CHAPTER 8 - MOVEMENT
Smooth muscle: A type of muscle found in the lining of the digestive tract, within arteries, and in the reproductive system; controlled by the autonomic nervous system. Smooth muscles move nutrients through the digestive tract, control blood pressure, and mix sperm with seminal fluid, among other tasks.
Striated muscle: A type of muscle named for its striped appearance; including cardiac and skeletal muscles.
Cardiac muscle: A type of striated muscle found in the heart.
Skeletal muscle: A type of striated muscle that is attached to bones and is responsible for the majority of body movements. The contraction of skeletal muscles is directly controlled by motor neurons originating in either the spinal cord or in the nuclei of the cranial nerves.
Muscle fiber: An individual muscle cell. When a molecule of ACh is bound to a receptor site in the muscle fiber membrane, sodium channels open. Sodium rushing into the muscle fiber depolarizes the cell and triggers an action potential. In Chapter 3, we observed that action potentials in neurons travel in only one direction—from the axon hillock down the length of the axon. In contrast, the action potential in a muscle fiber spreads out in two directions on either side of the receptor site. Each action potential produces a single contraction of the muscle fiber.
Twitch: The contraction of a single muscle fiber.
Myofibril: Along fiber strand running the length of a muscle fiber that is responsible for contraction.
Sarcomere: A myofibril segment bound on either side by a Z line and spanned by thin filaments.
Z line: A boundary line for each sarcomere within a myofibril.
Actin: A protein that makes up the thin filaments of the myofibril.
Myosin: A protein that makes up the thick filaments of the myofibril.
Muscle Fiber Contraction Muscle contractions are caused by the movement of the thick myosin filaments along the length of the thin actin filaments. As the filaments slide by each other, the Z lines move closer together, and the sarcomere shortens. As the sarcomeres shorten, the muscle contracts.
Troponin: The protein covering of an actin molecule that prevents the molecule from binding with myosin when a muscle is in the resting state.
Slow-twitch fiber A muscle fiber containing Type I myosin filaments and large numbers of mitochondria that contracts slowly using aerobic metabolism; primarily responsible for movement requiring endurance.
Fast-twitch fiber A muscle fiber containing Type IIa or Type IIb myosin filaments that contains few mitochondria, uses anaerobic metabolism, and contracts rapidly; primarily responsible for movement requiring explosive strength. Type IIb fibers can contract up to ten times faster than Type I fibers.
Most skeletal muscles contain mixtures of all three types of fibers but in different proportions. The postural muscles of the back, neck, and legs are dominated by slow-twitch fibers. The muscles of the arms and shoulders contain higher proportions of fast-twitch fibers.
WHY DO MUSCLES GET TIRED?
Researchers have discovered that after a muscle has contracted repeatedly, the calcium channels in the fibers’ internal organelles become. Without the normal ebb and flow of calcium in response to neural input to the fiber, the fiber begins to contract more weakly. In addition, the leaking calcium initiates the activity of enzymes that further damage the muscle. This same group of scientists is experimenting with a drug that will block the calcium leakage, which would help prevent muscle damage and maintain a muscle’s ability to contract.
Aerobic metabolism: A chemical processes that requires oxygen.
Anaerobic metabolism A chemical process that does not require oxygen.
Muscle enlargement occurs in response to muscle fiber damage. When fibers are damaged due to weightlifting or other strenuous activity, more actin and myosin filaments are produced. Lack of activity reverses this process quickly because filament proteins are either broken down faster or synthesized more slowly when muscles are not used. During space travel, in which lower gravity reduces the activity of major postural muscles, astronauts can lose as much as 20 percent of their muscle mass in as little as two weeks. Under certain circumstances, changes in muscle fiber type do occur. Endurance exercise tends to convert the very fast Type IIb fibers into Type IIa fibers.
THE EFFECTS OF AGING MUSCLES: When older adults move a muscle, the firing rates of the associated neurons is lower than the rate seen in younger adults, resulting in slower and weaker muscle responses. Youthful muscles have a more even distribution of slow- and fast- twitch fibers. In the elderly muscle, the types appear clustered together.
Antagonistic pair: Two opposing muscles, one a flexor and one an extensor, arranged at a joint.
