Chapter 13 nroc64.docx

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Chapter 13
SOMATIC MOTOR SYSTEM
- Smooth Muscle: lines the digestive tract, arteries, and related structures and is innervated by
nerve fibers from the autonomic nervous system, plays a role in peristalsis and the control of
blood pressure and blood flow
- Striated Muscle
Cardiac Muscle: heart muscle, contracts rhythmically even in the absence of any
innervations, innervations from ANS functions to accelerate or slow down heart rate
Skeletal Muscle: functions to move bones around joints, move eyes, control respiration,
facial expression and produce speech
- Each skeletal muscle enclosed in a connective tissue sheath that at the ends of the muscle, forms
the tendons
- Within each muscle are hundreds of muscle fibers : the cells of skeletal muscle each muscle
fiber is innervated by a single axon branch from the CNS
- Skeletal muscle is derived embryologically from 33 paired somites, these muscles, and the parts
of the nervous system that control them, are collectively called the somatic motor system
under voluntary control
- Example: Elbow Joint
Humerus bound by fibrous ligaments to radius and ulna
Flexors brachialis, whose tendons insert into the humerous at one end and into ulna
at the other end biceps brachii and coracobrachialis
Synergists: muscles working together
Extensors triceps brachii and anconeus
Antagonists: muscles working in opposite directions
- Axial Muscles: muscles responsible for movements of the trunk (maintains posture)
- Proximal/Girdle Muscles: move the shoulder, elbow, pelvis and knee (locomotion)
- Distal Muscles: move the hands feet and digits (manipulation of objects)
LOWER MOTOR NEURON
- Somatic musculature innervated by somatic motor neurons in the ventral horn of spinal cord
AKA lower motor neurons final common pathway for the control of behavior
Organization
- Axons of lower motor neurons bundle together to form ventral roots; each ventral root joins with
a dorsal root to form a spinal nerve that exists the cord through the notches between vertebrae
- 30 spinal nerves on each side
- Spinal segment consists of the motor neurons that provide fibers to one spinal nerve
- Segments: cervical (1-8), thoracic (1-12), lumbar (1-5), sacral (1-5)
- Motor neurons that innervate distal and proximal musculature found mainly in the cervical and
lumbar-sacral segments; those innervating axial musculature found at all levels
- Cells innervating axial muscles are medial to those innervating distal muscles
- Cells innervating flexors are dorsal to those innervating extensors
Alpha Motor Neurons (AMN)
- Two types of lower motor neurons: alpha and gamma
- AMN directly trigger the generation of force by muscles
- Motor Unit: one AMN + all the muscle fibers it innervates
- The collection of AMN that innervates a single muscle is called a motor neuron pool
- Graded Control of Muscle Contraction by AMN
nervous system uses several mechanisms to control force of muscle contraction in a
finely graded fashion:
1) varying the firing rate of motor neurons
AMN communicates with muscle fibers by releasing the neurotransmitter
acetylcholine (Ach) at the neuromuscular junction, causing an EPSP in the
muscle fiber large enough to trigger one postsynaptic action potential. This
causes a twitch rapid contracton and relaxation in the muscle fiber. High
frequency presynaptic activity causes temporal summation of postsynaptic
responses. Therefore, rate of firing of motor units important way the CNS grades
muscle contraction
2) Recruiting additional synergistic motor units
Extra tension provided by recruitment of active motor unit depends on how many
muscle fibers are in that unit. I.e. in antigravity muscles of the leg each motor
unit tends to be quite large with an innervations ratio of more than a 1000 muscle
fibers/single AMN
In general, muscles with a large number of small motor units are more finely
controlled by CNS; small motor units recruited first, largest last
Size principle: the orderly recruitment of motor neurons is due to variations in
AMN size
- Inputs to Alpha Motor Neurons
AMN excite skeletal muscles
Lower motor neurons are controlled by:
