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Unit 3 Communication and Control of the Human Body

PART 4: NERVOUS SYSTEM: SENSORY AND MOTOR PATHWAYS

READINGS AND RESOURCES Tortora Chapter 16

A. SENSATIONLearning Objectives Differentiate between general and special senses. Describe the process of sensation. Describe the three basic types of sensory receptors. Define the term sensory adaptation.

Sensation is the conscious awareness of sensory receptor stimulation. The brain constantly receives information from sensory receptors located throughout the body. Sensory receptors respond to stimuli by generating action potentials that are propagated along nerves to the spinal cord and brain. Generally speaking, sensations result when action potentials reach the cerebral cortex.

General and Special SensationsThe senses are the means by which the brain perceives information about the environment and the body. You are already familiar with 5 basic senses: smell, taste, sight, hearing, and touch. Each sense is unique, which allows us to distinguish one from another from another. This fact is built into our sensory pathway anatomy - a sensory neuron only carries information about one sense. For example, sensory neurons from the eye’s retina only carry visual information. Sensory neurons from the ear’s cochlea only carry auditory information. In anatomy, the types of sensation that our brains perceive are divided into two classes:

1. General senses are those with receptors distributed throughout the body. They provide sensory information about touch, pressure, itch, vibration, temperature, proprioception, and pain.

2. Special senses are those with receptors localized to specific sensory organs such as the eye (vision), ear (hearing and equilibrium), nose (smell) and tongue (taste).

The Process of Sensation For a sensation to arise, four events must occur:

1. Stimulation - A stimulus, or change in the environment, capable of initiating a nerve impulse by the nervous system must be present.

2. Transduction - A sensory receptor or sense organ must pick up the stimulus and transduce (convert) it to a nerve impulse by way of a generator potential.

J. Taylor Red River College 1


3. Conduction - The impulse(s) must be conducted along a neural pathway from the receptor or sense organ to the brain.

4. Translation. - A region of the CNS (usually the brain) must translate the impulse into a sensation.

Sensory ReceptorsSensory receptors are classified according to their structure, location, and type of stimulus they are sensitive to. For example, a touch receptor is classified as (1) an encapsulated nerve ending, (2), an exteroreceptor, and (3) a mechanoreceptor. The chart below describes these classification systems:

StructureUnencapsulated nerve endings

Bare dendrites of first-order sensory neurons

Encapsulated nerve endings

Dendrites of first-order sensory neurons enclosed in a connective tissue capsule

Sensory receptor cells Highly specialized separate cells that synapse with a first-order sensory neuron

LocationExteroreceptor Located at or near the body surfaceInteroreceptor Located in blood vessels, visceral organs, and

the nervous systemProprioreceptor Located in muscles, tendons, joints, and the

inner earStimulus TypeMechanoreceptor Detects mechanical stimuliThermoreceptor Detects changes in temperatureNociceptor Responds to painful stimuliPhotoreceptor Detects lightChemoreceptor Detects chemicals in mouth, nose, and body

fluidsOsmoreceptor Detects changes in osmotic pressure

A characteristic of most sensory receptors is adaptation, in which a receptor’s sensitivity decreases when exposed to a long-lasting stimulus. For example, when you first step into the shower, the water feels very hot. But, after a few minutes, the water does not feel as hot. Your thermoreceptors have adapted to the stimulus. Receptors vary in how quickly they adapt. Rapidly adapting receptors adapt quickly. Slowly adapting receptors adapt slowly.

B. SOMATIC SENSATIONS Define somatic sensation.



Classify somatic sensory receptors according to their structure, class, sensation, and adaptation rate.

Discuss some general aspects of pain perception.