Extensor: A muscle that acts to straighten a joint. To straighten the leg, the extensor muscles must contract while the flexors relax.
Flexor: A muscle that acts to bend a joint. To bend the knee, the flexor muscles of the thigh must contract while the extensor muscles relax
Distribution of Spinal Motor Neurons The spinal cord bulges in the segments that serve the arms and legs, due to the large number of alpha motor neurons located in the ventral parts of those segments. In comparison, relatively few alpha motor neurons serve the torso.
Alpha motor neuron: A spinal motor neuron directly responsible for signaling a muscle fiber to contract. These are large myelinated neurons capable of rapid signaling. The alpha motor neurons do not initiate movement on their own. Instead, these neurons are activated by input from other parts of the motor system. Alpha motor neurons receive input from three types of neurons: neurons from muscle spindles and Golgi tendon organs, neurons of the brainstem and motor cortex, and spinal interneurons. The alpha motor neurons that contract muscle fibers are at the lowest end of a chain of command for initiating movement. In addition to input from interneurons and stretch receptors, the alpha motor neurons receive direction from neurons located in the cerebellum, basal ganglia, red nucleus, brainstem, and cerebral cortex.
The alpha motor neurons form highly efficient connections with muscle fibers at a location called the…
Neuromuscular junction: A synapse formed between an alpha motor neuron axon terminal and a muscle fiber.
Motor unit: The combination of a single alpha motor neuron and all the muscle fibers that it innervates. A single motor unit includes either fast-or slow-twitch fibers but not a mixture of both.
Recruitment: The process of gradually activating more motor units as an increasing load is placed on a muscle.
As mentioned previously, a single action potential in the alpha motor neuron usually results in a single contraction in the associated muscle fiber. How, then, do we manage to produce muscular movements of different forces and durations?
There are two methods for controlling the force of our movements. The first method is to vary the firing rate of motor neurons. Rapid firing by the motor neuron produces a sustained contraction in the muscle fiber. Contractions last longer than action potentials, allowing temporal summation to occur at the neuromuscular junction. The muscle fiber responds with increasing contraction.
The second method for varying muscle responses is known as recruitment. As an increased load is placed on a muscle, as when you pick up a heavy object, more motor units are recruited to provide extra tension in the muscle. Recruitment proceeds according to the nature of the motor units. Smaller, slow-twitch units are recruited first, followed by the intermediate Type IIa units, and finally the largest, fastest Type IIb units. The smaller neurons that innervate slow-twitch fibers are more easily excited by synaptic input, which might account for them being recruited first. Recruiting the smaller motor units first ensures that the body uses the smallest amount of force and energy to get a job done.
Feedback from the Muscle Spindle Muscle spindles are specialized sensors that form part of a feedback loop from the muscle fibers to the spinal cord. If an object, a cup of coffee, for example, is placed in your outstretched hand, your hand might initially drop a bit in response to the weight of the cup before returning to its original position. You compensate for the added weight in your hand by increasing contraction in the muscles of your arm. This adjustment requires a precise feedback loop that measures how much your muscles have stretched.
Muscle spindle: A sensory structure that provides feedback regarding muscle stretch, serve as a source of information about muscle length.
Intrafusal muscle fiber: One of the fibers that make up a muscle spindle. Each muscle spindle has a dozen of these.
Extrafusal muscle fiber: One of the fibers outside the muscle spindle that is responsible for contracting the muscle. Muscle spindles lie parallel to the extrafusal fibers, so that when the muscle stretches, so do its associated spindles. Muscles needed for fine motor movements, such as those in the hand, have more muscle spindles than do muscles used primarily for force, such as those in the torso and legs. Larger numbers of muscle spindles provide the more precise feedback required for fine movements.
Ia sensory fiber: A large, fast sensory axon that connects a muscle spindle to neurons in the spinal cord. These fibers generate action potentials every time the muscle spindle stretches. Within the spinal cord, the Ia fibers synapse on interneurons and on the alpha motor neurons.
Example: Here’s how the system works. As the cup in our example is placed in your hand, your arm muscle will stretch due to the added weight. The stretching of your arm muscle will also stretch the muscle spindle. The Ia fibers surrounding the muscle spindle will sense the increased stretch and will excite the alpha motor neurons in the spinal cord. Excitation of the alpha motor neurons will cause the muscle to balance the stretch with further contraction. The cup is now safe.
