Nervous Tissue Will Kleinelp Associate Professor Department of Biology ©2006 Will Kleinelp Associate Professor Department of Biology ©2006.

Nervous TissueWill Kleinelp

Associate ProfessorDepartment of Biology

©2006

Will KleinelpAssociate Professor

Department of Biology©2006

Comprised of•brain•cranial nerves•spinal cord•spinal nerves•ganglia•enteric plexus •sensory receptors

Functions

•sense changes in internal and external environment via sensory receptors

•analyze sensory information, store information and decision making processes

•respond to stimuli via muscles glands or other nerves

Organization

Central nervous System (CNS)•brain•spinal cord

Peripheral nervous System (PNS)•cranial nerves•spinal nerves•sensory and motor components•ganglia•receptors

Peripheral Nervous System Subdivisions•somatic nervous system - voluntary•autonomic nervous system (ANS) - involuntary•sympathetic division•parasympathetic division•enteric nervous system - enteric plexi throughout GI tract

Peripheral Nervous System

Autonomic Nervous System

• Responsible for involuntary visceral motor activity• conducts impulses from the CNS to• cardiac muscle• smooth muscles• glands

• Two Divisions

SYMPATHETIC Engages body systems during activity

PARASYMPATHETIC Conserves energyantagonist of sympathetic system

Enteric Nervous System

• Sensory receptors and neurons located in GI tract and enteric plexuses

• Involuntary motor neurons in the enteric plexuses

• Effects smooth muscle, gastroitestinal and endocrine glands of the GI tract

Cells of the Nervous System• Supporting cells - neurogliasmaller thean neuronscomprise 1/2 mass of the brainapproximately 10x more numerous than neurons

• Neurons

NEUROGLIA

Astocytes •most abundant•contains radiating processes that

adhere neurons to nearby blood capillaries

•make exchanges between capillaries and neurons

•guide developing neurons•aid in synapse formation•aid in cleaning up leaked K ions•recapture neurotransmitters•aid in information processing in the

brain

NEUROGLIA

Microglia•contains thorny extensions•function to monitor health of

nearby neurons•migrate towards injured or

unhealthy neurons•converts macrophages to

destroy/remove neuronal debris and microorganisms

•only protecting within the CNS

NEUROGLIA

Ependymal Cells

•shaped from squamous to columnar•some ciliated• located in the central cavities of

brain and spinal cord•form a permeable barrier between

the CSF of these cavities and surrounding tissue

NEUROGLIA

Oligodendrocytes

•similar to astrocytes•position themselves along thick

neuron fibers in CNS•wrap processes around neuron•produce insulating myelin sheaths

NEUROGLIA

Satellite & Schwann Cells

•both located in the PNS•Satellite cells surround neuron cell bodies with unknown

function

•Schwann cells are also called neurolemmocytes•these surround the myelin sheath•aid in the formation of myelin similar to oligodendrocytes•aid to repair damaged PNS nerve fibers

THE NEURON•Functional cell of the nervous system

•transmit and conduct impulses from one part to another

•found as•afferent or sensory•efferent or motor• internuncial or association

CHARACTERISTICS•longevity - with adequate nutrition will last a lifetime•amitotic - once fucntional they lose the ability to divide and cannot be replaced...EXCEPT•olfactory epithelium•memory cells o the hippocampus•retain a high metabolic rate - require continuous and abundant supplies of glucose and oxygen

http://www.youtube.com/watch?v=sX87g3AHIbc

100,000,000,000

dendrites

nucleus

neurofibrils

axon terminals

synaptic end bulbs

myelin sheath

neurilemma

node of Ranvier

cell body

axon collateral

nissl bodies

axon cylinder

axonhillock

Multipolar Bipolar Unipolar

multiple dendrites; one axon; found in brain and spinal cord

one dendrite; one axon; fond in retina if eye, inner ear and olfactory area of the brain

begin as bipolar in fetus

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http://www.mpi-cbg.de/images/kinesin.mov