Somatic sensations arise from stimulation of sensory receptors embedded in the skin and hypodermis, mucous membranes, muscles and tendons, joints, and the inner ear. Some parts of the body surface are densely populated with somatic sensory receptors. Other parts are not. The tip of the tongue, lips, and fingertips have the highest density of sensory receptors. Our somatic sensations can be grouped into four categories:

1. Tactile sensations – touch, pressure, vibration, itch, and tickle2. Thermal sensations – cold and warm3. Proprioceptive sensations – kinesthesia and weight discrimination4. Pain sensations

The table below describes some somatic sensory receptors:

Receptor Type Description Receptor Class * Sensation Adaptation

Tactile ReceptorsTouch corpuscle Capsule surrounds mass of

dendrites in dermal papillae of hairless skin

(S) Encapsulated(L) Exteroreceptor(St) Mechanoreceptor

Touch, pressure, slow vibrations

Rapid

Hair root plexus Free nerve endings wrapped around hair follicles in skin

(S) Unencapsulated(L) Exteroreceptor(St) Mechanoreceptor

Touch Rapid

Lamellated corpuscle

Oval, layered capsule surrounds dendrites in dermis and sub-cutaneous layer

(S) Encapsulated(L) Exteroreceptor(St) Mechanoreceptor

Pressure and fast vibrations

Rapid

Itch receptor Free nerve endings in skin and mucous membranes

(S) Unencapsulated(L) Exteroreceptor(St) Mechanoreceptor

Itch Both slow and rapid

ThermoreceptorsWarm and cold receptors

Free nerve endings in skin and mucous membranes

(S) Unencapsulated(L) Exteroreceptor(St) Thermoreceptor

Warmth up to 48CCold down to 10C

Rapid

ProprioceptorsMuscle spindle Sensory nerve endings wrapped

around specialized muscle fibers

(S) Encapsulated(L) Proprioceptor(St) Mechanoreceptor

Muscle stretch and length

Slow

Tendon organ Capsule surrounds mass of dendrites at junction of tendon and muscle

(S) Encapsulated(L) Proprioceptor(St) Mechanoreceptor

Tendon stretch and tension

Slow

Joint kinesthetic receptors

Lamellated corpuscles, tendon organs, and free nerve endings within and around synovial

(S) Encapsulated(L) Proprioceptors(St)Mechanoreceptors

Joint position and movement

Rapid



joints

Pain receptorsNociceptor Free nerve endings in every

tissue of the body except the brain and cartilage

(S) Unencapsulated(L) Exteroreceptor(St) Nociceptor

Pain and extremes in temperature

Slow

* (S) structure; (L) location; (St) stimulus type

Find some examples of somatic sensory receptors in the following diagram:

Somatic Pain PerceptionPain is a vital sensation that warns us of actual or impending tissue damage. Pain receptors (nociceptors) are located in nearly every body tissue and respond to many stimuli: extremes of pressure, temperature and a host of chemicals released from damaged tissue (e.g. histamine, K+, ATP, acids, and bradykinin). To ensure that we to take protective action, pain receptors adapt slowly to stimuli, if at all.



Somatic pain is carried to the parietal lobe of the cerebral cortex by 2 major sensory pathways, spinothalamic and trigeminothalamic, which we will discuss later. Our perception of pain depends, in part, on the type of sensory neuron stimulated:

Sharp pain is conducted very quickly to the brain by myelinated nociceptive fibers. This type of pain is also referred to as fast, acute, and pricking pain. An example would be the pain felt by a needle puncture or knife cut to the skin.

Throbbing pain, in contrast, is conducted more slowly to the brain by unmyelinated nociceptive fibers. This type of pain is also referred to as slow, chronic, burning, and aching pain. An example would be the pain felt during a toothache.

Both types of somatic pain can be stimulated by the same event. For example, when you stub your toe, you first feel the sensation of sharp pain and then feel the slower, aching sensation of throbbing pain.

C. SOMATIC SENSORY PATHWAYS Discuss the general features of a somatic sensory pathway. Map out the posterior column-medial lemniscus pathway and state its

functions. Map out the spinothalamic pathway and state its functions. Map out the trigeminothalamic pathway and state its functions.

Most somatic sensory pathways conduct sensory information from somatic receptors to the primary somatosensory area of the cerebral cortex. The brain consciously perceives sensations carried by these pathways. Some somatic sensory pathways conduct sensory information to the cerebellum. These sensations are not consciously perceived. Our discussion will focus on pathways that conduct sensory information to the cerebral cortex.

