Chapter 10

The Nervous System

Introduction

• Types of neural tissue:• 1. Neurons – react to changes around them & send

impulses• 2. Neuroglia – support tissue with a variety of

functions• Functions of nervous system:• 1. Sensory – use sensory neurons to gather info.

inside & outside the body• 2. Motor – use motor neurons to help the body

react to stimuli• 3. Integrative – integrate signals from sensory &

motor neurons to produce thought, memory, etc.

Divisions of the Nervous System

• Central Nervous System (CNS) – consists of the brain & spinal cord

• Peripheral Nervous System (PNS) – consist of nerves that connect the CNS to other body parts

Structure of a Neuron

• Dendrites – pick upimpulses

• Cell body – contains cell parts

• Axon – sends impulses

• Schwann cells – wraparound the axon

• Myelin – lipid coveringformed by Schwann cells; speedsrate of impulse

• Axon terminals – end of axon

Structure of A Neuron

• Axon hillock – slight

elevation where

axon originates

• Node of Ranvier –

gap in myelin

Structure of A Neuron• Neurofibrils – network of fine threads

that extend into the axon; forsupport

• Nissl bodies – consist ofrough ER

Neurilemmal sheath – formed by the cytoplasm & nucleus of theSchwann cell that remain on the outside

Direction of Impulse

• Impulse always

travels from dendrites,

through cell body, &

down axon

• Axon synapses w/next

neuron or an effector

(muscle or gland)

Structural Classification of Neurons

• Bipolar – has 2 processes from the cell body, 1 at each end; in sense organs

• Unipolar – has 1 process from c.b. thatdivides into 2; in PNS

• Multipolar – have manyprocesses from c.b; in CNSand motor neurons.

Functional Classification of Neurons

• Sensory (afferent) – unipolar & carry impulses from body parts to brain or s.c.

• Interneurons (association neurons) – multipolar & in CNS; form links b/t other neurons

• Motor (efferent) – multipolar & carry impulses from brain or s.c. to muscle or gland

Types of Neuroglia

• Support tissue w/a variety of functions:

1. Astrocytes –star-shaped;

found b/t neurons & b.v.;

support, transport &

communication

b/t nerves & b.v.

• Transport glucose to

Neuron and store glycogen• Separate neurons from

each other.

Types of Neuroglia

2.Microglia – small w/few processes; found throughout CNS; support & phagocytosis

of harmful sub-

stances

Types of Neuroglia

3. Oligodendrocytes – resemble astrocytes but w/fewer processes; form myelin sheath in CNS

Types of Neuroglia

4. Ependyma – columnar & cuboidal shaped cells; form inner lining of brain & s.c.; provide a layer for diffusion to occur

Types of Neuroglia

Cell suicide

• Microglia can destroy cells that are old &/or damaged• A – healthy neuron• B – neuron being

destroyed & DNA breaking apart

• C – microglia removing debris

Nerve Impulse Cartoon

• Impulse Animation

Resting Potential• A resting neuron is one not sending an impulse & is in resting

potential• The cell membrane of this neuron is polarized b/c of an un=

distribution of ions on either side• Outside the neuron – greater concentration of Na+ ions• Inside the neuron – greater concentration of K+ ions & negatively

charged proteins

Resting Potential• K+ leak out of K+ channels at a slow rate leaving behind

negatively charged proteins• This makes the charge on the inside of the membrane

negative • The voltage meter (next pg.) shows a charge of -70 mv &

refers to the charge of a neuron in resting potential

Resting Potential

Movement of Ions• Ions follow the laws of diffusion (movement

from high to low concentrations) when moving thru membranes

• Ions enter & leave the membrane thru channels or gates that

are specific for

that ion

Ion Channels

• 3 types– Passive- always open– Ligand gated- opened by a chemical

compound. (neurotransmitter)– Voltage gated- opened in response to a

change in electric potential.

Resting Potential

The charge outside the cell is positive b/c:

1. the high concentration of Na+ ions

2. the movement of K+ ions to the outside

Resting Potential Animation

• Resting Potential Animation

Sodium Potassium Pump

• Membrane protein used for the active transport of Na+ and K+ across membrane.

• Requires ATP

• Removes 3 Na+ ions and accepts 2 K+ for every ATP molecule used.

• Maintains resting potential.

