold

Resting potential

Stimulus Strong depolarizing stimulus

Action potential

Depolarizations

Mem

bran

e po

tent

ial (

mV)

Mem

bran

e po

tent

ial (

mV)

Mem

bran

e po

tent

ial (

mV)

(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+

(b) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to Na+

(c) Action potential triggered by a depolarization that reaches the threshold

ACTION POTENTIALS• Both voltage-gated Na+ channels and voltage-gated K+ channels are involved in the production of action potential

• When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell

• As the action potential subsides, K+ channels open, and K+ flows out of the cell

• A refractory period follows during which a second action potential cannot be initiated.

Fig. 48.11

Action potential

Threshold

Resting potential

Time

Mem

bran

e po

tent

ial

(mV

)

+50

-100

-50

0

1

2

3

4

5 1

7

8

9

ACTION POTENTIALS• Can travel long distances

• Action potential is generated at the axon hillock where an electrical current depolarizes the neighboring region of the axon

• The speed increases with the diameter of the axon

• In vertebrates axons are myelinated which causes the speed of an action potential to increase

• Action potential jumps between the nodes of Ranvier in a process called saltatory conduction

Figs. 48.12 & 48.14

K+

K+

K+

K+

Na+

Na+

Na+

Action potential

Axon

Plasma membrane

Cytosol

Action potential

Action potential

2

1

3

Cell body

Schwann cell

Depolarized region (node of Ranvier)

Myelin sheath

Axon

NEURONS COMMUNICATE AT SYNAPSES• Electrical synapse - electrical current

flows directly from one cell to another via a gap junction

• Chemical synapse - a presynaptic neuron releases chemical neurotransmitters which are stored in the synaptic terminal

• When the action potential reaches the terminal neurotransmitters are released into the synaptic cleft where they bind to ligand-gated ion channels

• Binding of neurotransmitters causes ion channels to open generating a postsynaptic potential Figs. 48.15 & 48.16

Presynaptic cell Postsynaptic cell

Axon

Presynaptic membrane

Synaptic vesicle containing neurotransmitter

Postsynaptic membrane

Synaptic cleft

Voltage-gated Ca2+ channel

Ligand-gated ion channels

Ca2+

Na+

K+

2

1

3

4

Postsynaptic neuron

Synaptic terminals of pre- synaptic neurons

5 µ

m

POSTSYNAPTIC POTENTIALS• After release, neurotransmitters diffuse out of the synaptic cleft and may be taken up

by surrounding cells and degraded.

• Excitatory postsynaptic potentials (EPSPs) - most neurons have many synapses so a single EPSP is too small to trigger an action potential in the postsynaptic neuron

• Inhibitory postsynaptic potentials (IPSPs) - can counter the effect of an EPSP

Fig. 48.17

Terminal branch of presynaptic neuron

Postsynaptic neuron

Axon hillock

E1

E2

E1

E2

E1

E2

E1

E2

I! I! I! I!

0

-70

Mem

bran

e po

tent

ial (

mV

)

Threshold of axon of postsynaptic neuron

Resting potential

Action potential

Action potential

I!E1 E1 E1 E1 E1 + E2 E1 + I!

Subthreshold, no summation

(a) (b) Temporal summation (c) Spatial summation Spatial summation of EPSP and IPSP

(d)

E1

10

11

12

NEUROTRANSMITTERS• Same neurotransmitter can produce

different effects in different types of cells

• Acetylcholine - one of the most common in both vertebrates and invertebrates; can be inhibitory or excitatory

• Biogenic amines - epinephrine, norepinephrine, dopamine, and serotonin (active in the CNS and PNS)

• Neuropeptides - endorphins which impact perception of pain

• Amino acids and peptides

• Gasses - nitric oxide and carbon monoxide (regulators in the PNS)

NERVOUS SYSTEM ORGANIZATION

Fig. 49.2

Nerve net

(a) Hydra (cnidarian)

Radial nerve

Nerve ring

(b) Sea star (echinoderm)

Eyespot Brain Nerve cords Transverse nerve

Brain

Ventral nerve cord

Segmental ganglia

(c) Planarian (flatworm)

(d) Leech (annelid)

(h) Salamander (vertebrate)

(e) Insect (arthropod) (f) Chiton (mollusc) (g) Squid (mollusc)

Brain Brain

Brain

Ventral nerve cord

Segmental ganglia

Anterior nerve ring

Longitudinal nerve cords

Ganglia

Ganglia

Spinal cord (dorsal nerve cord)

Sensory ganglia

• Nerve net - series of interconnected nerve cells

• Nerves - bundles of axons of multiple nerve cells

Fig. 48.4

ORGANIZATION OF VERTEBRATE NERVOUS SYSTEM• Spinal cord conveys information from and to the brain

• Spinal cord also produces reflexes (body’s automatic response to a stimulus

Quadriceps muscle

Cell body of sensory neuron in dorsal root ganglion

Gray matter

White matter

Hamstring muscle

Spinal cord (cross section)

