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