Olfaction and gustation
AKA "chemosensory system"
- Signals are created in response to molecules
- Monitors chemical content of the environment
- Note: "pheromones" do NOT play a role in sexual attraction in humans (that we can observe)
Olfaction
Roof of nasal cavity contains olfactory epithelium
- High surface area "hairs" with a thin mucous coating
"Sniffing" allows air to carry molecules to the olfactory epithelium and activate olfactory neurons
Olfactory bulbs
Mirror symmetry of glomeruli (bulb clusters)
Organized chemotropically (in an unclear fashion)
Olfactory receptors last a number of weeks
Primary olfactory cortex
Less evolutionarily advanced vs vision and hearing
- Cellular structure of 3 layers vs 6
- Includes multiple structure-regions
Further dispersed through the limbic system
Brain regions of importance in smell
Amygdala
Hippocampus
Hypothalamus
Amygdala
Processes "pheromones" of other people, other species, and potentially harmful species, sends to hippocampus
Hippocampus
Stores memories related to smell
Hypothalamus
Motivational/emotional responses from smell
Gustation
Taste occurs via activation of taste receptors within the mouth
- Mostly located on tongue (taste buds, papillae)
- Some on palate, cheeks, epiglottis
Olfaction contributes ~ 80% of taste as well
Taste buds
Clusters of 50 - 100 receptor cells
Located around papillae
Gustatory receptors survive a few weeks
5 primary "flavors" with protein- coupled receptors
- Sweet, bitter, sour, salty, savory (umami)
Transmission of Gustation
3 Cranial Nerves --> Brainstem --> Solitary Nucleus (medulla)
Somatosensory System
Sensation derived from cutaneous stimulation (skin)
Touch, pressure, pain, temperature
Works with skeletal muscle to sense positional information
- NOTE: Pain will be discussed in detail later
Association cortexes
Posterior parietal association cortex
- Spatial information and attention
Dorsolateral prefrontal association cortex
- Decisions to initiate voluntary movement
Motor Cortex and Association
Directly anterior to central sulcus
- Brain region in posterior frontal lobe which sends output to muscles of the body
--- Directly involved in control of movement
--- Neurons in different regions connect to different muscles
- Remember the homunculus!
Cortical Areas' Role in Voluntary Movement
Prefrontal cortex: decides to move
Supplementary motor area and premotor area: movement is planned in conjunction with thalamus and basal ganglia
Primary motor cortex sends signals via lateral pathway
Lateral pathway signals spinal motor neurons to initia
Primary Motor Cortex Function
Orchestrated movements
Stimulating the same brain region can produce opposite movements
Mapped out somatotropically
Pyramidal vs Extrapyramidal - Descending Motor Pathways
Anatomy
- Goes from cortex to muscle
- Goes from basal ganglia and other structures of the central nervous system
Physiologic movements
- Voluntary
- Involuntary
Pyramidal neural system
Higher cognitive type movement
Cells from cerebral cortex
Move contralateral and form a corticospinal tract
Extrapyramidal Neural System
Posture, reflexes, and stereotypical (repetitive) movement
Fibers mostly are ipsilateral
Main contributors are basal ganglia and cerebellum
Movement and the Cerebellum
Receives information from motor cortexes, brain stem nuclei, and somatosensory & vestibular information
Complex internal structure with multiple cell types and lobes
Rapid, automated movements like swinging, dancing, etc
Removes tremors and jerking
Movement and the Basal Ganglia
Receives information from the cortical areas and transmit to cortex via thalamus
Involved in cognitive functions and motor output
Circuit to the motor cortex and association areas
Basal ganglia regions --> ventral anterior nucleus and ventrolateral nucleu
A little on pain
Noxious stimulus
Indicator of actual or potential bodily harm
Can be influenced by psychological state
Include multiple brain regions including the anterolateral system
What are muscles?
Cells that induce movement
Smooth
- Usually innervated subconsciously by PNS
- Cardiac, blood vessels, gut
Skeletal
- Attached to bones and structures
--- Movement of limbs
- Mostly voluntary
Skeletal Muscle Structure
Muscles attached to structures via tendons and ligaments
Muscle fibers
- Covered by sarcolemma
- Packed with myofibrils which respond to neural impulses by contracting
- Innervated by alpha motor neurons
Myofibrils
Sarcomeres are the segments that interlock
- Actin /thin filaments
- Myosin/ thick filaments
Striated with M lines and Z lines, with a myosin-only center (H zone)
Function by sliding over one another and "hooking" to create muscular contraction
Summary of events in Muscular Contraction
Proprioception
Your real 6th sense!
Systems involved in proprioception
Muscles, tendons, and joints
- Stretching, velocity, pressure
Visual system
- Seeing movement, orientation
Tactile senses
- Pressure on the skin
Vestibular system
- Fluid in semicircular canals that sense gravity and movement with hair cells, sacs, and ot
How muscles balance us
Specialized proprioreceptors: muscle spindles
- AKA "stretch receptors"
- Informs the CNS on position and contraction of skeletal muscles
- Initiation of reflexes when load bearing occurs
- Reflex arcs!
Reflex Arcs
Unconscious motor response to a sensory stimulus
May not directly involve the brain!
1. Receptor senses change
2. Sensory neuron carries impulse into CNS
3. Interneuron carries impulse within CNS
4. Motor neuron- carries impulse away from CNS
5. Effector
The spinal polysynaptic reflex
Stretch reflex
- Extension of muscle fibers excite stretch receptors
- Receptors relay information to spinal cord
- Alpha motor neurons contract
- Stretch is then counteracted
Withdrawal reflex
Polysynaptic reflex
Works similar to the stretch reflex
Sends excitatory input to interneurons in response to noxious stimuli
Other considerations
Golgi tendons
- Provide information to CNS about muscle tension
Muscle spindles
- Respond to muscle length
Reciprocal innervation
- Physically permits smooth movement
When one muscle contracts, opposing muscle relaxes