Bain Development, Plasticity, and Repair

Bain Development, Plasticity, and Repair

Development of the Brain

Sensory Systems

What do all the sensory systems have in common?

1. bilateral symmetry: there are two (1 on each side)
2. synaptic breaks: neuron to spine, neuron to brain (can be more than 2 neurons)
3. crossing-over: decussation (pons)
4. divergence of information: information enters the brain & then splits(ex: thalamus → cortex)
5. thalamic relay: except olfactory
6. cortical processing: specific cortical representation

1. sensation (sensory input)
2. perception

4 Steps to Sensory Perception

1. Stimulus- what is being sensed?

ex: light, touch, taste, sound

1. Receptor– made to receive the physical stimulus

        ex: light receptors only receive light stimulation

1. Transduction– how the stimulus becomes a sensation; conversion of physical energy into an electrochemical pattern in neurons
2. Coding– what parts of the brain processes the stimulation

Chapter 6 Vision

1. Sensation to Perception

1. Organisms have developed sensory systems that transduce (translate sensory information into neural signals) different information into action potentials that the nervous system can process
2. sensation= The process of obtaining information about the environment and transmitting it to the brain for processing.
3. Perception= The process of interpreting sensory signals sent to the brain.
4. Bottom-Up Processing vs Top-Down Processing

1. When analyzing how we move between sensation and perception, we describe two processes:
2. Bottom-Up Processing

1. Senses analyze the stimulus (bottom)
2. Analysis of the stimulus begins with the sense receptors and works up to the level of the brain and mind

1. Top-Down Processing

1. Brain-higher level thinking (top)
2. Information processing guided by higher-level of mental processes as we construct perceptions, drawing on our experience and expectations.

1. Vision is one of the most important sensory systems in humans, with about 50 percent of our cerebral cortex responding to visual information but only 3 percent for hearing and 11 percent for touch and pain
2. Visible light, or the energy we can see, is one form of electromagnetic radiation produced by the sun. there’s a small part of the electromagnetic spectrum we could see: 400-700 nm (violet to red).

1. Structure and Function of the Eye and Visual System

1. Eyes are in the bony orbits of the skull, which deflect blows and cushions the eye around fat
2. Eyelids can be opened and closed either voluntarily or involuntarily, to both clean and moisten the front of the eyeball while protecting it from incoming foreign materials
3. The eye is moistened and protected from foreign objects by tears, produced in the lacrimal gland
4. Sclera= the thick tough, white outer covering of the eyeball; helps the fluid-filled eyeball maintain shape
5. Cornea= a clear, curvy covering over the iris and pupil that helps protect the eye and begins focusing light onto the retina

1. Refract, or bend, light rays to form an image on the back of the eye
2. Blood-vessel free

1. Aqueous Humor= clear fluid located in the anterior chamber that nourishes the cornea and lens

1. highest number of nerve fibers (pain receptors) than any other body part

1. Pupil= hole that allows light to enter the eye

1. Light travels from the cornea and aqueous humor of the anterior chamber
2. formed by the circular muscles of the iris
3. Affected by emotional state through the Automatic Nervous System

1. Sympathetic (fight-or-flight) =>pupils dilate
2. Parasympathetic (rest-and-digest) => pupils constricted

1. Iris= colored part of the eye

1. (the muscles of the iris) Controls the amount of light that is allowed into the eye
2. Influenced on the amount of melanin pigment

1. Lens= a clear, flexible structure that adjusts the eye's focus, allowing us to see objects both near and far (accommodation)

1. Lack blood supply
2. Depends on the aqueous humor for nutrients

1. Retina= the layer of light sensitive cells lining the inner eyeball
2. Optic Nerve= the bundle of nerve fibers that carry information from the retina to the brain

[pic 1]

• photoreceptor= Specialized sensory cell in the retina that responds to light.

• Located in the deepest layer of the retina

• optic disk= The area in the retina where blood vessels and the optic nerve exit the eye.

• Contains no photoreceptors; causes blind spots

• macula= A 6 mm round area in the retina that is not covered by blood vessels and that is specialized for detailed vision.

• central vision= The ability to perceive visual stimuli (directly in front) focused on the macula of the retina.

• peripheral vision= The ability to perceive visual stimuli that are off to the side while looking straight ahead.
• Fovea= A small pit in the macula specialized for detailed vision.

