Week 5- Sensory System 1 (Visual System)

The 8 Sensory Systems

1. Taste

2. Smell

3. Vision

4. Hearing

5. Interoception

6. Vestibular

7. Proprioception

8. Tactile

Where do sensory systems receive information from?

The environment

Each sensory organ is responsive to a particular kind of _______.

Energy

• Mechanical

• Photic Thermal

Energies in the world must be converted into_________ _________ that relay the exact attributes of stimuli.

electrical signals

Type of Sensory System: Mechanical

Modality

• Touch

• Hearing

• Vestibular

• Joint

• Muscle

Adequate Stimuli

• Contact with or deformation of body surface

• Sound vibrations in air or water

• Head movement and orientation

• Position and movement

• Tension

Transduction

Receptor cells detect particular energies or chemicals

What is a change within the receptor cell in response to an outside energy called?

Graded/generator potential

Sensory Systems

• Receive information from the environment

• Exact attributes of stimuli

• Have a common organization

Common organization of sensory systems

• Receptor: Transduces energy into electrical stimulation (graded/generator potential or action potential)

• Chain of neurons

• Importance of the thalamus

• Sensory regions of the cortex

• Topographical organization

Functions of the Visual System

• Sight

• Eye movement control

  • CN 3, 4, and 6

  • Extraocular muscles

• Visual Perception:

  • Recognition of objects and the location of objects

• Information used in postural and limb movement control

Eye movement

• Visual and vestibular systems work together

2 Objectives

a. Keep eyes stable during head movement

b. Direct gaze as visual target

2 Types of Eye Movements

1. Conjugate Movements

2. Vergence Movements

Direction of Gaze is achieved by…

1. Fast saccadic

2. Smooth pursuits

3. Vergence Movements

Fast Saccadic

Fast eye movement to switch gaze from one object to another

Smooth Pursuits

Eye movements that follow a moving target

Vergence Movements

Moves the eyes towards or away from midline

Gaze stabilization is achieved by

• VOR (vestibulo-ocular reflex)

• Optokinetic nystagmus

VOR (Vestibulo-Ocular Reflex)

Adjusts eye position during fast head movements

• WHen the head moves in one direction, eyes move in the opposite direction

• “Doll’s eye reflex”

• Provides stable visual field

• VOR suppression helps maintain voluntary control over the VOR reflex

Optokinetic Nystagmus

Adjusts eye position during slow head movements (or when objects move relative to the head)

Non-neural Peripheral Structures

1. Pupil

2. Cornea

3. Iris

4. Lens

• Ciliary muscles

Pupil

Opening in the middle of the iris where light enters the eye

Cornea

Outer layer at the front of the eye, which bends and focuses the light rays

Iris

Controls the size of the pupil to regulate amount of light entering the eye

Lens

Changes in shape and fine-tunes the eye’s focus

Ciliary muscles

Helps adjust the shape of the lens

Neural peripheral structures

Converts (transduces) light into electrical signals via photoreceptors

1. Retina

2. Macula

3. Fovea

4. Nasal hemiretina

5. Temporal hemiretina

Retina

A thin sheet which lines the rear wall of the eyeball

Macula

Concave area

Fovea

Center part of concave area of the macula

• Location of highest concetration of cones

Nasal Hemiretina

Portion of retina that lies medial to fovea (closer to nose)

Temporal Hemiretina

Portion of retina that lies laterally to the fovea (closer to the temples)

What does the retina do?

Accomplishes transduction and does some elementary processing of visual information

Neural Structures: Retina

1. Photoreceptors

   1. Rods

   2. Cones

2. Bipolar cells

3. Ganglion Cells

4. Optic Nerve

5. Optic Disk

Photoreceptors

Transduce light into electrical signals

Rods

Detect dim light

Cones

Acuity & detec color

Bipolar cells

Link photoreceptors to ganglion cells

Ganglion cells

Output neurons

Optic Nerve

Axons join together to exit the retina

Optic Disk

Blind Spot: Area of no photoreceptors

What is the visual field?

• The amount of the world that can be seen at a point of time, without head or eye movements

• Inverted representation

Which fibers cross?

ONLY nasal fibers!

Central Visual Pathway

Optic Nerve

• Carries information from both visual fields

Optic Chiasm

• Where optic nerves (one from each eye) come together and cross

Optic Tract

• Conveys visual information from the chiasm to lateral geniculate nucleus (LGN) of the thalamus

  • Subcortical projections

    • Lateral Geniculate nucelus (LGN)

    • Superior colliculi
      
      

Optic Radiations

• Emerge from LGN and project to the primary visual cortex (V1)

Visual Cortices

Primary Visual Cortex (V1)

• Discriminates shape, size, texture of objects

Secondary Visual Areas (V2, V3, etc)

• Analyzes for colors & motion

Visual Association Areas

• Other areas of the cerebral cortex

  • Adjusts movements

  • Visually identify objects

• Dorsal Stream/ Action Stream

• Ventral Stream/ Perception Stream

Dorsal Stream/ Action Stream

This information is used to direct movement

• “Where is it?”

Ventral Stream/ Perception Stream

This information is used to recognize visual objects

• “What is it?”

