Chapter 16: Sensory, Motor, and Integrative Systems

Question 1

define sensation and discuss the components of sensation.

Answer 1

Sensation – the conscious or subconscious awareness of changes in the external or internal environment.

Perception – the conscious interpretation of sensations and is primarily a function of thecerebral cortex. No perception of some sensory information because it never reaches the cerebral cortex. Ex. Nerve impulses for BP propagate to the cardiovascular center in the medulla oblongata, so BP is not consciously perceived.

sensory modality – any of the specific sensory entities, such as vision, smell, taste, touch. AKA – Each unique type of sensation. A given sensory neuron carries info only for one sensory modality. Touch receptors do not transmit pain impulses, and vision impulses do not transmit sound.

general senses – refer to both somatic and visceral sensessomatic senses – include tactile sensations (touch, pressure, vibration, itch, tickle), thermal sensations (warm, cold), pain sensations, and proprioceptive sensations (which allow perception of both the static position of limbs ad body parts and movements of the limbs and head).

visceral senses – provide info about conditions within internal organs, Ex. Pressure, stretch, chemicals, nausea, hunger, temperature

special senses – include the sensory modalities of smell, taste, vision, hearing, and balance. process of sensation – begins in a sensory receptor (either specialized cell or dendrites of a sensory neuron) stimulus – a change in the environment that can activate certain sensory receptors.

Selectivity – characteristic of sensory receptors where a given sensory receptor responds vigorously to one particular kind of stimulus but weakly or not at all to any other stimulus.

4 steps for a sensation to arise:

• Stimulation of sensory receptor
• Transduction of the stimulus – sensory receptor transduces (converts) energy in a stimulus into a graded potential.
• Generation of nerve impulses – when a graded potential in a sensory neuron reaches threshold, it triggers one or more nerve impulses, which then propagate toward the CNS. Sensory neurons that conduct impulses from the PNS into the CNS are called first-order neurons
• Integration of sensory input – a particular region of the CNS receives and integrates the sensory nerve impulses. Conscious sensations or perceptions are integrated in the cerebral cortex .Senses seem to occur from the specific location (sight from eyes, sound from ears, pain from injury) because the impulses arrive in a specific region of the cerebral cortex which interprets the sensation as coming from the stimulated sensory receptors located there

Somatic sensory neuronshave cell bodies in the spinal cord or brain stem and axons terminating in neuromuscular junction.

Question 2

describe the different ways to classify sensory receptors.

Answer 2

1. types of sensory receptors based on microscopic features:

free nerve endings – of first-order sensory neurons. Bare dendrites. Lack any structural specialization. Ex. Receptors for pain, temp, tickle, itch, and some touch sensations

encapsulated nerve endings – of first-order sensory neurons. Dendrites enclosed in a connective tissue capsule that has a distinctive microscopic structure, ex. Lamellated corpuscles. Ex. Receptors for pressure, vibration, and some touch sensations. is modified in a manner that helps enhance the sensitivity of the receptor

separate cells – that synapse with first-order sensory neurons. Ex sensory receptors for special senses: hair cells for hearing and equilibrium in the inner ear, gustatory receptor cells in taste buds, and photoreceptors in the retina of the eye for vision

.generator potential – in response to a stimulus, produced by dendrites of free nerve endings encapsulated nerve endings, and the receptive part of olfactory receptors .When the generator potential is large enough to reach threshold, it triggers one or more nerve impulses in the axon of a first-order sensory neuron which propagates into the CNS. Generator potentials generate action potentials.

receptor potential – Produced by sensory receptors that are separate cells. Receptor potentials trigger release of neurotransmitter through exocytosis of synaptic vesicles. The released neurotransmitter diffuse across the synaptic cleft and produce a postsynaptic potential (PSP) in the first-order neuron. In turn, the PSPs may trigger one or more nerve impulses which propagate along the axon into the CNS. Amplitude of both kinds of potentials varies with intensity of the stimulus and large potentials trigger nerve impulses at high frequency and smaller potentials trigger nerve impulses at a lower frequency.

2. based on location and activating stimuli:

exteroceptors – located at or near the external surface of the body Sensitive to stimuli originating outside the body and provide the body with info about the external environment. Ex. Hearing, vision, smell, taste, touch, pressure, vibration, temp, and pain

Interoceptors – AKA visceroceptors – located in blood vessels, visceral organs, muscles, and the nervous system Monitor conditions in the internal environment Nerve impulses produced are usually not consciously perceived however activation of interceptors by strong stimuli may be felt as pain or pressure

Proprioceptors – located in muscles, tendons, joints, and the inner ear Provide information about body position, muscle length and tension, position and movement of joints.

