The Nervous System Chapter 44. Nervous System Organization All animals must be able to respond to environmental stimuli Sensory receptors – detect stimulus

The Nervous System

Chapter 44

Nervous System Organization

• All animals must be able to respond to environmental stimuli

• Sensory receptors – detect stimulus

• Motor effectors – respond to it

• Nervous system links the two– Consists of neurons and supporting cells

2


• Vertebrates have three types of neurons1. Sensory neurons (afferent neurons) carry

impulses to central nervous system (CNS)

2. Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands)

3. Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory)

3


• Central nervous system (CNS )– Brain and spinal cord

• Peripheral nervous system (PNS) – Sensory and motor neurons– Somatic NS stimulates skeletal muscles– Autonomic NS stimulates smooth and cardiac

muscles, as well as glands• Sympathetic and parasympathetic NS

– Counterbalance each other

4

5

6

PN

SC

NS

Brain and Spinal Cord

Sympathetic nervoussystem

"fight or flight"

Parasympathetic nervoussystem

"rest and repose"

Somatic nervoussystem

(voluntary)

Sensory neuronsregistering external

stimuli

Autonomic nervoussystem

(involuntary)

Sensory Pathways Motor Pathways

central nervous system (CNS)peripheral nervous system (PNS)

Sensory neuronsregistering external

stimuli


• Neurons have the same basic structure– Cell body

• Enlarged part containing nucleus

– Dendrites• Short, cytoplasmic extensions that receive stimuli

– Axon• Single, long extension that conducts impulses

away from cell body

7

8



• Neuroglia– Support neurons both structurally and

functionally– Schwann cells and oligodendrocytes produce

myelin sheaths surrounding axons– In the CNS, myelinated axons form white matter

• Dendrites/cell bodies form gray matter

– In the PNS, myelinated axons are bundled to form nerves

9

Nerve Impulse Transmission

• The inside of the cell is more negatively charged than the outside

1. Sodium–potassium pump • Brings two K+ into cell for every three Na+ it

pumps out

2. Ion leakage channels • Allow more K+ to diffuse out than Na+ to

diffuse in

10

11


• Two major forces act on ions in establishing the resting membrane potential1. Electrical potential produced by unequal

distribution of charges

2. Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other

12


• Sodium–potassium pump creates significant concentration gradient

• Concentration of K+ is much higher inside the cell• Membrane not permeable to negative ions• Leads to buildup of positive charges outside and

negative charges inside cell• Attractive force to bring K+ back inside cell• Equilibrium potential – balance between

diffusional force and electrical force

13

14



• Depolarization makes the membrane potential more positive

• Hyperpolarization makes it more negative• These small changes result in graded potentials• Size depends on either the strength of the

stimulus or the amount of ligand available to bind with their receptors

• Can reinforce or negate each other• Summation is the ability of graded potentials to

combine15

16



• Action potentials– Result when depolarization reaches the threshold

potential (–55 mV)– Depolarizations bring a neuron closer to the

threshold– Hyperpolarizations move the neuron further from

the threshold– Caused by voltage-gated ion channels

• Voltage-gated Na+ channels • Voltage-gated K+ channels

17


• Voltage-gated Na+ channels – Activation gate and inactivation gate– At rest, activation gate closed, inactivation gate open– Transient influx of Na+ causes the membrane to

depolarize

• Voltage-gated K+ channels– Single activation gate that is closed in the resting

state– K+ channel opens slowly– Efflux of K+ repolarizes the membrane

18


• The action potential has three phases– Rising, falling, and undershoot

• Action potentials are always separate, all-or-none events with the same amplitude

• Do not add up or interfere with each other

• Intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

19

20


• Propagation of action potentials– Each action potential, in its rising phase,

reflects a reversal in membrane polarity– Positive charges due to influx of Na+ can

depolarize the adjacent region to threshold – And so the next region produces its own action

potential– Meanwhile, the previous region repolarizes

back to the resting membrane potential• Signal does not go back toward cell body

21

22

23

Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.


• Two ways to increase velocity of conduction– Axon has a large diameter

• Less resistance to current flow• Found primarily in invertebrates

– Axon is myelinated • Action potential is only produced at the

nodes of Ranvier• Impulse jumps from node to node• Saltatory conduction

24

25


Synapses

• Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells

• Presynaptic cell transmits action potential

• Postsynaptic cell receives it

• Two basic types: electrical and chemical

26

• Electrical synapses– Involve direct cytoplasmic

connections between the two cells formed by gap junctions

– Relatively rare in vertebrates

• Chemical synapses– Have a synaptic cleft

between the two cells– End of presynaptic cell

contains synaptic vesicles packed with neurotransmitters

27

Synapses

• Chemical synapses– Action potential triggers influx of Ca2+ – Synaptic vesicles fuse with cell membrane– Neurotransmitter is released by exocytosis– Diffuses to other side of cleft and binds to

chemical- or ligand-gated receptor proteins– Produces graded potentials in the postsynaptic

membrane– Neurotransmitter action is terminated by

enzymatic cleavage or cellular uptake28

29

Synapses

Neurotransmitters

• Acetylcholine (ACh)– Crosses the

synapse between a motor neuron and a muscle fiber

– Neuromuscular junction

30

Neurotransmitters

• Amino acids– Glutamate

• Major excitatory neurotransmitter in the vertebrate CNS

• Glycine and GABA (-aminobutyric acid) are inhibitory neurotransmitters

– Open ligand-gated channels for Cl– – Produce a hyperpolarization called an inhibitory

postsynaptic potential (IPSP)

