Autonomic Nervous System & Skeletal Muscle Physiology: Key Concepts and Structures

Ganglion

A collection of neuronal cell bodies in the peripheral nervous system.

Preganglionic neuron

The first of two ANS lower motor neurons; cell body is located in brainstem or spinal cord & axon projects to autonomic ganglion.

(Post)Ganglionic neuron

The second of two ANS lower motor neurons; cell body is located in an autonomic ganglion & axon projects to effector/target.

Thoracolumbar division

Another name for the sympathetic division of the ANS; refers to location of preganglionic cell bodies in T1 - L2 lateral horns of the spinal cord.

Craniosacral division

Another name for the parasympathetic division of the ANS; refers to location of preganglionic cell bodies in the brainstem or S2-S4 lateral horns of the spinal cord.

Neuronal convergence

Occurs when numerous preganglionic axons synapse on and influence the activity of a single ganglionic neuron.

Neuronal divergence

Occurs when a single preganglionic axon branches to synapse on and influence the activity of numerous ganglionic neurons.

Dual innervation

Effectors of the ANS are innervated by postganglionic axons of both the sympathetic and parasympathetic divisions of the ANS.

Antagonistic control

The effects of the sympathetic and parasympathetic innervation of a single target oppose one another.

Somatic Nervous System

Voluntary control of skeletal muscles.

Autonomic Nervous System

Involuntary control of cardiac muscle, smooth muscle, and glands.

Hypothalamus

Region of the CNS that controls the autonomic nervous system.

Lower motor neurons

In the somatic nervous system, one lower motor neuron extends from the anterior horn of the spinal cord to the effector.

Ganglionic neuron

In the autonomic nervous system, a ganglionic neuron extends to the effector.

Excitation only

Response of the effector in the somatic nervous system.

Excitation or inhibition

Response of the effector in the autonomic nervous system.

Sympathetic Division

Maintains homeostasis during exercise, emergency, or stress. 'Fight or flight'.

Parasympathetic Division

Maintains homeostasis during rest. 'Rest and digest'.

Preganglionic Soma Location (Sympathetic)

Cell bodies located in lateral horns of T1-L2 spinal cord.

Preganglionic Soma Location (Parasympathetic)

Cell bodies located in cranial nerve nuclei in the brainstem and lateral horns of S2-S4 spinal cord.

Ganglia Location (Sympathetic)

Ganglia located close to the spinal cord - sympathetic trunk or prevertebral ganglia.

Ganglia Location (Parasympathetic)

Ganglia located close to or within effectors.

Preganglionic Axon Length (Sympathetic)

Long.

Preganglionic Axon Length (Parasympathetic)

Short.

Postganglionic Axon Length (Sympathetic)

Short.

Postganglionic Axon Length (Parasympathetic)

Long.

Degree of Response (Sympathetic)

Mass activation: stimulate many body systems simultaneously.

Degree of Response (Parasympathetic)

Local response: stimulate one or only a few body systems at one time.

Cranial Nerve III

Oculomotor.

Cranial Nerve VII

Facial.

Cranial Nerve IX

Glossopharyngeal.

Cranial Nerve X

Vagus.

Muscarinic Receptors

Receptors that are all effectors of parasympathetic targets.

Alpha Adrenergic Receptors

Receptors that are effectors of sympathetic targets.

Beta Adrenergic Receptors

Receptors that can be either excitatory or inhibitory, depending on subtype.

Nicotinic Receptors

Receptors located at the cell body of all ganglionic neurons in the ANS.

Acetylcholine

Neurotransmitter bound by muscarinic and nicotinic receptors.

Norepinephrine

Neurotransmitter bound by adrenergic receptors.

Ionotropic Receptors

Receptors that are typically excitatory.

Metabotropic Receptors

Receptors that can be either excitatory or inhibitory, depending on subtype.

Excitability

Ability to respond to stimulus, causing a change in membrane potential.

Conductivity

Ability to propagate an action potential along the plasma membrane.

Contractility

Ability to shorten.

Extensibility

Ability to lengthen/stretch.

Elasticity

Ability to return to original shape after contracting or elongating.

Skeletal Muscle

An organ consisting of muscle fibers, connective tissue, blood vessels, and nerves.

Fascicle

A bundle of muscle fibers.

Skeletal Muscle Fiber

Cells of skeletal muscle tissue; aka myofibers.

