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Chapter 12. GENERAL SENSORY SYSTEM

 

21.1 OBJECTIVES.

1. Define the general sensory system.
2. Define and classify the sensory receptors.
3. Describe the three neuron chain concept of the sensory system.
4. Identify higher centers for sensory perception.
5. Identify and describe major GP ascending tracts, including cerebellar pathways, and their major elements.
6. Discuss signs of GP deficiency and application of the postural reaction test in evaluation of GP system.
7. Identify and describe major ascending tracts of GSA.
8. Describe the nociceptive pathway and explain how nervous system modulate nociception.
9. List signs of GSA abnormality and describe sequence of deficit when the spinal cord is compressed.
10. Discuss neural projections of GVA.
11. Define the terms:
Visceral pain
Referred pain
Phantom pain
12. Explain neural mechanisms involved in panniculus, flexor and palpebral reflexes.

12.2 READING ASSIGNMENT.

deLahunta: pp 156-174; 311-315; 347-349
Jenkins: pp 124-131; 172-184; 212-216; 246; 318-319

12.3 GENERAL

12.3.1 SENSORY SYSTEM.
Sensory system convey environmental signals from different sources through sensory receptors and ascending tracts to specific thalamic nuclei and the sensory cortex where sensations are perceived.
In order to distinguish various aspect of the sensory system, the system is divided into 3 categories based on sources of the stimulation:
12.3.1.1.Exteroceptive System:
Sensitive to stimuli from external environment. It includes cutaneous, visual and auditory sensations.
12.3.1.2 Proprioceptive System:
It involves information about the relative position of body segments to one another and the position of body in space.
C. Interoceptive System:
It signals internal bodily events such as chemical, thermal and pressure changes from viscera as well as olfaction and gustation.
12.3.2 RECEPTORS:
Sensory receptors transform the stimulation into neural signals. Each receptor is sensitive to a specific modality of sensation - touch, temperature, pressure, movement, light, sound, chemicals and others. But it may react to other modalities as well. Receptors may be classified according to their:
Modality
Location
12.3.3.SENSORY NERVE FIBERS.
To reach the conscious level, sensory impulses are transmitted from the receptor through a 3 neuron chain of sensory path:
1. The first order sensory neurons are in the dorsal root ganglia or the sensory ganglia of cranial nerves.
2. The second order sensory neurons are in the dorsal gray column or various sensory nuclei of the brainstem.
3. The third order sensory neurons are in the thalamic nuclei.
The long ascending sensory tracts found in the spinal cord or the brainstem are formed by either the first or the second order neurons.
Some fibers contribute to formation of local reflex arcs.
12.3.4 HIGHER CENTERS.
The long ascending tracts relay sensory impulses to the higher centers found in:
Thalamus
Cerebral cortex - somesthetic and other sensory cortices

 

