Human Physiology Chapter 13
What is the purpose of alpha-gamma coactivation? Explain how it occurs.
Alpha-gamma coactivation allows muscle spindles to continue functioning when the muscle contracts. When the muscle contracts, the ends of the spindles also contract to maintain stretch on the central portion of the spindle.
feedforward response
Allows the body to anticipate a stimulus and begin the response. An example would be bracing one's self before a collision.
Spinal Reflexes
Are integrated in the spinal cord. These reflexes may be modulated by higher input from the brain, but they can occur without that input.
When a muscle is at its ________ length, the central region of each muscle spindle is stretched enough to activate the sensory fibers. As a result, sensory neurons from the spindles are ________ _________, sending a steady stream of action potentials to the CNS
resting; tonically active
Autonomic reflexes are also known as ________ ________ because they often involve the internal organs of the body. Some, such as urination and defecation, are spinal reflexes that can take place without input from the brain.
visceral reflexes
Movement can be loosely classified into three categories:
reflex movement, voluntary movement, and rhythmic movement
Briefly describe the control of voluntary movement, designating the parts of the brain that controls each step.
(1) Sensory input = Information comes from sensory neurons to sensory cortex. (2) Planning and decision-making: Motor cortex send information to Prefrontal cortex and Motor association areas. The information is relayed throught the basal ganglia and thalamus. (3) Coordination and timing: the motor cortex relays information to cerebellum for cerebellar input. (4) Execution: motor cortex communicates with brain stem and then the corticospinal tract to skeletal muscles. The basal ganglia relays through the thalamus and then to the brain stem to extrapyramidal tracts to influence on posture, balance, and gait. (5) Continuous feedback: sensory neurons continue to communicate with sensory cortex.
The control of voluntary movement can be divided into three steps:
(1) decisionmaking and planning, (cerebral cortex) (2) initiating the movement, (cerebral cortex) and (3) executing the movement.
Three levels of the nervous system control movement:
(1) the spinal cord, which integrates spinal reflexes and contains central pattern generators; (2) the brain stem and cerebellum, which control postural reflexes and hand and eye movements; and (3) the cerebral cortex and basal ganglia, which are responsible for voluntary movements.
At your last physical, your physician checked your patellar tendon reflex by tapping just below your knee while you sat quietly on the edge of the table. (a) What was she checking when she did this test? (b) What would happen if you were worried about falling off the table and were very tense? Where does this additional input to the efferent motor neurons originate? Are these modulatory neurons causing EPSPs or IPSPs [p. 277] at the spinal motor neuron? (c) Your physician notices that you are tense and asks you to count backward from 100 by 3's while she repeats the test. Why would carrying out this counting task enhance your reflex?
(a) Assessing the components that regulate limb movement, including quadriceps muscle, the nerves that control it, and the area of the spinal cord where the reflex integrates. (b) The reflex would probably be less apparent. The origin of this inhibition is the primary motor cortex. The inhibitory cells will produce IPSPs in the spinal motor neuron. (c) If the brain is distracted by some other task, the inhibitory signals will presumably stop.
Intrafusal Fibers
A connective tissue capsule that encloses a group of small muscle fibers that muscle fibers consist. are modified so that the ends are contractile but the central region lacks myofibrils. The contractile ends of the have their own innervation from gamma motor neurons. The noncontractile central region of each intrafusal fiber is wrapped by sensory nerve endings that are stimulated by stretch. These sensory neurons project to the spinal cord and synapse directly on alpha motor neurons innervating the muscle in which the spindles lie.
Tonic Activity
A continuous stream of action potentials that creates ongoing activity in the effector. For example, the tonic control of blood vessels is an example of a continuously active autonomic reflex
What distinguishes a stretch reflex from a crossed extensor reflex?
A stretch reflex is initiated by stretch and causes a reflex contraction. A crossed extensor reflex is a postural reflex initiated by withdrawal from a painful stimulus; the extensor muscles contract, but the corresponding flexors are inhibited.
Explain how the Golgi tendon organ works:
Contraction pulls collagen fibers within the Golgi tendon organ tight, pinching sensory endings of the afferent neurons and causing them to fire. Afferent input from activation of the Golgi tendon organ excites inhibitory interneurons in the spinal cord. The interneurons inhibit alpha motor neurons innervating the muscle, and muscle contraction decreases or ceases. This reflex slows muscle contraction as the force of contraction increases. In other instances, the Golgi tendon organs prevent excessive contraction that might injure the muscle.