Tendon

Thick, cordlike structure that attaches muscle to bone/ligament/fascia.

Aponeurosis

Flat sheet of dense regular connective tissue that attaches muscle to bone/ligament/fascia.

Sarcoplasm

Cytoplasm of a skeletal muscle fiber.

Sarcolemma

Plasma membrane of a skeletal muscle fiber.

T-tubule

Extensions of the sarcolemma that extend down into the muscle fiber.

Myofibril

Long, cylindrical structures filling the sarcoplasm; composed of bundles of contractile proteins.

Sarcoplasmic Reticulum

Intracellular membrane complex that fits around each myofibril like a sleeve.

Terminal Cisternae

Sacs at the ends of individual sections of sarcoplasmic reticulum that act as reservoirs for calcium.

Triad

A central T-tubule and two terminal cisternae that sit on either side of it.

Myofilaments

Contractile proteins bundled within myofibrils; includes thick and thin filaments.

Myosin

Major protein of the thick filament.

Actin

Contractile protein of the thin filament.

Tropomyosin

A rope-like protein in the thin filament that covers the myosin binding site on G-actin.

Troponin

A ball-like protein in the thin filament that contains a binding site for calcium.

Sarcomere

A repeating cylindrical unit composed of overlapping thick and thin filaments.

Motor Unit

A motor neuron and all the muscle fibers it innervates.

Neuromuscular Junction

The specific location where the axon terminal of a motor neuron innervates a skeletal muscle fiber.

Synaptic Knob

The axon terminal containing synaptic vesicles filled with acetylcholine.

Motor End Plate

Specialized region of the sarcolemma innervated by the motor neuron.

Acetylcholinesterase

An enzyme that breaks down acetylcholine.

Muscle fiber

An individual skeletal muscle fiber.

Myofilament

Consists of actin, troponin, and tropomyosin.

Endomysium

Connective tissue wrapping around a muscle fiber.

Perimysium

A bundle of skeletal fibers.

Epimysium

A skeletal muscle.

Thick cord

A structure formed by connective tissue.

Multinucleated

Skeletal muscle fibers are multinucleated because multiple embryonic muscle cells, called myoblasts, fuse to form a single muscle fiber.

Functional significance of multinucleated

Multiple nuclei help to coordinate necessary cellular processes due to high energy demands.

Z disc

Ends of each sarcomere; anchor for thin filaments.

I bands

Region containing only thin filaments.

A bands

Central region of sarcomere containing entire thick filament and partially overlapped thin filament.

H zone

Center of the A-band; containing only thick filaments.

M line

Very center of the H-zone; anchor for thick filaments.

Striations

Produced by the partial overlapping of thick and thin filaments in a sarcomere.

Thin filament

Consists of actin.

Thick filament

Consists of myosin.

Voltage-gated calcium channels

Open when an action potential arrives at the terminal and allows calcium to enter the synaptic knob and initiate ACh release.

Calcium pumps

Restore the concentration gradient for calcium, which is needed to initiate the release of ACh.

Acetylcholine Receptors

Chemically gated cation channels expressed on the motor end plate that establish the end plate potential.

Resting membrane potential

The electrical charge difference across the sarcolemma at rest, with a value of -90 mV.

Sodium (Na+)

More concentrated in the interstitial fluid when a skeletal muscle fiber is at rest.

Potassium (K+)

More concentrated in the cytosol when a skeletal muscle fiber is at rest.

Excitation

The first phase of muscle contraction associated with the neuromuscular junction.

Excitation - Contraction Coupling

The second phase of muscle contraction associated with the sarcomere.

Crossbridge Cycling

The third phase of muscle contraction associated with the sarcolemma and T-tubules.

Calcium ion channels

Open in the sarcoplasmic reticulum when T-tubule depolarization occurs.

End-plate potential

Stimulated by the entry of Na+ which depolarizes the sarcolemma locally.

Sliding filament mechanism

Thin filaments are pulled past thick filaments toward the center of the sarcomere.

Crossbridge

Formed when the myosin head binds actin after calcium binds to troponin.

Power Stroke

Occurs when ADP and Pi are released as the myosin head swivels, pulling the thin filament toward the center of the sarcomere.

ATP hydrolysis

Resets the myosin head into a 'cocked' position.

Skeletal muscle relaxation

Involves the dissociation of calcium from troponin and the breakdown of ACh by acetylcholinesterase.