3. based on type of stimulus detected:

mechanoreceptors – sensitive to mechanical stimuli such as the deformation, stretching or bending of cells. Provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. Also monitor stretching of blood vessels and internal organs.

Thermoreceptors – temperature changes Nociceptors – respond to painful stimuli resulting from physical or chemical damage to tissue

Photoreceptors – detect light that strikes the retina of the eye

Chemoreceptors – detect chemicals in the mouth (taste), nose (smell), and body fluids

Osmoreceptors – detect the osmotic pressure of body fluids adaptation in sensory receptors

adaptation – a characteristic of most sensory receptors in which the generator or receptor potential decreases in amplitude during a maintained, constant stimulus. Causes the frequency of nerve impulses in the first-order neuron to decrease. Therefore, the perception of a sensation may fade or disappear even though the stimulus persists. Ex, step into hot shower, water initially feels very hot, but soon the sensation decreases to one of comfortable warmth even though water temp didn’t change.

rapidly adapting receptor or phasic receptor – specialized for signaling changes in a stimulus. a. Ex. Receptors for pressure, touch, and smell.slowly adapting receptor or tonic receptor – continue to trigger nerve impulses as long as the stimulus persists a. Ex. Monitor stimuli associated with pain, body position, and chemical composition of the blood.

Question 3

describe the location and function of the somatic sensory receptors for tactile, thermal, and pain sensations.

Answer 3

1. somatic sensations – arise from stimulation of sensory receptors embedded in the skin or subcutaneous layer, in mucous membranes in the mouth, vagina, and anus, muscles, tendons, and joints, and in the inner ear. Distributed unevenly. Highest density in tip of tongue, lips, fingertips.
2. cutaneous sensations – somatic sensations that arise from stimulating the skin surface
3. tactile sensations – include touch, pressure, vibration, itch, and tickle. - arise by activation of some of the same types of receptors. Examples: encapsulated mechanoreceptors attached to large diameter myelinated A fibers for touch, pressure, vibration. Free nerve endings attached to small diameter unmyelinated C fibers for itch and tickle.
4. Touch
5. corpuscle of touch or Meissner corpuscle – rapid adapting. Located in dermal papillae of skin. Egg shaped mass of dendrites enclosed by a capsule of connective tissue. Generate nerve impulses mainly at the onset of a touch. Abundant in fingertips, hands, eyelids, tip of tongue, lips, nipples, soles, clitoris. tip of penis.
6. hair root plexus – rapid adapting. Found in hairy skin. Consist of free nerve endings wrapped around hair follicles. Detect movement on the skin surface that disturb hairs.
7. type I cutaneous mechanoreceptor or Merkel disc – slow adapting. AKA tactile discs. Saucer-shaped flattened free nerve endings that make contact with tactile epithelial cells of the stratum basale. Plentiful in fingertips, hands, lips, external genitalia.
8. type II cutaneous mechanoreceptor or Ruffini corpuscle – elongated encapsulated receptors deep in the dermis and in ligaments and tendons. Present in hands, abundance on soles, most sensitive to stretching that occurs as digits or limbs are moved.
9. Pressure – sustained sensation felt over a larger area than touch, occurs with deformation of deeper tissues. Receptors include corpuscles of touch, type I cutaneous mechanoreceptors, and lamellated corpuscles
   * lamellated or Pacinian corpuscle – rapid adapting. Large oval structure, composed of a multilayered connective tissue capsule that encloses a dendrite. Widely distributed in the body: dermis and subcut layer, submucosal tissues that underlie mucous and serous membranes, around joints, tendons, and muscles, in the periosteum, mammary glands, external genitalia, certain viscera, such as pancreas and urinary bladder.
10. Vibration – result from rapidly repetitive sensory signals from tactile receptors. Receptors are corpuscles of touch (lower-frequency vibrations) and lamellated corpuscles (higher frequency vibrations).
11. itch and tickle – itch from stimulation of free nerve endings from certain chemicals (ex. Mosquito bite) often because of a local inflammatory response. Tickle thought to be free nerve endings. Only occurs when someone else does it. Thought to be because impulses that conduct to and from the cerebellum when you are moving your own fingers and touching yourself that don’t occur when someone else is touching you.
12. phantom limb sensation – when a limb is amputated but the person still experiences sensations such as itching, pressure, tingling, or pain as if it were still there.

• Severed endings of sensory axons still present in the remaining stump. If activated, cerebral cortex interprets the sensation as coming from the sensory receptors in the non-existing limb
• Also thought to be caused by the cerebral cortex remapping to respond to stimuli from another body part. Might give rise to false sensory perceptions from the missing limb.