Somatosensation

Information about state of body (skin, musculoskeletal system, visceral organs)

3 Main Functions of Somatosensory Info

1. Exteroception

2. Proprioception

3. Interoception

Exteroception

Sense of direct interactions with the external world and its impact on the body

Proprioception

Sense of one’s own body, involving sensations from muscles, joints, and tendons

Interoception

Sense of the functions and state of internal body organs

Discriminative touch/ tactile perception

• Detection and perception of objects and materials that contact skin

• Contributes to development of body schema & praxis

• Associated with fine motor coordination

Social/affective touch

• Light, caressing touch

• Emotional bonding; contact and comfort

• Emotion and arousal regulation

Proprioception

Perception of:

• Joint & body movements

• Position of the body (body segments) in space

Provides a continuous flow of sensation from the muscles, joints & tendons. This tells us…

• Detection of joint position & movement (including rate & timing of movements)

• Spatial orientation of the body/body parts

• Amount of force our muscles are exerting

Why is tactile touch important?

• Physical safety

• Connection with the social world

• Exploration of the environment

• Development of body scheme and motor skills

• Influence on state regulation (arousal & activation)

Why is proprioception important?

• Physical safety

• Development of motor skills/ motor learning

• Refinement of movement

• Timing of movement

• Influence on state regulation (arousal & activation)

Social-Emotional Development: Animal Studies

• Harlow: Infant monkeys preferred a cuddly surrogate mother without food over a wire mother with food

• Mice separated from their mother’s show long-term changes in behavior

Social-Emotional Development: Humans

• Being held and the comfort it provides is important developmentally

• Early caregiver neglect may change the way the brain is wired

Primary Receptors in Somatosensation

Somatosensation or '“somesthesia”

• Bodily sensations of touch, pain, temperature, and limb position

Cutaneous Receptors

• All cutaneous receptors are stimulated by physical deformation of the receptor itself or the surrounding area

Proprioceptors

• Specialized mechanoreceptors located in the muscles, tendons, joints, and vestibular apparatus of the inner ear

Somatosensory Receptors

• Mechanoreceptors

• Proprioceptors

• Thermoreceptors

• Chemoreceptors

• Nociceptors

*Information from all of these may reach awareness, but much of the information is used to make automatic adjustments/reflexes

Touch

Mechanoreceptors

• Response to mechanical deformation of receptors

Body Part Position & Movement

Proprioceptors (mechanoreceptors)

• Responds to length of muscle, tension of joints and tendons, etc

Temperature

Thermoreceptors

• Responds to heat or cooling

Chemicals

Chemoreceptors

• Responds to substances released by cells

Pain

Nociceptors (subset of all above)

• Sensitive to stimuli that damage or can damage

Overview of Somatosensory Receptors

• Specialized component of the PNS, located at the distal end of peripheral neurons

• Function: Transduction of stimulus into an electrical signal

• Adaptation

  • Slow adapting/tonic

  • Fast adapting/phasic

Slow Adapting/Tonic

Receptor continues to fire during entire presentation of the stimulus

Fast adapting/Phasic

Receptor adapts quickly to stimulus and stops responding

• Phasic receptors

Receptor Density

Number of receptors within a given area of body surface

Receptive Field

Area of skin innervated by a single afferent neuron

Large

• Borders are difficult to define; often covers a large area

Small

• Borders are sharply defined; cover small area

Which body region tends to have a greater density of receptors than the other & smaller receptive fields.

Distal has greater density than proximal

Touch Receptors

Fine Touch

• Meissner’s Corpuscle

• Merkel’s Disk

• Pacinian Corpuscle

• Ruffini Corpuscle/ending

Meissner’s Corpuscle

Light touch & vibration, phasic

Merkel’s Disk

Pressure; tonic

Pacinian Corpuscle

Vibration & touch; phasic

Ruffini Corpuscle/ending

Skin stretch; tonic

Crude/coarse touch

Free nerve endings: pressure, tickle/itch

Proprioceptors

• Body part position (stationary & movement)

• Mechanoreceptors: joint, muscles

  • Muscle Spindle

  • Golgi Tendon Organ

3 Types of Sensory Pathways

1. Conscious Relay Pathway

2. Divergent Pathways

3. Unconscious Pathway

Conscious Relay Pathway

Information about location & type of stimulation to cortex

Divergent Pathway

Transmit info to many areas of the brainstem & cerebrum

Unconscious Pathway

Brings unconscious proprioceptive and other movement-related information to the cerebellum

3 Primary Components of Somatosensory Pathways

1. Receptors

2. Sensory Pathways

3. Brain Centers

Organization of Somatosensory Systems

• 3 Primary components of somatosensory pathways

• Longitudinal and parallel systems

• 3-neuron projection system

  • Only projection neurons are counted (not interneurons)

  • Often named for the origin and termination of the tract that contains the second neuron in the system

Primary Afferent Fiber

• First order neurons are pseudounipolar

  • Not receiving any info from other neurons

  • Cell Body: dorsal root ganglia

    • Distal/peripheral and proximal/central branches

Dorsal Root

Afferent sensory root that enters the spinal cord (dorsal horn)

Dorsal Horn

Gray matter

Dorsal column

White matter

Dermatomes

• The area of skin innervated by axons from cell bodies in a single dorsal root

• These are overlapping