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Biology - Nervous System


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What does the nervous system do?
It enables organisms to receive and respond to stimuli from their external and internal environments
What are neurons?
They are the functional units of the nervous system
What does a neuron do?
It converts stimuli into electrochemical signals that are conducted through the nervous system
What does a neuron look like?
It is an elongated cell consisting of dendrites, a cell body, and an axon
What are dendrites?
They are cytoplasmic extensions that receive information and transmit it toward the cell body
What is the cell body also known as?
What does it contain?
It contains the nucleus
What does it do?
It absorbs one wavelength
What does the axon hillock do?
It connects the cell body to the axon (nerve fiber), which is a long cellular process that transmits impulses away from the cell body
What are most mammalian axons ensheathed by?
Most are ensheathed by an insulting substance known as myelin
What is myelin produced by?
It is produced by cells known as glial cells
What do oligodendrocytes do?
They produce myelin in the central nervous system
What do Schwann cells produce?
They produce myelin in the peripheral nervous system
What are the gaps between segments of myelin known as?
Nodes of Ranvier
What do the axons ultimately end at?
Swellings known as synaptic terminals
What else are they called?
Synaptic boutons or knobs
What is released from these terminals?
Where are they released into?
The synapse or synaptic cleft
What is the synapse?
It is the gap between the axon terminals of one cell and the dendrites of the next cell
What is the function of neurons?
They are specialized to receive signals from sensory receptors or from other neurons in the body and transfer this information along the length of the axon
What are impulses called that travel the length of the axon and invade the nerve terminal?
They are called action potentials
What does this cause?
It causes the depolarization of the skeletal muscle
What is it called when the neuron is at rest, and there is a potential difference between the extracellular space and the intracellular space?
It is called the resting potential
When is a neuron polarized?
It is polarized even at rest, so always
What is this potential difference the result of?
It is the result of an unequal distribution of ions between the inside and outside of the cell
What is a typical resting membrane potential?
-70 millivolts
What does this mean?
It means that the inside of the neuron is more negative than the outside
What is this difference due to?
It is due to the selective ion permeability of the neuronal cell membrane
What is this difference maintained by?
It is maintained by the Na+/K+ pump
How is concentration of K+ inside the neuron compared to the outside?
It is higher inside the neuron than outside
What about the concentration Na+?
It is higher outside than inside
Where are negatively charged proteins trapped?
They are trapped inside the cell
Why is the resting potential created?
Because the neuron is selectively permeable to K+
What does this do to K+?
It causes K+ to diffuse down its concentration gradient
What does this do to the net charge inside the neuron?
It causes a net negative charge to exist inside
What does the transmission of action potentials lead to?
It leads to the disruption of ionic gradients
What does this mean the gradients must be restored by?
They must be restored by the Na+/K+ pump
How does this pump work?
Using ATP energy, it transports 3 Na+ out for every 2 K+ it transports in
What types of impulses does the nerve cell body receive from other cells?
It receives both excitatory and inhibitory impulses
What does it mean if the cell becomes sufficiently excited or depolarized?
It means the inside of the cell becomes less negative, and an action potential is generated
What is the minimum threshold membrane potential?
It is usually around -50mV, and it is the level at which an action potential is initiated
What do ion channels located in the nerve cell membrane do in response to these changes in voltage?
They open in response to the changes
What are they called because of this?
They are called voltage-gated ion channels
When does an action potential begin concerning voltage-gated Na+ channels?
It begins when voltage-gated Na+ channels open in response to depolarization
What does this allow?
It allows Na+ to rush down its electrochemical gradient
Where does it go?
It goes down the gradient into the cell
What happens to the voltage-gated Na+ channels then?
They close
What happens to voltage-gated K+ channels?
They open, allowing K+ to rush out down its electrochemical gradient
What does this do to the cell potential?
It returns the cell to a more negative potential
What is this process known as?
It is known as repolarization
What is hyperpolarization?
It is when a neuron may shoot past the resting potential and become even more negative inside than normal
What happens immediately following an action potential?
It may be very difficult or impossible to initiate another action potential
What is this period of time called?
It is called the refractory period
What is the action potential described as?
An all-or-none response
What does this imply?
It means that whenever the threshold membrane potential is reached, an action potential with a consistent size and duration is produced
How is neuronal information coded?
It is coded by the frequency and number of action potentials rather than the size of the action potential
If there is an adequate stimulus, where will the action potential first be initiated?
The axon hillock
What happens with Na+ at the axon hillock?
Na+ rushes into the neuron and diffusers to adjacent parts of the axon
When does this occur?
It occurs while previous segments are repolarizing
Where does this chain of events (depolarization followed by a subsequent repolarization) continue?
It continues along the length of the axon
Can axons propagate action potentials bidirectionally?
Theoretically they can, but information transfer only occurs in one direction
What direction is that?
From dendrite to synaptic terminal
Why is that?
Because photoreceptors are not present there
Can different axons propagate action potentials at different speeds?
