exam 4 --- textbook p.1

neuronal pathways carry sensory and motor information to and from the brain

All major spinal tracts are part of multineuron pathways that connect the brain to the body periphery. These great ascending and descending pathways contain not only spinal cord neurons but also parts of peripheral neurons and neurons in the brain.

The central regions of the intrafusal fibers lack myofilaments and are noncontractile. These regions are the receptive surfaces of the spindle. Two types of afferent endings send sensory inputs to the CNS:

Anulospiral endings (also called primary sensory endings): are the endings of large axons that wrap around the spindle center. They are stimulated by both the rate and degree of stretch. Flower spray endings (also called secondary sensory endings): are formed by smaller axons that supply the spindle ends. They are stimulated only by degree of stretch.

sensory areas

Areas concerned with conscious awareness of sensation, the sensory areas of the cortex, occur in the parietal, insular, temporal, and occipital lobes

The ability to modify the stretch reflex is important in many situations

As the speed and difficulty of a movement increase, the brain increases γ motor output to make the muscle spindles more sensitive.

processing at the circuit level

At the second level of integration, the circuit level, the task is to deliver impulses to the appropriate region of the cerebral cortex for localization and perception of the stimulus. ascending sensory pathways typically consist of a chain of three neurons called first-, second-, and third-order sensory neurons. The axons of first-order sensory neurons, whose cell bodies are in the dorsal root or cranial ganglia, link the receptor and circuit levels of processing. Central processes of first-order neurons branch diffusely when they enter the spinal cord. Some branches take part in local spinal cord reflexes. Others synapse with second-order sensory neurons, which then synapse with the third-order sensory neurons that take the message to the cerebral cortex. the different ascending pathways (spinothalamic, dorsal column-medial lemniscal, and spino-cerebellar) carry various types of information to different destinations in the brain.

Indirect motor pathways are complex and multisynaptic. They are most involved in regulating:

Axial muscles that maintain balance and posture. Muscles controlling coarse limb movements. Head, neck, and eye movements that follow objects in the visual field.

Whenever the muscle spindle is stretched, its associated sensory neurons transmit impulses at higher frequency to the spinal cord. The muscle spindle is stretched (and excited) in one of two ways:

By applying an external force that lengthens the entire muscle, such as when we carry a heavy weight or when antagonistic muscles contract (external stretch). By activating the γ motor neurons that stimulate the distal ends of the intrafusal fibers to contract, thereby stretching the middle of the spindle (internal stretch).

functional Anatomy of Muscle Spindles

Each muscle spindle consists of three to ten modified skeletal muscle fibers called intrafusal muscle fibers enclosed in a connective tissue capsule. extrafusal muscle fibers: effector fibers of the muscle.

other non-encapsulated nerve endings include

Epithelial tactile complexes (Merkel cells and discs): which lie at the junction of the epidermis and dermis, function as light pressure receptors. Certain disclike free nerve endings (Merkel discs) associate with a spiky, hemispherical epidermal cell, forming a complex. Hair follicle receptors: free nerve endings that wrap basketlike around hair follicles, are light touch receptors that detect bending of hairs. The tickle of a mosquito landing on your skin is mediated by hair follicle receptors.

there are three levels of motor control

In the motor system, we have motor endings serving effectors (muscle fibers) instead of sensory receptors, descending efferent circuits instead of ascending afferent circuits, and motor behavior instead of perception. However, as in sensory systems, the basic mechanisms of motor systems operate at three levels.

adaptation

Information about a stimulus—its strength, duration, and pattern—is encoded in the frequency of nerve impulses: the greater the frequency, the stronger the stimulus. Many but not all sensory receptors exhibit adaptation, a change in sensitivity (and nerve impulse generation) in the presence of a constant stimulus. -phasic receptors: are fast adapting, often giving bursts of impulses at the beginning and the end of the stimulus. Phasic receptors report changes in the internal or external environment. Examples are lamellar and tactile corpuscles. -tonic receptors: provide a sustained response with little or no adaptation. Nociceptors and most proprioceptors are tonic receptors because of the protective importance of their information.

premotor cortex

Just anterior to the precentral gyrus in the frontal lobe is the premotor cortex. Helps plan movements. This region selects and sequences basic motor movements into more complex tasks. Using highly processed sensory information received from other cortical areas, it can control voluntary actions that depend on sensory feedback. Coordinates the movement of several muscle groups either simultaneously or sequentially, mainly by sending activating impulses to the primary motor cortex.

