Biology 110
Study Notes Exam 2
- central nervous system (CNS): brain & spinal cord
- peripheral nervous system (PNS): cranial & spinal nerves & their branches
o somatic
nervous system
o autonomic
nervous system
- permits sensory input
- performs integration
- stimulates motor output
- neurons: primary cell of nervous tissue
o have 3 parts:
• cell body: contains nucleus & other organelles
• dendrite(s): receive information from other neurons & conduct information to cell body
• axon: conducts nerve impulses away from cell body
o 3 types of neurons:
• motor neurons: conduct impulses from CNS to effector organs (muscles or glands)
⋅ efferent neurons (take impulse away from CNS)
⋅ have short dendrites & long axon
• sensory neurons: conduct impulses from peripheral body parts to CNS
⋅ afferent neurons (take impulse toward CNS)
⋅ begin at receptors, which are sensory nerve endings sensitive to internal & external stimuli
• interneuron: found only in CNS; conducts impulses between parts of CNS
o fibers: dendrites & axons of neurons
o myelin sheath: Schwann cell wrapped around fibers outside CNS
• provides electrical insulation & much faster impulse conduction
• neurilemmal sheath: cytoplasm & nucleus of Schwann cell; promotes regeneration of injured fibers
• nodes of Ranvier: gaps between adjacent Schwann cells
• multiple sclerosis (MS): chronic autoimmune disease where body's immune cells attack & destroy myelin sheath (forms hardened scars or scleroses)
- neuroglial cells: support, protect & nourish neurons
o PNS neuroglial cells:
• Schwann cells: form myelin sheath in PNS
o CNS
neuroglial cells:
• microglial cells: phagocytes; destroy microbes (bacteria) & clear debris
• astrocytes: form bridges between neurons & capillaries; may deliver nutrients to neurons
• oligodendroglial cells: form myelin sheath in CNS
• ependymal cells: ciliated cells that circulate cerebrospinal fluid (CSF) in ventricles (cavities) in CNS
Nerve Impulses:
- inside of neuron is negative with respect to outside (due to negatively charged proteins in cell & Na+/K+ pump that pumps sodium out of cell)
- resting potential: charge (voltage) across membrane; normally -70mV
- depolarization: sodium flows into neuron; neuron becomes more positive inside
- action potential: if enough sodium ions flow in, nerve cell fires or sends nerve impulse (sodium ions) along axon
- repolarization: pump sends potassium ions out of cell to return negative resting potential
- in myelinated fibers, nerve impulse skips from one node of Ranvier to next (faster)
The Synapse:
- synapse: junction of axon of one neuron & another neuron (usually at dendrite)
o axon
membrane of first neuron is presynaptic membrane; dendrite membrane of second neuron is postsynaptic
membrane
o synaptic cleft: small gap between neurons in synapse
o neurotransmitters: chemicals that transmit impulse across synapse
• acetyl choline (ACh): neurotransmitter used throughout nervous system
⋅ broken down after signal reaches postsynaptic membrane by acetylcholinesterase (AChE)
• norepinephrine (NE): another common neurotransmitter
• serotonin & dopamine are neurotransmitters associated with behavior (mood, tension, learning memory)
• stimulatory neurotransmitters increase likelihood of impulse
• inhibitory neurotransmitters decrease likelihood of impulse
• Parkinson Disease is due to a deficiency of dopamine
⋅ characterized by wide-eyed, unblinking expression, muscular rigidity & involuntary tremors
• In Huntington Disease there is a decrease in the neurotransmitter GABA
⋅ autosomal dominant disorder due to chromosomal changes (trinucleotide repeats)
⋅ leads to deterioration of nervous system; constant thrashing movements & eventually insanity & death
• Alzheimer disease is due to loss of neurons that use ACh as neurotransmitter
Nerves: cordlike organs of PNS consisting of bundles of axons enclosed in layers of connective tissue
- sensory (afferent) nerves: carry impulses toward CNS
- motor (efferent) nerves: carry impulses away from CNS
- mixed nerves: contain both sensory & motor fibers (can be somatic &/or autonomic fibers)
- white matter: myelinated fibers in CNS
- gray matter: mostly nerve cell bodies & unmyelinated fibers in CNS
Reflexes & the Reflex Arc:
- reflexes: automatic, involuntary responses to stimuli
- control heart & breathing rate, body temperature, swallowing, sneezing, reactions to pain, etc.
