Section 1 Neurons and Nerve Impulses Chapter 49 What is a neuron? • The scientific name for a nerve cell • is made up of a cell body, dendrites, and axons. • A cell body is the nucleus of a neuron and most of its organelles. • Dendrites extend from the cell body and are covered by a membrane. • The axon is a long membrane-bound projection that transmits information away from the cell body in the form of electrical signals. • These electrical signals are called action potentials

More on the axon • The end of an axon is called the axon terminal – It may contact and communicate with: • Muscle cells • A gland cell • Another neuron • Axons are covered by a lipid membrane called the myelin sheath (ring a bell?) • It insulates the axon • It speeds up the transmission of action potentials • Produced by Schwann cells that surround the axon • Gaps in the myelin sheath are called the Nodes of Ranvier

Let’s label a neuron

Section 1 Neurons and Nerve Impulses Chapter 49 Neuron Communication • Neurons do not touch each other or other cells • There is a small gap between neurons called a synaptic cleft. That region or junction is called synapses. • This is where neurons communicate • The signaling activity of the nervous system is made up of electrical activity within neurons and chemical flow between neurons. • These synapses do not communicate by touch, but by releasing chemicals, or neurotransmitters, into the synaptic cleft. • At a synapse, the transmitting neuron is called the presynaptic neuron and the receiving cell is called a postsynaptic cell

Section 1 Neurons and Nerve Impulses Chapter 49 Nerve Impulses • All cells have an electrical charge inside them that is different from outside the cell • A membrane potential is a difference in the electrical charge across a cell membrane. • A membrane potential can change with an addition or removal of ions within the cell. • Ions move in and out of the cell by passing through proteins that act as ion channels. Whether the ion channels are open or closed affects the membrane potential. • Just like with batteries, membrane potentials are expressed as voltage

Section 1 Neurons and Nerve Impulses Chapter 49 Resting Potential • A neuron is at rest when it is not sending or receiving a signal. • When at rest, the neuron is more negatively charged on the inside than on the outside of the cell body. • Inside the cell has negatively charged proteins and K+ ions • Outside the cell, there is an accumulation of Na+ ions (due to the sodium potassium pump) • The electric potential across the cell membrane when the neuron is at rest is called the resting potential. It is about -70 millivolts.

Section 1 Neurons and Nerve Impulses Chapter 49 Action Potential • When a dendrite or cell body is stimulated, the permeability of the neuron’s membrane changes suddenly. • A stimulus triggers the cell membrane to suddenly become permeable to Na+ ions and they rush into the cell • The membrane potential reverses and the neuron’s cell body becomes more positively charged than the exterior of a cell. • This reverse in polarity begins an action potential.

Section 1 Neurons and Nerve Impulses Chapter 49 Action Potentials continued • After the first segment of the neuron is stimulated the segment next to the first will become stimulated. • The action potential will continue away from the cell body. • Soon the voltage gated channels for Na+ close and the voltage gated channels for K+ open. • Soon after being stimulated, the interior of the neuron begins to become more and more negative. • This signals the end of the action potential.

Section 1 Neurons and Nerve Impulses Chapter 49 Refractory Period • A neuron cannot generate another action potential until it has returned to its resting potential. • The period in which a neuron cannot send a signal is called the refractory period. • Returning the neuron to its resting potential requires energy. • Neurons need a continuous supply of ATP to keep the sodium potassium pump operating.

Section 1 Neurons and Nerve Impulses Chapter 49 Conduction of a Nerve Impulse

Section 1 Neurons and Nerve Impulses Chapter 49 Action Potential Click below to watch the Visual Concept. Visual Concept

Section 1 Neurons and Nerve Impulses Chapter 49 Communication Between Neurons • Once an action potential reaches the axon terminal, it releases neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors proteins and open the ion channels of the new neuron cell. • If enough ion channels are opened, the action potential will continue through the new neuron. If not, the nervous signal will be terminated. • After the neurotransmitters have opened the ion channels, they will be cleared out of the synaptic cleft by being reabsorbed by the neuron that released them or broken down by enzymes.

Section 1 Neurons and Nerve Impulses Chapter 49 Release of Neurotransmitter Click below to watch the Visual Concept. Visual Concept

Section 1 Neurons and Nerve Impulses Chapter 49 Synaptic Transmission

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