1. thermal sensations – coldness and warmth
2. Thermoreceptor – free nerve endings that have receptive fields about 1mm in diameter on the skin surface
3. cold receptor – located in the stratum basale of epidermis and attached to medium diameter myelinated A fibers (but a few attach to small diameter unmyelinated C fibers. Activated by temps from 10-40 degrees Celsius.
4. warm receptor – not as abundant as cold receptors. Located in dermis. Attached to small diameter unmyelinated C fibers. Activated by temps between 32-38 degrees Celsius.

• Temps below 10 and above 38 Celsius usually stimulate pain receptors, not thermoreceptors.
  1. Pain – signals the presence of noxious, tissue-damaging conditions. Subjective description and location may pinpoint underlying cause of disease.

1. Nociceptor – pain receptor. Free nerve endings found in every tissue in the body except the brain. Can be stimulated by intense thermal, mechanical or chemical stimuli. Tissue irritation or injury releases chemicals such as prostaglandins, kinins, and K+ ions that stimulate nociceptors. Pain can persist after painproducing stimulus is removed because pain-mediating chemicals linger and nociceptors exhibit very little adaptation. Pain can be caused by excessive distension of a structure, prolonged muscular contraction, muscle spasms, or Ischemia (inadequate blood flow to an organ) responds to stimuli resulting from physical or chemical damage to tissue
2. types of pain – two types: fast and slow
3. fast pain – perception occurs very rapidly, within 0.1 second after stimulus applied/ The nerve impulses propagate along medium diameter myelinated A fibers. AKA acute, sharp, or pricking pain. Not felt in deeper tissues of the body.
4. slow pain – begins a second or more after a stimulus is applied. Gradually increases in intensity over a period of several seconds or minutes. Impulses conduct along small diameter unmyelinated C fibers. May be excruciating, described as chronic, burning, aching, or throbbing. Can occur in both skin and deeper tissue or internal organs. Ex toothache.
5. superficial somatic pain – pain that arises from stimulation of receptors in the skin.
6. deep somatic pain – pain caused by stimulation of receptors in skeletal muscles, joints, tendons, and fascia.
7. visceral pain – stimulation of nociceptors in visceral organs. Can be severe if stimulation is diffuse (large area). Ex kidney stone distends a ureter, gallstone distends bile duct.
   
   • localization of pain
8. Fast pain is very localized to the stimulated area. Ex. Pin prick.
9. referred pain – pain that is felt at a site remote from the place of origin.
10. Ex visceral pain: felt in or just deep to the skin that overlies the stimulated organ, or in a surface area far away from the stimulated organ.
11. Generally, the visceral organ affected and area to which pain is referred are served by the same segment of the spinal cord. Ex. Heart attack can = pain in skin over heart and left arm.

Question 4

identify the receptors for proprioception and describe their functions.

Answer 4

proprioceptive sensations – AKA proprioception. Allow us to recognize that parts of our body belong to us. Body part locations and movements.

I. Kinesthesia – the perception of body movements. Made possible by nerve impulses generated by proprioceptors.

II. Proprioceptor – a receptor for proprioceptive sensations. Embedded in muscles (esp postural muscles). Tell us degree of muscle contraction, tension on tendons, position of joints.

• Adapt slowly, and only slightly.
• Brain constantly receives nerve impulses related to position of body parts, makes corrections for coordination.
• Allow weight discrimination: ability to determine heaviness of an object and amount of effort needed

muscle spindle – proprioceptors in skeletal muscles that monitor changes in length of skeletal muscles and participate in stretch reflexes. Interspersed in most skeletal muscles, parallel to them.

• Many in fine motor control muscles, ex. Fingers. Fewer in coarser, forceful movements, such as thigh muscles. Only muscle with no muscle spindles are tiny muscles of middle ear.
• Main function of muscle spindle: measure muscle length – stretch of a muscle.
• Sudden or prolonged stretching of central areas of the intrafusal muscle fibers stimulates the sensory nerve endings and propagate nerve impulses to CNS, arriving quickly at the somatic sensory areas of the
• cerebral cortex, allowing conscious perception of limb positions and movements.
• At the same time, impulses from muscle spindles pass to the cerebellum, where the input is used to coordinate muscle contractions.

muscle tone – Small degree of contraction that is present while the muscle is at rest. Set by brain by adjusting muscle spindle response to stretching

intrafusal muscle fibers – Make up a muscle spindle. 3-10 specialized muscle fibers, partially enclosed in a spindle shaped connective tissue capsule. Anchored to the endo and perimysium.

