chapter 4
twitch
a brief period of muscle activity produced by the muscle in response to a single nerve impulse
neuron
a cell specialized to transmit electrical signals
neurotransmitter
a chemical substance released by a neuron that diffuses across a small gap and activates receptors on the target cell
multiple motor unit summation
a method of varying the force produced by a muscle by activating different numbers of motor units within the same muscle
motor (efferent) neurons
a neuron that carries impulses from the central nervous system to the muscle
alpha (α) motor neuron
a neuron that controls skeletal muscle activity; it is composed of relatively short dendrites that receive the information, a cell body, and long axons that carry impulses from the cell body to the neuromuscular junction, which interfaces with the muscle fiber
acetylcholine
a neurotransmitter released at motor synapses and neuromuscular junctions, active in the transmission of nerve impulses
ligand
a neurotransmitter, hormone, or other chemical substance that interacts with a receptor protein
sensory-somatic nervous system
a part of the peripheral nervous system that controls our conscious awareness of the external environment and motor responses
size principle
a principle that explains how the nervous system recruits individual motor units in an orderly, predictable fashion from smaller to larger motor units
depolarization
a reduction in the polarity from resting membrane potential (−70 mV) to a more positive value (+30 mV) of a neuron's membrane
cerebrum
a region of the brain consisting of the left and right hemispheres that is important for control of conscious movements
receptor
a specialized site on the target cell that is activated by neurotransmitters; a protein that is found in a cell membrane or within the cytoplasm or cell nucleus that will bind with a ligand
excitability
ability of a neuron or muscle fiber to respond to an electrical impulse
asynchronous recruitment
alternating recruitment of motor units when force production needs are low
motor unit
an alpha motor neuron and its associated muscle fibers
motor cortex
an area in the frontal lobe of the brain responsible for primary motor control
Na+-K+ pump
an energy-dependent pumping system that restores the resting membrane potential by actively removing Na+ ions from the inside of the neuron and K+ ions from outside the neuron to the inside of the neuron
action potential (nerve impulse)
an impulse in the form of electrical energy that travels down a neuron as its membrane changes from −70 mV to +30 mV back to −70 mV due to the movement of electrically charged ions moving in and out of the cell
central nervous system
brain and spinal column
Schwann cells
cells that create and maintain the myelin sheath
saltatory conduction
conduction of nerve impulses in myelinated nerves in which the action potential "jumps" from node of Ranvier to node of Ranvier
local conduction
conduction of nerve impulses in unmyelinated nerves in which the ionic current flows along the entire length of the axon
myelinated
nerve axons possessing a myelin sheath
autonomic nerves (motor neurons)
nerves of the autonomic nervous system, outside of the central nervous system
peripheral nervous system
nerves that transmit information to and from the central nervous system
sensory (afferent) neurons
neurons that enter the spinal cord from the periphery and carry messages from sensory receptors to the central nervous system
axon
part of a neuron that carries an impulse from the cell body to another neuron or target tissue receptor (e.g., muscle); it is sometimes referred to as a nerve fiber
dendrite
part of a neuron that receives information (impulses) and sends it to the cell body
cerebellum
part of the unconscious brain; regulates muscle coordination and coordinates balance and normal posture
medulla oblongata
part of the unconscious brain; regulates the heart, breathing, and blood pressure and reflexes such as swallowing, hiccups, sneezing, and vomiting
repolarization
restoration of a membrane back to its original resting membrane potential (−70 mV) after depolarizing (+30 mV)
nodes of Ranvier
small gaps in the myelin sheath occurring at regular intervals along the axon that allow the action potential to jump from node to node, which allows faster impulse conduction and conserves energy
interneurons
special neurons located only in the central nervous system that connect one neuron to another neuron
homeostasis
the ability of an organism or cell to maintain internal equilibrium by adjusting its physiological processes to keep function within physiological limits at rest or during exercise
energy transformation
the conversion of one form of energy to another; in the transmission of an action potential, electrical energy is transformed into chemical energy to cross a synapse or neuromuscular junction
hypothalamus
the homeostatic center of the brain; regulates metabolic rate, body temperature, thirst, blood pressure, water balance, and endocrine function
cell body (soma)
the part of a neuron that contains the nucleus, mitochondria, ribosomes, and other cellular constituents
soma (cell body)
the part of a neuron that contains the nucleus, mitochondria, ribosomes, and other cellular constituents
axon hillock
the part of a neuron where the summation for incoming information is processed, if threshold is reached an impulse is transmitted down the axon
synapse
the point of connection, and communication, between two excitable cells
nervous impulse (action potential)
the stimulus in the form of electrical energy that travels down an axon due to the movement of electrically charged ions moving in and out of the axon
tetanus
the summation of nerve impulse twitches, resulting in the maximal force a motor unit can develop
all-or-none law
when a threshold level for activation is reached, all of the muscle fibers in a motor unit are activated; if the threshold of activation is not met, none of the muscle fibers are activated
myelin sheath
white covering high in lipid (fat) content that surrounds axons, provides insulation, and maintains electrical signal strength of the action potential as it travels down the axon