12.4 GENERAL PROPRIOCEPTION

The system dictates the state of position and movement of the body. Its receptors are limited to muscle, tendon and joints.
12.4.1 RECEPTORS.
Muscle spindle
Golgi tendon organ
Encapsulated nerve ending
Free nerve ending
12.4.2 DORSAL COLUMN - MEDIAL LEMNISCUS SYSTEM.
This is the GP pathway to the somesthetic cortex for conscious perception.
12.4.2.1 Spinal Nerve Involvement:
1. First order neurons are in the dorsal root ganglia. Axons enter the spinal cord and bifurcate into a short descending and a long ascending branches. Part of these long ascending branches form the dorsal column which consists of:
Fasciculus gracilis - made up of fibers entering the cord caudal to the mid-thoracic segments.
Fasciculus cuneatus - fibers entering the cord cranial to the mid- thoracic segments.
2. Second order neurons found in caudal medulla form 2 nuclei:
Nucleus gracilis
Medial cuneate nucleus
Their neurons receive inputs from the fasciculus gracilis and f. cuneatus. Their axons run first ventrally as the internal arcuate fibers, then cross over to become the medial lemniscus which ascend in the brainstem to reach the third order neurons.
3. Third order neurons are in the ventral caudal (posterior) lateral nucleus (VPL) of the thalamus. Their axons project to the primary sensory area of cerebral cortex.
12.4.2.2 Cranial nerve involvement:
1. First order neurons concentrate in the following sites:
Trigeminal ganglion
Mesencephalic nucleus of V
Geniculate ganglion
2. Second order neurons concentrate mostly in the pontine nucleus of the V cranial nerve. They receive inputs from the first order ganglia, and send their axons cross over the midline and ascend with the medial lemniscus as the trigeminal lemniscus.
3. Third order neurons are in the VPL of thalamus.
III. REFLEX CONNECTIONS.
Collaterals of the first order neurons terminate directly (monosynaptic) or indirectly through interneurons (multisynaptic) on LMNs to establish the reflex arcs. Intersegmental reflexes are mediated by the propriospinal tract.
IV. PROPRIOCEPTIVE CEREBELLAR PATHWAYS.
A. Function of Cerebellum:
1. Coordination and synergistic integration of the action of muscle concerned with movement.
2. Maintenance of strength and tome in muscle and other related functions.
3. Cerebellum does NOT initiate movement nor proprioceptive conscious recognition.
4. In order for cerebellum to carry out the above mentioned function, it must receive GP inputs.
B. Dorsal Spinocerebellar Tract:
1. First order neurons are in the T1 - L4 dorsal root ganglia. Their axons enter the dorsal gray column.
2. Second order neurons form the dorsal (thoracic) nucleus of Clarke. Their axons become the (ipsilateral) dorsal spinocerebellar tract. This tract enters cerebellum via the caudal cerebellar peduncle and terminates in vermis.
C. Ventral Spinocerebellar Tract:
1. First order neurons in the dorsal ganglia caudal to thoracic segments.
2. Second order neurons are in lateral and basal part of the dorsal gray column. Their axons cross over to become the contralateral ventral spinocerebellar tract. This tract enter cerebellum via the rostral peduncle. But recrosses the midline and terminates in the ipsilateral vermis.
D. Cuneocerebellar Tract:
First order neurons have their axons in the fasciculus cuneatus. Second order neurons are in the lateral cuneate nucleus. Their axons enter cerebellum by way of the ipsilateral caudal peduncle.
E. Cranial Spinocerebellar Tract:
Homologous to the ventral spinocerebellar tract, but the second order neurons originate form the C1 - T1 segments of spinal cord. The react enters cerebellum via the rostral and caudal peduncles.
V. SIGNS OF GP DEFICIENCY.
Ataxia (incoordination of muscle movement due to lack of sense of motion or position) is the major sign with lesions of GP. However, ataxia may be seen in cerebellar or vestibular dysfunction as well.
Posture of broad-based stance.
On moving, the limb may swing widely or cross over, sometime interface with other limbs.
"Knuckle over" may be seen.
GP deficiency is evaluated by postural reactions such as:
proprioceptive positioning response
hopping reaction

12.5 GENERAL SOMATIC AFFERENT

The system conveys signals from external environment. Sensory modalities involved are touch, pressure, temperature and pain.
 
12.5.1RECEPTORS.
Sensory receptors are found mostly in the skin or hypodermis.
Free nerve ending
Encapsulated nerve endings
Merkel's tactile disc
 
