NEUROPHYSIOLOGY

Sensory Pathways: I

Mesencephalon (cerebral peduncles)

Spinothalamic tract

Lower part of medulla oblongata

Reticular formation

Cervical part of spinal cord

Lateral spinothalamic tract: pain, temperature

Ventral (anterior) spinothalamic tract: touch, pressure

Lumbar part of spinal cord

FIGURE 2.29 SOMESTHETIC SYSTEM OF THE BODY•

Cerebral cortex: postcentral gyrus

Posterior limb of internal capsule

Ventral posterolateral (VPL) nucleus of thalamus

Medial lemniscus

Gracile nucleus

Cuneate nucleus

Fasciculus gracilis

Fasciculus cuneatus

Dorsal (posterior) spinal root ganglion

	Proprioception,	Large
	position
	Touch,	myelinated
		fibers
	pressure,
	vibration	Small
	Pain,
		myelinated
	temperature	and unmyelin-
		ated fibers

Lateral cervical nucleus

Spinocervical tract

©

Pain, temperature, and pressure sensations below the head ultimately are conveyed to the primary somatosensory cortex (postcentral gyrus) by the anterolateral system (spinothalamic and spinoreticular tracts). The fasciculus gracilis and cuneatus of the spinal lemniscal system convey proprioceptive, vibratory, and tactile sen-

sations to the thalamus (ventral posterolateral nucleus), whereas the lateral cervical system mediates some touch, vibratory, and proprioceptive sensations (blue and purple lines show these dual pathways). Ultimately, these fibers ascend as parallel pathways to the thalamus, synapse, and ascend to the cortex.

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Sensory Pathways: II

NEUROPHYSIOLOGY

Midbrain (cerebral peduncles)

Ventral trigeminal lemniscus

Pontine reticular formation

Pons

Medullary reticular formation

Spinal trigeminal tract

Spinal trigeminal nucleus

Cervical part of spinal cord

FIGURE 2.30 SOMESTHETIC SYSTEM OF THE HEAD•

Cerebral cortex: postcentral gyrus

Ventral posteromedial (VPM) nucleus of thalamus

Internal capsule

Dorsal trigeminal lemniscus

Trigeminal mesencephalic nucleus

Trigeminal motor nucleus

Principal sensory trigeminal nucleus

Touch, pressure

Pain, temperature

Proprioception

Trigeminal (semilunar) ganglion

Ophthalmic n.

Maxillary n.

Sensory root and

Motor root of mandibular n.

Facial (VII) n. Vagus (X) n.

Dorsolateral fasciculus (of Lissauer)

Substantia gelatinosa (Iamina II)

©

Nerve cells bodies for touch, pressure, pain, and temperature in the head are in the trigeminal (semilunar) ganglion of the trigeminal (CN V) nerve (blue and red lines in figure). Neuronal cell bodies mediating proprioception reside in the mesencephalic nucleus

of CN V (purple fibers). Most relay neurons project to the contralateral VPM nucleus of the thalamus and thence to the postcentral gyrus of the cerebral cortex, where they are somatotopically represented.

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NEUROPHYSIOLOGY

Sensory Pathways: III

C2

C3

C4

C5

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

L1

S2, 3 L2

L3

L4

	Schematic demarcation of
	dermatomes shown as distinct	C2
	segments. There is actually
	considerable overlap between	C3
	any two adjacent dermatomes	C4
		C5
		C6

	C7	T1
	C8
		T2
		T3
C6	C6	T4
		T5
		T6
T1		T7
		T8
	C5	T9
		T10
	C8	T11
		T12
	C7
		L1
		L2
		L3
	C6	L4
		L5

			S1
C8	C7	C8	S2
	C7

S3

S4

S5	S1
	S2
L5
L1
L2

L5

S1

L5

L4

Levels of principal dermatomes

C5	Clavicles
C5, 6, 7	Lateral parts of upper limbs
C8, T1	Medial sides of upper limbs
C6	Thumb
C6, 7, 8	Hand
C8	Ring and little fingers
T4	Level of nipples

	L3
	S1 S2
	L4
	S1
	L5
	L4
T10	Level of umbilicus
T12	Inguinal or groin regions
L1, 2, 3, 4	Anterior and inner surfaces of lower limbs
L4, 5, S1	Foot
L4	Medial side of great toe
S1, 2, L5	Posterior and outer surfaces of lower limbs
S1	Lateral margin of foot and little toe
S2, 3, 4	Perineum	©

FIGURE 2.31 DERMATOMES•

Sensory information below the head is localized to specific areas of the body, which reflect the distribution of peripheral sensory fibers that convey sensations to the spinal cord through the dorsal roots (sensory nerve cell bodies reside in the corresponding dorsal root ganglion). The area of skin subserved by afferent fibers of one

dorsal root is called a dermatome. This figure shows the dermatome segments and lists key dermatome levels used by clinicians. Variability and overlap occur, so all dermatome segments are only approximations.

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Visual System: Receptors

NEUROPHYSIOLOGY

A. Eyeball

Lens

Iris Cornea

Suspensory

ligament Ciliary body

Anterior Posterior chamber chamber

Ora containing serrata aqueous humor

Vitreous humor

Retina

Choroid

Sclera

Fovea

Optic nerve

Synaptic ending depolarized

C. Rod in dark

	Rhodopsin
	Metabolic
	energy
Current	Retinene
flow	+
	Opsin
Na+	Vitamin A
permeability
increased
	Circulation

FIGURE 2.32 VISUAL RECEPTORS•

B. Section through retina

D. Rod in light

Photons of light

Lumirhodopsin

Metarhodopsin

Retinene

+

Opsin

Vitamin A

Inner limiting membrane

Axons at surface of retina passing via optic nerve, chiasm, and tract to lateral geniculate body

Ganglion cell

Müller cell (supporting glial cell)

Amacrine cell Bipolar cell Horizontal cell Rod

Cone Pigment cells of choroid

Synaptic ending fully polarized

Synaptic bar

Nucleus

Centriole (basal body)

Na+ permeability decreased

©

The rods and cones of the retina transduce light into electrical signals. As illustrated for the rod, light is absorbed by rhodopsin, and through the second messenger cGMP (not shown), Na channels in the membrane close and the cell hyperpolarizes. Thus, in the

dark the cell is depolarized, but it is hyperpolarized in the light. This electrical response to light is distinct from other receptor responses, in which the response to a stimulus results in a depolarization of the receptor cell membrane.

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