THE NERVOUS SYSTEMCH 48 AND 49

• Designed to sense an animal’s external environment, initiate a response, and return the animal to HOMEOSTASIS

• Involves a series of cell-cell interactions

I. Nervous systems consist of circuits of neurons and supporting cells

• Single celled organisms can respond to environment

• Most multicelled animals need neurons• Simplest nervous system is in cnidarians,

radially symmetrical animals with nerve net• All bilaterally symmetrical animals have

centralized nervous system

What is the trend here? Importance?

II. The vertebrate nervous systemA. Organization of the nervous system • Central nervous system (CNS)– Brain and spine– Where integration takes place

• Peripheral nervous system (PNS)– Neurons that send info to CNS and receive info

from CNS– Sensory neurons: send info to CNS– Motor neurons: receive info from CNS

B. Neurons • Sensory (afferent) neurons – carry nerve impulses from the sensory receptors to the CNS – sensory receptors are located in: – skin, eyes, ears, mouth, nose

• Motor (efferent) neurons – Carry the nerve impulse from the CNS to effectors – Effectors are muscles and glands

 • interneurons– Located in the CNS – link sensory and motor neurons

Sense receptors send info to sense neurons which send info to interneurons which send info to motor neurons which send info to muscles or glands

1. Neuron structure and function

• Cell body: contains organelles • Dendrites: short extensions off cell body that

receive signals • Axon: Long extension off of the cell body that

relays the stimulus • synaptic terminal: sends info from one cell to

another thru neurotransmitters• synapse: space between two cells

• presynaptic cell sends info to postsynaptic cell

C. How information is sent thru the nervous system

• all cells have a difference in charge on either side of plasma membrane called a membrane potential

• Rest membrane potential: Membrane potential of unstimulated neuron

• Change in resting membrane potential results in transmission of nerve impulse

What do you notice about the resting membrane potential?

1. Forming the resting membrane potential

• Uses energy in active transport• [ K+] is highest in cell, [Na+] is highest out • Na+/K+ pumps use NRG of ATP to maintain

these K+ and Na+ gradients across the plasma membrane (active transport: low to high concentration using NRG)

• These concentration gradients represent chemical potential energy

2. Generating the action potential

• Stimulus from the environment causes the membrane of the nerve cell to depolarize: – Na+ diffuses into the nerve cell – K+ diffuses out

• As soon as this happens, the Na+/K+ pump restores the membrane potential

3. Propagating the action potential

• The action potential travels down the nerve cell from dendrites to axon in a wave of depolarization and restoration of the membrane potential

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__the_nerve_impulse.html  http://www.blackwellpublishing.com/matthews/actionp.html  http://wps.aw.com/bc_campbell_biology_8ap/

D. How the nerve impulse moves from one cell to another

• The space between two nerve cells is called a synapse

• Two nerves communicate with each other by synaptic signaling

• How: – When the action potential reaches the end of the

axon, it stimulates the release of neurotransmitters into the synapse

– The neurotransmitters bind to receptors on the postsynaptic cell to transmit the nerve impulse

http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__chemical_synapse__quiz_2_.html

E. What are neurotransmitters?

• Small molecules stored in vesicles in the presynaptic neuron

• Most are made in RER and golgi

• Types:– Dopamine: the “reward” neurotransmitter,

cocaine and meth increase dopamine release– Serotonin: the “feel good” neurotransmitter.

MDMA either increases or prevents degradation of serotonin. Low serotonin is associated with depression

– Acetylcholine: involved in muscle contraction. Broken down by acetylcholinesterase to control muscle contraction

• http://www.youtube.com/watch?v=UabDiuTtU0M&list=PLFCE4D99C4124A27A&index=50