SOMATIC SENSORY SYSTEM
Neuroanatomy of Sensation
Sensory homunculus
Lesions of the Somatosensory Cortex Hemisensory numbness to pain / temperature / touch Hemineglect / astereognosis Impaired position sense
Somatic Senses
Touch Temperature Pain Itch Proprioception Pathway
Figure 10-10: The somatosensory cortex
Somatic Pathways
Figure 10-9: Sensory pathways cross the body’s midline
Ascending Pathways of Nociceptive Information from the SC to the Thalamus and Cerebral Cortex 1. Spinothalamic tract – most prominent • • •
From contralateral side to the anterolateral white matter to the thalamus Electrical stimulation – severe pain Lesioning the system by cordotomy – reduced pain on the side opposite the spinal lesion
Ascending Pathways of Nociceptive Information from the SC to the Thalamus and Cerebral Cortex 2. Spinoreticular tract – many of the axons do not cross  They terminate in the reticular formation of the medulla and thalamus
Ascending Pathways of Nociceptive Information from the SC to the Thalamus and Cerebral Cortex 3. Spinomesencephalic Tract
Projects axons to the mesencephalon and periaqueductal gray matter via the Spinoparabrachial tract Parabrachial neuron projecting to the amygdala, a component of the limbic system. Thought to contribute to the affective component of pain. Most project to the lateral funiculus instead of the anterolateral quadrant so that if these fibers are spared, pain may persist/recur.
Ascending Pathways of Nociceptive Information from the SC to the Thalamus and Cerebral Cortex 4. Cervicothalamic tract Arise form neurons in the lateral cervical nucleus Found in the 2 upper cervical segments of the spinal cord Cross the midline and end up in the thalamus
Ascending Pathways of Nociceptive Information from the SC to the Thalamus and Cerebral Cortex 5. Spinohypothalamic tract  Projects directly to supraspinal autonomic control center  Thought to activate complex neuroendocrine and cardiovascular response
Peripheral Sensory Receptors  Structures that pick up sensory stimuli  Initiate signals in sensory axons
Peripheral Sensory Receptors Two main categories of sensory receptors Special nerve endings of sensory neurons Monitor general sensory information
Independent receptor cells – specialized epithelial cells or small neurons Monitor most types of special sensory information
Peripheral Sensory Receptors Sensory receptors also classified according to: Location Type of stimulus detected Structure
Classification by Location Exteroceptors – sensitive to stimuli arising from outside the body
Located at or near body surfaces Include receptors for touch, pressure, pain, and temperature
Interoceptors – (visceroceptors) receive stimuli from internal viscera Monitor a variety of stimuli
Proprioceptors – monitor degree of stretch Located in musculoskeletal organs
Classification by Modality Mechanoreceptors – respond to mechanical forces Thermoreceptors – respond to temperature changes Chemoreceptors – respond to chemicals in solution Photoreceptors – respond to light – located in the eye Nociceptors – respond to harmful stimuli that result in pain
Classification by Structure General sensory receptors
Widely distributed Nerve endings of sensory neurons monitor: Touch, pressure, vibration, stretch Pain, temperature, proprioception
Divided into two groups Free nerve endings Encapsulated nerve endings
Free Nerve Endings Abundant in epithelia and underlying connective tissue Respond to pain and temperature Monitor affective senses Two specialized types of free nerve endings Merkel discs – lie in the epidermis Slowly adapting receptors for light touch
Hair follicle receptors – wrap around hair follicles Rapidly adapting receptors
Unencapsulated Nerve Endings
Sensory Receptor Types
Figure 10-1: Sensory receptors
Encapsulated Nerve Endings Consist of one or more end fibers of sensory neurons Enclosed in connective tissue Mechanoreceptors Include four main types
Encapsulated Nerve Endings  Encapsulated nerve endings: dendrites with special supporting structures (mechanoreceptors and proprioceptors)
Encapsulated Nerve Endings
Meissner’s corpuscles Pacinian corpuscles Ruffini’s corpuscles Proprioceptors
Encapsulated Receptors
Touch
Figure 10-11: Touch-pressure receptors
Structure of Receptors in Skin
Proprioceptors  Monitor stretch in locomotory organs  Three types of proprioceptors
Three Types of Proprioceptors Muscle spindles – measure the changing length of a muscle Imbedded in the perimysium between muscle fascicles
Golgi tendon organs – located near the muscle-tendon junction Monitor tension within tendons
Joint kinesthetic receptors Sensory nerve endings within the joint capsules
Proprioceptors
Table 14.1
Sensory Fibers The dorsal root fiber axons that leave the ganglion and enter the cord are of different diameters depending on function, and the diameter determines the speed of conduction. The myelinated fibers are classified as Group A Group C fibers are unmyelinated.
