Nervous System

IB Biology

Nervous System

• In order to survive and reproduce an organism must respond rapidly and appropriately to environmental stimuli in order to send the “right” messages at the “right” time to the “right” places.

• The nervous system provides a “speedy” communication system so the organism can quickly respond to internal and external stimuli.

The Neuron• Neuron = Nerve cell– Functional unit of the nervous system

Receives incoming messages and sends it to the cell bodyPlays role in summation and transmission of the signal

Conveys outgoing message to other cells (neurons/muscles/glands)

Insulating layer that helps propagate the signalRelease neurotransmitters into the synapse (space between neuron and effector cell)

The Synapse• Connects presynaptic and postsynaptic cells

3 overlapping functions of NS

The Brain (integration) … cerebrumReasoning, Problem Solving

Sends commands to skeletal muscle

Receives and integrates signals from touch, pain, pressure, and temperature receptors

The proportion of somatosensory or motor cortex devoted to a particular part of the body is correlated with the relative importance of sensory or motor information for that part of the body

The Homunculus• This homunculus, or “little

human”, is a visual representation that shows the connection between different body parts and brain devotion to those areas. The bigger the body parts in the picture or model, the more brainpower there is dedicated to detecting sensatory input received by the somatosensory cortex.

3 overlapping functions

Central and Peripheral NS

1) Sensory Input• PNS – sensory receptors detect stimuli

2) Integration• CNS (Brain and spinal cord) makes an interpretation

and association with appropriate response

3) Motor output• PNS – motor receptors to effector cells (muscle cells

or glands)

Reflex vs. Response

Glial Cells

• Astrocytes– Structural and metabolic support– Tight junctions (blood brain barrier)

• Oligodendrocytes– Form insulating myelin sheaths around axons in

the CNS

• Schwann cells– Form insulating myelin sheaths around axons in

the PNS

Membrane Potential

Arises from:

1) Difference in ion concentration on the inside and outside of the cell membrane

2) Selective permeability of the membrane

Excitable Cells

• Cells that have the ability to generate large changes in their membrane potentials.• May result in an electrical impulse• Examples:–Neurons–Muscles

Which way do the ions “want” to diffuse?

Based on Concentration

• Sodium wants to go in

• Potassium wants to go out

Based on Charge

• Sodium wants to go in

• Potassium wants to go in

If Potassium were the only ion that could cross themembrane, which way would K+ go and what would be the result in terms of membrane potential?

It would become about – 85 mV. This is when theinflux of K+ down the electrical gradient is equalto the efflux of K+ down its concentration gradient.

Action Potential (Nerve Impulse)

• This is an “all or nothing response”

• “Gated” ion channels open and close in response to stimuli (pressure, light, chemical)–Only one kind of ion can pass through

–Chemically-gated ion channels

–Voltage-gated ion channels

Action Potential

• Nerve impulse

• Strong enough stimulus causes depolarization to reach threshold

Resting Potential (-70 mV inside)

Activation Gate opens due to stimulus

Voltage gated channels open: more Sodium (Na+) gates open in response to less negative charge

Potassium (K+) channels open (voltage gated

channel)

Inactivation gate (Na+) closes in

response to voltage

Undershoot = refractory (sodium channels are closed … no more action potentials)

How does a neuron get back to it’s resting state where there is more

Sodium outside and Potassium inside?

Sodium – Potassium Pump

3 Na+ out and 2 K+ in

Requires ATP!

Action Potential reaches the end

of the axon

http://www.bristol.ac.uk/synaptic/basics/basics-2.html

Nerve Impulse reaches synaptic terminal

• Depolarization causes voltage-gated Ca2+ channels to open

• Ca2+ enters the nueron and triggers synaptic vesicles to move to presynaptic membrane

• The vesicles release neurotransmitters into the synapse

Note: Voltage sensitive calcium channels open at the axon terminals and cause the synaptic vesicles to release neurotransmitters.

Neurotransmitters Relay Information

• If the neurotranmitter binds to and opens a Na+ channel, what will happen?– Depolarization … AP

• If the neurotransmitter binds to and opens a K+ channel, what will happen?– Hyperpolarization … no AP

When does the AP domino effect end?• When it reaches a muscle or gland for a

response.

What causes one stimulus to be stronger than another?

•More Action Potentials in the same amount of time.

Responding to Stimulus

Hyperpolarization – becomes even more negativeDepolarization – becomes less negativeThreshold – charge required to create action potential

Definitions:

a) presynaptic cell before synapse

b) postsynaptic cell after synapse

c) synaptic cleft separates pre/post

d) synaptic vesicle contains

neurotransmitters

e) neurotransmitter chemical messenger

released into synapse

f) presynaptic membrane synaptic terminal that

faces the cleft

g) postsynaptic membrane cell body or dendrite on

other side of synapse