• In humans and other primates, contains only one type of photoreceptor, the cones (photoreceptor cells in the retina that are responsible for daylight and color vision)
• Contains two types of photoreceptors, the cones and rods, in other mammals

• Epithelium= The pigmented layer of cells supporting the photoreceptors of the retina and absorb random light.

• Interior of the eye looks black when seen through the pupil
• When a bright light is pointed directly at the eyes, you could see the rich-blood supply within the retina.

• Rods-one type of photoreceptor cells in the retina and they respond to dim light

• Blind Spot- the area on the retina without receptors that respond to light. Therefore an image that falls on this region will NOT be seen.

Mechanical Senses: Touch, Vestibular Sensation, Pain and Hearing

Mechanical senses include: 1). somatosensation (touch); 2). vestibular sensation (balance); 3). pain; and 4). hearing

Stimuli are mechanical, and receptors (specialized neurons that receives specific stimuli) are mechanoreceptors.

1. somatosensation (touch)

1. such as vibration, pressure, and movement of the joints
2. Stimulus-> displacement of the skin causing bending of the receptors
3. Receptors-> 6 types of mechanoreceptors (in order from top to deepest)

1. Meissner corpuscles

1. located just below surface of skin
2. adapt rapidly, so respond to transient stimulation
3. sensitive to light touch, such as the fingers and lips
4. Purpose: to detect shape and textural changes in exploratory and discriminatory touch

1. ex: flutter, wind

1. Merkel discs

1. located deeper in skin
2. slow to adapt, so respond to steady pressure

1. ex. handholding and hugs

1. Ruffini Endings

1. slow to adapt
2. respond to skin stretch when joints move

1. Pacinian corpuscles                         

1. largest & most deeply situated
2. respond to vibration

1. free nerve endings = “nociceptors”

1. respond to temperature & pain

1. hair follicle receptors

1. movement of hairs

1. Transduction->

1. displacement of the skin changes the shape of the receptor
2. when the receptor changes shape, it stops resisting Na+ influx thru its membrane
3. Na+ crosses the membrane and depolarizes the receptor → action potential
4. sensory information is carried to the spinal cord on a spinal nerve

1. There are 31 pairs of spinal nerves
2. each pair represents of specific area of skin → dermatome

1. spinal nerves enter the spinal cord at the dorsal root ganglia, synapse onto another neuron, and are carried up to the brain to be processed

1. Coding->

1. Once inside the brain, the signal goes to the ventral posterior lateral nucleus (VPL), in the thalamus
2. The thalamus directs & sends the information to the correct area of the somatosensory cortex  (postcentral gyrus in the parietal lobe)

1. somatotopic organization
2. sensory homunculus

1. Summary: Touch computer=> skin moved=> Na+ enters the membrane=> depolarization=> action potential=> signal moves from the dorsal root ganglia and to the spine and midbrain=> VPL (Thalamus)=> postcentral gyrus in the parietal lobe (where somatosensory processes- feels sensations)

1. Vestibular Sensation

1. Vestibular organs are in the ear, and detects balance, position and movement of the head
2. Stimulus = movement with otolith (located inside saccule, utricle & semicircular canals- inner ear)
3. Receptors = hair cells

1. The saccule, utricle, & semicircular canals are filled with a jellylike substance.
2. When you shake your head, the jellylike in the canals moves and stimulates hair cells.        

1. Transduction

1. Movement of fluid and otoliths bends the membrane of the hair cells, reducing resistance to Na+ influx (Na+ floods the inside of the membrane) and causing the membrane to depolarize, creating action potential.
2. The information travels on the 8th cranial (vestibulocochlear) nerve to the brainstem (the pons)

1. Coding-

1. Brainstem (the pons) receives signal from 8th cranial nerve and sends to thalamus (where sensory information is relayed to the cerebral cortex; not processed)
2. The thalamus sends signal to angular gyrus (parietal lobe) and the cerebellum

1. Summary:

1. Move head=> jellylike substance in the inner ear and the otoliths moves, causing an influx of Na+ in the membrane=> Na+ floods the inside of the membrane, and resulting in the membrane to depolarize, creating action potential=> The signal travels on the 8th cranial (vestibulocochlear) nerve to the brainstem (the pons), which goes to the thalamus=> the thalamus sends a signal to the angular gyrus of the parietal lobe and the cerebellum.

