Muscle Physiology: Motor Units and Junctions

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Questions and Answers

What is the primary difference in the number of muscle cells per motor neuron between precise and gross body movements?

There is no difference in the number of muscle cells per neuron.

Precise movements have more muscle cells per neuron than gross movements.

Gross movements have significantly fewer muscle cells per neuron.

Precise movements have fewer muscle cells per neuron than gross movements. (correct)

How many individual skeletal muscle cells does a single motor neuron typically stimulate?

About 5 to 10 individual skeletal muscle cells.

Approximately 150 individual skeletal muscle cells. (correct)

Over 2,000 individual skeletal muscle cells.

About 500 individual skeletal muscle cells.

What term refers to the combination of a motor neuron and all the skeletal muscle cells it stimulates?

Motor unit (correct)

Neuromuscular unit

Skeletal synapse

Motor pool

What is the role of the synaptic cleft in the neuromuscular junction?

It separates the excitable cells from communicating directly. Signup and view all the answers

What structure forms the synapse between a motor neuron and a skeletal muscle cell?

Neuromuscular junction Signup and view all the answers

How does a motor neuron communicate with a skeletal muscle cell?

By releasing neurotransmitters across the synaptic cleft. Signup and view all the answers

What is determined by the total strength of any particular muscle?

The total number of muscle cells stimulated. Signup and view all the answers

In muscles that control gross body movements, how many muscle cells can be stimulated by a single motor neuron?

Possibly up to 2,000 muscle cells. Signup and view all the answers

What is the primary origin of smooth muscle in the walls of blood and lymphatic vessels?

Somatic mesoderm Signup and view all the answers

Which muscular disease at birth is characterized by the immobility of multiple joints?

Congenital arthrogryposis Signup and view all the answers

What type of cells do myoblasts become as they differentiate?

Spindle-shaped and mononucleated Signup and view all the answers

What is a possible cause of congenital torticollis?

Tearing of fibers during childbirth Signup and view all the answers

Which type of muscular disease may involve replacement of muscle with fat and fibrous tissue?

Congenital myopathy Signup and view all the answers

What type of muscle is derived from ectoderm?

Muscles of the iris Signup and view all the answers

What is the result of congenital arthrogryposis?

Movement limitations due to joint immobility Signup and view all the answers

What happens to myoblasts during muscle development?

They remain mononucleated and spindle-shaped. Signup and view all the answers

What structural element primarily connects adjacent cardiac muscle cells?

Gap junctions Signup and view all the answers

From which embryonic layer does cardiac muscle develop?

Splanchnic mesoderm Signup and view all the answers

What is the structure that forms as myoblasts adhere to one another during cardiac muscle development?

Intercalated discs Signup and view all the answers

Which of the following statements about smooth muscle is true?

It is characterized by fusiform shaped cells. Signup and view all the answers

What role do dense bodies play in smooth muscle cells?

They anchor actin filaments to the sarcolemma. Signup and view all the answers

How does smooth muscle contraction differ from skeletal muscle contraction?

Smooth muscle contraction involves a calcium-calmodulin complex. Signup and view all the answers

Which type of muscle lacks a T tubule system?

Smooth muscle Signup and view all the answers

What type of cells do myoblasts differentiate into during cardiac muscle development?

Cardiac myocytes Signup and view all the answers

What is a characteristic morphological feature of cardiac muscle cells?

Intercalated discs Signup and view all the answers

What is a characteristic of bipolar neurons?

They have one dendrite and one axon extending from the cell body. Signup and view all the answers

What type of muscle tissue is primarily located in blood vessels and various organs?

Smooth muscle Signup and view all the answers

Which disorder is classified under degeneration disorders?

Multiple sclerosis Signup and view all the answers

What role do astrocytes play in the central nervous system?

They provide structural and nutritional support to neurons. Signup and view all the answers

Which statement accurately describes the condition in synaptic transmission?

The presynaptic neuron absorbs the neurotransmitter after it is released. Signup and view all the answers

What is the primary function of myofilaments?

To facilitate muscle contraction. Signup and view all the answers

Which of the following is NOT a functional disorder?

Parkinson disease Signup and view all the answers

What differentiates neuronal structures such as axons and dendrites?