spinal reflexes are somatic reflexes mediated by the spinal cord

Many spinal reflexes occur without the direct involvement of higher brain centers. the brain is "advised" of most spinal reflex activity and can facilitate, inhibit, or adapt it, depending on the circumstances. continuous facilitating signals from the brain are required for normal spinal reflex activity. Exaggerated, distorted, or absent reflexes indicate degeneration or pathology of specific nervous system regions, often before other signs appear. The most commonly assessed reflexes are the stretch, flexor, and superficial reflexes.

classification by stimulus type

Mechanoreceptors: respond to mechanical force such as touch, pressure (including blood pressure), vibration, and stretch. Thermoreceptors: respond to temperature changes. Photoreceptors: such as those of the retina of the eye, respond to light. Chemoreceptors: respond to chemicals in solution (molecules smelled or tasted, or changes in blood or interstitial fluid chemistry). Nociceptors: respond to potentially damaging stimuli that result in pain. For example, searing heat, extreme cold, excessive pressure, and inflammatory chemicals are all interpreted as painful. These signals stimulate subtypes of thermoreceptors, mechanoreceptors, and chemoreceptors.

major features of sensory perception

Perceptual detection: is the ability to detect that a stimulus has occurred. This is the simplest level of perception. As a general rule, inputs from several receptors must be summed for perceptual detection to occur. Magnitude estimation: is the ability to detect how intense the stimulus is. Perceived intensity increases as stimulus intensity increases because of frequency coding. Spatial discrimination: allows us to identify the site or pattern of stimulation. A common tool for studying this quality in the laboratory is the two-point discrimination test. The test determines how close together two points on the skin can be and still be perceived as two points rather than as one. This test provides a crude map of the density of tactile receptors in the various regions of the skin. The distance between perceived points varies from less than 5 mm on highly sensitive body areas (tip of the tongue) to more than 50 mm on less sensitive areas (the back). Feature abstraction: is the mechanism by which a neuron or circuit is tuned to one feature, or property, of a stimulus in preference to others. Sensation usually involves an interplay of several stimulus features. Feature abstraction enables us to identify more complex aspects of a sensation. Quality discrimination: is the ability to differentiate the submodalities of a particular sensation. Each sensory modality has several qualities, or submodalities. Pattern recognition: is the ability to take in the scene around us and recognize a familiar pattern, an unfamiliar one, or one that has special significance for us.

Nonencapsulated (Free) Nerve Endings

Present nearly every-where in the body, nonencapsulated (free) nerve endings of sensory neurons are particularly abundant in epithelia and connective tissues. Most of these sensory fibers are nonmyelinated, small-diameter group C fibers, and their distal endings (the sensory terminals) usually have small knoblike swellings. Free nerve endings respond chiefly to painful stimuli and changes in temperature, but some respond to tissue movements caused by pressure as well.

stretch and tendon reflexes

Stretch and tendon reflexes help your nervous system smoothly coordinate the activity of your skeletal muscles. Two types of information about the current state of a muscle are required. The nervous system needs to know: -The length of the muscle. This information comes from the muscle spindles in skeletal muscles. -The amount of tension in the muscle and its associated tendons. Tendon organs provide this information. These two types of proprioceptors play an important role in spinal reflexes and also provide essential feedback to the cerebral cortex and cerebellum, so that the brain can compare what actually happened with what was supposed to happen.

most familiar clinical example of a stretch reflex is the knee-jerk reflex

Stretch reflexes can be elicited in any skeletal muscle by a sudden jolt to the tendon or the muscle itself. All stretch reflexes are monosynaptic and ipsilateral (ipsi = same; lateral = side). They involve a single synapse and motor activity on the same side of the body. A positive knee jerk (or a positive result for any other stretch reflex test) provides two important pieces of information. First, it proves that the sensory and motor connections between that muscle and the spinal cord are intact. Second, the vigor of the response indicates the degree of excitability of the spinal cord.

direct (pyramidal) pathways

The direct pathways originate mainly with the pyramidal cells located in the precentral gyri. These neurons send impulses through the brain stem via the large pyramidal (corticospinal) tracts. The direct pathways are so called because their axons descend without synapsing from the pyramidal cells to the spinal cord. There they synapse either with interneurons or with ventral horn motor neurons. Stimulation of the ventral horn neurons activates the skeletal muscles with which they are associated. The direct pathway primarily regulates fast and fine (or skilled) movements such as texting or playing an instrument.