gamma motor neurons – motor neurons contained in muscle spindles that terminate near both ends of the intrafusal fibers and adjust the tension in a muscle spindle to variations in the length of the muscle. Maintain sensitivity of the muscle spindle to stretching of the muscle

extrafusal muscle fibers – ordinary skeletal muscle fibers surrounding muscle spindles.

alpha motor neurons – large diameter A fibers that supply the extrafusal muscle fibers. Cell bodies of both gamma and alpha motor neurons are located in the anterior gray horn of the spinal cord.

tendon organ – a proprioceptive receptor, sensitive to changes in muscle tension and force of contraction. Slow adapting

• Protect tendons and associated muscles from excessive tension.
• Consists of a thin capsule of connective tissue that encloses a few tendon fascicles (bundles of collagen fibers) with one or more sensory nerve endings entwined around and among the collagen fibers
• Tension applied to muscle = tendon organ generate nerve impulses to CNS = tendon reflexes that decrease muscle tension by relaxing the muscle.
  
  • Free nerve endings and type II cutaneous mechanoreceptors in the joint capsules respond to pressure.

joint kinesthetic receptor – proprioceptive receptor located in a synovial joint, stimulated by joint movement.

• Small lamellated corpuscles in the connective tissue outside articular capsules respond to acceleration and deceleration of joints during movement
• Joint ligaments contain receptors similar to tendon organs that adjust reflex inhibition of the adjacent muscles when excessive strain is placed on the joint.

Question 5

describe the general components of a sensory pathway.

Answer 5

somatic sensory pathways - relay info from somatic sensory receptors to primary somatosensory area in the cerebral cortex and to the cerebellum. Consist of 1000’s of sets of three neurons:

sets of three neurons:

a. first order neurons – conduct impulses from somatic receptors into the brainstem or spinal cord.

• From the face, mouth, teeth and eyes, somatic sensory impulses propagate along cranial nerves into brain stem.
• From neck, trunk, limbs, and posterior aspect of head, somatic sensory impulses propagate along spinal nerves into the spinal cord.

b. second order neurons – conduct impulses from the brain stem and spinal cord to the thalamus.

Axons of second-order neurons decussate (cross over to opposite side) in the brain stem or spinal cord before ascending to the ventral posterior nucleus of the thalamus. Thus all somatic sensory info from one side of the body reaches the thalamus on the opposite side.

c. third order neurons – conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side.
d. relay stations – regions within the CNS where neurons synapse with other neurons that are a part of a particular sensory or motor pathway.

• Ex. Neurons of many sensory pathways synapse with neurons in the thalamus; therefore the thalamus functions as a major relay station.
• Many other regions of CNS, including brain stem and spinal cord, can function as relay stations.

Question 6

describe the neuronal components and functions of the posterior column-medial lemniscus, anterolateral, trigeminothalamic, and spinocerebellar pathways.

Answer 6

Somatic Sensory impulses ascend to the cerebral cortex via 3 general pathways:

posterior column-medial lemniscus pathway to the cortex – nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head ascend to the cerebral cortex along the posterior column-medial lemniscus pathway.

• Pathway named from the two white-matter tracts that convey the impulses: the posterior column of the spinal cord and medial lemniscus of brain stem.
• First-order neurons in this pathway extend from sensory receptors in limbs, trunk, neck, and posterior head into the spinal cord and ascend to the medulla oblongata on the same side of the body.
• The cell bodies of these first-order neurons are in the posterior root ganglia of spinal nerve.
• In the spinal cord, their axons form the posterior columns, which consist of two parts:
  
  • Gracile fasciculus – nerve impulses for touch, pressure, and vibration from the lower limbs and lower trunk propagate along axons in the gracile fasciculus and arrive at the gracile nucleus
  • Cuneate fasciculus – Nerve impulses for touch, pressure, vibration, and conscious proprioception from upper limbs, upper trunk, neck, and posterior head propagate along axons in the cuneate fasciculus and arrive at the cuneate nucleus.
• The axons synapse with the dendrites of second-order neurons whose cell bodies are located in the gracile nucleus or cuneate nucleus of the medulla.
• The axons of second-order neurons cross to the opposite side of the medulla and enter the medial lemniscus Thin ribbonlike projection tract that extends from the medulla to the ventral posterior nucleus of the thalamus.
• In the thalamus, axon terminals of second-order neurons synapse with third-order neurons, which project their axons to the primary somatosensory area of the cerebral cortex.
  
  • This pathway is also composed of three-neuron sets.

anterolateral or spinothalamic pathways to the cortex – nerve impulses for pain, temp, itch, and tickle from the limbs, trunk, neck, and posterior head ascend to the cerebral cortex along the anterolateral pathway.