Axons transmit signals away from the neuron body, dendrites receive signals. Signup and view all the answers

Which type of neuron is characterized by having one axon that branches into two distinct pathways?

Pseudounipolar Signup and view all the answers

Which neurons are primarily involved in sensory systems, such as the retina of the eye?

Bipolar neurons Signup and view all the answers

What type of glial cell is primarily responsible for regulating blood flow and providing nourishment to neurons?

Astrocytes Signup and view all the answers

Which description accurately represents the resting potential of a neuron?

The inside of the membrane is negatively charged compared to the outside. Signup and view all the answers

How do oligodendrocytes differ from Schwann cells in their function?

Oligodendrocytes myelinate multiple axons; Schwann cells myelinate only one axon. Signup and view all the answers

What happens to neurotransmitters after they are released from the presynaptic neuron?

They diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron. Signup and view all the answers

Which of the following statements regarding synaptic transmission is correct?

Neurotransmitters released bind to receptors on the postsynaptic neuron to initiate a response. Signup and view all the answers

Which glial cell type is primarily involved in the defense and immune response of the central nervous system?

Microglial cells Signup and view all the answers

What is the primary ion responsible for creating a negative charge inside a neuron at resting potential?

Potassium ions Signup and view all the answers

Study Notes

Motor Unit

A motor neuron stimulates a group of skeletal muscle cells.
One motor unit consists of one motor neuron and all of the skeletal muscle cells it stimulates.
The number of muscle cells per motor neuron varies based on the muscle's function:
- Muscles with precise movements (eye muscles) may have 2-3 muscle cells per motor neuron.
- Muscles for gross movements (gluteal muscles) could have as many as 2,000 muscle cells per motor neuron.

The Neuromuscular Junction

The specialized region where motor neurons communicate with muscle cells is a synapse.
A small gap, the synaptic cleft, separates the motor neuron from the muscle cell.
Motor neurons use neurotransmitters to communicate with muscle cells across the synaptic cleft.
The synapse between a motor neuron and a skeletal muscle cell is called the neuromuscular junction.
Motor neurons branch into axon terminals that form synapses with groups of muscle cells, forming motor units.
Gap junctions are essential components for transmitting signals between cardiac muscle cells.

Cardiac Muscle Development

Arises from the splanchnic mesoderm, the tissue surrounding the developing heart.
Cardiac myoblasts differentiate from the primordial myocardium.
Myoblasts adhere to each other, forming intercalated discs.

Smooth Muscles

Location: found in the walls of blood vessels, ducts, GI system, respiratory system, urogenital system, etc.
Characteristics:
- Short, fusiform cells.
- Mononucleated with the nucleus centrally located.
- Lack T Tubules, but have sarcolemma invaginations analogous to T tubules.
- Sarcolemma is the cytoplasmic membrane of the muscle cells.
Contractile Apparatus:
- Composed of myofilaments (actin and myosin) that are not regularly arranged, oriented obliquely to the long axis of the cell.
- Actin filaments attach to the sarcolemma via dense bodies, which function like Z disks.
- Gap junctions are present for coordinated contraction.
Calcium Regulation of Contraction:
- Calcium-calmodulin complex leads to the phosphorylation of myosin, initiating contraction.

Smooth Muscle Development

Differentiates from splanchnic mesoderm surrounding the endoderm of the primordial gut and its derivatives.
Smooth muscle in blood and lymphatic vessel walls derives from somatic mesoderm.
Muscles of the iris (sphincter and dilator pupillae) and myoepithelial cells in glands derive from ectoderm.
Myoblasts become spindle-shaped, remain mononucleated, and don't fuse.

Common Muscular Diseases

Congenital Arthrogryposis:
- Failure of normal muscle development leading to joint immobility.
- Muscles are replaced by fat and fibrous tissue.
Congenital Torticollis (Wryneck):
- Fixed rotation and tilting of the head due to fibrosis and shortening of the sternocleidomastoid muscle.
- Can result from tearing of sternocleidomastoid muscle fibers during childbirth.

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Description

Explore the essential concepts of motor units and the neuromuscular junction in skeletal muscle physiology. Learn how motor neurons stimulate muscle cells and the significance of various muscle cell arrangements for movement. This quiz covers key terminology and functions related to motor units and neurotransmission.