12.3 The Function of Nervous Tissue

•  the major functions of the nervous system: sensation, integration, and response.
•  Testing the Water: 

1.  The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus.
2.   The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). 
3.  The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. 
4. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals.
5.  An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. 
6. Another synapse passes the information along to the next neuron. 
7.  The sensory pathway ends when the signal reaches the cerebral cortex. 
8. After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex.
9. The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. 
10.  The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.

• Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor .
• The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a graded potential
• . If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the axon. You have learned about this type of signaling before, with respect to the interaction of nerves and muscles at the neuromuscular junction. The voltage at which such a signal is generated is called the threshold, and the resulting electrical signal is called an action potential. 
• In this example, the action potential travels—a process known as propagation—along the axon from the axon hillock to the axon terminals and into the synaptic end bulbs.
• When this signal reaches the end bulbs, it causes the release of a signaling molecule called a neurotransmitter

When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins.

• If graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. 

The upper motor neuron is in this region, called the precentral gyrus of the frontal cortex, which has an axon that extends all the way down the spinal cord. 

• At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a lower motor neuron.
• This second motor neuron is responsible for causing muscle fibers to contract.
• The axon terminates on muscle fibers at the neuromuscular junction.
• Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential.
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