Sensory Fibers Aα are the thickest and fastest of the myelinated afferent (sensory) axons, having a diameter ranging from around 12-20 mm and conduction velocities ranging from 70-120 m/sec. The Ia muscle afferents are an example of Aα fibers
Sensory Fibers The myelinated Aß afferents range in diameter from 5-14 μm and have conduction velocities ranging from 25-70 m/sec. The Group II muscle mechanoreceptor afferents are an example of the Aß afferent type
Sensory Fibers Group III afferents, and are the thinnest of the myelinated sensory afferents, having axon diameters ranging from 2-7 μm and conduction velocities of 10-30 m/sec. These afferents subserve temperature, pain, and crude touch and pressure
Sensory Fibers Group IV fibers (C) are unmyelinated, have diameters ranging from 1-5 μm and conduction velocities of less than 2.5 m/ sec. They subserve temperature and (together with Aδ) subserve afferent transmission of pain.
Peripheral Nerves  The spinal nerve roots constitute the initial segment of the peripheral nervous system  In their further course, the fibers of the spinal nerve roots of multiple segments form plexuses, from which they are then distributed to the peripheral nerves.
Peripheral Nerves  The areas innervated by the nerve roots thus differ from those innervated by the peripheral nerves  The sensory component of a spinal nerve root innervates a characteristic segmental area of skin, which is called a dermatome.
Brachial Plexus  The complex structure of the brachial plexus and the redistribution of individual radicular elements within it make it very difficult to localize brachial plexus lesions exactly based on the neurological findings alone
Upper brachial plexus lesion(Erb−Duchenne palsy)  This type of lesion involves the fibers originating in the C5 and C6 nerve roots.
Lower brachial plexus lesion (Dejerine−Klumpke palsy) This type of lesion involves the fibers originating in the C8 and T1 roots. Its prominent findings include weakness of the intrinsic muscles of the hand, sometimes also of the long flexors of the fingers, and rarely of the wrist flexors. There is always a sensory deficit involving the ulnar edge of the forearm, hand, and fingers
Diseases of the Peripheral Nerves of the Upper Limbs Suprascapular N. (C4−C6) Anatomy. This nerve innervates the supraspinatus and infraspinatus mm. It reaches them after passing through the scapular notch and then running dorsally. It receives sensory branches from the shoulder joint, but not from the skin.
Diseases of the Peripheral Nerves of the Upper Limbs Axillary N. (C5−C6) Anatomy. This nerve provides motor innervation to the deltoid and teres minor mm. and sensory innervation to a palm-sized patch of skin on the proximal, lateral surface of the upper arm ( superior lateral brachial cutaneous n.)
Diseases of the Peripheral Nerves of the Upper Limbs Musculocutaneous N. (C5−C7) Anatomy. This nerve innervates the biceps brachii and coracobrachialis mm. and a portion of the brachialis m. Its sensory terminal branch, the lateral antebrachial cutaneous n., innervates the skin on the radial side of the forearm
Diseases of the Peripheral Nerves of the Upper Limbs
Radial N. (C5−C8) Anatomy. It provides motor innervation to the triceps brachii, brachioradialis, and supinator mm., and all of the extensors of the wrist, thumb, and fingers Its sensory innervation is to the dorsal skin of the upper arm and forearm as well as the dorsum of the hand, with an autonomic zone located between the first and second metacarpal bones.
Diseases of the Peripheral Nerves of the Upper Limbs  Median N. (C5−T1)  Its sensory innervation is to the radial side of the palm, the volar surface of the fingers from the thumb to the radial half of the fourth finger, and the dorsal surface of the terminal phalanges of these fingers.
