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Ocular Reflex Responses (continued)

C. Pupillary Dark Response

The pupils normally dilate (increase in size) when it is dark (i.e., when light is removed). This response involves the relaxation of the iris sphincter and contraction of the iris dilator. The iris dilator is controlled by the sympathetic nervous system.

The pupillary dark reflex neural circuit: The pathway controlling pupil dilation involves the

• retina and the optic tract fibers terminating on neurons in the hypothalamus and the
  
• axons of the hypothalamic neurons that descend to the spinal cord to end on the
  
• sympathetic preganglionic neurons in the lateral horn of spinal cord segments T1 to T3, which send their axons out the spinal cord to end on the
  
• sympathetic neurons in the superior cervical ganglion, which send their
  
• sympathetic postganglionic axons in the long ciliary nerve to the
  
• iris dilator.
  

Axons from the superior cervical ganglion also innervate the face vasculature, sweat and lachrymal glands and the eyelid tarsal muscles. When the superior cervical ganglion or its axons are damaged, a constellation of symptoms, known as Horner's syndrome, result. This syndrome is characterized by miosis (pupil constriction), anhidrosis (loss of sweating), pseudoptosis (mild eyelid droop), enopthalmosis (sunken eye) and flushing of the face.

D. The Accommodation Response

The accommodation response is elicited when the viewer directs his eyes from a distant (greater than 30 ft. away) object to a nearby object (Nolte, Figure 17-40, Pg. 447). The stimulus is an “out-of-focus” image. The accommodation (near point) response is consensual (i.e., it involves the actions of the muscles of both eyes). The accommodation response involves three actions:

Pupil accommodation: The action of the iris sphincter was covered in the section on the pupillary light reflex. During accommodation, pupil constriction utilizes the "pin-hole" effect and increases the depth of focus of the eye by blocking the light scattered by the periphery of the cornea (Nolte, Figure 17-39, Pg. 447). The iris sphincter is innervated by the postganglionic parasympathetic axons (short ciliary nerve fibers) of the ciliary ganglion (Figure 7.3).

Lens accommodation: Lens accommodation increases the curvature of the lens, which increases its refractive (focusing) power. The ciliary muscles are responsible for the lens accommodation response. They control the tension on the zonules, which are attached to the elastic lens capsule at one end and anchored to the ciliary body at the other end (Figure 7.4).

Figure 7.4 The ciliary muscles, which control the position of the ciliary processes and the tension on the zonule, control the shape of the lens. The ciliary muscles function as a sphincter and when contracted pull the ciliary body toward the lens to decrease tension on the zonules (see Figure 7.5). The decreased tension allows the lens to increase its curvature and refractive (focusing) power. When the ciliary muscle is relaxed, the ciliary body is not pulled toward the lens, and the tension on the zonules is higher. High tension on the zonules pulls radially on the lens capsule and flattens the lens for distance vision. The ciliary muscles are innervated by the postganglionic parasympathetic axons (short ciliary nerve fibers) of the ciliary ganglion

Figure 7.5 The accommodation response of the lens: comparing the lens shape during near vision (contraction of the ciliary muscle during accommodation) with lens shape during distance vision (relaxation of the ciliary muscle).

Convergence in accommodation: When shifting one's view from a distant object to a nearby object, the eyes converge (are directed nasally) to keep the object's image focused on the foveae of the two eyes. This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles. The medial rectus attaches to the medial aspect of the eye and its contraction directs the eye nasally (adducts the eye). The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve.

The accommodation neural circuit: The circuitry of the accommodation response is more complex than that of the pupillary light reflex (Figure 7.6).

The afferent limb of the circuit includes the

• retina (with the retinal ganglion axons in the optic nerve, chiasm and tract),
  
• lateral geniculate body (with axons in the optic radiations), and
  
• visual cortex.
  

Ocular motor control neurons are interposed between the afferent and efferent limbs of this circuit and include the

• visual association cortex, which
  
  • determines the image is "out-of-focus"
    
  • sends corrective signals via the internal capsule and crus cerebri to the
    
• supraoculomotor nuclei, which
  
  • is located immediately superior to the oculomotor nuclei
    
  • generates motor control signals that initiate the accommodation response
    
  • sends these control signals bilaterally to the oculomotor complex.
    

The efferent limb of this system has two components: the

• Edinger-Westphal nucleus, which
  
  • sends its axons in the oculomotor nerve to
    
  • control the ciliary ganglion, which
    
    • sends it axons in the short ciliary nerve to
      
    • control the iris sphincter and the ciliary muscle/zonules/lens of the eye
      
• oculomotor neurons, which
  
  • sends its axons in the oculomotor nerve to
    
  • control the medial rectus
    
  • converge the two eyes.
    

Figure 7.6 The accommodation pathway includes the afferent limb, which consists of the entire visual pathway; the higher motor control structures, which includes an area in the visual association cortex and the supraoculomotor area; and the efferent limb, which includes the oculomotor nuclei and ciliary ganglion. The lines ending with an arrow indicate axons terminating in the structure at the tip of the arrow. The lines beginning with a dot indicate axons originating in the structure containing the dot. During accommodation three motor responses occur: convergence (medial rectus contracts to direct the eye nasally), pupil constriction (iris sphincter contracts to decrease the iris aperture) and lens accommodation (ciliary muscles contract to decrease tension on the zonules).