Dorsal column-medial lemniscal pathways

The dorsal column-medial lemniscal pathways mediate precise, straight-through transmission of inputs from a single type (or a few related types) of sensory receptor that can be localized precisely on the body surface, such as discriminative touch and vibrations. It also transmits information from proprioceptors. These pathways are formed by the paired tracts of the dorsal white column of the spinal cord—fasciculus cuneatus and fasciculus gracilis—and the medial lemniscus. The medial lemniscus arises in the medulla and terminates in the thalamus. From the thalamus, impulses are forwarded to specific areas of the somatosensory cortex.

innervation of visceral muscle and glands

The junctions between autonomic motor endings and their effectors (smooth and cardiac muscle and glands) are much simpler than the junctions formed between somatic fibers and skeletal muscle cells. The autonomic motor axons branch repeatedly, each branch forming synapses en passant ("synapses in passing") with its effector cells. Instead of a cluster of bulblike terminals, an axon ending serving smooth muscle or a gland (but not cardiac muscle) has a series of varicosities, knoblike swellings containing mitochondria and synaptic vesicles, that make it look like a string of beads

adjusting muscle spindle sensitivity

The motor supply to the muscle spindle allows the brain to voluntarily modify the stretch reflex response and the firing rate of α motor neurons. When the γ neurons are vigorously stimulated by impulses from the brain, the spindle is stretched and highly sensitive, and muscle contraction force is maintained or increased. When the γ motor neurons are inhibited, the spindle resembles a loose rubber band and is nonresponsive, and the extrafusal muscles relax.

plantar reflex

The plantar reflex tests the integrity of the spinal cord from L4 to S2 and indirectly determines if the corticospinal tracts are functioning properly. To elicit the plantar reflex, draw a blunt object from heel to toe along the lateral aspect of the plantar surface (sole) of the foot. The normal response is for the toes to flex downward (curl).

general organization of the somatosensory system

The somatosensory system—the part of the sensory system serving the body wall and limbs—receives inputs from exteroceptors, proprioceptors, and interoceptors. Consequently, it transmits information about several different sensory modalities, or types of sensation. Three main levels of neural integration operate in the somatosensory (or any sensory) system.

the projection level

The spinal cord is under the direct control of the projection level of motor control. The projection level consists of neurons acting through the direct and indirect motor pathways, which use descending projection fibers. Upper motor neurons of the motor cortex initiate the direct (pyramidal) pathways. Axons of direct pathway neurons produce discrete voluntary movements of the skeletal muscles. Brain stem motor nuclei oversee the indirect pathways. Axons of these pathways help control reflex and CPG-controlled motor actions, modifying and controlling the activity of the segmental apparatus. Projection motor pathways convey information to lower motor neurons, and send a copy of that information as internal feedback to higher command levels, continually informing them of what is supposed to happen.

Spinothalamic pathways

The spinothalamic pathways receive input from many different types of sensory receptors and make multiple synapses in the brain stem. These pathways consist of the lateral and ventral (anterior) spinothalamic tracts. Their fibers cross over in the spinal cord. The fibers in these pathways primarily transmit impulses for pain and temperature, but also for coarse touch and pressure. All are sensations that we are aware of but have difficulty localizing precisely on the body surface.

spinocerebellar pathways

The third ascending pathway consists of the ventral and dorsal spinocerebellar tracts. They convey information about muscle or tendon stretch to the cerebellum, which uses this information to coordinate skeletal muscle activity. As noted earlier, these pathways do not contribute to conscious sensation. The fibers of the spinocerebellar pathways either do not decussate or else cross over twice (thus "undoing" the decussation).

simple receptors of the general senses

The widely distributed general sensory receptors are involved in tactile sensation (a mix of touch, pressure, stretch, and vibration), temperature monitoring, and pain, as well as the "muscle sense" provided by proprioceptors. one type of receptor can respond to several different kinds of stimuli.

the precommend level

Two other systems of brain neurons, located in the cerebellum and basal nuclei, regulate motor activity. They precisely start or stop movements, coordinate movements with posture, block unwanted movements, and monitor muscle tone. Collectively called precommand areas, these systems control the outputs of the cortex and brain stem motor centers and stand at the highest level of the motor hierarchy. The key center for "online" sensorimotor integration and control is the cerebellum: The cerebellum lacks direct connections to the spinal cord. It acts on motor pathways through the projection areas of the brain stem and on the motor cortex via the thalamus to fine-tune motor activity. The basal nuclei receive inputs from all cortical areas and send their output back mainly to premotor and prefrontal cortical areas via the thalamus. Compared to the cerebellum, the basal nuclei appear to be involved in more complex aspects of motor control. Under resting conditions, the basal nuclei inhibit various motor centers of the brain. When the motor centers are released from inhibition, coordinated motions can begin.