1. Pain

1. Purpose: evoked by a harmful stimulus, that directs attention
2. Stimulus is noxious (e.g. too much pressure, too much heat or cold, etc)
3. This is subjective; pressure is relative to an individual.
4. Receptors = free nerve endings

1. Are nociceptors (type of receptor that responds to harmful or noxious stimuli)
2. Generally unmyelinated fibers; thus slow transmission of information (myelin increases the speed of the signal by having the signal “jump” between the nodes of ranvier; without myelin, the signal will have to travel down the axon which makes it slower).

1. Unmyelinated fibers=> For dull, aching types of pain
2. Myelinated fibers=> for quick, sharp pains

1. Respond to temperature and pain

1. Capsaicin- a chemical, that’s found in hot peppers, also stimulates these receptors

1. Transduction

1. noxious displacement of the skin, e.g. a pinch on the arm, changes the shape of the nociceptor’s membrane (free nerve endings), reducing its resistance to Na+
2. Na+ crosses the membrane; depolarizing the nociceptor, and causing action potential
3. A spinal nerve carries the signal to the spinal cord on the dorsal root ganglion.
4. In the spinal cord, the nociceptor releases neurotransmitter onto the synapses of another neuron, which carries the signal to the brain on the contralateral side (opposite side) of the spinal cord

1. glutamate for mild pain
2. substance P for intense pain

1. Coding

1. There are 2 pain pathways:

1. Where is the pain?

1. Spinal cord=> VPL nucleus of the thalamus=> somatosensory cortex

1. How do I feel about the pain?

1. Spinal cord=> VPL nucleus of the thalamus=> Limbic system’s emotional response [Amygdala (relating to fear), Hippocampus (memory), and Cingulate cortex (frontal lobe)]

1. Summary:

1. Your hand accidently touched the hot burner on the stove=> the stove was hot (a noxious stimulus which too much heat was applied to the skin)=> touching the stove caused a change in the shape of the nociceptor’s membrane (free nerve ending)=> caused a influx of Na+ across the membrane=> depolarizing the nociceptor and causing action potential=> A spinal nerve carries the signal to the spinal cord on the dorsal root ganglion=> In the spinal cord, the nociceptor releases neurotransmitter (either glutamate for mild pain or substance p for intense pain) onto the synapses of another neuron=> the signal is carried to the brain on the contralateral side (opposite side) of the spinal cord=> travels to the VPL nucleus of the thalamus=> somatosensory cortex (for “where is the pain?”) and limbic system’s emotional response (for “how do I feel about the pain?”)

1. Pain Inhibition

1. Gate Theory- the spinal cord areas that receive messages from pain receptors also receive input from axons descending (coming) from the brain.

1. non-pain stimuli can increase or decrease perception of the intensity of pain
2. ex: distracted from pain, to escape pain

1. Endorphins from the periaqueductal gray bind to opioid receptors in the spinal cord, which decrease the release of substance P (intense pain)

1. Hearing

1. sound waves are periodic compressions (that’s what you hear) of air or water
2. properties of sound waves:

1. amplitude → the intensity of a sound wave

1. how loud a sound is (decibels); the distance between the peak & the trough

1. Frequency→ compressions per second

1. the pitch of a sound; measured in Hz, cycles per second

1. Receptors=> hair cells inside the cochlea
2. Parts of the ear (as we describe how sound travels through the ear)

1. Outer ear

1. “funnels” the sound waves into the ear
2. Pinna

1. helps with sound localization
2. moveable in some species (ex: dogs)

1. To show emotions in animals who can’t show facial emotions

1. not essential for hearing

1. auditory canal- tube into the ear

1. middle ear

1. Purpose: “impedance matching”; increases the pressure of the sound waves, to match the rate it was moving in the air, in order to move the fluid in the cochlea
2. tympanic membrane- “eardrum”

1. vibrates when sound waves hit it, at the same frequency as the sound waves that contact it.