31

32

Neurotransmitters

• Biogenic amines– Epinephrine (adrenaline) and norepinephrine

are responsible for the “fight or flight” response

– Dopamine is used in some areas of the brain that control body movements

– Serotonin is involved in the regulation of sleep

33

Neurotransmitters

• Neuropeptides– Substance P is released from sensory

neurons activated by painful stimuli– Intensity of pain perception depends on

enkephalins and endorphins– Nitric oxide (NO)

• A gas – produced as needed from arginine• Causes smooth muscle relaxation

34

Drug Addiction

• Habituation– Prolonged exposure to a stimulus may cause

cells to lose the ability to respond to it– Cell decreases the number of receptors

because there is an abundance of neurotransmitters

– In long-term drug use, means that more of the drug is needed to obtain the same effect

35

Drug Addiction

• Cocaine – Affects neurons in the brain’s “pleasure

pathways” (limbic system) – Binds dopamine transporters and prevents

the reuptake of dopamine– Dopamine survives longer in the synapse and

fires pleasure pathways more and more

36

37

Drug Addiction

• Nicotine– Binds directly to a specific receptor on

postsynaptic neurons of the brain– Binds to a receptor for acetylcholine– Brain adjusts to prolonged exposure by

“turning down the volume” by• Making fewer receptors to which nicotine binds• Altering the pattern of activation of the nicotine

receptors

38

Cerebrum

• The increase in brain size in mammals reflects the great enlargement of the cerebrum

• Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum

• Each hemisphere receives sensory input from the opposite side

• Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes

39

40

Cerebrum

Cerebrum

• Cerebral cortex– Outer layer of the cerebrum– Contains about 10% of all neurons in brain– Highly convoluted surface

• Increases threefold the surface area of the human brain

– Divided into three regions, each with a specific function

41

Cerebrum

• Cerebral cortex• Primary motor cortex – movement control• Primary somatosensory cortex – sensory

control • Association cortex – higher mental functions • Basal ganglia

• Aggregates of neuron cell bodies – gray matter• Participate in the control of body movements

42

43

44

Each of these regions of the cerebral cortex is associated with a different region of the body

Other Brain Structures

• Thalamus– Integrates visual, auditory, and somatosensory

information

• Hypothalamus– Integrates visceral activities – Controls pituitary gland

• Limbic system– Hypothalamus, hippocampus, and amygdala– Responsible for emotional responses

45

Complex Functions of the Brain

• Sleep and arousal– One section of reticular formation is the

reticular-activating system • Controls consciousness and alertness

– Brain state can be monitored by means of an electroencephalogram (EEG)

• Records electrical activity

46


• Language– Left hemisphere is “dominant” hemisphere

• Different regions control various language activities• Adept at sequential reasoning

– Right hemisphere is adept at spatial reasoning

• Primarily involved in musical ability• Nondominant hemisphere is also important for the

consolidation of memories of nonverbal experiences

47

48


• Memory– Appears dispersed across the brain– Short-term memory is stored in the form of

transient neural excitations– Long-term memory appears to involve

structural changes in neural connections– Two parts of the temporal lobes, the

hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory

49


• Alzheimer disease – Condition where memory and thought become

dysfunctional– Two causes have been proposed

1. Nerve cells are killed from the outside in– External protein: -amyloid

2. Nerve cells are killed from the inside out– Internal proteins: tau (

50

Spinal Cord

• Cable of neurons extending from the brain down through the backbone

• Enclosed and protected by the vertebral column and the meninges

51

Spinal Cord

• 2 zones– Inner zone is gray matter

• Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia

– Outer zone is white matter• Contains cables of sensory axons in the dorsal

columns and motor axons in the ventral columns

52

Spinal Cord

• It serves as the body’s “information highway”– Relays messages between the body and the

brain

• It also functions in reflexes– The knee-jerk reflex is monosynaptic– However, most reflexes in vertebrates involve

a single interneuron

53

54

Knee-jerk reflex is monosynaptic

55Most reflexes in vertebrates involve a single interneuron

The Peripheral Nervous System

• Consists of nerves and ganglia– Nerves are bundles of axons

bound by connective tissue– Ganglia are aggregates of

neuron cell bodies

• Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells

56

The Peripheral Nervous System

• Sensory neurons– Axons enter the dorsal surface of the spinal

cord and form dorsal root of spinal nerve– Cell bodies are grouped outside the spinal

cord in dorsal root ganglia

• Motor neurons– Axons leave from the ventral surface and form

ventral root of spinal nerve– Cell bodies are located in the spinal cord

57

The Autonomic Nervous System

• Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata

• In both, efferent motor pathway has 2 neurons– Preganglionic neuron – exits the CNS and

synapses at an autonomic ganglion– Postganglionic neuron – exits the ganglion and

regulates visceral effectors• Smooth or cardiac muscle or glands

58

59

The Autonomic Nervous System

• Sympathetic division– Preganglionic neurons originate in the

thoracic and lumbar regions of spinal cord – Most axons synapse in two parallel chains of

ganglia right outside the spinal cord

• Parasympathetic division– Preganglionic neurons originate in the brain

and sacral regions of spinal cord – Axons terminate in ganglia near or even

within internal organs

60

61

Documents

The Nervous System Chapter 44. Nervous System Organization All animals must be able to respond to environmental stimuli Sensory receptors – detect stimulus