Nervous System and Nervous Tissue

Nervous System

Master control and communication

Functions (system level and cell level)

Sensory input – monitoring stimuli

Integration – interpretation of sensory input

Motor output – response to stimuli

Cellular v. System level

Dendrites: input

Cell body: integration

Axon: output

PNS

PNS

CNS

Nervous System Organization

Central nervous system (CNS)

Form: Brain and spinal cord

Function: Integration and command center

Peripheral nervous system (PNS)

Form: Paired spinal and cranial nerves

Function: Carries messages to and from the spinal cord and brain

Central Nervous System

Peripheral Nervous System

Motor (efferent)Sensory(afferent)

Somatic(voluntary)

Autonomic(involuntary)

Parasympathetic (Stop! )

Sympathetic(Action! Go!)

Peripheral Nervous System (PNS)

INPUTS: Sensory (afferent) division

Sensory afferent fibers – from skin, skeletal muscles, and joints to the brain

Visceral afferent fibers – from visceral organs to the brain

OUTPUTS: Motor (efferent) division

Transmits impulses from the CNS to effector organs

Motor Division Organization

Somatic nervous system (SNS)

Conscious control of skeletal muscles

Autonomic nervous system (ANS)

Regulates involuntary muscle (smooth and cardiac) and glands

▪ Sympathetic (Stimulates = Go!)

▪ Parasympathetic (Conserves = Stop!)

Nervous System Cell Types

Neurons

Transmit electrical signals

Neuroglia (“nerve glue”)

Supporting cells

Neuroglia in the CNS

Astrocytes

Microglia

Ependymal cells

Oligodendrocytes

Neuroglia in the PNS

Satellite cells

Schwann cells

Neurons

Structural units of the nervous system

Long-lived (100+ years)

Amitotic (no centrioles = can’t divide)

High metabolic rate (glucose gobblers!)

Neuron Classification (function)

Sensory (afferent)

transmit impulses toward the CNS

Motor (efferent)

transmit impulses away from the CNS

Interneurons (association neurons)

shuttle signals through CNS pathways

(b)

(a)

Dendrites Cell body

Nissl bodies

Axon terminals(secretory component)

Axon hillock

Node of Ranvier

Impulsedirection

Schwann cell

Neuron cell body

Dendriticspine

Neuron (nerve cell)

Nerve Cell Body (Soma)

Contains nucleus and nucleolus

Major biosynthetic center

Focal point for the outgrowth of neuronal processes (dendrites and axons)

Axon hillock – where axons arise

Neuronal processes (fibers)

Dendrites Numerous Short and tapering Diffusely branched Contain “spines” where synapses form

Axons One per cell Long (up to 4 ft. in length) Form synapses at terminals (release neurotransmitters) Anterograde and retrograde transport (out and back!)

Supporting Cells: Neuroglia

Provide a supportive scaffolding for neurons

Segregate and insulate neurons

Guide young neurons to the proper connections

Promote health and growth

Help regulate neurotransmitter levels

Phagocytosis

Astrocytes

Most abundant and versatile

Cling to neurons and synaptic endings

Cover capillaries (blood-brain barrier)

Support and brace neurons

Guide migration of young neurons

Control the chemical environment

Microglia

Monitor health of neurons Transform into macrophages to remove cellular

debris, microbes and dead neurons

NOTE: Normal immune system cells can’t enter CNS

Ependymal Cells

Shape: squamous to columnar (often ciliated)

Location: Line the central cavities of the brain and spinal column

Function: Circulate cerebrospinal fluid

Oligodendrocytes

Wrap CNS axons like a jelly roll

Form insulating myelin sheath

Schwann Cells and Satellite Cells

Schwann cells

Surround axons of the PNS

Form insulating myelin sheath

Satellite cells

Surround neuron cell bodies

Nodes of Ranvier

Myelin Sheath and Neurilemma

Myelin Sheath White, fatty sheath protects long axons

Electrically insulates fibers

Increases the speed of nerve impulses

Neurilemma remaining nucleus and cytoplasm of a

Schwann cell

Axons of the CNS

Both myelinated and unmyelinated fibers are present

Oligodendrocytes insulate up to 60 axons each

White matter: dense collections of myelinated fibers

Gray matter: mostly soma and unmyelinated fibers

Action Potentials (nerve impulse)