Gamma motor neuron: A small spinal neuron that innervates the muscle spindles. Without the gamma motor neurons, the intrafusal fibers could not provide accurate information about how far the muscle was stretched. The gamma motor neurons cause a small contraction of the spindle at nearly the same time that the alpha motor neurons contract the extrafusal fibers. In this way, the spindle matches the length of the muscle, and the Ia fibers can provide continuous feedback.
Golgi tendon organ: A structure located in the tendons of muscles that provides information about muscle contraction. Gives feedback regarding the degree of muscle contraction, or force.
Ib sensory fiber A small, slower Alpha-alpha (Aα) sensory axon that connects the Golgi tendon organs to neurons in the spinal cord. The Ib fibers from the Golgi tendon organs enter the spinal cord and form synapses with spinal interneurons. In turn, these interneurons form inhibitory synapses on alpha motor neurons.
To understand how the Golgi tendon organ feedback loop works, we return to the example of holding a coffee cup steady. The Ia fibers from muscle spindles in your fingers and arm sense the stretch needed to hold the cup and activate the alpha motor neurons. The Golgi tendon organs respond to the resulting increase in muscle tension by sending signals to the spinal interneurons via the Ib sensory fibers. In response to this input, the interneurons inhibit the alpha motor neurons, and muscle contraction is reduced. However, the reduced muscle contraction results in less Golgi tendon organ activity, less input to the spinal interneurons, and less inhibition of the alpha motor neurons. The muscle contracts again. Not only does this system help prevent damage to the muscle fibers from too much contraction, but it also maintains the steady control over muscle tension that we need, particularly for fine motor movements.
Feedback from Joints In addition to receiving feedback about muscle length and tension, we also receive information about position and movement from mechanoreceptors in the tissues surrounding each joint. These mechanoreceptors respond primarily to movement of the joint, and they are relatively quiet when the joint is at rest. Receptors located in the skin near joints also supply information about movement and position.
Monosynaptic reflex: A spinal reflex, such as the patellar reflex, that requires the action of only one synapse between sensory and motor neurons.
Polysynaptic reflex: A spinal reflex that requires interaction at more than one synapse.
Reciprocal inhibition: A polysynaptic reflex that prevents the simultaneous contraction of flexors and extensors serving the same joint.
Flexion reflex: A polysynaptic spinal reflex that produces withdrawal of a limb from a painful stimulus. It’s a good thing that the spinal cord, rather than the brain, manages this function. By the time the brain perceived the problem, generated solutions, evaluated solutions, and implemented solutions, your hand would be in bad shape. The flexion reflex begins as sensory neurons transmit information about the painful stimulus to interneurons in the spinal cord. The interneurons excite the alpha motor neurons serving the flexor muscles of the affected limb. At the same time, alpha motor neurons serving the opposing muscle, the extensor, are inhibited. As a result, your hand is successfully pulled back from the heat source.
Babinski sign: A polysynaptic flexion reflex present in infants and in adults with neural damage, in which stroking the foot causes the toes to spread with the big toe pointing upward.
Lateral pathway: A large collection of axons that originates in the cerebral cortex, synapses on either the red nucleus or alpha motor neurons, and controls voluntary movements of the hands, feet, and outer limbs.
Cell bodies giving rise to the axons of the lateral pathwayare located either in the primary motor cortex of the frontal lobe or in the red nucleus of the midbrain. The two components of the lateral pathway are the corticospinal tract and the rubrospinal tract. As its name implies, the corticospinal tract originates in the motor cortex of the brain. These fibers are some of the fastest and longest in the central nervous system. The fibers of the rubrospinal tract originate in the red nucleus. Consequently, the motor cortex exerts both direct control (via the corticospinal tract) and indirect control (via the rubrospinal tract) on the alpha motor neurons of the spinal cord.
Ventromedial pathway: A spinal motor pathway originating in the brainstem and carrying commands for subconscious, automatic movements of the neck and torso. You use the ventromedial pathway for behaviors such as maintaining posture and muscle tone and moving the head in response to visual stimuli.