Somatic sensory pathways share a few general features:1. Decussation. The pathways cross from one side of the CNS to the

other (decussate) at some point along their journey.2. Relay. The pathways consist of a chain of 3 neurons (a relay). The

neurons are named:a. First-order neurons. They conduct information from somatic

sensory receptors to second-order neurons. In most pathways, cell bodies of first-order neurons are located in dorsal root ganglia.

b. Second-order neurons. A type of interneuron, they conduct sensory information from first-order neurons to third-order neurons. Depending on the pathway, cell bodies of second-order neurons are located in the brain stem or spinal cord.



c. Third-order neurons. A type of interneuron, they conduct sensory information from second-order neurons to neurons making up the primary somatosensory area of the cerebral cortex. Cell bodies of third-order neurons are located in the thalamus.

3. Symmetry. All pathways are paired symmetrically (right and left) with a member of the pair present on each side of the spinal cord and brain.

4. Somatotopy. There is a precise spatial relationship among the neurons making up each pathway that reflects the orderly mapping of the body. For example, in the posterior column – medial lemniscus pathway, sensory neuron fibers carrying information from the upper limbs are located in the lateral portions of the posterior columns of the spinal cord, whereas sensory neurons carrying information from the lower limbs are located in the medial portions of the posterior columns. Somatotopy is also present in the primary somatosensory area of the cerebral cortex (see Figure 16.8 for verification).

There are several somatic sensory pathways. We will concentrate on (1) posterior column – medial lemniscus pathways (2) spinothalamic pathways, and (3) trigeminothalamic pathways. But before we describe these pathways, let’s review the internal anatomy of the spinal cord. Note that the illustration does not demonstrate symmetry: only right-sided sensory pathways and left-sided motor pathways are depicted. We will concentrate, for now, on the right-sided sensory pathways.

Posterior Column-Medial Lemniscus Pathways



Posterior column-medial lemniscus pathways conduct nerve impulses for touch, pressure, vibration, and conscious proprioception from the limbs, trunk, neck, and posterior head to the primary somatosensory area of the cerebral cortex. These pathways are formed by white matter tracts in the spinal cord and brainstem:

1. Posterior white column. Located in spinal cord. Each is subdivided into:a. Gracile fasciculus. Medial part of posterior column; conveys nerve

impulses from lower limbs and trunk.b. Cuneate fasciculus. Lateral part of posterior column; conveys

impulses from upper limbs, upper trunk, posterior head, and neck.2. Medial lemniscus tract. Begins in brainstem; ends at thalamus.

The pathways are composed of three neurons:1. First-order neurons from one side of the body form the spinal cord’s

posterior column on the same side and end in the medulla oblongata, where they synapse with second-order neurons.

2. Second-order neurons cross over in the medulla oblongata and enter the medial lemniscus on the opposite side of the brain stem. They extend to the thalamus where they synapse with third-order neurons.

3. Third order neurons transmit nerve impulses from the thalamus to the primary somatosensory cortex on the side opposite the site of stimulation.

Spinothalamic PathwaySpinothalamic pathways conduct nerve impulses for pain, cold, warmth, and itch from the limbs, trunk, neck, and posterior head to the primary somatosensory area of the cerebral cortex. These pathways are formed by white matter tracts in the lateral spinal cord and brainstem. The pathways are composed of three `neurons:

1. First-order neurons from one side of the body synapse with second-order neurons in the spinal cord’s posterior gray horn on the same side of the body.

2. Second-order neurons cross over in the spinal cord and enter the spinothalamic tract on the opposite side of the spinal cord. They extend to the thalamus where they synapse with third-order neurons.

3. Third-order neurons transmit nerve impulses from the thalamus to the primary somatosensory cortex on the side opposite the site of



stimulation.

