Download Skeletal Muscle

Muscular System
There are three different types of muscle:
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Skeletal Muscle
o Well-organized body tissue, composed in a complex array of smaller
and smaller structures
o Attaches to and covers the bony skeleton, moves skeleton
o Composed of many units called muscle fascicles
 fascicles are bound together by a type of connective tissue
called fascia
 Smaller strands called myofibrils organize muscle fibers
 Move as skeletal muscle contracts. It is the interaction of the
myofibrils as they slide and pull along side each other that gives
skeletal muscle its functional ability to do work and move things.
o Myofibrils compose muscle fibers, muscle fibers make-up muscle
fascicles, and muscle fascicles are bound together by fascia to compose
skeletal muscle.

Smooth Muscle
o Found in the walls of the body organs
o Involuntary (functions without conscious control)
o Muscle fibers are much smaller than Skeletal muscle fibers
 The muscle fibers are gathered into dense sheets or bands
 Densely and irregularly packed together, most often so that the
narrower portion of one fiber lies against the wider portion of its
neighbor
 Fibers have connections

The plasma membranes of two neighboring fibers
form gap junctions that act as low resistance pathway for
the rapid spread of electrical signals throughout the tissue.
The multi-unit smooth muscle fibers have no
interconnecting bridges: they are mingled with connective
tissue fibers.

Cardiac Muscle
o Found only in the heart, making up most of the heart wall
o Contains specialized conduction system that controls our heartbeat
o Involuntary muscle
 highly branched with cells connected by overlapping projections
of the sarcolemma called intercalated discs.

These discs contain desmosomes and gap
junctions
 cardiac muscle is autorhythmic, generating its own action
potential, which spreads rapidly throughout muscle tissue by
electrical synapses across the gap junctions.
o Throughout our life, it contracts some 70 times per minute pumping
about 5 liters of blood each minute.
How do muscles contract 2
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The cell membrane of a muscle cell is called the sarcolemma, and this membrane, like that of
neurons, maintains a membrane potential. So, impulses travel along muscle cell membranes just
as they do along nerve cell membranes.
Acetylcholine: One of the most common neurotransmitters of the vertebrate nervous system; a
chemical (CH3COOCH2CH2N+(CH3)3) that transmits impulses between the ends of two
adjacent nerves or neuromuscular junctions.
Neuromuscular Junctions: The specialized junction between a nerve ending and a muscle,
which is a type of synapse.
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What is a reflex?
A reflex is a response to a perturbing stimulus that acts to return the body to homeostasis. This
may be subconscious as in the regulation of blood sugar by the pancreatic hormones, may be
somewhat noticeable as in shivering in response to a drop in body temperature; or may be quite
obvious as in stepping on a nail and immediately withdrawing your foot.
Examples of reflexes that protect the body

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Blinking
Sneezing
Coughing
DTR; stands for deep tendon reflex
A reflex arc refers to the neural pathway that a nerve impulse follows. The receptor at the
end of a sensory neuron reacts to a stimulus. The sensory (afferent) neuron conducts nerve
impulses along an afferent pathway towards the central nervous system (CNS). The integration
center consists of one or more synapses in the CNS. A motor (efferent) neuron conducts a
nerve impulse along an efferent pathway from the integration center to an effector. An effector
responds to the efferent impulses by contracting (if the effector is a muscle fiber) or secreting a
product (if the effector is a gland). Sensory neuron (input) and a motor neuron (output)The
sensory neuron (such as a pain receptor in the skin) detects the stimuli and sends a signal towards
the CNS.
This type of reflex is the "withdrawal" reflex and is monosynaptic, meaning only one
synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest
reflex arc and the integration center is the synapse itself. Polysynaptic reflexes are more complex
and more common. They involve interneurons which are found in the CNS. More complex
reflexes may have their integration center in the spinal cord, in the brainstem, or in the cerebrum
where conscious thoughts are initiated
On average, humans have a reaction time of 0.25 seconds to a visual stimulus, 0.17
for an audio stimulus, and
0.15 seconds for a touch stimulus (2).
Stretch reflexes are a special type of muscle reflex which protect the
muscle against increases in length which may tear or damage muscle fibers.
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Visceral reflexes (also known as autonomic reflexes) refer to reflexes
such as those which affect the gland secretion or smooth muscle of the
cardiac system to contract. The main purpose of the visceral reflexes is to
ensure that the involuntary process of the body are in full operating
condition and can be tailored to react to a given situation. Heart rate,
respiratory rate, blood flow, and digestion are just examples of the types
of visceral reflexes that require constant monitoring by the internal body
systems. Reflexes like coughing, swallowing, sneezing, or vomiting are
considered to autonomic, however, these reflexes require at least some
participation of the skeletal muscles.
Somatic reflexes refer to those that include some participation of the
skeletal muscles. Somatic reflexes are segregated into three different
types, each named after the response that they produce. For instance, the
stretch reflex requires only the participation of two neurons and one
synapse in the reflexive pathway. Thus, this is referred to as a
monosynaptic reflex arc. Neuromuscular spindles initiate a minor
stretching within their receptors. A sensory neuron along the spinal
column picks up this stretching, and a synapse occurs within the gray
column (anterior) with a motor neuron. The motor unit that corresponds
with all this stretching is then activated, and specified muscle fibers
automatically contract to cause stretching. The receptors and the effector
organs that are involved in this action are situated along the same side of
the spinal column, and thus this particular reflex receives the term
ipsilateral arc. An additional example of an ipsilateral arc is the knee-jerk
reflex.
withdrawal reflex, also known as a flexor reflex, is a polysynaptic reflex
arc. Sensory and motor neurons are part of this reflex, as are association
neurons. Pain is the most obvious stimuli that initiates the flexor reflex, as
painful stimuli sets the whole reflex into action
How does the nervous system and muscular system interact?
o
Receptors in muscles provide brain with info about body position and
movement
o
Brain controls contraction of skeletal muscles
o
nervous system regulates speed at which food moves through digest track