- sensory receptor: distal end of sensory neuron; responds to stimulus
- sensory neuron: carries impulse from receptor to axon terminals in gray matter of spinal cord (or brain stem)
o ganglia: collections of neuron cell bodies associated with nerves in PNS
o dorsal root ganglion: cell bodies of sensory nerves entering spinal cord
- integration center: within CNS gray matter, signal travels across synapse or through interneuron, generating a response
- motor neuron: response impulse is sent from sensory neuron or interneuron through motor neuron to effector
- effector: muscle or gland that carries out response (reflex)
- stretch
reflex: causes contraction of skeletal
muscle in response to muscle stretching
- tendon reflex: causes muscle relaxation in response to increasing muscle tension (to prevent tendons from tearing)
- flexor
(withdrawal) reflex: causes muscle
contraction to move body region away from painful stimulus
o plantar
flexion reflex: curling under of toes in
response to stimulation of lateral outer margin of toe
• damage
to descending motor pathways causes great toe extension (Babiski sign),
although a normal response in infants
Central Nervous System
Brain:
- Brain
Stem:
o midbrain: between diencephalon & pons
• cerebral aqueduct: connects third & fourth ventricles
• corpora quadrigemina: midbrain nuclei
• contains
visual , auditory & touch reflex centers
o pons: between midbrain & medulla oblongata
• conduction tracts (pons = bridge); complete pathways between higher brain centers & spinal cord; relays between motor cortex & cerebellum
• works with medulla to regulate breathing rate
o medulla oblongata: most inferior part of brain stem; blends with spinal cord
• tracts cross over to opposite side before entering spinal cord... cerebral hemispheres control voluntary movements of muscles on opposite side of body
• sensory & visceral motor nuclei (control heart rate & blood vessel diameter, breathing rate, vomiting, coughing...)
- Diencephalon: central core of forebrain; surrounded by cerebral hemispheres
o thalamus: most (80%) of diencephalon & forms walls of third ventricle
• central relay station: sorts & edits sensory information & relays to appropriate area of sensory cortex & association areas
• most inputs to cerebral cortex travel through thalamic nuclei (emotion, visceral & motor activity...)
o hypothalamus: below thalamus; merges with midbrain inferiorly
• connected to pituitary gland by stalk called infundibulum
• main visceral control center; controls homeostasis
⋅ roles:
autonomic control center, center
for emotional response, body
temperature regulation, regulation of food intake, regulation of water balance
& thirst, regulation of sleep-wake cycles, control of endocrine system
function
o epithalamus: dorsal area of diencephalons, forms roof of third ventricle
• pineal
gland: secretes hormone melatonin - helps in regulation of
sleep-wake cycle
• choroid
plexus: secretes CSF
- Reticular Formation: extends through core of brainstem
o reticular activating system: maintains cerebral cortical alertness
o filters out repetitive stimuli
o motor nuclei help regulate skeletal & visceral muscle activity
- Cerebellum: dorsal to pons & medulla; inferior to occipital lobes; ~ 11% of total brain mass
o bilaterally symmetrical with two hemispheres connected medially by vermis
o surface gray matter & deep white matter
o processes information from cerebral motor cortex & from sensory pathways
o sends instructions to cerebral motor cortex & motor centers to regulate balance, posture & coordinated skeletal muscle movement
- Cerebral
Hemispheres:
o superior part of brain; ~ 83% of total brain mass
o 3 regions: cerebral cortex (gray matter), white matter & basal nuclei
o gyri: elevated ridges of brain tissue, separated by shallow grooves called sulci
o fissures: deeper grooves separating larger regions of brain
o deep sulci divide cerebral hemisphere into 5 lobes: frontal, parietal, temporal, occipital & insula
o cerebral cortex: conscious mind; awareness, communication, memory & understanding & initiation of voluntary movements
• billions of neurons; ~ 40% of total brain mass
• Brodmann areas: 52 structural regions of cortex
• cortex has 3 types of functional areas:
⋅ motor areas: control voluntary motor functions
⋅ sensory areas: conscious awareness of sensation
⋅ association areas: integrate information for voluntary action
• each hemisphere controls sensory & motor functions of opposite side of body
• cerebral palsy: spastic weakness of arms & legs due to damage to motor areas of cerebral cortex from a lack of oxygen during birth