IMPORTANCE OF THE CEREBELLUM: Additional descending control of the motor system originates in the cerebellum. Although the cerebellum does not appear to initiate movement, it plays a very important role in the sequencing of complex movements. As you ride a bicycle or shoot a basket, your cerebellum is coordinating the contraction and relaxation of muscles at just the right time. One common example of poor cerebellar function occurs when a person drinks alcohol. The cerebellum is one of the first structures in the brain to show the effects of alcohol, leading to a lack of balance and coordination. How does the cerebellum help us coordinate sequenced movements? It appears that the cerebellum is able to inform the motor cortex about such factors as the direction, force, and timing required to carry out a skilled movement. The cerebellum is constantly comparing the cortex’s intended movements with what actually happened.
IMPORTANCE OF THE BASAL GANGLIA:
The basal ganglia participate in the choice and initiation of voluntary movements. The basal ganglia inhibit the activity of the thalamus. As a result, you might think of the basal ganglia as a gate or filter for intentional activity. Only those voluntary actions that pass the basal ganglia with sufficient strength will be implemented. A number of disorders result from abnormalities in the basal ganglia, including Parkinson’s disease and Huntington’s disease. These are cases in which motor activity is either lower (Parkinson’s) or higher (Huntington’s).
MPORTANCE OF THE (PRIMARY) MOTOR CORTEX:
Cortex located in the precentral (before the central sulcus) gyrus has been identified as primary motor cortex, the main source of voluntary motor control.
Supplementary motor area (SMA): Motor area located in the gyrus rostral to the precentral gyrus; involved with managing complex sequences of movement.
Pre-SMA: A motor area located in the gyrus rostral to the precentral gyrus; this area participates in holding a motor plan until it can be implemented; formerly referred to as the premotor area (PMA). Pre-SMA activity helps us bridge delays between the planning and initiation of movement.
The anterior cingulate cortex (ACC): appears to coordinate an organism’s history of reward with the selection of voluntary movement.
THE INITIATION OF VOLUNTARY MOVEMENT:
1. Decision to make a movement originates in prefrontal cortex and parietal lobe.
2. Movement is planned in SMA and Pre-SMA, incorporating input from the thalamus and basal ganglia.
3. Primary motor cortex sends signals via lateral pathway.
4. Lateral pathway carries signals to spinal motor neurons, which initiate muscle contractions.
Pyramidal cell A large, pyramid-shaped neuron found in the output layers (Layers III and V) of the cerebral cortex, including primary motor cortex. These pyramidal cell axons are an important source of input to the brainstem and to spinal motor neurons.
Mirror neurons: Although the exact functions of mirror neurons remain unknown, they appear to participate in a number of essential processes. Because human mirror neurons have been identified in Broca’s area, additional speculation on the role of these neurons in the evolution of language has been proposed. Language could have originally developed out of systems of gesture and imitation made possible by mirror neurons. Mirror neurons might also form the basis for imitation, empathy, and theory of mind (TOM), the ability to predict and understand the thoughts of others. Mirror neuron activity might allow us to simulate others’ thoughts subconsciously by literally putting ourselves mentally in another person’s shoes.
Toxins: A variety of toxic substances interfere with movement by acting on synapses within the motor system. Many of these effects take place at the neuromuscular junction, which uses the neurotransmitter acetylcholine (ACh). Toxins that are cholinergic agonists boost the activity of ACh at the neuromuscular junction, affecting muscle tone.
Myastheniagravis An autoimmune condition caused by the degeneration of ACh receptors at the neuromuscular junction, resulting in muscle weakness and fatigue.
Muscular dystrophy: A group of diseases characterized by extreme muscle development followed by muscle wasting, due to abnormalities in the protein dystrophin.
Polio: A contagious viral disease that attacks the spinal motor neurons, producing paralysis.
Quadriplegia: if the damage occurs in the cervical, or neck, region of the spinal cord it will result in the loss of movement in both arms and legs. If the damage occurs in the lumbar region of the lower back, the person will experience paraplegia, the loss of movement in the legs.
A “cocktail” containing stem cells derived from the brains of adult mice, growth hormones, and anti-inflammatory drugs helped improve the mobility of rats with spinal injuries. It is still quite controversial.
