Sensation starts with the activation of a sensory ending, such as the thermoreceptor in the skin sensing the temperature of the water. The sensory endings in the skin initiate an electrical signal that travels along the sensory axon within a nerve into the spinal cord, where it synapses with a neuron in the gray matter of the spinal cord. The temperature information represented in that electrical signal is passed to the next neuron by a chemical signal that diffuses across the small gap of the synapse and initiates a new electrical signal in the target cell. That signal travels through the sensory pathway to the brain, passing through the thalamus, where conscious perception of the water temperature is made possible by the cerebral cortex. Following integration of that information with other cognitive processes and sensory information, the brain sends a command back down to the spinal cord to initiate a motor response by controlling a skeletal muscle. The motor pathway is composed of two cells, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the cerebral cortex and synapses on a cell in the gray matter of the spinal cord. The lower motor neuron is that cell in the gray matter of the spinal cord and its axon extends into the periphery where it synapses with a skeletal muscle in a neuromuscular junction.
action potential
change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
cerebral cortex
outermost layer of gray matter in the brain, where conscious perception takes place
graded potential
change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it
lower motor neuron
second neuron in the motor command pathway that is directly connected to the skeletal muscle
neurotransmitter
chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell
precentral gyrus of the frontal cortex
region of the cerebral cortex responsible for generating motor commands, where the upper motor neuron cell body is located
propagation
movement of an action potential along the length of an axon
thalamus
region of the central nervous system that acts as a relay for sensory pathways
thermoreceptor
type of sensory receptor capable of transducing temperature stimuli into neural action potentials
threshold
membrane voltage at which an action potential is initiated
upper motor neuron
first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord
1. If a thermoreceptor is sensitive to temperature sensations, what would a chemoreceptor be sensitive to?
A) light
B) sound
C) molecules
D) vibration
C
2. Which of these locations is where the greatest level of integration is taking place in the example of testing the temperature of the shower?
A) skeletal muscle
B) spinal cord
C) thalamus
D) cerebral cortex
D
3. How long does all the signaling through the sensory pathway, within the central nervous system, and through the motor command pathway take?
A) 1 to 2 minutes
B) 1 to 2 seconds
C) fraction of a second
D) varies with graded potential
C
4. What is the target of an upper motor neuron?
A) cerebral cortex
B) lower motor neuron
C) skeletal muscle
D) thalamus
B
1. Sensory fibers, or pathways, are referred to as “afferent.” Motor fibers, or pathways, are referred to as “efferent.” What can you infer about the meaning of these two terms (afferent and efferent) in a structural or anatomical context?
Afferent means “toward,” as in sensory information traveling from the periphery into the CNS. Efferent means “away from,” as in motor commands that travel from the brain down the spinal cord and out into the periphery.
2. If a person has a motor disorder and cannot move their arm voluntarily, but their muscles have tone, which motor neuron—upper or lower—is probably affected? Explain why.
The upper motor neuron would be affected because it is carrying the command from the brain down.