In general, somatosensory information travels

along three main pathways on each side of the spinal cord. Two of these pathways (the dorsal column-medial lemniscaland spinothalamic pathways) transmit impulses via the thalamus to the sensory cortex for conscious interpretation. Collectively the inputs of these sister tracts provide discriminative touch and conscious proprioception. Both pathways decussate—the first in the medulla and the second in the spinal cord.

lamellar corpuscles

also called Pacinian corpuscles, are scattered deep in the dermis, and in subcutaneous tissue underlying the skin. Although they are mechanoreceptors stimulated by deep pressure, they respond only when the pressure is first applied, and thus are best suited to monitoring vibration (an "on/off" pressure stimulus). They are the largest corpuscular receptors. Some are over 3 mm long and half as wide and are visible to the naked eye as white, egg-shaped bodies. In section, a lamellar corpuscle resembles a cut onion. Its single dendrite is surrounded by a capsule containing up to 60 layers of collagen fibers and flattened supporting cells.

interoceptors

also called visceroceptors, respond to stimuli within the body, such as from the internal viscera and blood vessels. Interoceptors monitor a variety of stimuli, including chemical changes, tissue stretch, and temperature. Sometimes their activity causes us to feel pain, discomfort, hunger, or thirst. However, we are usually unaware of their workings.

Superficial Reflexes

are elicited by gentle cutaneous stimulation, such as that produced by stroking the skin with a tongue depressor. These clinically important reflexes depend both on functional upper motor pathways and on cord-level reflex arcs. The best known are the abdominal and plantar reflexes.

Receptors for the special senses (vision, hearing, equilibrium, smell, and taste)

are housed in complex sense organs.

tendon organs

are proprioceptors located in tendons, close to the junction between the skeletal muscle and the tendon. They consist of small bundles of tendon (collagen) fibers enclosed in a layered capsule, with sensory terminals coiling between and around the fibers. When muscle contraction stretches the tendon fibers, the resulting compression of the nerve fibers activates the tendon organs. This initiates a reflex that causes the contracting muscle to relax.

joint kinesthetic receptors

are proprioceptors that monitor stretch in the articular capsules that enclose synovial joints. This receptor category contains at least four receptor types: lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors resembling tendon organs. Together these receptors provide information on joint position and motion (kines = movement), a sensation of which we are highly conscious.

exteroceptors

are sensitive to stimuli arising outside the body (extero = outside), so most exteroceptors are near or at the body surface. They include touch, pressure, pain, and temperature receptors in the skin and most receptors of the special senses (vision, hearing, equilibrium, smell, and taste).

Tactile corpuscles or Meissner's corpuscles

are small receptors in which a few spiraling sensory terminals are surrounded by Schwann cells and then by a thin egg-shaped connective tissue capsule. Tactile corpuscles are found just beneath the epidermis in the dermal papillae and are especially numerous in sensitive and hairless skin areas such as the nipples, fingertips, and soles of the feet. They are receptors for discriminative touch, and apparently play the same role in sensing light touch in hairless skin that hair follicle receptors do in hairy skin.

sensory receptors

are specialized to respond to changes in their environment, which are called stimuli. Typically, activation of a sensory receptor by an adequate stimulus results in graded potentials that in turn trigger nerve impulses along the afferent PNS fibers coursing to the CNS. Sensation (awareness of the stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

motor areas

control voluntary movement, lie in the posterior part of the frontal lobes: primary motor cortex, premotor cortex, Broca's area, and the frontal eye field.

