The Nervous System

• 2 categories in nervous system.• Central nervous system (CNS) –

brain, spinal cord• Peripheral nervous system (PNS) –

nerves outside CNS.• 2 divisions of PNS – somatic

(voluntary), autonomic (involuntary)

http://lumen.georgetown.edu/faculty/che3/bvl/images/nervous.jpg

• Autonomic divided into 2: 1sympathetic (during stress), 2parasympathetic (normal functioning)

http://www.drstandley.com/images/nervous5.bmp

Nerve Tissue

• 2 types of cells:1neurons (responsive cells that conduct impulses at fast speeds)

2neuroglia (support, maintain neurons)

http://www.greenspine.ca/media/neurons_and_glial_cells.jpg

5 types of neuroglia• 1astrocytes (anchor neurons)• 2ependymal (form cerebrospinal

fluid) • 3microglia (eat invading

microorganisms) • 4oligodendrocytes (provide

insulation around CNS – myelin)• 5Schwann cells (insulation

around PNS)

http://www.nrc-cnrc.gc.ca/education/images/bio/gallery/pl_astrocytes.jpg

Neuron made up of… • 1cell body (cytoplasm, nucleus,

organelles)• 2dendrites (branching extensions

from cell body – receive impulses)

• 3axons (conducts impulse away from body)

http://pami.uwaterloo.ca/~gsdharwa/b_c_i/neuron.gif

• Axons enclosed with Schwann cells forming layers rich in fat.

• Provides insulation – myelin.• Insulation not continuous (gaps –

nodes of Ranvier)

http://www.sirinet.net/~jgjohnso/neuronsandsynapsel.gif

• Gray matter is made of unmyelinated fibers – shorter with no myelin

• White matter is made of myelinated fibers – can be longer and have myelin

http://cache.eb.com/eb/image?id=54745&rendTypeId=4

Types of Neurons

• Structurally, 3 different types.• 1Multipolar – many dendrites –

carry impulses to skeletal muscle.

• 2Bipolar – single dendrite, single axon – special sensory areas (ears, eyes)

• 3Unipolar – one nerve fiber (from skin to spinal cord)

http://homepage.psy.utexas.edu/homepage/class/Psy332/Salinas/Cells/multipolar.gif

• Functionally, 3 different types of neurons.

• 1Sensory (afferent) – carry from body to CNS)

• 2Association (interneurons) – links between neurons

• 3Motor (efferent) – from CNS to body

http://www.nsbri.org/HumanPhysSpace/focus7/f7-290.jpg

Impulse Transmission

• Difference in voltage across cell membranes.

• Openings in membrane are called ion channels. They regulate movement of ions.

• Greatest influence – Na+, K+

http://www2.montana.edu/cftr/images/IonChannel2.gif

• Sodium-potassium pump – transports sodium out of cell, potassium inside.

• Causes concentration gradient – ions actively move across cell membrane through ion channels

• Every 3 Na+ pumped out, 2 K+ back in.

•Outside of the membrane accumulates positive ions (potassium leaks out faster)

•Resting membrane potential – no impulses are transmitted

• Potential on inside -70 mV.• Change in membrane permeability

to sodium – ions flow outward; inside becomes more positive – depolarization (+ 30 mV)

• Restored to normal – repolarization – potassium inward through ion channels, sodium channels close.

•Depolarization followed by repolarization – impulse sent down axon.

•Nerve impulse – wave of ion reversals (changing charge of membrane)

http://scienceblogs.com/clock/upload/2006/06/ActionPotential.jpg

• Myelinated fibers- Conduct impulses faster than nonmyelinated fibers

• Node of Ranvier – gaps in axon of myelinated fibers

• Impulse jumps across myelin sheath from node to node – fastest conduction in body.

http://www.getbodysmart.com/ap/nervoussystem/celltypes/menu/image.gif

Types of Stimuli

• All-or-none – either impulse conducted or not.

• Threshold – minimum strength of stimulus needed for action potential.

• Subthreshold – no action potential.• Series of subthreshold – summation

(lead to action potential)

http://www.octc.kctcs.edu/gcaplan/anat/images/Image370.gif

• Gap between adjacent neurons - Synapse

• Neuron that sends impulse – presynaptic neuron; recieves impulse – postsynaptic neruon

• Axon of presynaptic – bulb with synaptic vescicles (contains neurotransmitters)

•Synaptic Cleft – Small space between the terminal end of an axon and the next neuron or muscle.

http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg

2 effects of neurotransmitters• 1Excitatory – increase membrane

permeability to sodium ions (cause action potential) – accetylcholine, norepinephrine

• 2Inhibitory – lowers chance of impulse crossing synapse – endorphins, GABA (inhibit pain)

http://www.ainenn.org/images/bio-sinapse.PNG

Central Nervous System

• 1Spinal cord – from base of brain to 1st, 2nd lumbar vertebrae.

• Enters through foramen magnum of skull.

• Protected by vertebral column, fluid, and meninges (layers of membrane)

Direction of Impulses

• Ascending tract (up towards brain)– sensory information

• Descending tract (away from brain)– motor information

• Spinal cord also serves for reflexes – rapid response to emergency.

Reflex arc

• Reflex arc- Receptor (generates action potential) sends message along sensory neuron to CNS (spinal cord).

• Examples: Withdrawal reflex, patellar reflex, vomiting- smooth muscle reflex, heart rate- cardiac muscle reflex

http://webanatomy.net/anatomy/reflex_arc.jpg

The Brain

• 3 major regions: 1forebrain, 2midbrain, 3hindbrain.

• Forebrain – cerebrum, diencephalon• Midbrain – below diencephalon• Hindbrain – pons, medulla

oblongata, cerebellum.• Brain stem – midbrain, pons,

medulla

http://www.dhushara.com/book/brainp/brainil/brain.jpg

Cerebrum

• Cerebrum – higher brain – conscious thought, memory, learning.

• Divided into left and right cerebral hemispheres.

• Wrinkled structure (convolutions) – result from rapid growth during development.

http://www.web-books.com/eLibrary/Medicine/Physiology/Nervous/cerebrum_lobes.jpg

• Foldings project upward – gyri; downward – sulci.

• Deep grove – fissure; 2 major ones – longitudinal (divides hemispheres), transverse (cerebrum from cerebellum)

http://cas.bellarmine.edu/tietjen/HumanBioogy/giri_and_sulci.gif

Peripheral Nervous System

• Nerves, ganglia, sensory receptors.

• Communication between CNS and other areas of body.

• Nerve – composed of more than 1 type of tissue; responsible for transporting nerve impulses.

http://www.nrc-cnrc.gc.ca/education/images/bio/gallery/pl_nerve.jpg

• Nerves with…• sensory fibers – afferent nerves• motor fibers – efferent nerves • both – mixed nerves• Ganglia – clusters of neurons

outside CNS

http://content.answers.com/main/content/wp/en/f/f0/ReflexArc1.jpg

• Sensory receptors – respond to stimuli (changes in environment)

• Most endings of dendrites from sensory neurons.

• Also found in special sensory organs (eye, taste bud, etc)