• First order neurons connect a receptor of the limbs, trunk, neck, or posterior head with the spinal cord.
• The cell bodies of the first order neurons are in the posterior root ganglion.
• The axons terminals of the first order neurons synapse with second order neurons whose cell bodies are located in the posterior gray horn of the spinal cord.
• The axons of the second order neurons cross to the opposite side of the spinal cord and then pass upward to the brain stem as the spinothalamic tract.
• The axons of second order neurons end in the ventral posterior nucleus of the thalamus, where they synapse with third order neurons.
• The axons of the third order neurons project to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.

trigeminothalamic pathway and tract – nerve impulses for most somatic sensations from the face, nasal cavity, oral cavity, and teeth ascend to the cerebral cortex along the trigeminothalamic pathway.

• First order neurons extend from somatic sensory receptors in the face, nasal cavity, oral cavity, and teeth into the pons through the trigeminal (V) nerves.
• The cell bodies of these first order neurons are in the trigeminal ganglion.
• The axon terminals of some first order neurons synapse with second order neurons in the pons.
• The axons of other first order neurons descend into the medulla to synapse with second order neurons
• The axons of second order neurons cross to the opposite side of the pons and medulla and then ascend as the trigeminothalamic tract to the ventral posterior nucleus of the thalamus.
• In the thalamus, axon terminals of second order neurons synapse with third order neurons which project their axons to the primary somatosensory area on the same side of the cerebral cortex as the thalamus.

somatic sensory pathways to the cerebellum

• Two tracts in the spinal cord are the major routes proprioceptive impulses take to reach the cerebellum: posterior spinocerebellar tract and anterior spinocerebellar tract.
• Not consciously perceived but sensory impulses conveyed to the cerebellum along these two pathways are critical for posture, balance, and coordination of skilled movements.

Question 7

explain the basis for mapping the primary somatosensory area.

Answer 7

mapping of the somatosensory area (located in the postcentral gyrus – area behind the central gyrus (fissure) in the brain)

• Specific areas of the cerebral cortex receive somatic sensory input from particular parts of the body
• Precise localization of somatic sensations occurs when nerve impulses arrive at the primary somatosensory area which occupies the postcentral gyri of the parietal lobes of the cerebral cortex.
• Not all body parts represented equally: the more sensory receptors, the larger the area in the cerebral cortex that is dedicated to that body part Lips = large amount sensory receptors; Trunk = small amount sensory receptors.
• Sensory homunculus – distorted somatic sensory map of the body.

Question 8

identify the locations and functions of the different types of neurons that regulate lower motor neurons.

Answer 8

somatic motor pathways – pathway that carries info from the cerebral cortex, basal nuclei, and cerebellum that stimulates contraction of skeletal muscles. Consist of 4 distinct but highly interactive neural circuits.

lower motor neuron (LMN) - motor neurons that extend out of the brain stem and spinal cord to innervate skeletal muscles in the body.

• Have their cell bodies in the brain stem and spinal cord
• From the brain stem, axons of LMN’s extend through cranial nerves to innervate skeletal muscles of the face and head.
• From the spinal cord, axons of LMN’s extend through spinal nerves to innervate skeletal muscles of the limbs and trunk.
• Only LMNs provide output from the CNS to skeletal muscle. For this reason, AKA final common pathway.

local circuit neurons – interneurons located near motor neuron cell bodies in the brain stem and spinal cord.

• Input arrives at lower motor neurons from these local circuit neurons close by.
• Receive input from somatic sensory receptors, such as nociceptors and muscle spindles, as well as from higher brain centers.
• Help coordinate rhythmic activity in specific muscle groups, such as alternating flexion and extension of the lower limbs during walking.

upper motor neuron (UMN) - send input to local circuit and lower motor neurons.

• Most upper motor neurons synapse with local circuit neurons which in turn synapse with lower motor neurons (a few upper synapse directly with lower)
• UMNs from the cerebral cortex are essential for the execution of voluntary movements of the body.
• Other UMNs originate in motor centers of the brain stem: the red nucleus, the vestibular nucleus, the superior colliculus, and the reticular formation.
• UMNs from the brain stem regulate muscle tone, control postural muscles, and help maintain balance and orientation of the head and body.
• Both the basal nuclei and cerebellum exert influence on UMNs
• Basal Nuclei neurons – assist movement by providing input to UMNs. Neural circuits interconnect to basal nuclei with motor areas of the cerebral cortex (via the thalamus) and the brain
• stem. There circuits help initiate and terminate movements, suppress unwanted movements, and establish a normal level of muscle tone.

