Nervous System

Chapter 9

Pages 211-257

Chapter 9 Wordbytes

1. af- = toward 11. -ferrent = carried2. arachn- = spider 12. gangli- = swelling3. astro- = star 13. -glia = glue4. auto- = self 14. mening- =membrane 5. dendro- = tree 15. micro- = small6. di- = 2, through 16. neuro- = nerve7. ef- = away from 17. –oid = similar to8. encephalo- = brain 18. oligo- = few9. enter- = intestines 19. peri- = around10. epen- = above 20. somat- = body

Nervous System Overview

Master controller and communicator for the body

Responsible for all behavior 3 functions:

Sensory input monitors changes inside/outside of body

Integration processes and interprets, then decides what should be done

Motor output causes a response in effector organs

Organization—2 main parts:

1. Central Nervous System (CNS) = brain and spinal cord

Interprets incoming sensory info. and dictates motor responses

2. Peripheral Nervous System (PNS) = nerves from brain & in spinal cord

INPUT-Afferent or Sensory division OUTPUT- Efferent or Motor division Subdivided: Somatic (SNS—from CNS to

skeletal muscles=voluntary) & Autonomic (ANS—regulate smooth & cardiac muscle, glands=involuntary)

Major structures:

Histology

Highly cellular—densely packed & tightly intertwined

2 types of cells:1. Neuron= nerve cell

• Specialized for signal carrying & information processing

2. Neuroglia cells support, nourish & protect neurons• Neuroglia critical for homeostasis of interstitial fluid

around neurons

Supporting cells (Neuroglia)

~ half the volume of CNS Cells smaller than neurons Can multiply and divide and fill in

brain areas Do not conduct nerve impulses

Supporting Cells in CNS

Astrocytes most abundant and most versatile; blood-brain barrier

Oligodendrocytes have fewer branches; produce insulating myelin sheath in CNS

Microglia ovoid cells with thorny processes; provide defense (because immunity cells not allowed in CNS)

Ependymal cells squamous/columnar cells with cilia; produce cerebrospinal fluid (CSF)

Supporting Cells in PNS

Schwann cells PNS cell support; produce & maintain myelin sheath, regenerate PNS axons

Satellite cells in PNS ganglia; support neurons in ganglia, regulate exchange of materials between neurons and interstitial fluid

Neuron Characteristics

They conduct nerve impulses from one part of the body to another

They have extreme longevity live/function for a lifetime

They are amitotic lose their ability to divide

They have a high metabolic rate = need O2 and glucose

Neuronal Structure Cell body nucleus, cytoplasm with typical

organelles; most within CNS = protected by cranial bones & vertebrae

Dendrites short, highly branched input structures emerging from cell body = high surface area to receive signals

Axon may be short or long, only one per neuron; conducts away from cell body toward another neuron or effector Emerges at cone-shaped axon hillock

Axon terminals at end of axon with synaptic bulbs

Figure 9.3

Pg. 391

(Neurilemma)

= impulse direction

Myelination

Axons covered with a myelin sheath Many layered lipid & protein creating insulations Increases speed of nerve conduction. Formed by:

• Schwann cells in PNS (pg. 393 fig. 11.5)• Oligodendrocytes in CNS

Nodes of Ranvier= gaps in the myelin Nodes are important for signal conduction

Some diseases destroy myelin multiple sclerosis & Tay-Sachs

Gray and White Matter

White matter- primarily myelinated axons Gray matter- nerve cell bodies, dendrites,

unmyelinated axons, axon terminals & neurogliaSpinal cord gray matter is centrally located

Classification of Neurons

1. Structural according to # of processes:

Multipolar 3 or more; most common, especially in CNS

Bipolar 2 processes (an axon and a dendrite) that extend from opposite sides; found in special sense organs

Unipolar 1 process that divides like a T; found in ganglia in PNS

2. Functional according to the direction impulse travels (Table 11.1)

Sensory (afferent) neurons transmit impulses from sensory receptors toward or into the CNS; mostly unipolar, with cell bodies in ganglia outside CNS

Motor (efferent) neurons carry impulses away from CNS to muscles and glands; multipolar, usually with cell bodies in CNS

Interneurons (association neurons) between motor & sensory neurons; most in CNS; 99% of neurons in body; mostly multipolar