The pathway pattern that brings information from many neurons into a smaller number of neurons is known as _______________ .
Convergence
proprioceptors
Located in skeletal muscles, joint capsules, and ligaments. They monitor the position of our limbs in space, our movements, and the effort we exert in lifting objects. The input signal from proprioceptors goes to the CNS through sensory neurons.
Neural reflexes can be classified in several ways:
Efferent division that controls the effector: a. Somatic motor neurons control skeletal muscles. b. Autonomic neurons control smooth and cardiac muscle, glands, and adipose tissue. 2. Integrating region within the central nervous system: a. Spinal reflexes do not require input from the brain. b. Cranial reflexes are integrated within the brain. 3. Time at which the reflex develops: a. Innate (inborn) reflexes are genetically determined. b. Learned (conditioned) reflexes are acquired through experience. 4. The number of neurons in the reflex pathway: a. Monosynaptic reflexes have only two neurons: one afferent (sensory) and one efferent. Only somatic motor reflexes can be monosynaptic. b. Polysynaptic reflexes include one or more interneurons between the afferent and efferent neurons. All autonomic reflexes are polysynaptic because they have three neurons: one afferent and two efferent.
Reflex movements require input from the cerebral cortex.
False
Crossed extensor reflex
Flexion reflexes, particularly in the legs, are usually accompanied by a postural reflex that helps maintain balance when one foot is lifted from the ground. For example, the quick withdrawal of the right foot from a painful stimulus (a tack) is matched by extension of the left leg so that it can support the sudden shift in weight. The extensors contract in the supporting left leg and relax in the withdrawing right leg, while the opposite occurs in the flexor muscles.
Joint Receptors
Found in the capsules and ligaments around joints in the body. They are stimulated by mechanical distortion that accompanies changes in the relative positioning of bones linked by flexible joints. Sensory information from joint receptors is integrated primarily in the cerebellum.
Postural Reflexes
Help us maintain body position as we stand or move through space. These reflexes are integrated in the brain stem. They require continuous sensory input from visual and vestibular (inner ear) sensory systems and from the muscles themselves. Muscle, tendon, and joint receptors provide information about proprioception, the positions of various body parts relative to one another.
Convergence
Integrates the input from multiple sources to modify the response.
Flexion reflex requires more time than a stretch reflex (such as the knee jerk reflex). Why?
It is a polysynaptic rather than a monosynaptic reflex.
Neurotransmitter release will decrease when M's neurotransmitter hyperpolarizes P.
Modulatory neuron M synapses on the axon terminal of neuron P, just before P synapses with the effector organ. If M is an inhibitory neuron, what happens to neurotransmitter release by P? What effect does M's neurotransmitter have on the postsynaptic membrane potential of P? (Hint: Draw this pathway.)
Three types of proprioceptors are found in the body:
Muscle spindles, Golgi tendon organs, and joint receptors.
central pattern generators (CPGs)
Networks of CNS interneurons established in brainstem or spinal cord. When this organization is triggered by stimulus from brain, a command neuron, it produces rhythmic, oscillating commands to the musculature as if it were defining a sequence of right-left-right activities These commands occur even if the sensory nerves are cut suggesting that the organization is truly central in origin
Divergence
Pathways allows a single stimulus to affect multiple targets.
Cranial Reflexes
Reflexes integrated in the brain.
Learned or Conditioned Reflex
Reflexes that are acquired through experience. The example of Pavlov's dogs salivating upon hearing a bell is the classic example.
Somatic Reflexes
Reflexes that involve somatic motor neurons and skeletal muscles.
Innate Reflexes
Reflexes that we are born with, and they are genetically determined. One example is the knee jerk, or patellar tendon reflex: when the patellar tendon at the lower edge of the kneecap is stretched with a tap from a reflex hammer, the lower leg kicks out.
Autonomic Reflexes
Reflexes whose responses are controlled by autonomic neurons .
List the general steps of a reflex pathway, including the anatomical structures in the nervous system that correspond to each step.
