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Chapter 10Nervous System I

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Divisions of the Nervous System

• Central Nervous System

•Brain and spinal cord

•All sensations have to be relayed here to be acted on

•Muscle & gland stimulation

•Control center for the entire system

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• Peripheral Nervous System• Connection between

and CNS & receptors, muscles, and glands

• Nerves• Cranial nerves• Spinal nerves

Divisions of the Nervous System

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Divisions of Peripheral Nervous System

• Sensory Division•Afferent System• Picks up sensory information and delivers it to the CNS

• Motor Division•Efferent System• Carries information to muscles and glands

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• Divisions of the Motor Division

Divisions of Peripheral Nervous System

– Autonomic (ANS) • Carries information to smooth muscle, cardiac muscle, and glands• Involuntary

– Somatic (SNS)• Carries information to skeletal muscle• Voluntary

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Divisions Nervous System

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Functions of Nervous System

Sensory Function• sensory receptors gather information• information is carried to the CNS

Integrative Function• sensory information used to create

• sensations• memory• thoughts• decisions

Motor Function• decisions are acted upon • impulses are carried to effectors

Most rapid means of maintaining homeostasis in your body

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Two Types of Cells• Neurons• Conduct nerve

impulses from one part of the body to another• Information

Processing Units

• Neuroglial cells

Nervous System Cytology

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Neuron Structure

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• Cell Body• Large nucleus

surrounded by granular cytoplasm

Neuron Structure

• Dendrites• Thick branched divisions of cell body• Bring nerve impulses toward the cell body

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• Axon• Usually a single,

longer process that conducts nerve impulses from the cell body

• Terminates at another neuron, muscle or gland

• May be up to a meter

Neuron Structure

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• Axon • Axon terminal

•End of an axon with many branching fibers

•End expands to form synaptic end bulb

• Nerve Fiber•Common name for

an axon and its myelin sheath

Neuron Structure

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• Myelin sheath• Formed by neuroglial

cell

• Phospholipid segment that wraps around axon

• Provides protection for axon

• Increases speed of impulse along axon

Myelination of Axons

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• Myelin sheath• Schwann cells

•Form myelin sheath•Found ONLY in

peripheral nervous system

•Assist in repair of damaged axons, provides tube for axon or dendrite to grow

Myelination of Axons

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• Myelin sheath• Production begins

during 1st year of life

• Amount increases from birth to maturity

• This is why adults react quicker to certain stimuli

Myelination of Axons

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• Myelin sheath• Nodes of Ranvier

• Segments on axon that are not myelinated

• Gaps in sheath

Myelination of Axons

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Myelination of Axons

White Matter• contains myelinated axons

Gray Matter• contains unmyelinated structures• cell bodies, dendrites

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Classification of Neurons – Structural Differences

Bipolar• Two processes

• One axon, one dendrite

• Eyes, ears, nose

Unipolar• One process that branches in two• Ganglia• Specialized masses of nerve tissue outside brain & spinal cord

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• Multipolar

• Many processes• Only one axon

• Most neurons of CNS

Classification of Neurons – Structural Differences

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Classification of Neurons – Functional Differences

Sensory Neurons• Afferent• Carry impulse to CNS• Dendrites act as sensory receptors• Most are unipolar• Some are bipolar

Interneurons• Link neurons• Multipolar• In CNS

Motor Neurons• multipolar• carry impulses away from CNS• Carry impulses to effectors (muscles or glands)

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Classification of Neurons – Functional Differences

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Types of Neuroglial Cellsin the PNS

Schwann Cells• Produce myelin found on peripheral myelinated neurons• Speed neurotransmission

Satellite Cells• Support ganglia in PNS

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Types of Neuroglial Cellsin the CNS

Astrocytes• CNS• Scar tissue in CNS• Regulate ion concentrations (K+)• Induce synapse formation• Connect neurons to blood vessels

Oligodendrocytes• Form myelin in CNS• Myelinating cell

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• Microglia• CNS• Phagocytic cell• Proliferate w/ CNS

injury

Types of Neuroglial Cellsin the CNS

• Ependyma• CNS• Cuboidal or columnar• Ciliated• Line central canal of spinal cord• Line ventricles of brain

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The Synapse

• Junction between two neurons

• Also called synaptic clefts

• Essential in homeostasis because of the ability to transmit some impulses while inhibiting others

• Brain disease & many psychiatric disorders result from bad synaptic communication

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• 2 Types• Electrical & Chemical• Most in CNS are chemical

The Synapse

• Function• Neuron secretes neurotransmitters across synaptic cleft• Post-synaptic neuron has receptors to match transmitter• When they match, impulse continues

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Resting Membrane Potential

• Inside is negative relative to the outside• Polarized membrane• Due to distribution of ions• Unequal distribution of Na+ and K+ ions• K+ 28x greater inside• Na+

14x greater outside

• Na+/K+ pump

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• Fights osmosis• Transports 3 Na+ out

& 2 K+ into a resting neuron

• Active Process

Sodium/Potassium Pump

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Local Potential Changes

• Caused by various stimuli• Temperature changes• Light• Pressure

• Environmental changes affect the membrane potential by opening a gated ion channel•Allows Na+ to diffuse in & K+ to diffuse out

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Local Potential Changes

• If membrane potential becomes more negative, it has hyperpolarized

• If membrane potential becomes less negative, it has depolarized

• Summation can lead to threshold stimulus that starts an action potential• Multiple impulses often needed to reach threshold stimulus

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Local Potential Changes

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Action Potentials

• At rest membrane is polarized

• Na+ channels open and membrane depolarizes

• Na+ enters cell

• K+ leaves cytoplasm and membrane repolarizes

• Threshold stimulus reached

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Action Potentials

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Action Potentials

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All-or-None Response

• If a neuron responds at all, it responds completely

• A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon

• All impulses carried on an axon are the same strength

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Refractory Period

• Absolute – Time when threshold stimulus does not start another action potential

• Relative – Time when stronger threshold stimulus can start another action potential

• Under normal conditions each fiber may conduct 10-500 impulses per second• Larger neurons conduct up to 2500 per second

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• Nerve cell membrane maintains resting potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients

Impulse Conduction

• Neurons receive stimulation, causing local potentials, which may sum to reach threshold

• Sodium channels in a local region of the membrane open

• Sodium ions diffuse inward, depolarizing the membrane

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• Potassium channels in the membrane open

Impulse Conduction

• Potassium ions diffuse outward, repolarizing the membrane

• The resulting action potential causes an electric current that stimulates adjacent portions of the membrane

• Series of action potentials occurs sequentially along the length of the axon as a nerve impulse

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Saltatory Conduction

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• Impulse along myelinated fiber

Saltatory Conduction

• Impulse mechanism is the same, BUT impulse skips from one node to the next.

• Ionic current flows through extra-cellular fluid & triggers impulse at next node

• Nodes of Ranvier allow generation of action potentials and conduction

• Sheath inhibits movement of ions

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• Valuable to Homeostasis• Speed of impulse greatly increased

• Low ATP expenditure by Na-K pump due to little exposed membrane

Saltatory Conduction

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Clinical Application

Drug Addiction

• Occurs because of the complex interaction of neurons, drugs, and individual behaviors

• Understanding how neurotransmitters fit receptors can help explain the actions of certain drugs

• Drugs have different mechanisms of action

• Several questions remain about the biological effects of addiction, such as why some individuals become addicted and others do not