Sensory neuron Motor neuron Interneuron

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14

15

VERTEBRATE NERVOUS SYSTEM• Cephalization and distinct CNS

and PNS

• Brain provides integrative power that allows for complex behavior

• Spinal cord integrates simple responses to stimuli and conveys information to and from the brain

• Central canal of the spinal cord and four ventricles of the brian are hollow because they are derived from the dorsal embryonic nerve cord and filled with cerebrospinal fluid

Figs. 49.4

& 49.5

Central nervous system (CNS)

Brain

Spinal cord

Peripheral nervous system (PNS)

Cranial nerves

Ganglia outside CNS

Spinal nerves

Gray matter

White matter

Ventricles

SUPPORTING CELLS• Gilia - essential for structural integrity of the nervous system and for

normal functioning neurons

• Astrocytes - provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters in the CNS

• Oligodendrocytes (CNS) and Schwann cells (PNS) - are gila that form the myelin sheaths around axons of many vertebrate neurons

Fig. 49.6

CNS PNS VENTRICLE Cilia

Neuron Astrocyte

Oligodendrocyte

Capillary Ependymal cell

Schwann cell

Microglial cell LM 50

µm

PERIPHERAL NERVOUS SYSTEM• PNS transmits information to and from the

CNS

• Cranial nerves originate in the brain and terminates in the organs of the head

• Spinal nerves originates in the spinal cord and extend the parts of the body below the head

• PNS is divided into two functional components:

• Motor system - carries signals to skeletal muscles

• Autonomic nervous systems - regulates the internal environment in an involuntary manner; divided into the sympathetic, parasympathetic, and enteric divisions

Fig. 49.7

Efferent neurons Afferent neurons

Central Nervous System

(information processing)

Peripheral Nervous System

Sensory receptors

Internal and external

stimuli

Autonomic nervous system

Motor system

Control of skeletal muscle

Sympathetic division

Parasympathetic division

Enteric division

Control of smooth muscles, cardiac muscles, glands

16

17

18

AUTONOMIC NERVOUS SYSTEM• Sympathetic and

parasympathetic are antagonistic

• Sympathetic - fight-or-flight

• Parasympathetic - return to self-maintenance functions

• Enteric - controls activity of digestive tract, pancreas, and gallbaldder

Fig. 49.8Parasympathetic division

Action on target organs: Constricts pupil

of eye

Stimulates salivary gland secretion

Constricts bronchi in lungs

Slows heart

Stimulates activity of stomach and

intestines

Stimulates activity of pancreas

Stimulates gallbladder

Promotes emptying of bladder

Promotes erection of genitalia

Cervical

Thoracic

Lumbar

Synapse Sacral

Sympathetic ganglia

Sympathetic division

Action on target organs:

Dilates pupil of eye

Accelerates heart

Inhibits salivary gland secretion

Relaxes bronchi in lungs

Inhibits activity of stomach and intestines

Inhibits activity of pancreas

Stimulates glucose release from liver;

inhibits gallbladder

Stimulates adrenal medulla

Inhibits emptying of bladder

Promotes ejaculation and vaginal contractions

DEVELOPMENT OF BRAIN• Brain develops from the forebrain, midbrain, and hindbrain

• Week 5 - give brain regions form from original three

• Biggest changes happens in the forebrain (gives rise to the cerebrum)

Fig. 49.9

Embryonic brain regions Brain structures in child and adult

Forebrain

Midbrain

Hindbrain

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Cerebrum (includes cerebral cortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

Midbrain

Forebrain

Hindbrain

Telencephalon

Diencephalon

Mesencephalon Metencephalon

Myelencephalon

Spinal cord

Cerebrum Diencephalon

Midbrain

Pons Medulla oblongata

Cerebellum Spinal cord

Child Embryo at 5 weeks Embryo at 1 month

Adult brain viewed from the rear

Cerebellum

Basal nuclei Cerebrum

Left cerebral hemisphere

Right cerebral hemisphere

Cerebral cortex

Corpus callosum

Diencephalon Thalamus Pineal gland Hypothalamus Pituitary gland

Spinal cord

Brainstem

Midbrain

Pons

Medulla oblongata

AROUSAL AND SLEEP

• Controlled by brainstem

• Regulates the amount and type of information that reaches the cerebral cortex and affects alertness

• Hormone melatonin is released by the pineal glad and plays a role in bird and mammal sleep cycles

• Sleep is essential (may play a role in memory)

• Biological clock can direct gene expression and is usually synchronized to light and dark cycles

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20

21

EMOTIONS

• Limbic system - ring of structures around the brainstem

• Includes three parts of the cerebral cortex: amygdala, hippocampus, and olfactory bulb

• These structures interact with with the neocortex to mediate primary emotions and attach “feelings” to survival-related functions