Cerebellar Neurons – aid movement by controlling activity of UMN. Neural circuits interconnect the cerebellum with motor areas of the cerebral cortex (via the thalamus) and the brain stem. A prime function of the cerebellum is to monitor difference between intended movements and movements actually performed. Then it issues commands to UMN to reduce errors in movements. The cerebellum coordinates body movements and helps maintain normal posture and balance.

primary motor area (located in the precentral gyrus) - region of cerebral cortex that controls specific muscles or groups of muscles.

• A major control region for the execution of voluntary movements.
• Different muscles are represented unequally in the primary motor area.
• More area devoted to muscles involved in skilled, complex, or delicate movements.
• This distorted map of the body is called the motor homunculus. Similar but not identical somatosensory and somatic motor representation for body parts

Question 9

explain how the cerebral cortex, brainstem, basal nuclei, and cerebellum contribute to body movement.

Answer 9

roles of the basal nuclei – play major role in initiation and termination of movements

• Caudate nucleus and the putamen – two parts of the basal nuclei that receive input from sensory, association, and motor areas of the cerebral cortex and from the substantia nigra
• Output from the basal nuclei comes from the globus pallidus and substantia nigra, which send feedback signals to the upper motor cortex by way of the thalamus.
• This circuit, from cortex to basal nuclei to thalamus to cortex, appears to function in initiating and terminating movements.
• Neurons in the putamen generate impulses just before body movements occur and neurons in the caudate nucleus generate impulses just before eye movements occur
• The basal nuclei suppress unwanted movements by their inhibitory effects on the thalamus and superior colliculus.
• The basal nuclei influence muscle tone. The globus pallidus sends impulses into the reticular formation that reduce muscle tone. Damage or destruction of some basal nuclei connections causes a generalized increase in muscle tone.
• The basal nuclei influence many aspects of cortical function, including sensory, limbic, cognitive, and linguistic functions. Ex. The basal nuclei help initiate and terminated some cognitive processes, such as attention, memory, and planning. In addition, the basal nuclei may act with the limbic system to regulate emotional behaviors.
• Disorders of the basal nuclei include Parkinson’s disease, Huntington disease (inherited) and Tourette Syndrome (unknown cause). Also schizophrenia and OCD involve dysfunction between neural circuits and basal nuclei and limbic system.

roles of the cerebellum – maintains proper balance and posture

Also active in learning and performing rapid, coordinated highly skilled movements. Ex. Hitting golf ball, speaking, swimming.

Cerebellar function involves 4 activities:

1. Monitoring intentions for movements - the cerebellum receives nerve impulses from the motor cortex and basal nuclei via the pontine nuclei in the pons regarding what movements are planned.
2. Monitoring actual movement - the cerebellum receives input from proprioceptors in joints and muscles that reveals what is actually happening. These nerve impulses travel in the anterior and posterior spinocerebellar tracts. Nerve impulses from the vestibular (equilibrium sensing) apparatus in the inner ear and from the eyes also enter the cerebellum
3. Comparing command signals with sensory information – the cerebellum compares intentions for movement with the actual movement performed.
4. Sending out corrective feedback – if there is a discrepancy between intended and actual movement, the cerebellum sends feedback to upper motor neurons. This information travels via the thalamus to UMNs in the cerebral cortex but goes directly to UMNs in brain stem motor centers. As movements occur, the cerebellum continuously provides error corrections to upper motor neurons, which decreases errors and smooths the motion. Over longer periods, it also contributes to learning new motor skills. Ex. Tennis – to connect with ball, you must move arm but stop at exactly the right time. Before you even hit the ball, the cerebellum has sent nerve impulses to the cerebral cortex and basal nuclei informing them where your swing must stop. In response to cerebellar impulses, the cortex and basal nuclei transmit motor impulses to opposing body muscles to stop the swing.

Question 10

compare the locations and functions of the direct and indirect motor pathways.

Answer 10

final common pathway – lower motor neurons which send all input to the skeletal muscles. Receive input from upper motor neurons from cerebral cortex and upper motor neurons from brain stem.

direct motor pathways or pyramidal pathways

– consist of axons that descend from pyramidal cells.

• Pyramidal cells – upper motor neurons with pyramid shaped cell bodies located in the primary motor area and the premotor area of the cerebral cortex.
• Direct motor pathways consist of the corticospinal pathways and the corticobulbar pathway.