Electrical impulses carried along the length of axons

Always the same regardless of stimulus

Based on changes in ion concentrations across plasma membrane

This is HOW the nervous system functions

Electricity Definitions

Voltage (V) potential energy from separation of charges (+ and -) For neurons, measured in millivolts

Current (I) the flow of electrical charge between two points

Insulator substance with high electrical resistance

Think myelin sheath!

Ion Channels

Passive (leakage) channels: always open

Voltage-gated channels: open and close in response to membrane potential

Ligand-gated (chemically gated) channels: open when a specific neurotransmitter binds

Mechanically gated channels: open and close in response to physical forces

Let’s review! The sodium-potassium pump

Voltage-Gated Channels

Chemically Gated Channels

Gated Channels

When gated channels are open: Ions move along electro-chemical gradients

▪ Takes into account charge differences

▪ Takes into account concentration differences

An electrical current is created

Voltage changes across the membrane

Measuring Membrane Potential

Resting Membrane Potential

Resting membrane potential (–70 mV) The inside of a cell membrane has more negative charges than

outside the membrane

Major differences are in Na+ and K+

Changes in Membrane Potential

Depolarization the inside of the membrane becomes less

negative

Hyperpolarization the inside of the membrane becomes more

negative than the resting potential

Repolarization the membrane returns to its resting membrane

potential

Action Potentials = nerve impulse

Principal means of neural communication A brief reversal of membrane polarity All or nothing event Maintain their strength over distance Generated only by muscle cells and neurons

Phases of an Action Potential

1. Resting state

2. Depolarization

3. Repolarization

4. Hyperpolarization

5. Return to resting potential

Action Potential: Resting State

Na+ and K+ GATED channels are closed

Each Na+ channel has two voltage-regulated gates Activation gates Inactivation gates

Action Potential: Depolarization

Na+ permeability increases; membrane potential reverses Na+ gates are opened, but K+ gates are closed Threshold: critical level of depolarization (-55 to -50 mV) Once threshold is passed,action potential fires

Action Potential: Repolarization

Sodium inactivation gates close

Voltage-sensitive K+ gates open

K+ rushes out

Interior of the neuronis negative again

Action Potential: Hyperpolarization

Potassium gates remain open Excess K+ leaves cell

Membrane becomes hyperpolarized Neuron is

insensitive to stimuli until resting potential is restored

Return to resting potential:Sodium-potassium pump

Repolarization ONLY restores the electrical differences

across the membrane

DOES NOT restore the resting ionic conditions

Sodium-potassium pump restores ionic conditions More sodium outside

More potassium inside

Na+

Na+

Potassiumchannel

Sodiumchannel

1 Resting state

2 Depolarization

3 Repolarization

4 Hyperpolarization

Activationgates

Inactivation gateK+

K+

Na+

K+

Na+

K+

ACTION!

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

http://www.youtube.com/watch?v=SCasruJT-DU

http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf

http://www.blackwellpublishing.com/matthews/channel.html

Absolute and Relative Refractory Periods

Refractory Periods

Absolute refractory period (NO WAY! NO HOW!) Neuron CANNOT generate an action potential

Ensures that each action potential is separate event

Enforces one-way transmission of nerve impulses

Relative refractory period (Well, maybe…) Threshold is elevated

Only strong stimuli can generate action potentials

Propagation of an Action Potential

–70

+30

(a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms

Voltageat 2 ms

Voltageat 4 ms

Voltageat 0 ms

Resting potentialPeak of action potentialHyperpolarization

Mem

bra

ne

po

ten

tial

(m

V))

Action Potential Frequency

Stronger stimuli generate more frequent action potentials

How fast does a signal travel?