Figs. 49.13 & 49.14

Hypothalamus

Thalamus

Olfactory bulb

Amygdala Hippocampus

Nucleus accumbens Amygdala

Happy music Sad music

LANGUAGE AND SPEECH• Broca’s area - area in

frontal lobe that is active when speech is generated

• Wernicke’s area - area in temporal lobe that is active when speech is heard

• Left hemisphere is more adept at language, math, logic

• Right hemisphere is stronger at pattern recognition, nonverbal thinking, and emotional processing

Hearing words

Speaking words

Seeing words

Generating words

Max

Min

Fig. 49.16

CEREBRAL CORTEX• Has four lobes: frontal, parietal, temporal, and occipital

• Somatosensory cortex and motor cortex - neurons are distributed according to the part of the body that generates sensory input or receives motor input

• Lateralization - competing functions segregate and displace each other in the cortex of left and right hemispheres

• Left hemisphere - language, math, logical operations

• Right hemisphere - pattern recognition, nonverbal thinking , and emotional processing

Fig. 49.17

Frontal lobe Parietal lobe

Primary motor cortex

Primary somatosensory cortex

Genitalia Toes

Abdominal organs

Tongue

Jaw Lips

Face

Eye Brow

Neck

Thumb

Fingers Hand

Wrist

Forearm

Elbow

Shoulder Trunk

Hip

Knee

Tongue Pharynx

Jaw Gums Teeth

Lips

Face

Nose

Eye

Thumb Fingers

Hand Forearm

Elbow

U

pper arm

Head

Neck

Trunk H

ip Leg

22

23

24

MEMORY AND LEARNING• Frontal lobes - site of short-

term memory

• Interact with hippocampus and amygdala to consolidate long-term memory

• Areas of the cerebral cortex are involved in the storing and retrieving of words and images

• Neural plasticity - describes ability of nervous system to be modified after birth Fig. 49.19

N2

N1

N2

N1

(a) Synapses are strengthened or weakened in response to activity.

(b) If two synapses are often active at the same time, the strength of the postsynaptic response may increase at both synapses.

MEMORY AND LEARNING

• Short-term memory - accessed via the hippocampus

• Long-term memory - stored in cerebral cortex

• Long-term potentiation - involves increase in strength of synaptic transmission

PRESYNAPTIC NEURON

Glutamate Mg2+

Ca2+ Na+

NMDA receptor (closed)

Stored AMPA receptor

NMDA receptor (open)

POSTSYNAPTIC NEURON

(a) Synapse prior to long-term potentiation (LTP)

(b) Establishing LTP

(c) Synapse exhibiting LTP

Depolarization Action potential

2

1

3

1

2

3

4

Fig. 49.20

STEM CELLS IN THE BRAIN

• Adult human brain contains neural stem cells

• Play a role in learning and memory

Fig. 49.21

25

26

27

CNS INJURIES

• CNS (unlike PNS) cannot repair itself when damaged or diseased

• Receptor binding of adjacent nerve cells triggers a signal transduction pathway which may cause an axon to grow toward or away from the source of a signal

• Neural stem cells have the ability to differentiate into mature neurons and may hold promise for repairing damage

CNS DISORDERS• Schizophrenia - (about 1% of world’s population)

• Characterized by hallucinations, delusions, blunted emotions

• Treatments focus on brain pathways that use dopamine as a neurotransmitter

• Depression: bipolar disorder (manic and depressive phases) and major depression (persistent low mood)

• Treatments involve drugs like Prozac and lithium

• Alzheimer’s disease - mental deterioration characterized by confusion and memory loss

• Caused by neurofibrillary tangles and plaques in the brain

• Parkinson’s disease - caused by death of dopamine-secreting neurons; characterized by difficulty initiating movements

Genes shared with relatives of person with schizophrenia

12.5% (3rd-degree relative) 25% (2nd-degree relative) 50% (1st-degree relative) 100%

50

40

30

20

10

0

Relationship to person with schizophrenia

Ris

k of

dev

elop

ing

schi

zoph

reni

a (%

)

Indi

vidu

al,

gene

ral

popu

latio

n Fi

rst c

ousi

n

Unc

le/a

unt

Nep

hew

/ ni

ece

Frat

erna

l tw

in

Iden

tical

tw

in

Gra

ndch

ild

Hal

f sib

ling

Pare

nt

Full

sibl

ing

Chi

ld

Fig.s 49.22 & 49.24

Amyloid plaque Neurofibrillary tangle 20 µm

DRUG ADDICTION

• Some drugs are addictive because they increase activity of the brain’s reward system

• Addictive drugs enhance the activity of the dopamine pathway

• Drug addiction leads to long-lasting changes in the reward circuitry that causes a craving for the drug

Nicotine stimulates dopamine- releasing VTA neuron.

Inhibitory neuron

Dopamine- releasing VTA neuron

Cerebral neuron of reward pathway

Opium and heroin decrease activity of inhibitory neuron.

Cocaine and amphetamines block removal of dopamine from synaptic cleft.

Reward system response

Fig. 48.23

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