1. Corticospinal pathways – conduct impulses for the control of muscles of the limbs and trunk.

1. Corticospinal tracts – formed by axons of upper motor neurons in the cerebral cortex

• Descend through the internal capsule of the cerebrum and the cerebral peduncle of the midbrain.
• In the medulla oblongata, the axons bundles of the corticospinal tracts form the ventral bulges knows as the pyramids.
• 90% of corticospinal axons cross over (decussate) in the medulla oblongata and then descend into the spinal cord where they synapse with a local circuit neuron or a lower motor neuron
• The remaining 10% eventually cross over at the spinal cord levels where they synapse with a local circuit neuron or lower motor neuron.

2 types of corticospinal tracts – the lateral corticospinal tract and the anterior corticospinal tract

a. Lateral corticospinal tract – formed by the corticospinal axons that decussate in the medulla

• Located in the lateral white column of spinal cord
• Axons synapse with local circuit neurons or lower motor neurons in the anterior gray horn of the spinal cord.
• Axons of these lower motor neurons exit the spinal cord in the anterior roots of spinal nerves and terminate in distal limb skeletal muscles These distal muscles are responsible for precise, agile, and highly skilled movements of the hands and feet.
• Anterior corticospinal tract – formed by axons that do not decussate in the medulla
• Located in the anterior white column of the spinal cord
• At each spinal cord level, some axons decussate via the anterior white commissure.
• Then they synapse with local circuit neurons or lower motor neurons in the anterior gray horn.
• Axons of these lower motor neurons exit the spinal cord in the anterior roots of spinal nerves.
• They terminate in skeletal muscles that control movements of the trunk and proximal parts of the limbs

2. Corticobulbar Pathway – conduct impulses for the control of skeletal muscles in the head.

Corticobulbar tract – formed by axons of upper motor neurons from the cerebral cortex which descend along with the corticospinal tracts through the internal capsule of the cerebrum and cerebral peduncle of the midbrain. Some axons of the corticobulbar tract decussate, others do not.

The axons terminate in the motor nuclei of 9 pairs of cranial nerves in the brain stem:

• Oculomotor III
• Trochlear IV
• Trigeminal V
• Abducens VI
• Facial VII
• Glossopharyngeal XII
• Vagus X
• Accessory XI
• Hypoglossal XII

ALS – Amyotrophic Lateral Sclerosis – progressive degenerative disease that attacks motor areas of cerebral cortex, axons of upper motor neurons in the lateral white columns (corticospinal and rubrospinal tracts) and lower motor neuron cell bodies. Causes progressive muscle weakness and atrophy. 15% hereditary cases, the rest sporadic, unknown causes, usual suspects: virus, autoimmune, free radicals, toxins, trauma, deficiency of nerve growth factor.

indirect motor pathways or extrapyramidal pathways

– include all somatic motor tracts other than the corticospinal and corticobulbar tracts.

I. Axons of upper motor neurons that give rise to the indirect motor pathways descend from various nuclei of the brain stem into 5 major tracts of the spinal cord and terminate on local circuit neurons or lower motor neurons.

These tracts are the:

a. Rubrospinal – conveys nerve impulses from red nucleus (which receives input from cerebral cortex and cerebellum) to contralateral skeletal muscles that govern precise, voluntary movements of distal parts of upper limbs
b. Tectospinal – conveys nerve impulses from superior colliculus to contralateral skeletal muscles that reflexively move head, eyes, and trunk in response to visual or auditory stimuli.
c. Vestibulospinal – conveys nerve impulses from vestibular nucleus (which receives input about head movements from inner ear) to ipsilateral skeletal muscles of trunk and proximal parts of limbs for maintaining posture and balance in response to head movements.
d. Lateral and Medial reticulospinal – conveys nerve impulses from reticular formation to ipsilateral skeletal muscles of trunk and proximal parts of limbs for maintaining posture and regulating muscle tone in response to ongoing body movements.

Question 11

compare the integrative cerebral functions of wakefulness and sleep coma, and learning and memory, and language.

Answer 11

integrative functions of the cerebrum – include sleep and wakefulness, learning and memory, and emotional responses (other than the role of limbic system as in Chapter 14)

wakefulness and sleep

a. circadian rhythm – established by the suprachiasmatic nucleus of the hypothalamus

reticular activating system (RAS) - a portion of the reticular formation. When the RAS is active, many nerve impulses are transmitted to widespread areas of the cerebral cortex, both directly and through the thalamus resulting in a generalized increase in cortical activity.

• Arousal – awakening from sleep. Also involves increased activity in the RAS.
  
  • For arousal to occur, the RAS must be stimulated.
  • Many sensory stimuli can activate the RAS: Pain, touch and pressure on the skin, movements of the limbs, bright light, or sounds.
  • Once the RAS is activated, the cerebral cortex is also activated and arousal occurs.

