The Nervous System

Chapter 44

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Nervous System Organization

All animals must be able to respond to environmental stimuli

-Sensory receptors = Detect stimulus

-Motor effectors = Respond to it

-The nervous system links the two

-Consists of neurons and supporting cells

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Types of Neurons

Vertebrates have3 types of neurons-Sensory neurons to CNS(afferent neurons)

-Motor neurons (efferent neurons) to effectors (muscles and glands)

-Interneurons (association neurons) provide associative functions

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A Neuron

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

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A Neuron’s Charge The inside of the cell is more negatively charged than the

outside because of: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

When a neuron is not being stimulated, it maintains a resting Potential @ -70mv

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Sodium-potassium pump Helps create a Neuron’s charge

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Synapses

Synapses are intercellular junctions

Two basic types: electrical and chemical

Electrical synapses involve direct cytoplasmic connections between the two cells formed by gap junctions

-Relatively rare in vertebrates

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Synapses

Chemical synapses have a synaptic cleft between the two cells

-End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

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Neurons Begin to Communicate @ Synapses

Synapses are intercellular junctions-Presynaptic cell transmits action potential-Postsynaptic cell receives it

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More Facts About the Synapse

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

-Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

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Neurotransmitters

Acetylcholine (ACh)

-Crosses the synapse between a motor neuron and a muscle fiber

-Neuromuscular junction

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Neurotransmitters

Acetylcholine (ACh)

-Binds to ligand-gated receptor in the postsynaptic membrane

-Produces a depolarization called an excitatory postsynaptic potential (EPSP)

-Stimulates muscle contraction

-Acetylcholinesterase (AChE) degrades ACh

-Causes muscle relaxation

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Excitatory and Inhibitory Neurotransmitters

Amino acids

-Glutamate is the 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)

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Neurotransmitters

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Nerve Impulse Transmission

Chemically-gated or ligand-gated channels

-Ligands are hormones or neurotransmitters

-Induce opening and cause changes in cell membrane permeability

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Nerve Impulse Transmission

Depolarization makes the membrane potential more positive, whereas a hyperpolarization makes it more negative

-These small changes result in graded potentials

-Can reinforce or negate each other

Summation is the ability of graded potentials to combine

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Synaptic Integration

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Synaptic Integration

Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body

-Small EPSPs add together to bring the membrane potential closer to the threshold

-IPSPs subtract from the depolarizing effect of EPSPs

-And will therefore deter the membrane potential from reaching threshold

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Synaptic Integration

There are two ways that the membrane can reach the threshold voltage

-Spatial summation

-Many different dendrites produce EPSPs

-Temporal summation

-One dendrite produces repeated EPSPs

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Nerve Impulse Transmission

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Nerve Impulse Transmission

Action potentials result when depolarization reaches the threshold potential about -55mV

Remember resting potential is -70mV so Positive charge has to come in to get it to be -55mV!!

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Nerve Impulse Transmission

When the threshold voltage is reached, sodium channels open rapidly

-Transient influx of Na+ causes the membrane to depolarize

In contrast, potassium channel opens slowly

-Efflux of K+ repolarizes the membrane

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The intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

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-The action potential is caused by voltage-gated ion channels

-Two different channels are used:-Voltage-gated Na+ channels -Voltage-gated K+ channels

Propagation of an Action Potential

-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

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Neurons are supported both structurally and functionally by cells called neuroglia -Schwann cells (PNS) and oligodendrocytes (CNS) produce myelin sheaths surrounding axons

-In the CNS, myelinated axons form white matter -In the CNS dendrites/cell bodies form gray matter

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

Nervous System Organization

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Nerve Impulse Transmission

Two ways to increase velocity of conduction:1. Axon has a large diameter

-Less resistance to current flow-Found primarily in invertebrates

2. Axon is myelinated -Action potential is only produced at the

nodes of Ranvier-Impulse jumps from node to node

-Saltatory conduction

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Nerve Impulse Transmission

Insulation allows for charge, accumulated at a distance, to be

detected by neighbor-Na+channels

Action potential is only produced at the nodes of Ranvier

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Drug Addiction

Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it

-This process is called habituation

-The cell decreases the number of receptors because there is an

abundance of neurotransmitters

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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

-Prolonged exposure triggers the limbic system neurons to reduce receptor numbers

-The cocaine user is now addicted

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Drug Addiction

Nicotine binds directly to a specific receptor on postsynaptic neurons of the brain

-Brain adjusts to prolonged exposure by “turning down the volume” in two ways:

1. Making fewer nicotine receptors

2. Altering the sensitivity to stimulation by neurotransmitters

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Nervous System Organization

The CNS consists of the brain and spinal cord

The Peripheral Nervous System (PNS) consists of 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

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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

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Vertebrate BrainsAll vertebrate brains have three basic divisions:-Hindbrain or rhombencephalon-Midbrain or mesencephalon-Forebrain or prosencephalon

In fishes,-Hindbrain = Largest portion -Midbrain = Processes visual information -Forebrain = Processes olfactory information

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Vertebrate BrainsThe relative sizes of different brain regions have changed as

vertebrates evolved

Forebrain became the dominant feature...Neuronal correlate to: learning, association, emotions.

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Compare Brain Diagrams

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Vertebrate BrainsForebrain is composed of 2 elements:

-Diencephalon

-Thalamus: Integration and relay center

-Hypothalamus: Participates in basic drives & emotions; controls pituitary gland

-Telencephalon (“end brain”)

-Devoted largely to associative activity

-ALSO Called the CEREBRUM in mammals

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Cerebral Cortex is the outer layer of the Cerebrum

Cerebral cortex

-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

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Cerebral Cortex 3 Regions

Cerebral cortex1Primary motor cortex: Movement control

2Primary somatosensory cortex: Sensory control

3Association cortex: Higher mental functions

Basal ganglia

-Aggregates of neuron cell bodies

-Form islands of grey matter within the cerebrum’s white matter

Increase in brain size in mammals reflects the great enlargement of the cerebrum

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Sensory/Motor Homunculus

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Complex Functions of the Brain

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

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Complex Functions of the Brain

Alzheimer disease is a 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 (

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Spinal CordThe spinal cord is a cable of neurons

extending from the brain down through the backbone

-Enclosed and protected by the vertebral column and the meninges

-It serves as the body’s “information highway”

-Relays messages between

the body and the brain

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The Peripheral Nervous System

The PNS consists of nerves and ganglia

-Nerves are bundles of axons bound by connective tissue

-Ganglia are aggregates of neuron cell bodies

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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

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Cutaneous Spinal Reflex (in Somatic NervousSystem)

Use “SAME DAVE” Acronym

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The Peripheral Nervous System

PNS is divided into the:

Somatic System &

Autonomic System

.

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Quadricepsmuscle

(effector)

Spinal cord

Dorsal rootganglion

Graymatter

Whitematter

Monosynapticsynapse

Sensoryneuro

Nerve fiberStretch receptor(muscle spindle)

Skeletalmuscle

Stimulus

Response

Motor neuron

Somatic SystemIn GENERAL, The Somatic System is

responsible for skeletal muscle voluntary movements & reflexes.

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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

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Efferent Ganglia outside of spinal cord

The Autonomic Nervous System

General Model Seen In Both Sympathetic &

Parasympathetic Division