Lesson 6 - Structure and Signaling

The Nervous System

Central Nervous System Peripheral Nervous System

Brain Spinal Cord Somatic Autonomic

Sensory MotorParasympathetic

Sympathetic

Nervous System

• Afferent (sensory) Neurons – Carry impulses toward the CNS

• Efferent (Motor) Neurons – Carry impulses from the CNS

• Interneurons – conduct impulses within the spinal cord (between afferent and efferent) (Syn. Association, Internuncial)

• Ganglia are small masses of nervous tissue located outside the brain and spinal cord.

Sensory Neurons

• INPUT From sensory organs to the brain and spinal cord.

Somatosensory neuron - spinalVision, hearing, taste and smell -

cranial

SpinalCord

BrainSensoryNeuron

Touch receptors in skin

Motor Neurons

• OUTPUT From the brain and spinal cord to muscles and glands

SpinalCord

Brain

MotorNeuron

Motor neurons in spinal cord

Interneurons

• PROCESSING Relay information between other neurons

SpinalCord

Brain

Inter-Neurons

Interneurons in brain

Divisions of the PNS

• Somatic Nervous System (SNS)– Sensory neurons that convey information from

sensory receptors in the head, body wall and limbs to the CNS

– Motor neurons from the CNS that conduct impulses to the skeletal (voluntary) muscles only.

Divisions of the PNS

• Autonomic Nervous System (ANS)– Sensory neurons convey information from

receptors in the viscera (internal organs), to the CNS.

– Motor neurons then convey information from the CNS to smooth muscle, cardiac muscle, glands, etc.

– Motor functions in the ANS are not normally under conscious control; they are involuntary.

Neuron Resting Potential

• Uneven concentrations of Na+ (outside) and K+ (inside) on either side of neuron membrane results in the inside of the neuron being 70 mV less positive than the outside

• Text, pg. 419

A sodium-potassium pump maintains resting membrane potential after ions “leak” down their concentration gradient

- 3 Na+ ions are actively pumped out while 2 K+ ions are pumped in.

ACTION POTENTIAL• Nerve signals are transmitted by action

potentials that are abrupt, pulse-like changes in the membrane potential that last a few ten thousandths of a second.

• Action potentials can be divided into three phases: the resting or polarized state, depolarization, and repolarization

• The amplitude of an action potential is nearly constant and is not related to the size of the stimulus, so action potentials are all-or-nothing events.

Action Potential

Saltatory Conduction

Terminology• Synapse

– Region at which neurons come nearly together to communicate. (neuron or effector organ)

• Synaptic Cleft– Gap between neurons (at a synapse)

– Impulses can not propagate across a cleft

• Synaptic Vesicle– Packets of neurotransmitter in presynaptic neuron

• Presynaptic Neuron– Neuron sending a signal (before the synapse)

• Postsynaptic Neuron– Neuron receiving a signal (after the synapse)

Neurotransmitters

5 general criteria: 1) synthesized and released by neurons 2) released at the nerve terminal in a 'chemically identifiable' form 3) the chemical should reproduce the activity of the presynaptic neuron 4) can be blocked by competitive antagonist based on concentration 5) active mechanisms to stop the function of the neurotransmitter

Classical transmitters are small molecules (often amino acid based)

Non-classical transmitters can be peptides or even gasses

5 Steps of Neurotransmission

1) synthesis of the neurotransmitter

precursors and enzymes should be in the correct place

2) storage of neurotransmitter OR precursor

often stored in presynaptic vesicles

5 Steps of Neurotransmission3) release of the neurotransmitter

generally by vesicle fusion

4) binding to target receptor

ionotropic receptors open ion channels

metabotropic receptors modulate other signals

5 Steps of Neurotransmission5) termination of the signal

active termination caused by reuptake or chemical breakdown*For e.g. acetylcholine is broken down by . . .

passive termination uses diffusion

Types of Neurotransmitters

Acetylcholine + muscles, learning, memory

Serotonin (a derivative of tryptophan)

+ sleep, relaxation, self esteem, too little = depression, perception

Norepinephrine (aka noradrenaline)

+ stress and fight/flight response, sympathetic NS:+BP & heart rate

Dopamine + prolactin (milk production), involved in pleasure, movement

Endorphins (-) pain, involved in pleasure

GABA (gamma aminobutyric acid)

(-) anxiety, too little in parts of brain can lead to epilepsy

Glutamate Most common NT, memory, toxic

Sympathetic component prepares body for stress [neurotransmitter used=norepinephrine]•Diverts blood from internal organs to skeletal muscles, heart & brain

Parasympathetic brings things back to normal [neurotransmitter used=acetylcholine]

•Work in conjunction/opposition to each other

–Ex. “on” / “off” switches

“FLIGHT OR FIGHT RESPONSE”

You come across a bear on your walk to school…what

happens?• Sympathetic nervous system does

what?• Increases heart rate• Increases breathing rate• Dilates bronchioles• Dilates pupils• Inhibits digestion