Amyotrophic lateral sclerosis (ALS): A disease in which motor neurons of the spinal cord and brainstem progressively deteriorate, leading to death.
Parkinson’s disease: A degenerative disease characterized by difficulty in moving, muscular tremors, and frozen facial expressions. Parkinson’s disease also affects the peripheral nervous system because the number of noradrenergic terminals in the heart is also reduced by the disease. The direct causes of Parkinson’s disease are quite clear. This disease occurs when the dopaminergic neurons of the substantia nigra in the brainstem begin to degenerate. As we discussed previously, the substantia nigra forms close connections with the basal ganglia in the cerebral hemispheres. The end result of degeneration in the substantia nigra is a lack of typical dopaminergic activity in the basal ganglia. Because the basal ganglia are intimately involved with the production of voluntary movements, it should come as no surprise that the patients show great difficulties in voluntary movement.
Drinking caffeinated coffee reduces the odds of developing the disease.
The traditional treatment for Parkinson’s disease is the medication levodopa, or L-dopa. L-dopa is a precursor in the synthesis of dopamine, so additional L-dopa should help the neurons in the substantia nigra manufacture more of the neurotransmitter. However, L-dopa loses its effectiveness as the numbers of substantia nigra neurons decrease and feedback loops inhibit the further production of dopamine.
Huntington’s disease: A genetic disorder beginning in middle age that results in jerky, involuntary movements and progresses to psychosis and premature death. As the disease progresses, cognitive symptoms such as depression, hallucination, and delusion occur. Fifteen to twenty years after the onset of symptoms, the patient dies.
The cause of Huntington’s disease is simple and well understood. The Huntingtin gene on Chromosome 4, named after George Huntington, encodes the brain protein huntingtin. At the end of the Huntingtin gene is a codon, or sequence that encodes an amino acid, that can repeat between 6 and more than 100 times. Most people have between 10 and 15 repeats of this sequence. A person having fewer than 35 repeats will remain healthy, but a person with 39 or more repeats will develop Huntington’s disease. Higher numbers of repeats are correlated with an earlier onset of symptoms.
CHAPTER 10 – SEXUAL BEHAVIOR
Turner syndrome: A condition caused by an XO genotype, characterized by frequent abnormalities of the ovaries and infertility.
Klinefelter syndrome: A condition in males caused by an XXY genotype, characterized by frequent problems with fertility, secondary sex characteristics, and verbal skills.
Gonads: The internal organs, ovaries in females and testes in males, that produce reproductive cells (eggs and sperm) and secrete sex hormones.
External genitalia The external sexual organs, including the penis and scrotum in males and the labia, clitoris, and lower third of the vagina in females.
Intersex: A conditionin which elements of both male and female development occur in the same fetus.
Ovaries: Female gonads; the source of ova and sex hormones.
Testes: Male gonads; source of sperm and sex hormones.
Sex-determining region of the Y chromosome (SRY): A gene located on the short arm of the Y chromosome that encodes for testis-determining factor.
Testis-determining factor: A protein encoded by the SRY gene on the Y chromosome that turns the primordial gonads into testes.
Wolffian system: The internal system that develops into seminal vesicles, vas deferens, and the prostate gland in males.
Mülleriansystem: The internal system that develops into a uterus, fallopian tubes, and the upper two thirds of the vagina in the absence of anti-Müllerian hormone.
Testosterone: Anandrogen produced primarily in the testes.
Anti-Müllerian hormone: A hormone secreted by fetal testes that causes the degeneration of the Müllerian system.
Androgen: Asteroid hormone that develops and maintains typically masculine characteristics.
Androgen insensitivity syndrome (AIS): A condition in which a genetic male fetus lacks androgen receptors, which leads to the development of female external genitalia and typically female gender identity and sexual behavior.
Gender identity: The sense of being male or female, independent of genetic sex or physical appearance.
Congenital adrenal hyperplasia (CAH): A condition in which a fetus is exposed to higher- than-normal androgens, resulting in masculinization of external genitalia and some cognitive behaviors in affected females.
Secondary sex characteristics: Characteristics related to sex that appear at puberty, including deepening voice and facial hair growth in males and widening hips and breast development in females.