• Broca's area: near base of primary motor area
⋅ present in one hemisphere only (generally left)
⋅ special motor speech area - controls muscles of tongue, throat & lips during (& possibly in planning of) speech
• stroke: blood supply to brain temporarily halted due to burst blood vessel or blood clot
⋅ can damage Broca's area - often speech problems after stroke
- Basal Nuclei (basal ganglia): subcortical nuclei deep within cerebral white matter
o regulate motor control (muscle movements, & perhaps also attention & cognition)
o amygdala: on tail of caudate nucleus; part of limbic system
- Limbic
System: regions of the medial aspects of
each cerebral hemisphere & diencephalon encircling brain stem (limbus =
ring) & linked by the fornix
o emotional brain
o amygdala: recognizes angry or fearful facial expressions, assesses danger & elicits fear response
o cingulate gyrus: regulation of expression of emotions & feelings of frustration
o hippocampus: plays a role in storing information in long-term memory
Spinal Cord:
Anatomy of Spinal Cord:
- 31 pairs of spinal nerves attach to spinal cord by paired roots
- spinal nerves: communicate between spinal cord & body tissues served
• dorsal roots carry sensory tracts to spinal cord
• ventral roots send motor tracts away from spinal cord
- central gray matter (unmyelinated fibers): anterior, posterior & lateral horns
- outer white matter forms columns
o ascending& descending tracts: carry nerve impulses through CNS
Functions:
- center for thousands of reflex arcs using spinal nerves
- communication between brain & PNS
Meninges & Cerebrospinal Fluid (CSF)
- meninges: 3 connective tissue membranes just external to spinal cord (& brain)
o dura
mater: outer layer
o arachnoid: middle layer; subarachnoid space
contains large capillaries & CSF
o pia
mater: innermost
layer; just superficial to spinal cord
- cerebrospinal fluid: formed by choroid plexuses hanging from the roof of the ventricles in the brain
o within ventricles of brain & central canal of spinal cord
o supplies CNS with nutrients from blood & returns wastes to blood
Peripheral Nervous System (PNS)
Somatic Nervous System: includes all nerves that serve the musculoskeletal system & exterior sense organs (skin)
- exterior sense organs bear sensory receptors that receive external stimuli & send impulses to CNS
- CNS response is sent through motor neurons to effectors (skeletal muscles)
Cranial Nerves: know names, #'s & basic functions
Spinal Nerves: 31 pairs
- 8 pairs of cervical; 12 pairs of thoracic; 5 pairs of lumbar ; 5 pairs of sacral; 1 pair of coccygeal
- for each impulse there is 1 autonomic ganglion & 2 motor neurons
- first motor neuron has cell body in CNS & preganglionic fiber
- second motor neuron has cell body within ganglion & postganglionic fiber
o short preganglionic fibers arise from thoracolumbar (thoracic & lumbar) region of spinal cord
o long postganglionic fibers supply the visceral organs, as well as smooth muscle of structures of the skin (sweat glands, arrector pili muscles) & smooth muscle of all arteries and veins
o postganglionic fibers use neurotransmitter norepinephrine
o "fight or flight" system... activated during emergency or threatening/stressful situations; rapid heart & breathing rate, greatly decreased gastrointestinal & urinary tract activity, pupils dilated. Visceral blood vessels are constricted & blood is shunted to active skeletal muscles
- Parasympathetic Division:
o long preganglionic fibers arise from cranial nerves II, VII, IX & X & sacral region of spinal cord (craniosacral outflow)
o short postganglionic fibers
o neurons use neurotransmitter acetylcholine
o "resting & digesting" system... low energy use (low blood pressure, heart rate & respiratory rate), active digestion of food & elimination of waste; pupils of eyes are constricted & lenses accommodated for close vision
Effects of aging:
- by age 60, thousands of neurons are lost daily, particularly in cerebral cortex
- neurotransmitter production decreases (slower synaptic transmission)
- increased incidence of Alzheimer disease
Skin: contains sensory receptors for touch, pressure, pain & temperature
- touch receptors: Meissner's corpuscles, Merkel disks & Krause end bulbs
o Meissner's corpuscles concentrated in fingertips, palms, lips, tongue, nipples, penis & clitoris
- pressure receptors: Pacinian corpuscles (deep in dermis) & Ruffini endings (encapsulated networks of nerve fibers)
- temperature & pain receptors: free nerve endings in epidermis
Visceral Organs: receptors that aid in homeostasis