processing at the receptor level

generating a signal: for sensation to occur, a stimulus must excite a receptor and action potentials must reach the CNS. For this to happen: -The stimulus energy must match the specificity of the receptor. For example, a touch receptor may be sensitive to mechanical pressure, stretch, and vibration, but not to light energy (which is the province of receptors in the eye). The more complex the sensory receptor, the more specific it is. -The stimulus must be applied within a sensory receptor's receptive field—the area the receptor monitors. Typically, the smaller the receptive field, the greater the ability of the brain to accurately localize the stimulus site. -The stimulus energy must be converted into the energy of a graded potential, a process called transduction. This graded potential may be depolarizing or hyperpolarizing, similar to the EPSPs or IPSPs generated at postsynaptic membranes in response to neurotransmitter binding (p. 417). Receptors can produce one of two types of graded potentials. When the receptor region is part of a sensory neuron (as with free dendrites or the encapsulated receptors of most general sense receptors), the graded potential is called a generator potential because it generates action potentials in a sensory neuron. When the receptor is a separate cell (as in most special senses), the graded potential is called a receptor potentialbecause it occurs in a separate receptor cell. The receptor potential changes the amount of neurotransmitter released by the receptor cell onto the sensory neuron. The neurotransmitters then generate graded potentials in the sensory neuron. -Graded potentials in the first-order sensory neuron must reach threshold so that voltage-gated sodium channels on the axon are opened and nerve impulses are generated and propagated to the CNS.

When muscle tension increases substantially during contraction or passive stretching,

high-threshold tendon organs may be activated. Afferent impulses are transmitted to the spinal cord, and then to the cerebellum, where the information is used to adjust muscle tension. Simultaneously, motor neurons in spinal cord circuits supplying the contracting muscle are inhibited and antagonist muscles are activated, a phenomenon called reciprocal activation. As a result, the contracting muscle relaxes as its antagonist is activated.

An inborn (intrinsic) reflex

is a rapid, predictable motor response to a stimulus. It is unlearned, unpremeditated, and involuntary, and is built into our neural anatomy. Reflexes prevent us from having to think about all the little details of staying upright, intact, and alive—helping us maintain posture, avoid pain, and control visceral activities. many visceral reflexes are regulated by the subconscious lower regions of the CNS, specifically the brain stem and spinal cord.

The cerebral cortex

is at the highest level of our conscious motor pathways, but it is not the ultimate planner and coordinator of complex motor behaviors. The cerebellum and basal nuclei (ganglia) play this role and are therefore at the top of the motor control hierarchy. Motor control exerted by lower levels is mediated by reflex arcs in some cases, but complex motor activity, such as walking and swimming, depends on more complex patterns.

sharp pain

is carried by the smallest of the myelinated sensory fibers, the A delta fibers, while burning pain is carried more slowly by small nonmyelinated C fibers. Both types of fibers release the neurotransmitters glutamate and substance P, which activate second-order sensory neurons. Axons from these second-order neurons ascend to the brain via the spinothalamic tract and other pathways. you might not notice the pain - The brain has its own pain-suppressing analgesic system. The endogenous opioids such as endorphins and enkephalins play a key role in this system.

cerebral cortex

is the "executive suite" of the nervous system, where our conscious mind is found. It enables us to be aware of ourselves and our sensations, to communicate, remember, understand, and initiate voluntary movements. composed of gray matter: neuron cell bodies, dendrites, associated glia and blood vessels, but no fiber tracts. It contains billions of neurons arranged in six layers.

proprioceptors

like interoceptors, respond to internal stimuli. However, their location is much more restricted. Proprioceptors occur in skeletal muscles, tendons, joints, and ligaments and in connective tissue coverings of bones and muscles. (Some authorities include the equilibrium receptors of the inner ear in this class.) Proprioceptors constantly advise the brain of our body movements by monitoring how much the organs containing these receptors are stretched.

primary motor cortex

located in the precentral gyrus of the frontal lobe of each hemisphere. Large neurons, called pyramidal cells, in these gyri allow us to consciously control the precise or skilled voluntary movements of our skeletal muscles. -Their long axons, which project to the spinal cord, form the massive voluntary motor tracts called pyramidal tracts or corticospinal tracts. -All other descending motor tracts come from brain stem nuclei and consist of chains of two or more neurons. The entire body is represented spatially in the primary motor cortex of each hemisphere. Such mapping of the body in CNS structures is called somatotopy.

crossed-extensor reflex

often accompanies the flexor reflex in weight-bearing limbs and is particularly important in maintaining balance. It is a complex spinal reflex consisting of an ipsilateral withdrawal reflex and a contralateral extensor reflex. Incoming afferent fibers synapse with interneurons that control the flexor withdrawal response on the same side of the body and with other interneurons that control the extensor muscles on the opposite side.

perception of pain

pain is invaluable because it warns us of actual or impending tissue damage and motivates us to take protective action. Pain receptors are activated by extremes of pressure and temperature as well as a veritable soup of chemicals released from injured tissue.