Trigeminothalamic pathwaysTrigeminothalamic pathways conduct nerve impulses for most somatic sensations (tactile, thermal, pain, and proprioceptive) from the face, nasal cavity, oral cavity, and teeth to the primary somatosensory area of the cerebral cortex. These pathways are formed by white matter tracts in the brain only. The spinal cord is not involved! The pathways are composed of three neurons:

1. First-order neurons from one side of the head synapse with second-order neurons in the pons and medulla on the same side of the head.

2. Second-order neurons cross over in the brain stem and enter the trigeminothalamic tract on the opposite side of the brain stem. They extend to the thalamus where they synapse with third-order neurons.

3. Third-order neurons transmit nerve impulses from the thalamus to the primary somatosensory cortex on the side opposite the site of stimulation.



Posterior Column – Medial Lemniscus Pathways

SpinothalamicPathways Trigeminothalamic Pathways



SOMATIC SENSORY PATHWAYS

Posterior Column-Medial Lemniscus Pathway

Spinothalamic Pathway

Sensations carried

Beginning of pathway

End of pathway

Number of neurons in pathwayNeuron that crosses over

Location of crossover

First order neuronAxon

Cell body

Second order neuronAxon

Cell body

Third order neuronAxon

Cell body



D. SOMATIC MOTOR PATHWAYS Discuss the general features of a somatic motor pathway. Differentiate between direct and indirect somatic motor pathways Map out the corticospinal pathways and state their functions. Describe the corticobulbar pathway and state its functions.

Somatic motor pathways conduct motor information from the cerebral cortex and brain stem to skeletal muscle. Somatic motor pathways have similar general features as sensory pathways:

1. Decussation

2. Relay. The pathways consist of a chain of 2 neurons (a relay). The neurons are named:

a. Upper motor neurons. Extend from the primary motor area of the cerebral cortex or brain stem and synapse with lower motor neurons.

b. Lower motor neurons. Extend from the brain stem or spinal cord and synapse with skeletal muscle. Lower motor neurons that originate in the brain stem exit the CNS via cranial nerves. Lower motor neurons that originate in the spinal cord exit the CNS via spinal nerves.

3. Symmetry

4. Somatotopy

Upper motor neurons extend from the brain to lower motor neurons via two types of somatic motor pathways:

1. Direct somatic motor pathways provide input to lower motor neurons via upper motor neurons that extend directly from the cerebral cortex.

2. Indirect somatic motor pathways provide input to lower motor neurons via upper motor neurons that extend from motor centers in the brain stem.

We will concentrate our discussion on direct somatic motor pathways.



Direct Motor PathwaysDirect motor pathways convey impulses from the cerebral cortex that result in precise voluntary muscle movements. The direct pathways include the corticospinal tracts and corticobulbar tracts.

The corticospinal tracts promote voluntary movements of the limbs and trunks.

1. The lateral corticospinal tract conveys nerve impulses from the primary motor cortex to skeletal muscles on opposite side of the body for precise, voluntary movements of the distal parts of the limbs (e.g. hands and fingers). Axons of upper motor neurons descend from the precentral gyrus

of the cortex into the medulla oblongata. Here, 90% decussate and then enter the contralateral side of the spinal cord to form this tract.

At their level of termination, the upper motor neurons end in the anterior gray horn on the same side. They provide input to lower motor neurons, which innervate skeletal muscles.

2. The anterior corticospinal tract conveys nerve impulses from the motor cortex to skeletal muscles on opposite side of body for movements of the trunks and proximal parts of the limbs. Axons of upper motor neurons descend from the cortex to the

medulla oblongata. Here, the 10% that do not decussate enter the spinal cord and form this tract.

At their level of termination, these upper motor neurons decussate and end in the anterior gray horn on the opposite side of the body. They provide input to lower motor neurons, which innervate skeletal muscles.



Lateral Corticospinal Pathway Anterior Corticospinal Pathway



The corticobulbar tracts promote voluntary movements of the head and neck to coordinate precise voluntary movements including movements of the eyes, tongue, and neck, chewing, facial expressions, and speech. See Figure 16.11 for a map of the corticobulbar pathway.


lesson plan sheet 1€¦ · web viewthe corticobulbar tracts promote voluntary movements of the...

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