Diseases of the Peripheral Nerves of the Upper Limbs  Ulnar N. (C8−T1)  A sensory branch arising from the ulnar n. in the distal third of the forearm innervates the skin on the ulnar side of the dorsum of the hand, as well as on the dorsal surface of the fifth finger and the ulnar half of the fourth finger.
Lumbar Plexus
Diseases of the Peripheral Nervesof the Lower Limbs Genitofemoral and Ilioinguinal Nn. (L1−L2) Typical deficits. Lesions of these nerves cause local pain in the groin (ilioinguinal nerve syndrome), a sensory deficit in the corresponding zone(s) of cutaneous innervation, and sometimes, in men, loss of the cremaster reflex (because the afferent arm of the reflex loop is interrupted).
Diseases of the Peripheral Nervesof the Lower Limbs Lateral Femoral Cutaneous N. (L2−L3) Anatomy. This purely sensory nerve passes through the three layers of the abdominal wall and then penetrates the inguinal ligament, usually at a point three finger breadths medial to the anterior superior iliac spine, to emerge onto the anterior fascia of the thigh.
Diseases of the Peripheral Nervesof the Lower Limbs  It provides sensory innervation to a palmsized area of skin on the anterolateral surface of the thigh  Typical deficits. The lateral femoral cutaneous n. is vulnerable to injury at the point where it penetrates the inguinal ligament.
Diseases of the Peripheral Nervesof the Lower Limbs The resulting clinical disturbance is an entrapment neuropathy called meralgia paresthetica, characterized by burning pain in the cutaneous distribution of the nerve. The pain is better when the hip is Flexed, it isworse on hyperextension of the leg (reverse Lasègue sign).
Diseases of the Peripheral Nerves of the Lower Limbs
Femoral N. (L1−L4) It provides sensory innervation by way of anterior cutaneous branches to the anterior surface of the thigh, and, through its terminal branch, the saphenous n., to the medial quadrant of the anterior surface of the lower leg.
Diseases of the Peripheral Nerves of the Lower Limbs
Obturator N. (L3−L4) Its sensory innervation is to a small area of skin at the medial aspect of the thigh, just above the knee. Sometimes, irritation of the obturator nerve trunk can produce pain in this area as the sole clinical manifestation. This is called the Howship−Romberg phenomenon.
Diseases of the Peripheral Nerves of the Lower Limbs Sciatic N. (L4−S3) Anatomy. The sciatic n. is the common trunk of the fibular (peroneal or common peroneal) and tibial nn. It is the longest and thickest nerve in the human body. It gives cutaneous branches to the buttock and the posterior surface of the thigh (the inferior cluneal nn. and the posterior femoral cutaneous n.)
Diseases of the Peripheral Nerves of the Lower Limbs Fibular N. (L4−S2) Anatomy. The fibular (peroneal or common
peroneal) n., after it separates from the tibial portion of the sciatic n., travels to the lateral margin of the popliteal fossa, winds around the fibular neck, and then enters into the body of the peroneus longus muscle, where it divides into the superficial and deep fibular (peroneal) nn.
Diseases of the Peripheral Nerves of the Lower Limbs
 The superficial fibular (peroneal) n. provides motor innervation to the fibular (peroneal) muscles and sensory innervation to the lateral surface of the lower leg and the dorsum of the foot, with the exception of the space between the first and second toes (the first  interosseous space).
Diseases of the Peripheral Nerves of the Lower Limbs
Tibial N. (L4−S3) Anatomy. This nerve, derived from the medial portion of the sciatic n., innervates the plantar flexors of the foot and toes in the lower leg, as well as all of the intrinsic muscles of the foot, except those on the dorsum. It provides sensory innervation to the heel and sole
Sensory Examination
• Light touch, temperature, pain. • Joint position sense. • Vibration. • Double simultaneous stimulation to test “Negligence”/ “Extinction” • Graphesthesia. • Stereognosis.
Dorsal Column Modalities
Position Sense Joint position sense 2-point discrimination Pressure Texture Touch localization Sense of weight Graphesthesia
Sensory Abnormalities Anaesthesia – complete loss of sensation Hypesthesia – Reduced sensation Analgesia – Complete lack of pain sensation Hypalgesia – Reduced pain sensation Hyperalgesia – Excessive pain sensation Allodynia – Painful sensation with nonpainful stimulus