Sensor (sensory receptor), input signal (sensory afferent neuron), integrating center (central nervous system), output signal (autonomic or somatic motor neuron), targets (muscles, glands, some adipose tissue).
alpha-gamma coactivation
Simultaneous activation of alpha and gamma motor neurons so that during muscle contraction, the intrafusal fibers continue to monitor tension in the muscle.
alpha motor neurons
Somatic motor neurons that innervate skeletal muscle contractile fibers.
Using the standard steps of a reflex pathway (stimulus, receptor, and so forth), draw a reflex map of the stretch reflex.
Stimulus = Muscle Stretch. Sensor (sensory receptor) = Muscle Spindles. input signal (sensory afferent neuron) = Increased afferent signals to the spinal cord. integrating center (central nervous system) = Spinal Cord. output signal (autonomic or somatic motor neuron) = Increased efferent output through alpha motor neurons. targets (muscles, glands, some adipose tissue) = Muscle, which contracts. Negative Feedback = Firing rate of afferent sensory neuron decreases.
Describe the crossed extension reflex briefly as seen in the "tack example".
Stimulus = Painful stimulus Receptor= activates nociceptor. Input= Primary sensory neuron enters spinal cord (integration center) and diverges. Output= One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity). Response = Withdrawal reflex pulls foot away from painful stimulus (flexion reflex: extensors inhibited, flexors contract). Crossed extensor reflex supports body as weight shifts away from painful stimulus (extensors contract, flexors inhibited).
THE PATELLAR TENDON (KNEE JERK) REFLEX
Stimulus: = Tap to tendon stretches muscle. Sensor (sensory receptor): = Muscle spindle stretches and fires. input signal (sensory afferent neuron): = afferent neurons send sensory information. integrating center (central nervous system): = spinal cord. SIGNAL DIVERGES: 1- output signal (autonomic or somatic motor neuron): = Somatic Motor neurons 1- targets (muscles, glands, some adipose tissue): = Quadriceps contracts, swinging lower leg forward. 2- output signal (autonomic or somatic motor neuron): =Interneuron inhibiting somatic motor neuron 2- targets (muscles, glands, some adipose tissue): = Hamstring stays relaxed, allowing extension of leg (reciprocal inhibition).
Draw a reflex map of the flexion reflex initiated by a painful stimulus to the sole of a foot.
Stimulus: = foot contacts the point of the tack or something else painful. Sensor (sensory receptor): + nociceptors (pain receptors) in the foot. input signal (sensory afferent neuron): = afferent neurons send sensory information. integrating center (central nervous system): = spinal cord. SIGNAL DIVERGES: 1- output signal (autonomic or somatic motor neuron): = Somatic Motor neurons 1- targets (muscles, glands, some adipose tissue): = Quadriceps contracts, swinging lower leg forward. 2- output signal (autonomic or somatic motor neuron): =Interneuron inhibiting somatic motor neuron 2- targets (muscles, glands, some adipose tissue): = Hamstring stays relaxed, allowing extension of leg (reciprocal inhibition). 3- output signal (autonomic or somatic motor neuron) = Somatic motor neuron 3- targets (muscles, glands, some adipose tissue): = contraction of hip flexor muscles
Rhythmic Movements
Such as walking or running, are a combination of reflex movements and voluntary movements. They are initiated and terminated by input from the cerebral cortex, but once activated, networks of CNS interneurons called central pattern generators (CPGs) maintain the spontaneous repetitive activity. Changes in rhythmic activity, such as changing from walking to skipping, are also initiated by input from the cerebral cortex.
Behaviors such as movement require knowledge of:
The body's position in space (where am I?), a decision on what movement should be executed (what shall I do?), a plan for executing the movement (how shall I do it?), and the ability to hold the plan in memory long enough to carry it out (now, what was I just doing?).
myotatic unit
The collection of pathways controlling a single joint.
Reflex
The integration of sensory information into an involuntary response. All neural reflexes begin with a stimulus that activates a sensory receptor. The sensor sends information in the form of action potentials through sensory afferent neurons to the CNS
Reflex Movement
The least complex and are integrated primarily in the spinal cord. However, like other spinal reflexes, reflex movements can be modulated by input from higher brain centers. In addition, the sensory input that initiates reflex movements, such as the input from muscle spindles and Golgi tendon organs, goes to the brain and participates in the coordination of voluntary movements and postural reflexes.
