Sensory Receptors and Cranial Nerves Lecture Notes

Neurophysiology of Reflexes

  • Hyperreflexivity:

    • Definition: An overactive or overresponsive reflex response.

  • Hypoactivity:

    • Definition: A decreased reflex response.

  • Types of Reflexes:

    • Somatic Reflex:

      • Targets skeletal muscles via spinal nerves.

    • Spinal Cord Reflex:

      • Involves spinal nerves that have synapses only in the Central Nervous System (CNS).

    • Stretch Reflex:

      • A specific spinal cord reflex that helps maintain body position by correcting muscle stretch.

    • Deep Tendon Reflex:

      • Another spinal cord reflex that helps maintain body position by correcting muscle over-contraction.

  • Key Terminology:

    • Acuity:

      • The ability to discriminate between stimuli.

    • Punctate Distribution:

      • Describes sensory receptors in the skin that are clustered and not evenly distributed.

    • Rapidly Adapting Receptors:

      • Respond to a new or changing stimulus but not to a maintained stimulus.

    • Receptive Field:

      • The specific area of sensory space where a stimulus triggers the firing of a sensory neuron.

    • Slowly Adapting Receptors:

      • Maintain their response to a constant stimulus over time.

    • Two-Point Threshold:

      • The smallest separation between two points on the skin that can be perceived as two distinct stimuli rather than one. A higher threshold indicates a larger receptive field and reduced acuity.

Classification of Sensory Receptors and Senses

  • Classification by Location of Receptor:

    • Exteroreceptors:

      • Located on the body surface, receive stimuli from outside the body (e.g., touch and vision).

    • Interoreceptors (Visceroceptors):

      • Located within the body, detect internal stimuli such as blood pressure and cerebrospinal fluid (CSF) pH.

    • Proprioceptors:

      • Detect stimuli related to body position in space; contribute to posture, balance, and muscle tone; found in muscles, joints, and the inner ear.

  • Classification by Type of Stimulus:

    • Mechanoreceptors:

      • Stimulated by mechanical forces causing tissue deformation (e.g., touch and hearing).

    • Thermoreceptors:

      • Detect changes in temperature.

    • Photoreceptors:

      • Stimulated by light (e.g., vision).

    • Chemoreceptors:

      • Detect chemicals in a solution (e.g., taste, smell, oxygen).

    • Nociceptors:

      • Detect pain stimuli.

  • Classification by Distribution of Receptors:

    • General Senses:

      • Widely distributed throughout the body, including skin, skeletal muscle, and internal organs; sense pain, touch, temperature, and proprioception.

    • Special Senses:

      • Densely located in complex sensory organs within the head; include vision, taste, smell, hearing, and proprioception (inner ear).

Cranial Nerves Overview

  • General Characteristics:

    • 12 pairs of cranial nerves originate from nuclei in the brain, primarily affect the head and neck.

    • Numbered from front to back based on their location in the brain.

    • Functions categorized as sensory, motor, or parasympathetic.

Specific Cranial Nerves

  1. Olfactory Nerve (I):

    • Type: Sensory only

    • Function: Responsible for the sense of smell.

  2. Optic Nerve (II):

    • Type: Sensory only

    • Function: Responsible for vision.

    • Pathways:

      • Optic Chiasm: Where optic nerves cross; damage can affect peripheral vision.

      • Optic Tract: Damage can affect half of the visual field.

  3. Oculomotor Nerve (III):

    • Type: Motor only

    • Functions:

      • Somatic motor: Controls most extrinsic eye muscles (superior rectus, inferior rectus, medial rectus, inferior oblique) and raises eyelid.

      • Parasympathetic: Contracts intrinsic eye muscles, constricts pupil size, alters lens curvature for focusing on near objects.

  4. Trochlear Nerve (IV):

    • Type: Motor only

    • Function: Contracts superior oblique muscles of the eye.

  5. Trigeminal Nerve (V):

    • Type: Mixed (sensory & motor)

    • Functions:

      • Sensory: Provides sensation to the upper, middle, and lower face.

      • Motor: Innervates muscles associated with mandible.

  6. Abducens Nerve (VI):

    • Type: Motor only

    • Function: Contracts lateral rectus muscles of the eye to abduct the eye.

  7. Facial Nerve (VII):

    • Type: Mixed (sensory & motor)

    • Functions:

      • Sensory: Taste from the anterior 2/3 of the tongue.

      • Somatic motor: Controls facial expression.

      • Parasympathetic: Stimulates lacrimal glands and salivary glands.

  8. Vestibulocochlear Nerve (VIII):

    • Type: Sensory only

    • Functions:

      • Receives information from the inner ear.

        • Vestibular Apparatus: Responsible for balance and head position relative to gravity.

        • Cochlea: Responsible for hearing.

  9. Glossopharyngeal Nerve (IX):

    • Type: Mixed (sensory & motor)

    • Functions:

      • Sensory: Taste from posterior 1/3 of tongue; sensory input from the back of the mouth and pharynx (throat).

      • Somatic motor: Inputs to throat for swallowing.

      • Parasympathetic: Stimulates salivary glands.

  10. Vagus Nerve (X):

    • Type: Mixed (sensory & motor)

    • Characteristics:

      • Only cranial nerve to extend beyond the head/neck region.

    • Functions:

      • Sensory: Input from lower throat, thoracic organs, abdominal organs.

      • Somatic motor: Partial input to tongue and throat.

      • Parasympathetic: Various functions, e.g., decreases heart rate and increases digestion.

  11. Accessory Nerve (XI):

    • Type: Motor only

    • Functions:

      • Contraction of sternocleidomastoid and trapezius upper fibers.

  12. Hypoglossal Nerve (XII):

    • Type: Motor only

    • Functions:

      • Contraction of intrinsic tongue muscles and extrinsic tongue muscles.