Tendon organs help to prevent muscles and tendons from

tearing when they are subjected to potentially damaging stretching force. Tendon organs also function at normal muscle tensions, helping to ensure smooth onset and termination of muscle contraction.

The third pathway, the spinocerebellar pathway,

terminates in the cerebellum, and does not contribute to sensory perception.

During voluntary skeletal muscle contraction,

the muscle shortens. If the intrafusal muscle fibers didn't contract along with the extrafusal fibers, the muscle spindle would go slack and cease generating action potentials. At this point it would be unable to signal further changes in muscle length, so it would be useless. α-γ coactivation prevents this from happening. Descending fibers of motor pathways synapse with both α and γ motor neurons so that motor impulses are simultaneously sent to the large extrafusal fibers and to muscle spindle intrafusal fibers. Stimulating the intrafusal fibers maintains the spindle's tension (and sensitivity) during muscle contraction, so that the brain continues to be notified of changes in the muscle length.

Branches of the afferent fibers also synapse with interneurons that inhibit motor neurons controlling antagonistic muscles

the resulting inhibition is reciprocal inhibition. Consequently, the stretch stimulus causes the antagonists to relax so that they cannot resist the shortening of the "stretched" muscle caused by the main reflex arc.

three levels of motor control

the segmental level, projection level, and precommand level.

innervation of skeletal muscle

the terminals of somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells. As each axon branch reaches its target, which is a single muscle fiber, the ending splits into a cluster of axon terminalsthat branch treelike over the junctional folds of the sarcolemma of the muscle fiber. The axon terminals contain mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

The intrafusal muscle fibers have contractile regions at their ends

which are the only areas containing actin and myosin myofilaments. These regions are innervated by gamma (γ) efferent fibers that arise from small motor neurons in the ventral horn of the spinal cord. These γ motor fibers, which maintain spindle sensitivity, are distinct from the alpha (α) efferent fibers of the large alpha (α) motor neurons that stimulate the extrafusal muscle fibers to contract.

first order neurons

whose cell bodies reside in a ganglion (dorsal root or cranial), conduct impulses from the cutaneous receptors of the skin and from proprioceptors to the spinal cord or brain stem, where they synapse with second-order neurons. Impulses from the facial area are transmitted by cranial nerves, and spinal nerves conduct somatic sensory impulses from the rest of the body to the CNS.

four generalizations of the cerebral cortex

-The cerebral cortex contains three kinds of functional areas: motor areas, sensory areas, and association areas. As you read about these areas, do not confuse the sensory and motor areas of the cortex with sensory and motor neurons. All neurons in the cortex are interneurons. -Each hemisphere is chiefly concerned with the sensory and motor functions of the contralateral (opposite) side of the body. -Although largely symmetrical in structure, the two hemispheres are not entirely equal in function. Instead, there is lateralization (specialization) of cortical functions. -And finally, keep in mind that our approach is a gross oversimplification. No functional area of the cortex acts alone, and conscious behavior involves the entire cortex in one way or another.

three basic levels of neural integration in sensory systems

1. receptor level: sensory reception and transmission to the CNS. 2. circuit level: processing in ascending pathways. 3. perceptual level: processing in cortical sensory centers.

components of a reflex arc

1. receptor: the site of the stimulus action. 2. sensory neuron: transmits afferent impulses to the CNS. 3. integration center: in simple reflex arcs, the integration center may be a single synapse between a sensory neuron and. motor neuron. More complex reflex arcs involve multiple synapses with chains of interneurons. 4. motor neuron: conducts efferent impulses from the integration center to an effector organ. 5. effector: muscle fiber or gland cell that responds to the efferent impulses (by contracting or secreting).

the flexor and crossed-extensor reflexes

A painful stimulus initiates the flexor, or withdrawal, reflex, which causes automatic withdrawal of the threatened body part from the stimulus. Flexor reflexes are ipsilateral and polysynaptic, the latter a necessity when several muscles must be recruited to withdraw the injured body part. they override the spinal pathways and prevent any other reflexes from using them at the same time.

