Notes The Nervous System Chapter 35 Section 2

Notes The Nervous System Chapter 35 Section 2

Nervous System = Communication Function: Receives Stimuli Interprets information Causes a response -Controls and coordinates bodily functions -Responds to stimuli (inside or outside the body)

What is the basic functional unit of the nervous system? Neuron! What does the neuron do? Carries messages throughout the body How does it carry the messages? By conducting electrical signals What are these signals called? Nerve impulses!

What does a neuron look like?

Neuron Anatomy Three parts to a NEURON: Cell body: Large, central portion of the neuron where all organelle are located What is its job? Interpret incoming signals Cell Body

Neuron Anatomy 2. Dendrites: Short, highly branched fibers what is its job? Carries impulses toward the cell body Referred to as the afferent process

Neuron Anatomy 3. Axon: Long, slightly branched fiber What is its job? Carry impulses away from the cell body Referred to as the efferent process

The Axon Axon Hillock fiber Axon Terminals

Additional Parts of a Neuron Neuron Anatomy Additional Parts of a Neuron Dendrite Cell body Axon terminal Myelin sheath Nodes of Ranvier Axon Nucleus Structure of a Typical Neuron

Neuron anatomy What do you remember? dendrites 3 1 2 4 5 6 7 Myelin sheath nucleus Direction of impulse Cell body axon Nodes of Ranvier Axon terminals

Three Types of Neurons II. Motor Neurons: Carry impulses to muscles and glands Cause a response to some stimuli III. Interneurons: Connect sensory and motor neurons Allow for quick response (reflex action)

Three types of neurons

Neurons What is grey matter? What is white matter? Collective cell bodies and dendrites of all neurons What is white matter? Myelinated nerve fibers Axons of all neurons Can be approximately one meter in length White matter Gray matter

What is a Nerve? Bundle of axons held together by connective tissue. What color is a nerve? White! Why? Because axons are white matter and they compose nerves

How are nerves held together? Connective tissue What is this connective tissue called? Neuroglial cells (nerve glue) Approximately half of the volume of the brain is composed of neuroglial cells Most brain tumors develop in mesoglial cells – NOT neurons

What do neuroglial cells do? Support the axons Insulate the electrical impulses Like electrical tape insulates electric wires this prevents “leaking” of electric signals

An example of a neuroglial cell Schwann cell This wraps around the axon in multiple layers It is composed of a fatty material called… Myelin Regular breaks in the myelin sheath are called… Node of Ranvier

Impulse speed Some neurons are fast, good conductors of impulses Other neurons are slow, poor conductors of impulses What distinguishes these two types? Diameter of the axon Myelination Fastest axons have a large diameter and are myelinated. How much faster? Up to 100 times faster! (Reflexes are this type)

The Nerve Impulse How are messages (impulses) carried by the nervous system? As electrical and chemical signals. How does the impulse develop? Charged particles (ions) move across the cell membrane A neuron is ready to transmit an impulse when it is in the resting state. Resting membrane potential is -70 mV

Resting Membrane Potential The inside of the cell has a negative charge as compared to the outside of the cell membrane. How does this charge difference develop? There are more Na+ (sodium) ions outside and fewer K+ (potassium) ions inside.

The Nerve Impulse How does the impulse begin? The neuron is stimulated by another neuron or by stimuli from the environment.

The Nerve Impulse If the stimulus is strong enough, it reaches threshold level. This stimulates an impulse. If the stimulus is not strong enough, no impulse occurs (all-or-none principal)

The Nerve Impulse What happens if threshold is reached? An action potential (nerve impulse) begins What is an action potential? Rapid reversal of membrane potential in response to a stimulus How does this happen? Sodium channels open allowing Na+ to flood into the cell. The membrane potential rises to +30 mV (rising phase) as inside of cell becomes more positive

The Nerve Impulse Please, please, tell us what happens next… When the membrane potential reaches +30 mV, the sodium channels close. Potassium channels open and K+ flows out of the cell. This causes the membrane potential to become more negative again (falling phase).

The Nerve Impulse +50 Membrane potential (mV) -70 Na+ channels close 2 K+ channels open, K+ begins to leave cell K+ leaves cell Membrane potential (mV) 1 Na+ channels open, Na+ enters cell Threshold of excitation K+ channels close 3 -70 Excess K+ outside diffuses away

The Nerve Impulse

The Nerve Impulse Action potential +40 Depolarization Repolarization Depolarization Repolarization Voltage (mV) Failed initiations Threshold -55 Resting state -70 Stimulus Refractory period 0 1 2 3 4 5 Time (ms)

The Nerve Impulse What happens when the action potential passes down the axon? The resting potential is restored via the sodium/potassium pump. Now the membrane is ready to transmit another impulse. extracellular fluid cytoplasm Na+ ATP ADP K+

The Nerve Impulse An action potential only moves in one direction down the axon. From axon hillock, thru axon fiber to the axon terminal. The action potential is regenerated at each Node of Ranvier down the length of the axon.

The Nerve Impulse What happens when the impulse reaches the axon terminal? It must pass thru the synapse. What is a synapse? It is the gap between two neurons, or between a neuron and organ (effector). Why is there a gap? So the neurons don’t short each other out!!

The Synapse How do neurons pass the impulse across the synapse? Using chemicals called neurotransmitters What do neurotransmitters do? Stimulate the dendrites of other neurons or membranes of other cells. If a dendrite is stimulated, it sends its message to the cell body and the message is passed on If a muscle or gland is stimulated, a reaction occurs in that organ.

The Synapse

The Synapse Model of the Synapse Axon of presynaptic cell synaptic vesicles Axon of presynaptic cell neurotransmitters receptors for glycoprotein synaptic cleft Glycoprotein (ligands) Model of the Synapse

The Synapse

Neuron Smear: Identification: Note distinctive shape of neuron, with long processes (dendrites and/or axons, 5) extending out from main cell body. Features to Know: The large, irregularly shaped cell body (3) contains a darker nucleus (2), which contains an even darker-staining nucleolus (1). There are also numerous supporting glial cells, though only their small dark nuclei (4) are easily seen.

Myelin sheath Nerve Neuron

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