- stretch receptors in lungs respond to lung expansion; receptors in aorta & carotid arteries respond to pH & oxygen levels in blood; osmoreceptors in hypothalamus respond to solute concentration in blood
- pain receptors in internal organs contact neurons in spinal cord also contacted by pain receptors in skin
- referred pain: pain in internal organ felt in skin (e.g.: pain in heart is felt in skin of left shoulder & arm)
Muscles & Joints:
- proprioceptors are located within joints & associated tendons & ligaments that respond to stretch, pressure & pain
- stretch receptors in muscle spindles respond to muscle stretch
o knee-jerk reflex: when patellar tendon at knee is tapped in crossed leg, muscles & tendons in thigh are stretched & respond by contracting
Chemoreceptors: taste in food chemicals & chemical odors in air are sensed by chemoreceptors in the mouth & nose
- taste buds within papillae on surface of tongue respond to 4 types of taste (sweet, salty, sour & bitter)
- chemicals in foods bind to microvilli in epithelial taste bud cells & generate nerve impulses that are sent to the gustatory cortex of the brain
- olfactory cells for smell are located at the roof of the nasal cavity
- olfactory receptor cells are neurons that have olfactory cilia covered by a coat of mucus that dissolves airborne chemicals (odors)
- there are perhaps more than 1000 types of receptors; the specific odor detected depends on the combination of olfactory cells stimulated
- the olfactory cells stimulate neurons in the olfactory bulbs, which send the stimulus to the olfactory cortex of the brain
- taste is possibly about 80% smell... when olfactory receptors are blocked (nasal congestion), taste of food is partially to mostly blocked
Photoreceptors: light-sensitive receptors located in the eyes within the orbits of the skull
- eyebrows & eyelashes: divert sweat & debris from around eyes
- eyelids: thin skin covered folds of epithelium supported by connective tissue; contraction of the orbicularis oculi muscle closes eyelids & contraction of the levator palpebrae superioris opens eyelids
- conjunctiva: transparent mucus membrane that lines the eyelids & reflects back over the anterior surface of the eyeballs (except cornea)
- lacrimal
gland: lies within
the orbit above the eye; releases a moisturizing lacrimal secretion (tears)
containing mucus, antibodies & the antimicrobial enzymes
- extrinsic eye muscles:
o lateral rectus: moves eye laterally (control by CN VI)
o medial rectus: moves eye medially (control by CN III)
o superior rectus: elevates eye (control by CN III)
o inferior rectus: depresses eye (control by CN III)
o inferior oblique: elevates eye & turns it laterally (control by CN III)
o superior oblique: depresses eye & turns it laterally (control by CN IV)
- structure
of the eye
o sclera: white of eye
• cornea: anterior 1/6 of sclera; transparent CT
o choroid: highly vascular dark brown membrane; blood vessels supply nutrients to all tunics
• melanin from melanocytes absorb light & prevent scattering
• ciliary body: contains smooth muscle bundles (ciliary muscles) that control lens shape
• lens: binconvex, transparent structure attached to ciliary body
⋅ divides eye into anterior cavity in front of lens & posterior cavity behind lens
⋅ anterior cavity is filled with watery aqueous humor; posterior cavity is filled with gel-like vitreous humor
⋅ glaucoma: due to faulty drainage & thus buildup of aqueous humor in anterior chamber; pressure buildup can damage photoreceptors & lead to partial or total blindness
• iris: lies between cornea & lens; has round central opening called pupil
• pupil opens & closes to control light entry into eye; controlled by smooth muscle in iris
o retina: consists of 2 layers:
• outer pigmented layer: contains phagocytic pigmented epithelial cells that absorb light & prevent scattering
• inner neural layer: 3 main types of neurons
⋅ photoreceptors: rods & cones
* rods: respond to dim light; blurry shades of gray
* cones: respond to bright light; sharp, color vision
⋅ bipolar cells: link between photoreceptors & ganglion cells
⋅ ganglion cells: receive input from bipolar cells & their axons leave eye as optic nerve
• blind spot (optic disc): location on retina where the optic nerve exits eye
• fovea centralis: only cones present; region of greatest visual acuity
- rods: contain rhodopsin, which consists of the pigment retinal (a vitamin A derivative) linked to a form of opsin (protein)
o rods are sensitive to faint light (night) & motion, but do not detect color or fine detail
o hence, at night objects are seen as blurry & in shades of gray