When a nerve impulse reaches an axon terminal,

ACh is released by exocytosis, diffuses across the fluid-filled synaptic cleft, and attaches to ACh receptors on the sarcolemma at the junction. ACh binding opens ligand-gated channels that allow both Na+ and K+ to pass. Because more Na+ enters the cell than K+ leaves, the muscle cell interior at that point depolarizes. The resulting graded potential is called an end plate potential.

the stretch reflex

By sending commands to the motor neurons, the brain essentially sets a muscle's length. The stretch reflex makes sure that the muscle stays at that length. The stretch reflex is important for maintaining muscle tone and adjusting it reflexively. It is most important in the large extensor muscles that sustain upright posture and in postural muscles of the trunk.

there are four key points in regard to spinal tracts and pathways:

Decussation: Most pathways cross from one side of the CNS to the other (decussate) at some point along their journey. Relay: Most pathways consist of a chain of two or three neurons (a relay) that contribute to successive tracts of the pathway. Somatotopy: Most pathways exhibit somatotopy, a precise spatial relationship among the tract fibers that reflects the orderly mapping of the body. For example, fibers transmitting pain and temperature information from sensory receptors in superior body regions lie medial to those from inferior body regions within the same tract. Symmetry: All pathways and tracts are paired symmetrically (right and left), with a member of the pair present on each side of the spinal cord or brain.

Broca's area

Lies anterior to the inferior region of the premotor area. Considered to be (1) present in one hemisphere only (usually the left) and (2) a special motor speech area that directs the muscles involved in speech production. Broca's area also becomes active as we prepare to speak and even as we think about (plan) many voluntary motor activities other than speech.

the reflex arc enables rapid and predictable responses

Many of the body's control systems are reflexes, which can be either inborn or learned.

Heat or cold outside the range of thermoreceptors activates nociceptors and is perceived as painful

Nociceptors also respond to pinch and chemicals released from damaged tissue. A key player in detecting painful stimuli is a plasma membrane protein called the vanilloid receptor.

receptors, ascending pathways, and cerebral cortex process sensory information

Our survival depends not only on sensation (awareness of changes in the internal and external environments) but also on perception (conscious interpretation of those stimuli).

classification by location

Receptors can be grouped into three receptor classes according to either their location or the location of the activating stimulus: exteroceptors, interoceptors, and proprioceptors.

processing at the perceptual level

Sensory input is interpreted in the cerebral cortex. The ability to identify and appreciate sensations depends on the location of the target neurons in the sensory cortex, not on the nature of the message. The brain always interprets the activity of a specific sensory receptor ("who") as a specific sensation, no matter how it is activated.

the tendon reflex

Stretch reflexes cause muscle contraction in response to increased muscle length (stretch). The polysynaptic tendon reflexes, on the other hand, produce exactly the opposite effect: Muscles relax and lengthen in response to tension.

abdominal reflexes

Stroking the skin of the lateral abdomen above, to the side, or below the umbilicus induces a reflex contraction of the abdominal muscles in which the umbilicus moves toward the stimulated site. These reflexes, called abdominal reflexes, check the integrity of the spinal cord and ventral rami from T8 to T12.

ascending pathways to the brain

The ascending pathways conduct sensory impulses upward, typically through chains of three successive neurons (first-, second-, and third-order neurons) to various areas of the brain.

In general, the pathways to and from the head are similar to those of the trunk and limbs, with two exceptions:

The axons of the tracts servicing the head are located in cranial nerves. The cell bodies are located in the brain stem rather than in the spinal cord.

descending pathways and tracts

The descending pathways that deliver efferent impulses from the brain to the spinal cord are divided into two groups: (1) the direct pathways, which are the pyramidal tracts, and (2) the indirect pathways, essentially all others.

indirect pathways

The indirect pathways include brain stem motor nuclei and all motor pathways except the pyramidal pathways. These pathways were formerly lumped together as the extrapyramidal system because their nuclei of origin were presumed to be independent of ("extra to") the pyramidal tracts. pyramidal tract neurons are now known to project to and influence the activity of most "extrapyramidal" nuclei, so modern anatomists refer to them as indirect, or multineuronal, pathways, or simply use the names of the individual motor pathways.

classification by receptor structure

The overwhelming majority of sensory receptors belong to the general senses and are simply the modified dendritic endings of sensory neurons. They are found throughout the body and monitor most types of general sensory information.