Action Potential• An abrupt change in the electrical potential across the cell

membrane that occurs after a stimulus (a.k.a. nerve impulse):1. Resting neuron stimulated (remember – a resting neuron is

polarized)2. Na+ channels open & Na+ move into membrane; charge inside cell

becomes + (+30mv) & neuron is depolarized 3. Na+ channels close & K+ channels open; K+ move out & charge

reverts back to negative (-70mv); cell is repolarized

Resting Potential → Action Potential

A)Restingpotential(polarized)

B)Action potential A.P. in the1st regionstimulates adjacent region (de-polarized)

C)1st region repolarized

Action Potential Animation

• Action Potential Animation

Graphing Action Potential

After repolarization

a brief period of

delay occurs when

Na+ gates cannot

temporarily open;

called refractory

period

Graphing Action Potential

Hyperpolarization when the cell

becomes morenegative than -70mv; depends onwhich ions are allowed to enter the

cell, + or – ions (i.e.Cl- ions)

Threshold – the minimum amt. of stimulusrequired to cause an action potential

Impulse Conduction• Saltatory conduction – impulse jumps from 1 node of

Ranvier to another; why?• Myelin covering – mostly lipids which prevent flow of ions• channels - are located at nodes of Ranvier for ions to

diffuse in & out• Myelinated axons (white matter) - conduct impulses

faster than unmyelinated axons (gray matter)

Saltatory Conduction Animation

• Animation

The Synapse

• Junction b/t 2 neurons• Presynaptic neuron –

occurs before the syapse• Postsynaptic neuron –

occurs after the synapse• Synaptic knob –

enlargement of axon terminal

• Synaptic vesicles – store ntm

• Synaptic cleft – spaceb/t neurons

Actual Synapse

Events at the Synapse

• Action potential travels down presynaptic neuron & arrives at synapse

• Synaptic knob becomes morepermeable to Ca+ & theydiffuse inward

• This causes vesicles to release ntm

• Ntm causes A.P. to enterpostsynaptic neuron

• A.P. continues to travel downpostsynaptic neuron

The Synapse

Types of Neurotransmitters

• The nervous system produces approx. 30 different types of ntm

• Some open ion channels, others close them• Monoamines: Neuropeptides:

- epinephrine - endorphins

- norepinephrine - enkephalins

- dopamine - substance P

- serotonin Acetylcholine (ACh)

Effects of Ntms

• Epinephrine & norepinephrine – hormones when released in blood, but ntm in the n.s.; stimulate autonomic n.s.; incr. HR, resp. rate, etc.; “fight-or-flight” response

• Dopamine – excitatory or inhibitory; create a sense of well-being; insufficient levels associated with Parkinson’s disease

• Serotonin – inhibitory; insufficient levels associated with insomnia

• Endorphins & enkephalins – generally inhibitory & influence mood; released under stress to reducepain (blocks substance P)

• Substance P – excitatory; helps in perception of pain• ACh – stimulates muscles to contract

Synaptic Potentials

• Ion channels that respond to ntm are called chemically gated channels (as opposed to those that are voltage-gated & are involved in sending A.P.)

• Changes in chem. gated channels create local changes called synaptic potentials (a small, temporary change in the potential charge of a neuron)

• They allow one neuron to influence another

The Synapse

Synaptic Potentials

• 2 types:

1. Excitatory postsynaptic potential (EPSP) – occurs when the neuron is depolarized (or becomes less negative), but the charge is subthreshold (<+30mv).

A true A.P. won’t occur, but will be more likely to occur if the neuron receives more subthreshold stimuli

Synaptic Potentials

2. Inhibitory postsynaptic potential (IPSP) occurs when the neuron is hyperpolarized (or becomes more negative than -70mv). An A.P. will be less likely to occur.

• The type of ntm secreted will decide the effect that occurs.

Effects of Ntm on Synaptic Potentials

• If a ntm opens Na+ channels & Na+ diffuse in, the membrane is depolarized (EPSP)

• If a ntm opens K+ channels & K+ diffuse out, the membrane is hyperpolarized (IPSP)

• A neuron can receive EPSP’s & IPSP’s simultaneously; the neuron responds to the algebraic sum of the + and - charges

Synaptic Potential vs. Action Potential

• 2 differences:1. P.S.P. are graded (depends on amt. of ntm) & their effect adds up (called summation) whereas A.P. are all-or-none2. P.S.P. decr. in intensity w/incr. distance from synapse

• Facilitation – when a neuron receives subthreshold stimuli & gets closer to sending an A.P.

Convergence vs. Divergence

• Convergence – impulses from

2 or more fibers converge on

a single neuron (summation

will occur)• Divergence – when outgoing

impulses are divided onto several

branches of an axon

Convergence vs. Divergence

Importance of Ions

• Ca+ are needed for the release of ntm• Ca+ are also needed to close Na+

channels• Insufficient Ca+ levels result in channels

remaining open & impulses repeatedly transmitted; results in tetany

• May occur in pregnancy (as fetus uses maternal Ca+), when diet lacks Ca+ or Vit D during dehydration

Importance of Ions

• An incr. in extracellular K+ causes neuron to be less negative; threshold is reached sooner & neurons are very excitable; may result in convulsions

• A decr. in extracellular K+ causes neuron to be more neg.; does not allow an A.P. to occur & muscles may become paralyzed

Resting Potential

Action Potential

Saltatory Conduction

EPSP

IPSP

Convergence vs. Divergence