Polysynaptic Reflexes
The most common reflex, three or more neurons in the pathway (and at least two synapses), leading to their designation. Polysynaptic reflexes may be quite complex, with extensive branching in the CNS to form networks involving multiple interneurons. May display traits such as divergence and convergence. The modification in polysynaptic pathways may involve excitation or inhibition
How is Parkinson's treated?
The primary current treatment for Parkinson's is administration of drugs designed to enhance dopamine activity in the brain. Dopamine cannot cross the blood-brain barrier, so patients take l-dopa, a precursor of dopamine that crosses the blood-brain barrier, then is metabolized to dopamine. Other treatments include dopamine agonists and inhibitors of enzymes that break down dopamine, such as MAO. In severe cases, selected parts of the brain may be destroyed to reduce tremors and rigidity. Experimental treatments include transplants of dopamine secreting neurons.
Monosynaptic reflex
The simplest reflex, named for the single synapse between the two neurons in the pathway: a sensory afferent neuron (often just called a sensory afferent) and an efferent somatic motor neuron. These two neurons synapse in the spinal cord, allowing a signal initiated at the receptor to go directly from the sensory neuron to the motor neuron. (The synapse between the somatic motor neuron and its muscle target is ignored.) The patellar tendon reflex is an example.
Reciprocal inhibition
The simultaneous contraction of one muscle and the relaxation of its antagonist to allow movement to take place
What role does the limbic system play in reflexes?
The site of primitive drives such as sex, fear, rage, aggression, and hunger—has been called the "visceral brain" because of its role in these emotionally driven reflexes. Emotion-linked autonomic reflexes include such things as urination, defecation, blushing, blanching, and piloerection.
Muscle Tone
The state of balanced muscle tension that makes normal posture, coordination, and movement possible. Even a muscle at rest maintains a certain level of tension.
What purpose does the thalamus play in movement?
The thalamus relays and modifies signals as they pass from the spinal cord, basal ganglia, and cerebellum to the cerebral cortex
How are contractions inhibited in skeletal muscle?
There is no inhibitory neuron that synapses on skeletal muscles to cause them to relax. Instead, relaxation results from the absence of excitatory input by the somatic motor neuron. Inhibition and excitation of somatic motor neurons and their associated skeletal muscles must occur at synapses within the CNS.
Golgi tendon organ
These receptors are found at the junction of tendons and muscle fibers, placing them in series with the muscle fibers. They respond primarily to muscle tension created during an isometric contraction and are relatively insensitive to muscle stretch. Their reflexes cause relaxation, the opposite of the reflex contraction caused by muscle spindle reflexes. Composed of free nerve endings that wind between collagen fibers inside a connective tissue capsule.
Piloerection
Tiny muscles in the hair follicles pull the shaft of the hair erect. "Goosebumps".
Spinal reflexes are often modulated by excitatory or inhibitory signals from the brain, carried by descending tracts from higher brain centers.
True
How many synapses occur in the simplest autonomic reflexes? Where do the synapses occur?
Two neuron-neuron synapses in the spinal cord and the autonomic ganglion, and one neuron-target synapse.
If a cell hyperpolarizes, does its membrane potential become more positive or more negative? Does the potential move closer to threshold or farther from threshold?
Upon hyperpolarization, the membrane potential becomes more negative and moves farther from threshold.
stretch reflex
What reflex is initiated by stretching of muscle spindles and causes a contraction of the stimulated muscle and inhibits its antagonist. Muscle spindles are anchored in parallel to the extrafusal muscle fibers. Any movement that increases muscle length also stretches the muscle spindles and causes their sensory fibers to fire more rapidly. This creates a reflex contraction of the muscle, which prevents damage from overstretching
As you pick up a heavy weight, which of the following are active in your biceps muscle: alpha motor neurons, gamma motor neurons, muscle spindle afferents, Golgi tendon organ afferent neurons?
When you pick up a weight, alpha and gamma neurons, spindle afferents, and Golgi tendon organ afferents are all active.
corticospinal tract
a group of interneurons controlling voluntary movement that run from the motor cortex to the spinal cord, where they synapse directly onto somatic motor neurons. Most of these descending pathways cross to the opposite side of the body in a region of the medulla known as the pyramids. Consequently, this pathway is sometimes called the pyramidal tract.