Motor pathways involve two neurons, referred to as the upper and lower motor neurons:

Upper motor neurons are the pyramidal cells of the motor cortex and the neurons of subcortical motor nuclei. Lower motor neurons are the ventral horn motor neurons. These directly innervate the skeletal muscles (their effectors).

visceral and referred pain

Visceral pain results from noxious stimulation of receptors in the organs of the thorax and abdominal cavity. The fact that visceral pain afferents travel along the same pathways as somatic pain fibers helps explain the phenomenon of referred pain, in which pain stimuli arising in one part of the body are perceived as coming from another part.

pain tolerance

We all have the same pain threshold—that is, we begin to perceive pain at roughly the same stimulus intensity. However, our tolerance to pain varies widely. When we say that someone is "sensitive" to pain, we mean that the person has a low pain tolerance rather than a low pain threshold. A number of genes help determine a person's pain tolerance and response to pain medications. The genetics of pain is currently an area of intense research, aimed at allowing an individual's genes to determine the best pain treatment.

autonomic (visceral) reflexes

activate visceral effectors (smooth or cardiac muscle or glands).

second-order neurons

cell bodies reside in the dorsal horn of the spinal cord or in medullary nuclei. They transmit impulses to the thalamus or to the cerebellum where they synapse.

sensory receptors are activated by

changes in the internal or external environment

Encapsulated Nerve Endings

consist of one or more fiber terminals of sensory neurons enclosed in a connective tissue capsule. Virtually all encapsulated receptors are mechanoreceptors, but they vary greatly in shape, size, and distribution in the body. They include tactile corpuscles, lamellar corpuscles, bulbous corpuscles, muscle spindles, tendon organs, and joint kinesthetic receptors.

The autonomic synaptic vesicles typically contain

either acetylcholine or norepinephrine, both of which act indirectly on their targets via second messengers. Consequently, visceral motor responses tend to be slower than those induced by somatic motor fibers, which directly open ion channels.

third-order synapses

have cell bodies in the thalamus. They relay impulses to the somatosensory cortex of the cerebrum. (There are no third-order neurons in the cerebellum.)

frontal eye field

located partially in and anterior to the premotor cortex and superior to Broca's area. This cortical region controls voluntary movement of the eyes.

peripheral motor endings connect nerves to their effectors

motor endings, the PNS elements that activate effectors by releasing neurotransmitters.

bulbous corpuscles

or Ruffini endings, which lie in the dermis, subcutaneous tissue, and joint capsules, contain a spray of receptor endings enclosed by a flattened capsule. They bear a striking resemblance to tendon organs (which monitor tendon stretch) and probably play a similar role in other dense connective tissues where they respond to deep and continuous pressure.

muscle spindles

re fusiform (spindle-shaped) proprioceptors found throughout the perimysium that wraps individual fascicles of skeletal muscle. Each muscle spindle consists of a bundle of modified skeletal muscle fibers, called intrafusal fibers, enclosed in a connective tissue capsule. Muscle spindles detect muscle stretch and initiate a reflex that resists the stretch.

learned (acquired) reflex

results from practice or repetition.

The end plate potential

spreads to adjacent areas of the membrane where it triggers the opening of voltage-gated sodium channels. This event causes an action potential to propagate along the sarcolemma, which stimulates the muscle fiber to contract. The synaptic cleft at neuromuscular junctions contains acetylcholinesterase, the enzyme that breaks down ACh.

The motor innervation of the body is contralateral:

the left primary motor gyrus controls muscles on the right side of the body, and vice versa.

the segment level

the lowest level of the motor hierarchy. consists of reflexes and spinal cord circuits that control automatic movements. A segmental circuit activates a network of ventral horn neurons in a group of cord segments, causing them to stimulate specific groups of muscles. Circuits that control locomotion and other specific and oft-repeated motor activities are called central pattern generators (CPGs). CPGs consist of networks of oscillating inhibitory and excitatory neurons, which set crude rhythms and alternating patterns of movement.

reflexes are classified functionally as somatic reflexes if

they activate skeletal muscle

Cells in both the basal nuclei and the cerebellum are involved in

unconscious planning and discharge in advance of willed movements (which is why this is called the precommand level). The precommand areas control the motor cortex and provide its readiness to initiate a voluntary act. The conscious cortex then chooses to act or not act, but the groundwork has already been laid.

Overall, the reticulospinal and vestibulospinal tracts maintain balance by

varying the tone of postural muscles. The rubrospinal tracts control flexor muscles, whereas the tectospinal tracts and the superior colliculi mediate head movements in response to visual stimuli.

how stretch reflex works

when stretch activates sensory neurons of muscle spindles, they transmit impulses at a higher frequency to the spinal cord. There the sensory neurons synapse directly with a motor neurons, which rapidly excite the extrafusal muscle fibers of the stretched muscle. The reflexive muscle contraction that follows resists further muscle stretching.