a) Dorsal root ganglion cells with axons that innervate a specialized apparatus embedded
within the muscle, muscle spindle (provides input about muscle length)
b) Upper motor neurons in the motor cortex and brain stem (important for initiation and
control of voluntary movement)
c) Interneurons in the spinal cord (largest source, may be excitatory or inhibitory)
Types of Motor Neurons
- Red muscle fibers characterized by: large number of mitochondria and enzymes specialized for
oxidative energy metabolism, slow to contract but can sustain contraction for a long time without
fatigue, typically found in antigravity muscles of leg
- White muscle fibers; contain fewer mitochondria, rely mainly on anaerobic metabolism, contract
rapidly and powerfully, fatigue rapidly, typical of muscles involved in escape reflexes i.e. arm
muscles
- Each motor unit contains muscle fibers of a single type
- Fast motor units: contain rapidly fatiguing white fibers, motor neurons are generally bigger,
larger diameter, faster-conducting axons, generate occasional high-frequency bursts of action
potentials (30-60)
- Slow motor units: contain slowly fatiguing red fibers, smaller-diameter, more slowly conducting
axons, relatively steady low-frequency activity (10-20 impulses/second)
- Neuromuscular Matchmaking
The properties of the muscle are determined solely by the type of innervations it gets i.e.
if it receives a synaptic contact from a fast motor neuron, it becomes a fast fiber
Muscle phenotype change can also be induced by changing activity in the motor neuron
from fast pattern to slow pattern
Muscle fibers also changed by simply varying the absolute amount of activity i.e.
hypertrophy (exaggerated growth) or atropy (degeneration) of muscle fibers
EXCITATION-CONTRACTION COUPLING
- By process of excitation-contraction coupling, the action potential, the excitation, triggers the
release of Ca2+ from an organelle inside the muscle fiber, which leads to contraction of the fiber
- Relaxation occurs when the Ca2+ levels are lowered by reuptake into the organelle
Muscle Fiber Structure
- Muscle fibers are enclosed by an excitable cell membrane called sarcolemma
- Within muscle fiber are myofibrils, which contract in response to an action potential sweeping
down the sarcolemma
- Myofibrils are surrounded by the sarcoplasmic reticulum (SR), an intracellular sac that stores
Ca2+
- Action potentials sweeping along the sarcolemma gain access to SR via a network of tunnels
called T tubules lumen of each tubule is continuous with the extracellular fluid
- Special coupling between T tubule and SR: a voltage-sensitive cluster of four calcium channels,
tetrad, in the T tubule membrane is linked a calcium release channels in the SR
- Arrival of action potential in T tubule membrane causes a conformational change in the voltage-
sensitive tetrad of channels, which opens the calcium release channel in the SR membrane
- Some Ca2+ flows through tetrad channels, and even more from calcium release channel resulting
in increase in free Ca2+ within the cytosol causing myofibril to contract
Muscle Contraction
- Myofibril is divided into segments by disks called Z lines
- Sarcomere is a segment comprised of two Z lines and the myofibril in between
- Thin filaments are anchored to each side of the Z lines
- Between and among two sets of thin filaments are a series of fibers called thick filaments
- Muscle contraction occurs when thin filaments slide along the thick filaments bringing adjacent Z
lines toward one another
- Myosin: thick filament protein
- Actin: thin filament protein
- Exposed heads of myosin bind actin filaments and then undergo a conformational change causing
them to pivot, causing the thick filament to move wrt thin filament
- Myosin heads disengage and uncock to repeat the process using ATP
- When muscle is at rest, myosin cannot interact with actin because myosin attachment sites on
actin molecule are covered by the protein troponin
- Ca2+ initiates muscle contraction by binding to troponin, allowing myosin to bind to actin
- Contraction continues as long as Ca2+ and ATP are available
- Relaxation occurs when the Ca2+ is sequestered by SR
- Reuptake of Ca2+ by SR involves a calcium pump which also requires ATP