Estrogen A steroid hormone that develops and maintains typically female characteristics.
Estradiol: An estrogen hormone synthesized primarily in the ovaries.
Gonadotropin-releasing hormone (GnRH): A hormone released by the hypothalamus that stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by the anterior pituitary gland.
Follicle-stimulating hormone (FSH): A hormone released by the anterior pituitary that stimulates the development of eggs in the ovaries and sperm in the testes.
Luteinizing hormone (LH): A hormone released by the anterior pituitary that signals the male testes to produce testosterone and that regulates the menstrual cycle in females.
· Testis-determining factor turns the primordial gonads into testes. In the absence of testis-determining factor, ovaries will develop. Prenatal androgens promote the development of male internal organs and masculinize the external genitalia. Anti-Müllerian hormone prevents the development of female organs.
· At puberty, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) promote the release of testosterone by the testes and estradiol by the ovaries, leading to the development of secondary sex characteristics.
Follicle: One of several clusters of cells in the ovary each of which contains an egg cell.
Ovum: A female reproductive cell, or egg.
Hypothalamic Control of the Pituitary Gland: By secreting GnRH, the hypothalamus stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by the anterior pituitary gland. In males, LH signals the testes to produce testosterone. Both testosterone and FSH are necessary for producing mature sperm. In females, LH and FSH control the menstrual cycle.
The Human Menstrual Cycle The menstrual cycle is tightly regulated by the release of GnRH from the hypothalamus, LH and FSH by the anterior pituitary gland, and estrogens and progesterone from the follicles and corpus luteum.
Ovulation: The process of releasing a mature egg from the ovary.
Corpusluteum: A yellow mass of cells in the ovary formed by a ruptured follicle that has released an egg.
Progesterone: A hormone produced in the corpus luteum that prevents the development of additional follicles and promotes the growth of the uterine lining.
Premenstural dysphoric disorder (PMDD) A condition in which premenstrual mood changes are unusually severe.
Estrus: A regularly occurring period of sexual desire and fertility in some mammals.
Sexually dimorphic: Displaying structural differences between the sexes.
Sexually dimorphic nucleus of the POA (SDN-POA) A nucleus in the preoptic area of the hypothalamus that is larger in male rats than in female rats.
Interstitial nuclei of the anterior hypothalamus (INAH): A collection of four small nuclei in the anterior hypothalamus, two of which (INAH-2 and INAH-3) appear to be sexually dimorphic. The size of INAH-3 might be associated with male sexual orientation.
Spinal nucleus of the bulbocavernosus (SNB): Motor neurons in the spinal cord that innervate the male rat’s bulbocavernosus muscles in the penis.
Aromatization: A chemical reaction resulting in an aromatic compound, characterized by a six-carbon ring; for example, the enzyme aromatase transforms testosterone into estradiol.
Alpha fetoprotein: A substance circulated by rats that deactivates estradiol and prevents maternal estradiol from masculinizing female pups.
INAH-3 Size Correlates with Sexual Orientation Simon LeVay reported that INAH-3 is smaller among women and homosexual men than among heterosexual men. (a) This image shows the location of INAH-3. The micrograph in (b) is taken from a heterosexual man, whereas the micrograph in (c) is taken from a man who was homosexual.
Research on finger length and otoacoustic emissions suggest that prenatal exposure to hormones may influence sexual orientation. Genetics appear to have some role in the development of sexual orientation.
Major histocompatibility complex (MHC) gene: Agene that encodes our immune system’s ability to recognize intruders; might account for female human preferences for male odors.
Oxytocin: A hormone, released by the posterior pituitary gland, that stimulates uterine contractions, releases milk, and participates in social bonding, including romantic love and parenting behavior.
Brain Activity and Love: Bartels and Zeki (2000) compared people’s responses to photos of friends and photos of lovers. When viewing lovers, areas of the brain associated with reward and the activity of oxytocin and vasopressin showed increased activity. Areas of the brain that are associated with negative emotion, social judgment, and assessing other people’s intentions and emotions were less active when viewing a loved one. Love appears to be a push–pull mechanism in which bonding activates the reward system, and social distance is reduced by silencing social judgment.