Neurophysiology

Neurons are highly irritable = responsive to stimuli

When stimulated, an electrical impulse (action potential) is conducted along its axonAction potential underlies all functional

activities of the nervous system

Action Potentials

Action potentials = nerve impulses Require a membrane potential

electrical charge difference across cell membrane – like a battery

Ion Channels allow ions to move by diffusion = current

If no action potential then resting cell has resting membrane potential

Ion Channels

Allow specific ions to diffuse across membraneMove from high concentration to low or toward area of opposite charge

Leakage channels Gated channels- require trigger to open Voltage- Gated channels respond to a

change in membrane potential

Resting Membrane Potential

Leakage channels Cytosol high in K+ & interstitial fluid high in

Na+ (sodium –potassium pumps) Leakage lets K+ through easily and Na+

poorly inside is negative relative to outside actual value depends on the relative leakage

channel numbers

Figure 9.4

Graded Potentials

Short-lived, local changes to membrane potential

Cause current flows that decrease with distance

Magnitude varies with strength of stimulus

Action Potential (AP)

Generated by neurons and muscle cells

Series of active events Channels actively open & close Some initial event is required to reach

a voltage threshold (~ = - 55 mv) Stimulus = any event bringing

membrane to threshold

Action Potential

Resting statevoltage-gated channels closed

Depolarizing phase- membrane potential rises and becomes

positive Repolarizing phase-

potential restored to resting value ( PNa, PK)

UndershootPotassium permeability continues

Figure 9.5

Active Events

Stimulus to reach threshold Na+ channel opens=> Na+ ions enter=> positive potential=> Causes K+ channel opening => repolarization

All- or –None Phenomenon

This sequence is always the same If threshold then the same size of changes

occur no larger or smaller APs Stimulus must reach threshold to start After one AP there is a short period before next

can be triggered= absolute refractory period each AP is a separate, all-or-none event; enforces one-way transmission of AP

Conduction of Nerve Impulses

Each section triggers next locally Refractory period keeps it going the right

direction unmyelinated fiber- continuous conduction With myelin- saltatory conduction

Can only be triggered at nodes of Ranvier Myelinated fibers faster & larger neurons faster

Figure 9.6a

Figure 9.6b

The Syanpse

Synapse (to clasp or join)- junction that mediates information transfer from 1 neuron to another or from a neuron to an effector cell

Axodendritic or axosomatic synapses – most synapses occur between the axonal ending of a neuron and the dendrites or cell body of other neurons

Synaptic Transmission – Electrical synapse

Sequence of events at synapse Triggered by voltage change of the Action

Potential Sending neuron = presynaptic Receiving neuron = postsynaptic Space between = synaptic cleft Neurotransmitter carries signal across cleft

Events at Synapse – Chemical synapse

AP arrives at presynaptic end bulb=> Opens voltage gated Ca2+ channels=>

Ca2+ flows into cell increased Ca2+ concentration => exocytosis of synaptic vesicles=> Neurotransmitter released into cleft Diffuse across and bind to receptors in

postsynaptic cell membrane

Synaptic Transmission

Binding at receptors Chemical trigger of ion channels May depolarize or hyperpolarize postsynaptic

cell membrane If threshold reached at axon hillock then

postsynaptic cell action potential results

Synaptic Transmission

Finally the neurotransmitter must be removed from the cleft-

Diffusion away Destroyed by enzymes in cleft Transport back into presynaptic cell Neuroglia destruction

Figure 9.7

Neurotransmitters

AcetylCholine (Ach)- common in PNS Biogenic amines - Norepinephrine (NE), Dopamine (DA),

serotonin, Histamine

Amino Acids- Glutamate, Aspartate, gamma aminobutyric

acid (GABA), glycine Neuropeptides – endorphins Novel Messengers - ATP/ Nitric oxide (NO)/

Carbon monoxide (CO)

Development of Neurons

P. 422-424 Neuroblasts Growth cone Programmed cell death

Web sites:

http://www.sciencecases.org/chin/chin.asp http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html http://www.getbodysmart.com/ap/nervoussystem/menu/menu.h

tml http://www.bbc.co.uk/science/humanbody/body/interactives/3dji

gsaw_02/index.swf?startPosition=nervous http://learn.genetics.utah.edu/units/addiction/reward/madneuro

n.cfm http://www.gpc.edu/~bbrown/peril/neurons/level1.htm