Kinesin Movement

http://msjensen.cehd.umn.edu/WEBANATOMY/nervous/nerv_neuron_1_m.htm

http://msjensen.cehd.umn.edu/WEBANATOMY/nervous/nerv_neuron_2_m.htm

Review

http://www.youtube.com/watch?v=DF04XPBj5uc&NR=1

References

Ion Channels

Ion channels control the movement of ions through the neuronal cell membrane. Ion channels are

•selective•passive or active•regionally located•functionally unique

Selective Ion Channels

Selective channels select ions for passage based on the charge of the ion, the size of the ion and how much water the ion can attract and hold around it

Ion Channels

Ion channels are either active or passive

Active channels have gates that either open or close the channel

passive channels or leakage channels are always open and allow ions to flow continuously. These channels are located throughout all parts of the neuron.

http://staff.jccc.net/aalarabi/Movies/protein_channels_membrane.mov

Voltage Regulated Channels

When a neuron is at rest the voltage regulated gates are closed.

During an action potential, the voltage across the membrane changes with the flux of Na-K causing the voltage channels to open and close

Ions move through open channels

Found in axon hillock, nodes of Ranvier, all along unmyelinated axons

Chemically Regulated Channels or Ligand Gates

Some neurons have active channels that contain chemically controlled gates.

Neurotransmitters such as acetylcholine (ACH) and GABA bind to chemically gated channels causing them to open. This then permits ions to move across the membrane. Located on dendrites and cell body.

Uniqueness of Ion Channels

Passive channels

Chemically gated channels

Voltage gated channels

Passive channels are responsible for resting membrane potential

Chemically gated channels are responsible for synaptic potentials or the incoming signals to a neuron

Voltage gated channels are responsible for the generation and propagation of an action potential - the outgoing signal from the neuron

Impulse Conduction

RESTING POTENTIALNaK

REVERSAL POTENTIAL

SODIUM CHANNEL

POTASSIUM CHANNELGATE

REPOLARIZATION

IMPULSE CONDUCTION

SynapsesSynapses

IMPULSE CONDUCTION

IMPULSE TRANSMISSION

• When one neuron forms a gap junction with another neuron, an electrical synapse is made. Electrical current in the form of ions, flows directly from one neuron to another through the gap junction. These synapses are ALWAYS excitatory

ELECTRICAL SYNAPSES

• at a chemical synapse the neuronal membranes are separated by a gap called the synaptic cleft. Electrical current CANNOT flow directly from one neuron to another. A chemical called a neurotransmitter is released from the sending neuron and carries the signal to the next neuron.

CHEMICAL SYNAPSES

Parts of the Chemical Synapse

Synaptic vesicles

presynaptic neuron

postsynaptic neuronsynaptic cleft

SYNAPTIC TRANSMISSIONSYNAPTIC TRANSMISSION

nerve impulse

voltage gated Ca channel

synaptic vesicles

synaptic cleft

postsynaptic neuron

neurotransmitters

1. impulse moves down neuron to axon terminal

2. impulse reaches voltage gated Ca channels and the wave of depolarization causes gate to open and Ca ions to move in

3. Ca influx recruits synaptic vesicles to migrate to presynaptic neurons membrane

postsynaptic neuron

4.synaptic vesicles release neurotransmitter into synaptic cleft

5.neurotransmitters attaches to specific receptor sites on closed ligand gated channel

6.neurotransmitteractivates channel to open and causes an influx of Na ions triggering an postsynaptic action potential