o when light strikes rhodopsin, it breaks down & generates a nerve impulse
o in dim light, the pupils dilate to allow more light to reach the retina; at the same time, rhodopsin forms to improve vision -> 2 delays to adjustment to dim light
- cones: contain one of 3 opsins with retinal, allowing absorption of 3 different colors of light (red, green & blue)
o cones are sensitive to bright light & detect fine detail & color
o color observed depends on combination of cones (RGB) stimulated
o color blindness: usually, one type of cone is deficient (red or green cone deficiency is most common -> red-green color-blindness)
- stereoscopic vision: each eye forms image from different angle & sends independent stimulus to brain
o optic nerve fibers cross at optic chiasma, & one hemisphere of brain receives information from both eyes about the same part of an object
o the
2 hemispheres share information to arrive at complete 3D image
o accommodation: bulging of lens by contraction of ciliary muscle to view close objects (no accommodation is required to view distant objects)
o if accommodation is not enough to focus image, prescription lenses may be required
o cataract: lens becomes opaque (perhaps due to oxidation of lens proteins) & unable to transmit light to retina (lens can be surgically replaced with plastic lens)
- external ear: consists of pinna (auricle or visible portion of ear) & external auditory canal
o ceruminous glands: modified sweat glands in upper wall of canal that secrete earwax, which protects against particle entry into middle & inner ear
- middle ear: begins at tympanic membrane (eardrum) & ends at 2 small openings (oval & round window) to inner ear
o contains 3 ossicles (small bones) suspended by ligaments
• malleus (hammer) - attaches to eardrum
• incus (anvil)
• stapes (stirrup) - attaches to oval window of vestibule (inner ear)
o the anterior wall of the middle ear contains an opening to the auditory (eustachian) tube (leads to nasopharynx) that allows pressure equalization
o otitis media: middle ear infection causing inflammation & blockage
• myringotomy: incision in eardrum, followed by insertion of tube to equalize pressure (can be surgically removed later)
- inner ear:
o vestibule: central region composed of 2 sacs - the saccule & utricle
• contains static equilibrium receptors called maculae
o semicircular canals: 3 rounded tubes projecting from utricle through swellings called ampullae
• ampullae contain dynamic equilibrium receptors called cristae ampullaris
o cochlea: snail-shaped chamber extending from the saccule
• contains cochlear duct housing the organ of Corti, which contains receptors for hearing
- Hearing:
o sound waves enter the auditory canal & cause the tympanic membrane to vibrate
o the ossicles receive the vibration, amplify & transmit it to the oval window, causing pressure waves in the cochlear fluid
o the pressure waves move from the vestibular to cochlear to tympanic canal within the cochlea, causing bulging of the round window & reverberation of fluid
o the fluid movement moves the cilia of the hair cells within the organ of Corti, initiating nerve impulses which are transmitted along the cochlear branch of the vestibulocochlear nerve to the auditory cortex of the temporal lobe of the brain, which interprets the sound
- Equilibrium:
o dynamic equilibrium: required when a person is in angular or rotational motion
• fluid within the semicircular canals flows over the ampullae, causing displacement of the cilia of hair cells
• the hair cells generate impulses that are transmitted along the vestibular branch of the vestibulocochlear nerve to the brain
• continuous fluid movement in semicircular canals can cause motion sickness
o static equilibrium: required when the body moves horizontally or vertically
• otoliths (calcium carbonate granules) in the vestibule are displaced from the otolithic membrane, & the membrane sags
• the sagging membrane bends the cilia of the hair cells, which generate impulses that are transmitted along the vestibular branch of the vestibulocochlear nerve to the brain
Effects of Aging:
- the need for eyeglasses & hearing aids increases with age
- incidence of cataracts, glaucoma & age-related macular degeneration increases with age
- overgrowth of the stapes (otosclerosis) & atrophy of the organ of Corti (presbycusis) can lead to the development of hearing loss later in life
Endocrine System: a system of small glands scattered throughout the body that influences the metabolic activities of cells through hormones
- Hormones: chemical messen