Parkinson's disease
a progressive neurological disorder characterized by abnormal movements, speech difficulties, and cognitive changes. These signs and symptoms are associated with loss of neurons in the basal ganglia that release the neurotransmitter dopamine. One abnormal sign that most Parkinson patients have is tremors in the hands, arms, and legs, particularly at rest. In addition, they have difficulty initiating movement and walk slowly with stooped posture and shuffling gait. They lose facial expression, fail to blink (the reptilian stare), and may develop depression, sleep disturbances, and personality changes.
The ______ ______ is in charge of postural reflexes and hand and eye movements. It also gets commands from the __________, the part of the brain responsible for "fine-tuning" movement. The result is reflex movement. However, some sensory information is sent through ascending pathways to sensory areas of the __________, where it can be used to plan voluntary movements.
brain stem; cerebellum; cortex
Some reflexes are integrated in the brain, primarily in the ____________, __________, and ______ ______. These regions contain centers that coordinate body functions needed to maintain homeostasis, such as heart rate, blood pressure, breathing, eating, water balance, and maintenance of body temperature . The _______ ________ also contains the integrating centers for autonomic reflexes such as salivating, vomiting, sneezing, coughing, swallowing, and gagging.
hypothalamus; thalamus; brain stem; brain stem
Stretching a skeletal muscle causes sensory neurons to (increase/ decrease) their rate of firing, causing the muscle to contract, thereby relieving the stretch. Why is this a useful reflex?
increase. This reflex prevents damage from overstretching.
extrapyramidal tract or the extrapyramidal system
more primitive motor system regulated by basal ganglia; Allows for gross movement without conscious thought
A key feature of many reflex pathways is __________ feedback.
negative
When the axon terminal of a modulatory neuron (cell M) terminates close to the axon terminal of a presynaptic cell (cell P) and decreases the amount of neurotransmitter released by cell P, the resulting type of modulation is called .
presynaptic inhibition
Autonomic reflexes are all _____________, with at least one synapse in the CNS between the sensory neuron and the preganglionic autonomic neuron, and an additional synapse in the ganglion between the preganglionic and postganglionic neurons.
polysynaptic
The inhibitory interneurons suppress activity in the motor neurons controlling the hamstrings during the knee jerk reflex, it is a __________ reflex.
polysynaptic
Flexion Reflexes
polysynaptic reflex pathways that cause an arm or leg to be pulled away from a noxious stimulus, such as a pinprick or a hot stove. These reflexes, like the reciprocal inhibition reflex just described, rely on divergent pathways in the spinal cord.
Proprioceptors such as muscle spindles, Golgi tendon organs, and joint capsule receptors provide information to the:
spinal cord, brain stem, and cerebellum
Some autonomic reflexes are spinal reflexes; others are integrated in the brain. List some examples of each.
spinal reflexes: urination and defacation; cranial reflexes: control of rate, blood pressure, and body temperature.
Muscle Spindles
stretch receptors that send information to the spinal cord and brain about muscle length and changes in muscle length. They are small, elongated structures scattered among and arranged parallel to the contractile extrafusal muscle fibers. Every skeletal muscle in the body has many muscle spindles (except one muscle in the jaw).
The Golgi tendon organ responds to both __________ and , ____________ although elicits the stronger response. Its activation _______________ (increases/decreases) muscle contraction via the ___________ neuron.
stretch; contraction; contraction; decreases; alpha motor neuron.
extrafusal muscle fibers
surround muscle fibers supplied by alpha motor neurons. Action potentials in alpha motor neurons cause extrafusal fibers to contract.
Voluntary Movements
the most complex type of movement. They require integration at the cerebral cortex, and they can be initiated at will without external stimuli. Learned voluntary movements improve with practice, and some even become involuntary, like reflexes. . "Muscle memory" is the name dancers and athletes give the ability of the unconscious brain to reproduce voluntary, learned movements and positions.
Urination
the normal process of excreting urine. It may be voluntarily initiated by conscious thought. Or it may be inhibited by emotion or a stressful situation, such as the presence of other people (a syndrome known as "bashful bladder"). Often, the higher control of a spinal reflex is a learned response. The toilet training we master as toddlers is an example of a learned reflex that the CNS uses to modulate the simple spinal reflex of urination.
