Overview of Neurons, Synapses & Nervous System

Ch 48, 49 (8th ed.); Ch 48 (7th ed.)

Neurons: nerve cells that transfer information within the body

Two types of signals: long distance – electrical signals short distance chemical signals

Glial cells: support nerve cells

Neurons transfer different types of information: control heart rate, coordinate hand-eye

movement, record memories, generate dreams

Higher order processing is carried out by groups of neurons: ganglia, brain

Connection by neurons specify the information transmitted

Sensory neurons transmit information from sensors that detect stimuli: external stimuli – light,

sound, touch, heat small, taste

internal stimuli – blood pressure, carbon dioxide level, muscle tension

Integration centers: analyze and interpret the sensation obtained from sensory input

Motor neurons: exit the processing centers and trigger muscle or gland activity

Information processing

Central nervous system (CNS): Brain and longitudinal nerve cord (spinal cord)

Peripheral nervous system (PNS): neurons that carry information in an out of the CNS

Peripheral nervoussystem (PNS)

Cranialnerves

Brain

Central nervoussystem (CNS)

GangliaoutsideCNS

Spinalnerves

Spinal cord

Neuron structure: Cell body: contains nucleus and

organelles Dendrites: branched extensions of

the cell body that receive signals Axon: single extension that

transmits signals to other cells Axon hillock: cone shaped

extension where it joins the cell body

Dendrites

Stimulus

Nucleus

Cellbody

Axonhillock

Presynapticcell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitter

Synapse: junction where one neuron transmits information to another neuron or effector cell or organ Synaptic terminal: branch of the

axon that forms the specialized connection

Neurotransmitters: chemical messengers that send information from the transmitting neuron (presynaptic cell) to the receiving neuron (postsynaptic cell)

Dendrites

Stimulus

Nucleus

Cellbody

Axonhillock

Presynapticcell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitter

Transmission of electrical signal: changing K+, Na+ and Cl- concentrations inside

and outside the cell

Resting potential: membrane potential of a resting neuron negative inside

the membrane positive

outside the membrane.

OUTSIDECELL

[K+] [Na+] [Cl–]

INSIDECELL

[K+] [Na+] [Cl–]

Action potential: rapid change in membrane potential of an

excitable cell it is triggered by a stimulus, caused by opening

and closing of voltage sensitive gates in sodium and potassium ion channels

Action potential is generated as Na+ ions flow in at one location: causes depolarization

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Depolarization triggers action potential in the neighboring region and the previous region gets repolarized as K+ flows out

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

Depolarization and repolarization continues down the axon; propagation of action potential down the length of the axon

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

ActionpotentialK+

K+

Na+

Myelin sheath: Schwann cells wrap around the axon forming

layers of myelin insulates, gaps are known as nodes of Ranvier

Axon

Schwanncell

Myelin sheathNodes ofRanvier

Node of Ranvier

Schwanncell

Nucleus ofSchwann cell

Layers of myelinAxon

0.1 µm

Chemical synapse: 1. action potential depolarizes the plasma membrane of the synaptic terminal2. opens voltage-gated calcium channels in the membrane; influx of Ca++

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

3. elevated Ca++ causes synaptic vesicles to fuse with presynaptic membrane

4. vesicles release neurotransmitters into synaptic cleft

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

5. neurotransmitters bind to ion channels and open them

6. neurotransmitters are released and ion channels close

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

Neurotransmitters: Acetylcholine biogenic amines (serotonin, dopamine) amino acids gases

Nervous system in animals: sessile and slow-moving – simple sense organs

with no cephalization

active and predatory – sophisticated nervous system with cephalization and corresponding well developed sense organs

Cnidarians: diffused nerve net Sea stars: radial nerves connected to nerve

ring

(a) Hydra (cnidarian)

Nerve net

Nervering

Radialnerve

(b) Sea star (echinoderm)

Bilaterally symmetrical bodies show cephalization: arthropods, squids, salamanders

(e) Insect (arthropod)

Segmentalganglia

Ventralnerve cord

Brain

(g) Squid (mollusc)

Ganglia

Brain

Brain

Spinalcord(dorsalnervecord)

Sensoryganglia

(h) Salamander (vertebrate)

Whitematter

Cell body ofsensory neuron indorsal rootganglion

Spinal cord(cross section)

Graymatter

Hamstringmuscle

Quadricepsmuscle

Sensory neuron

Motor neuronInterneuron

Spinal cord transmits information to and from the brain and also has nerve circuits that produce reflexes

Cerebrospinal fluid circulates through central canal ventricles in the brain cushions the brain

Brain and spinal cord had gray and white matter Gray matter: neuron cell bodies, dendrites and

unmyelinated axon White matter: myelinated axons

Functional hierarchy in PNS

Efferentneurons

Locomotion

Motorsystem

Autonomicnervous system

Afferent(sensory) neurons

PNS

Hearing

CirculationGas exchange DigestionHormone

action

Entericdivision

Sympatheticdivision

Parasympatheticdivision

Vertebrate brain Pons (part of brainstem), cerebellum

Spinal cord

Cerebrum (includes cerebral cortex, white matter,basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Medulla oblongata (part of brainstem)

Pituitarygland

Cerebrum

Cerebellum

Central canal

Diencephalon:

Hypothalamus

Thalamus

Pineal gland(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Regionalizations in the vertebrate brain

Speech

Occipital lobe

Vision

Temporal lobe

Frontal lobeParietal lobe

Somatosensoryassociationarea

Frontalassociationarea

Visualassociationarea

Reading

Taste

Hearing

Auditoryassociationarea

Speech

Smell

Mo

tor

cort

exS

omat

osen

sory

cor

tex