1. ossicles- series of connected bones

1. hammer, anvil, & stirrup (moving from outside to inside)
2. the tympanic membrane (eardrum) hits the hammer

1. Inner ear

1. oval window- a membrane connecting the stirrup and the cochlea

1. receives vibrations from the stirrup
2. transmits waves through the fluid-filled inner ear

1. cochlea- filled with fluid

1. stirrup hits the oval window
2. the vibrations from the oval window cause pressure changes in the fluid within the cochlea

1. basilar membrane- runs through the cochlea

1. has hair cells

1. tectorial membrane- in the cochlea

1. sits on top of hair cells

1. hair cells- auditory receptors!!!

1. in the cochlea
2. sandwiched between the basilar membrane & the tectorial membrane
3. hair cells have cilia that move in response to the vibrations of the fluid
4. hair cells excite the 8th cranial nerve

1. Transduction

1. fluid pressing on hair cells (remember, a hair cell is a sensory neuron) displaces its membrane, allowing Na+ to enter and depolarize it = action potential
2. the signal is carried to the brain by the 8th cranial nerve (vestibulocochlear nerve- balance and hearing)

1. Coding

1. inside the brain, the signal goes to the medial geniculate nucleus (MGN) of the thalamus(synapse)
2. the thalamus relays the signal to the primary auditory cortex (temporal lobe) for processing
3. primary auditory cortex

1. located in the temporal lobe
2. each hemisphere receives most of its information from the opposite ear
3. tonotopic organization
4. each cell in the primary auditory cortex responds best to a specific tone
5. cells of similar frequencies are grouped together

1. Summary

1. Someone whispers in your ear. The sound wave created travels to you ear. It travels past the outer ear (pinna and auditory canal) by localizing and focusing the sound wave within the ear. The sound waves hits the tympanic membrane (ear drum) which begins to vibrate. The vibrating tympanic membrane hits the first ossicles in the middle ear, hammer. The stirrups, which is the last of three ossicles in the middle ear, hits the oval window. The vibrations from the oval window causes the fluid in the cochlea to change, and the hair cells (which are located between the basilar and tectorial membrane) to move. The stimulation causes an influx of Na+ in the membrane, which the membrane became depolarized and action potential to occur. The signal from the action potential is carried to the brain by the 8th cranial nerve (vestibulocochlear nerve), and arrives at the MGN of the thalamus. The MGN  (of the thalamus) relays the signal to the primary auditory cortex (which is located in the temporal lobe) for processing.
2. primary auditory cortex

1. located in the temporal lobe

1. each hemisphere receives most of its
2. information from the opposite ear

1. tonotopic organization

1. each cell in the primary auditory cortex         responds best to a specific tone
2. cells of similar frequencies are grouped         together
3. responds best to a specific tone
4. cells of similar frequencies are grouped together
5. The anterior portion of the primary auditory cortex prefers lower frequencies (lower pitch)
6. Similar to somatotopic organization in the parietal lobe

1. How do we make sense of sound?

1. Frequency Theory

1. basilar membrane vibrates in synchrony with sound waves, causing hair cells to produce action potentials at the same frequency
2. for low frequency (<100 Hz)

1. ex: Hz = number of action potentials

1. problem: doesn’t explain how we hear high frequency sounds, since refractory period prevents hair cells from sending action potentials fast enough

1. Place Theory

1. basilar membrane acts like piano strings, each section “tuned” to a specific frequency, and only vibrates at that frequency
2. for high frequency (>5000 Hz)
3. problem: basilar membrane is continuous, so different sections of it can’t resonate individually

1. Volley Theory

1. For medium frequency (100-5000 Hz)
2. groups of hair cells are “tuned” to a particular phase of a wave and signal when they sense it.         Together, they signal (or “volley”) collective impulses

1. deafness

1. conductive (middle ear) deafness

1. ossicles stiffen and fail to transmit properly; stirrup can’t hit the oval window effectively
2. why? disease, infections, or tumerous bone growth
3. surgery & hearing aids can amplify the stimulus

1. nerve (inner ear) deafness

1. due to damage to hair cells, cochlea, auditory nerve or loss of elasticity of basilar membrane
2. why? inherited, prenatal problems, or childhood disorders
3. hearing aids can’t help

1. Why do my ear pop?

1. Air pressure decreases as the plane rises.  The air trapped in the inner ear pushes out on the tympanic membrane.

1. What is the point of cerumen (ear wax)?

1. Wax has similar functions as hairs in your nose: to stop dirt and bacteria from entering or blocking the ear
2. Made in the outer portion of the auditory canal

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