Consciousness – a state of wakefulness that results after arousal.

learning and memory

Learning – the ability to acquire new information or skills through instruction or experience

Memory – the process by which info acquired through learning is stored and retrieved.

Plasticity - capability for change associated with learning. The ability for the body to make an experience become part of memory by producing persistent structural and functional changes that represent the experience in the brain.

• Parts of brain known to be involved in memory: association areas of frontal, parietal, occipital, and temporal lobes, parts of the limbic system (esp hippocampus and amygdala) and the diencephalon.
• Primary somatosensory and primary motor areas in brain also exhibit plasticity.
• If a particular body part is used more intensively or in a newly learned activity, (ex. Reading Braille), the cortical areas devoted to that body part gradually expand.

Stages of memory:

a. Immediate memory – ability to recall ongoing experience for a few seconds. Provides a perspective to the present time that allows us to know where we are and what we are doing.
b. Short-term memory – the temporary ability to recall a few pieces of info for seconds to minutes. Ex. Look up a phone number, cross room, dial phone number.

• If the number or info has no special significance, it is usually forgotten within a few seconds.
• Brain areas involved in immediate and short term memory: hippocampus, the mammillary bodies, and two nuclei of the thalamus: anterior and medial nuclei.
• Some evidence states short term memory depends more on electrical and chemical events in brain than on structural changes.

c. Long-term memory – more permanent type of memory which lasts from days to years.

• Info in long term memory can usually be recalled for use whenever needed.
• Info that can be expressed by language are apparently stored in wide regions of the cerebral cortex.
• Memories for motor skills are stored in the basal nuclei and cerebellum as well as in the cerebral cortex.
• Long term memory is eventually lost if not being recalled. Only 1% of all conscious information is stored as long-term memory.

memory consolidation - the reinforcement that results from the frequent retrieval of a piece of information.

Amnesia – lack or loss of memory. Anterograde amnesia – memory loss for events that occur after the trauma or disease that caused the condition AKA inability to form new memories. Retrograde amnesia – memory loss for events that occurred before the trauma or disease AKA inability to recall past events.

long term potentiation - prolonged, enhanced synaptic transmission that occurs at certain synapses within the hippocampus of the brain; believed to underlie some aspects of memory.

After a brief period of high-frequency stimulation, synaptic activity is enhanced for hours or weeks.

• Neurotransmitter released is glutamate which acts on NMDA glutamate receptors on post synaptic neurons.
• In some cases, induction of LTP depends on release of nitric oxide (NO) from the post synaptic neurons after they have been activated by glutamate. The NO in turn diffuses into the presynaptic neurons and causes LTP.

Sleep – a state of altered consciousness or partial unconsciousness from which you can be aroused. It is essential but the exact functions of sleep are unclear. Sleep deprivation impairs attention, learning, and performance. Normal sleep consists of two components: NREM and REM.

NREM sleep – non-rapid eye movement

a. Consists of 4 gradually merging steps.
b. Typically takes less than 1 hour to go from stage 1 to 4.
1. Stage one – transition stage between wakefulness and sleep
1. Normally lasts 1-7 minutes.
2. The person is relaxed with eyes closed and has fleeting thoughts
3. People awakened during this stage often say they have not been sleeping
2. Stage two – light sleep
1. First stage of true sleep
2. Person is a little more difficult to awaken
3. Fragments of dreams may be experienced, eyes may slowly roll from side to side
3. Stage three – period of moderately deep sleep
1. Body temp and blood pressure decrease
2. Difficult to awaken the person
3. This stage occurs about 20 mins after falling asleep
4. Stage four – deepest level of sleep
1. Brain metabolism decreases significantly and body temp drops slightly but most reflexes are intact and muscle tone is only slightly decreased.
2. When sleepwalking occurs, it is during this stage.

REM sleep – rapid eye movement

a. 3-5 episodes during a typical 7-8 hour sleep period
b. The eyes move rapidly back and forth under closed eyelids.
c. The person may rapidly ascend through stages 3 and 2 before entering REM sleep.
d. First episode of REM sleep lasts 10-20 mins. Then another interval of NREM sleep follows.
e. REM occurs approx q90 mins and gradually lengthen.
f. Last REM lasts about 50 mins
g. In adults, total REM is usually 90-120 mins.
h. REM and NREM sleep alternate throughout the night.
i. As we age, total time spent sleeping decreases, and percentage of REM sleep decreases.
j. Infant REM = up to 50%
k. 2 year old REM = 35%
l. Adult 25%
m. Higher REM thought to be due to maturation of the brain during sleep. Neuronal activity is high during REM sleep, brain blood flow and oxygen use are higher during REM sleep that during intense mental or physical activity while awake.