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SYNAPTIC INTEGRATION

EPSP

EPSP & Excitatory Synapses

IPSP

IPSP & Inhibitory Synapses

SPATIAL SUMMATION

SPATIAL SUMMATION

TEMPORAL SUMMATION

TEMPORAL SUMMATION

NEURONAL POOLS

DIVERGENT CIRCUITSSAME PATHWAY - AMPLIFICATION CIRCUIT

CONVERGENT CIRCUITS

MULTIPLE SOURCES - CONCENTRATION CIRCUITS

CONVERGENT CIRCUITS

SINGLE SOURCE

REVERBERATING CIRCUITS

PARALLEL AFTER-DISCHARGE CIRCUITS

o whole system works in a predictable all-or-nothing manner. o One neuron stimulates the next, which stimulates the next, and so on, eventually causing a specific, anticipated response. o examples of serial processing are spinal reflexes, and o straight-through sensory pathways from receptors to the brain

Serial Processing

• inputs are segregated into many pathways, and information delivered by each pathway is dealt with simultaneously by different parts of the neural circuitry. • smelling a pickle (the input) may cause you to remember picking cucumbers on a farm; or it may remind you that you don’t like pickles or that you must buy some at the market; or perhaps it will call to mind all these thoughts.• For each person, parallel processing triggers some pathways that are unique. • The same stimulus—pickle smell, in our example—promotes many responses beyond simple awareness of the smell. • Parallel processing is not repetitious because the circuits do different things with the information, and each “channel” is decoded in relation to all the others to produce a total picture.

Parallel Processing

http://www.youtube.com/watch?v=YwN9aCobCy8

References

http://youtube.com/watch?v=32aeRcWkLS8

NeurotransmittersNeurotransmitters

✴ Major NT found in PNS neuromuscular junction where it excites skeletal muscle, and inhibits cardiac muscle

✴ is also found in the CNS✴ is degraded via acetylcholinesterase

Acetylcholine

Glutamate✴ Most common excitatory NT in the CNS✴ removed by active pumping back into the presynaptic end bulb

Gamma amino butyric acid (GABA)✴ Most common inhibitory NT in the CNS✴ Anti-anxiety drugs enhance the action of GABA

Norepinephrine✴ In the brain NE effects sleep and moods. ✴ In the PNS, NE and epinephrine are main neurotransmitters of sympathetic postganglion

synapses✴ NE and epinephrine are also produced by the adrenal glands and are responsible for flight

or flight responses✴ this is a biogenic amine and is degraded enzymatically by monoamine oxidase

Dopamine✴ Found in the brain is involved in emotional responses and skeletal muscle movement✴ this is a biogenic amine and is degraded enzymatically by monoamine oxidase

Seratonin✴ Found in the brain is involved in temperature regulation, sleeping, sensory perception and moods

✴ this is a biogenic amine and is degraded enzymatically by monoamine oxidase

Neurotransmitters Neurotransmitters ContinuedContinued

Neuropeptides✴ widespread throughout CNS and PNS✴ some act as hormones

Endorphins and Enkephalins✴ involved in natural pain killing✴ biochemical derivatives of morphine and heroin✴ produce euphoric effects

Nitric Oxide

✴ is not produce by vesicles like other NE✴ is lipid soluble and diffuses out of the neuron✴ acts to cause changes in cellular enzymes instead of a membrane receptor✴ causes blood vessel vasodilation✴ Phagocytes release NO in higher toxic levels to destroy invading cells

Neurotransmitter RemovalNT removal is absolutely necessary for normal synaptic function.

If the NT lingers at the postsynpatic neuron, muscle fiber or gland it would cause continuous stimulation of the postsynaptic neuron.

NT are removed via

•Diffusion - some NT naturally diffuse away from receptors after their action closing the gates

•Enzymatic degradation - most NT are degraded by enzymes released at the postsynaptic neuron. Enzymes are released once the gate has been activated

•Uptake by cells - many NT are actively transported into the neuron that released them via neurotransmitter transporters.

•Some NT are blocked from reuptake by therapeutic drugs. Neuromodulators as Welbutin and Prozac are selective seratonin reuptake inhibitors (SSRI). By preventing reuptake, stimulation is prolonged.