Velocity determined by Axon diameter

▪ the larger the diameter, the faster the impulse

Presence of a myelin sheath

▪ Myleinated neurons have much faster impulses

▪ Why? Node-jumping! (Saltatory conduction)

Saltatory Conduction

Multiple Sclerosis (MS)

Cause: Autoimmune disease with symptoms appearing in young adults (women at highest risk)

UNKNOWN environmental and genetic factors

Symptoms: visual disturbances, weakness, loss of muscular control, incontinence

Physiology Myelin sheaths in the CNS are destroyed, producing a

hardened lesion (scleroses)

Shunting and short-circuiting of nerve impulses occurs

Alternating periods of relapse and remission

Multiple Sclerosis

Treatment Drugs that modify immune response

Prognosis Medications can prevent symptoms from worsening

Reduce complications

Reduce disability

HOWEVER, not all drugs work long-term in all patients

Synapses

Junction for cell cell communication

Neuron neuron

Neuron effector cell

Presynaptic neuron

Conducts impulses toward the synapse

Postsynaptic neuron/cell

Receives signal

May/may not act on signal

Synapses

Electrical Synapses (fast)

Less common Resemble gap junctions Allow direct ion flow cell cell

Important in the CNS Neural development

Synchronization of activity

Emotions and memory

Electrical Synapses

Chemical Synapses (slower)

Most common

Excitatory or inhibitory

Communication by neurotransmitters

Presynaptic neuron releases neurotransmitter

Postsynaptic neuron has membrane-bound receptors

Neurotransmitters must be recycled, removed or degraded after release

vesiclescontaining Neurotransmitter

Synapticcleft

Ion channel(closed)

Ion channel (open)

Axon terminal of presynaptic neuron

Postsynapticmembrane

Ion channel closed

Ion channel open

Neurotransmitter

Receptor

Postsynapticmembrane

Degradedneurotransmitter

Na+Ca2+

1

2

34

5

Action

potential

Chemical Synapses

NOTE: Ion channels are chemically gated, not voltage-gated

Neurotransmitter Diversity

Acetylcholine (ACh) Biogenic amines (dopamine, serotonin) Amino acids (glutamate, GABA) Peptides (endorphins, enkephalins) Novel messengers

ATP

Nitric oxide (why Viagra works!)

Carbon monoxide

Neurotransmitter Actions

Direct Alter ion channels Rapid response Important in sensory-motor coordination Ex.) ACh, GABA, glutamate

Indirect Work via second messengers and G-proteins Slower action Important in memory, learning, and autonomic

nervous system Ex.) dopamine, serotonin, norepinephrine

Postsynaptic Potentials

EPSP: excitatory postsynaptic potentials Cell is depolarized

Ex.) glutatmate

IPSP: inhibitory postsynaptic potentials Cell is hyperpolarized

Ex.) GABA

Postsynaptic Potentials

Will the postsynaptic cell fire? It depends on…

Which neurotransmitter is released

The amount of neurotransmitter released

The length of time the neurotransmitter is bound to receptors

If threshold isn’t reached, no action potential

Summation

Spatial summation Multiple potentials arrive at the same time

Number of IPSPs v. EPSPs determine if action potential is generated

Temporal summation Multiple potentials arrive at different times

Time intervals determine if action potential is generated

Summation

NOTE: This is oversimplified. One neuron can receive inputs from thousands of other neurons.

Neuronal signaling and health

Depression Often linked to altered levels of serotonin

Treated with SSRIs (selective serotonin reuptake inhibitors)

Provides greater signal from less neurotransmitter

WARNING: Suicide risk can actually increase in some patients, particularly adolescents and young adults.

Neuronal signaling and health

Addiction Dopamine is essential in “reward” pathways

▪ Triggers pleasurable sensations

▪ Involved in both drug and alcohol addiction

Glutamate is essential in memory pathways

▪ May trigger relapses

Neuronal signaling and health

BoTox = botulinum toxin Works by blocking acetylcholine release at

neuromuscular junction

Facial muscles can’t contract, wrinkles disappear

Also used for many spastic disorders

Local anaesthesia Most block sodium channels, so action potentials

aren’t generated