Amplitude: Measured in decibels (dB), which determines the loudness of a sound.
Frequency: Measured in Hertz (Hz), which determines the pitch of a sound.
The Hearing Process: Audition
Parts of the Ear
Outer Ear:
Pinna: The visible part of the ear that collects sound waves traveling through the air.
Auditory Canal: The canal leading inward after the pinna.
Tympanic Membrane (Eardrum): Vibrates when sound waves strike it.
Middle Ear:
Consists of three small bones (ossicles):
Hammer (Malleus)
Anvil (Incus)
Stirrup (Stapes)
These bones transmit vibrations from the tympanic membrane to the inner ear.
The stirrup connects to the Oval Window.
Inner Ear:
Cochlea: A fluid-filled, snail-shaped structure containing nerve endings.
Cilia (Hair-like cells): These are the nerve endings within the cochlea.
Transduction in Hearing
As sound waves enter the cochlea, they travel across the fluid and vibrate the cilia (hair cells).
This vibration converts the sound wave into a neural impulse.
This process, where sensory stimuli are converted into neural impulses, is called transduction.
Analogy: In the eye, rods and cones are the nerve endings that convert light waves into neural impulses. Similarly, in the ear, the cilia are the nerve endings responsible for transduction.
Like other senses, as individuals age, these nerves in the ear can deteriorate, leading to hearing loss.
Summary of Sound Pathway
Sound waves enter through the pinna.
Travel down the auditory canal.
Vibrate the tympanic membrane (outer ear).
Travel through the hammer, anvil, and stirrup.
Vibrate off the oval window (middle ear).
Reach the cochlea (inner ear), where cilia (nerve endings) convert the sound wave into a neural impulse (transduction).
The Auditory Nerve and Brain Processing
The neural impulse is carried from the cochlea to the brain by the auditory nerve.
Analogy: Similar to how the optic nerve carries visual impulses from the eye.
The impulse is then processed in the temporal lobe of the brain.
Hearing Loss
Conductive Hearing Loss:
Caused by damage to the outer or middle ear (e.g., a perforated tympanic membrane).
This type of hearing loss can often be repaired.
Sensorineural Hearing Loss:
Caused by damage to the inner ear (cochlea) or the auditory nerve.
Often permanent and not repairable, frequently occurring due to the deterioration of nerves with aging.
The Sense of Touch and Pain
Processing Location: Touch, pain, pressure, and temperature are processed in the sensory cortex located in the parietal lobe of the brain.
Phantom Limb Sensation: A phenomenon where individuals who have lost a limb still perceive sensations in the missing limb.
This demonstrates that the sensation is processed in the brain's sensory cortex, which remains active even after the limb is gone, rather than in the limb itself.
Gate Control Theory of Pain: This theory suggests that a 'gate' or channel in the spinal cord controls pain signals.
It proposes that the spinal cord contains a neurological “gate” that either blocks pain signals or allows them to pass on to the brain. This gate is normally closed off.
The Sense of Taste: Gustation
Gustation is the technical term for the sense of taste.
Taste signals are processed by taste buds.
Sensory Interaction
Definition: The principle that one sense can influence another.
Examples:
When you have a cold and your nose is blocked, your sense of smell (olfaction) is impaired, which in turn significantly reduces your ability to taste.
The visual appearance of food can influence its perceived taste. If something looks unappetizing, it can reduce your desire to eat it, regardless of its actual taste.
In all senses, nerve endings convert the stimulus into a neural impulse, which is where transduction occurs.
Kinesthetic Sense and Proprioception
Kinesthetic Sense: The sense of body position and movement without needing to look.
Receptors communicate with the brain, allowing movements like raising a hand or walking without visual input (e.g., muscles in the abdomen helping maintain upright posture).
Proprioception: Specifically refers to the sensation of movement and position of one's body parts.
Proprioceptors: Sensory receptors responsible for proprioception.
Impact of Loss: Individuals whose proprioceptors are destroyed (e.g., by a virus with a high fever) lose the communication between their body and brain.
They must consciously look at their limbs (e.g., feet to walk, stomach to sit upright, arm to raise it) to initiate and control movements, as their brain no longer receives automatic feedback on limb position and movement. An example was shown of a man who lost his proprioception below the neck but regained mobility with assistance from a neurophysiologist. He cannot sense where his limbs are or how they are moving without visual input. This highlights that while the brain processes these sensations, the initial input from the proprioceptors is crucial. The brain part eventually slows down and deteriorates over time without these inputs, but remains active initially. Both kinesthetic sense and proprioception contribute to our overall sense of body awareness and control. Without them, even basic movements become challenging and require constant visual monitoring. This illustrates that our brain is processing these sensations, but it is reliant on the input from these receptors. Loss of these receptors leads to severe motor impairment requiring constant visual compensation, demonstrating the critical role of sensory input in motor control and body awareness.