12.5.2 REFLEX CONNECTIONS.
They form multisynaptic reflex arcs. In the spinal cord, intersegmental reflex arcs involve the propriospinal fibers.
Flexor reflex and panniculus reflex are examples of important reflexes used in neurological diagnosis.
12.5.3 ASCENDING GSA PATHWAYS.
12.5.3.1 Dorsal column - Medial lemniscus:
This tract is for fine touch and vibration.
12.5.3.2 Spinothalamic Tract: for coarse touch, temperature and pain.
1. First order neurons are in the dorsal root ganglia. Nerve fibers are relatively small caliber fibers (A-delta or C fibers).
2. Second order neurons (in man) are located in basal portion of the dorsal gray column. Their fibers cross over and become the contralateral spinothalamic tract. They terminate in the thalamus.
3. Third order neurons are in the ventral caudal (posterial) lateral nucleus (VPL) of thalamus. Their axons pass through the internal capsule and terminate in the primary sensory area of the cortex.
4. Comparative Anatomy of the Tract -In animals, evidences indicate that the spinothalamic tract is a multisynaptic and bilateral tract.
Not all of the spinothalamic fibers terminate in the thalamus. Many of them end up in the brainstem reticular formation to become the spinoreticular fibers. Others end in elsewhere of the brainstem.
12.5.3.3 Ascending Reticular Activision System (ARAS):
This system is formed chiefly by the above mentioned spinoreticular fibers. ARAS brings in sensory information for development of arousal, alertness and wakefulness into the reticular formation. Disruption of ARAS may result in clinical conditions known as: depression, stupor or coma.
12.5.3.4 Spinocervical Tract: a significant tract only in the dog and cat.
First order neurons in dorsal root ganglia
Second order neurons in Lamina IV of the cord
Third order neurons in lateral cervical nucleus. Their axons become part of the contralateral medial lemniscus.
Fourth order neurons in VPL
12.5.3.5 Trigeminal GSA:
1. For conscious projection -
Receptors are found in the skin, mucous membrane of the head, and teeth.
First order neurons are in the trigeminal ganglion. Their axons form the spinal tract of V.
Second order neurons are in the pontine nucleus of V (for touch and GP) and the spinal nucleus of V (for temperature and pain). Their axons enter the contralateral medial lemniscus.
Third order neurons are in the VPL
2. For reflex - involving the first order neurons, interneurons in the brainstem and LMNs in GSE, SVE cranial nerve nuclei.
Important reflexes are corneal and palpebral reflexes.
 
12.5.4 PAIN PERCEPTION AND MODULATION.
12.5.4.1Nociceptive Pathway:
Nociceptors are mostly free nerve endings.
First order neurons have small caliber fibers (type IV or C fibers).
Second order neurons are in the laminae I, II , IV and V. Their axons ascend in the spinothalamic, spinoreticular, spinocervical tracts or the dorsal column.
Third order neurons are in the VPL or the central lateral nucleus of the thalamus. Their axons project to the somatosensory cortex and the limbic system.
In brainstem, the raphe nuclei and periaqueductal gray have rich inputs from the secondary order nociceptiv neurons.
12.5.4.2. Neural Modulation of Nociception:
Central nervous system not only can perceive pain, but also attenuate nociceptive responses that signal pain.
1. Gate control theory -
In the dorsal gray column of spinal cord non-nociceptive impulses transmitted by the larger caliber fibers can modulate nociceptive impulses carried by the smaller nerve fibers.
2. Descending control of pain -
Neurons in the periaqueductal gray are richly endowed with opiate receptors. Some of their axons project to the raphe nuclei.
Neurons in the raphe magnum project their serotonergic fibers to the dorsal gray column.
These serotonergic fibers inhibit activity of pain transmission neurons. They may also facilitate the inhibitory (enkephalinergic) neurons which exert their effect on the first and the second order nociceptive neurons.
12.5.5 GSA DEFICIENCY.
12.5.5.1. Alteration in Sensation of a Dermatome:
This can be used to localize a lesion involving GSA to a spinal nerve or a dorsal root.
Dermatome is an area of skin supplied by sensory fibers from a single dorsal root ganglion. Other than the head and limbs, dermatomes extend from the dorsal to ventral midline. Dermatome overlap (esp. for touch sensation) so that an area of skin is usually supplied by more than one spinal nerve.
12.5.5.2 Signs of Abnormality:
1. Segmental sensory signs -
anesthesia
hyperesthesia
loss of reflexes
2. Tract lesions - with compressive lesions of the spinal cord, sequence of deficits are:
propeioceptive deficits
superficial pain sensation and voluntary motor activity (are often lost at the same time)
deep pain sensation (is the last to be lost)

12.6 GENERAL VISCERAL AFFERENT

12.6.1 VISCERAL SENSATION.
Temperature
Chemicals
Pressure
Pain
12.6.2 RECEPTORS.
Free nerve endings
Encapsulated endings
12.6.3 GVA ASCENDING PROJECTION.
12.6.3.1 First Order Neurons:
Their cell bodies are in the VII (geniculate) and the IX & X (distal) ganglia and in the dorsal root ganglia. Their axons innervate the following organs:
1. Facial nerve - middle ear, auditory tube, soft palate and pharynx.
2. Glossopharyngeal nerve - pharynx, epiglottial GSA receptors.