They can
How is the speed of impulses related to the axon?
The greater the diameter of the axon and the more heavily it is myelinated, the faster the impulses will travel
How does myelin increase the conduction velocity?
It insulates segments of the axon, so that the membrane is permeable to ions only in the nodes of Ranvier
Well how does this make the impulses faster?
In this way, the action potential “jumps” from node to node
What is this process called?
It is called salutatory conduction
What is the synapse a gap between?
It is a gap between the axon terminal of one neuron and the dendrites of another neuron
What is the axon terminal of the first neuron known as?
It is known as the presynaptic neuron
What are the dendrites of another neuron known as?
They are known as the postsynaptic neuron
What else can neurons communicate with other than neurons?
They can communicate with postsynaptic cells such as cells in the muscles or glands
What are these referred to as?
Effector cells
What are the vast majority of synapses called in the human?
Chemical synapses
In chemical synapses, the nerve terminal contains thousands of membrane-bound vesicles full of chemical messengers known as what?
What happens when the action potential arrives at the nerve terminal and depolarizes it?
The synaptic vesicles fuse with the presynaptic membrane and release neurotransmitter into the synapse via a calcium-dependent process of exocytosis
What does the neurotransmitter diffuse across?
It diffuses across the synapse and acts on receptor proteins embedded in postsynaptic membrane
What type of effect can the neurotransmitter have on the receptor?
It depends on the receptor, but it could be an excitatory or an inhibitory effect on the postsynaptic cell
How is neurotransmitter removed from the synapse?
It is removed in a variety of ways
What are some examples?
It increases blood pressure and heart rate, it increases blood flow to skeletal muscles and it decreases gut motility
What is this called?
It is known as an uptake carrier
What happens with an uptake carrier?
It may be reused or degraded
What else can degrade it?
Enzymes located in the synapse
What is an example of a degradating enzyme?
Acetylcholinesterase is one example
What is another thing that is may do?
It may simply diffuse out of the synapse
What are afferent neurons?
They are neurons that carry information about the eternal or internal environment to the brain or spinal cord
What are efferent neurons?
They carry commands from the brain or spinal cord to various parts of the body such as muscles or glands
What are interneurons?
They participate only in local circuits
What are nerves?
They are essentially bundles of axons covered with connective tissue
What can a nerve carry?
It may carry only sensory fibers, only motor fibers, or a mixture of the two
What is a nerve that carries only sensory fibers called?
A sensory nerve
What about ones that carry only motor fibers?
Those are called motor nerves
What if they carry both types?
They are called mixed nerves
What happens when neuronal cell bodies cluster together?
They are called ganglia in the periphery
What are they called if they are in the central nervous system?
They are called nuclei
How many systems in the nervous system itself divided into?
It is divided into two major systems, the central nervous system and the peripheral nervous system
What does the central nervous system consist of?
It consists of the brain and spinal cord
What is the brain?
It is a jelly-like mass of neurons that resides in the skull
What does it function as?
It functions to interpret sensory information, form motor plans, and it also functions in cognitive function
What does the brain consist of?
It consists of gray matter and white matter
What is gray matter?
It is cell bodies
What about white matter?
That is myelinated axons
What can the brain be divided into?
3 sections, the forebrain, midbrain, and hindbrain
What does the forebrain consist of?
It consists of the telencephalon and the diencephalons
What does the telencephalon consist of?
It consists of the right and left hemispheres
What can each hemisphere be divided into?
Four different lobes
What are the four lobes?
They are the frontal, parietal, temporal, and occipital
What is a major component of the telencephalon?
It is the cerebral cortex
What is the cerebral cortex?
It is the highly convoluted gray matter that can be seen on the surface of the brain
What does the cortex do?
It processes and integrates sensory input and motor responses and is important for memory and creative thought
What do the right and left cerebal cortices communicate through?
They communicate with each other through the corpus callosum
What does the diencephalons contain?
It contains the thalamus and hypothalamus
What is the thalamus?
It is a relay and integration center for the spinal cord and cerebral cortex
What does the hypothalamus control?
It controls visceral functions such as hunger, thirst, sex drive, water balance, blood pressure, and temperature regulation
What else does it have a role in?
It has a role in the control of the endocrine system
What is the midbrain?
It is a relay center for visual and auditory impulses
What does it play a role in?
It plays an important role in motor control
What does the hindbrain do?
It is the posterior part of the brain and consists of the cerebellum, the pons, and the medulla
What does the cerebellum do?
It helps modulate motor impulses initiated by the motor cortex, and it is important in the maintenance of balance, hand-eye coordination, and the timing of rapid movements
What is a function of the pons?
It is to act as a relay center to allow the cortex to communicate with the cerebellum
What does the medulla do?
It controls many vital functions such as breathing, heart rate, and gastrointestinal activity
Together, what do the midbrain, pons, and the medulla constitute?
They constitute the brainstem
What about the spinal cord?
The spinal cord is an elongated structure continuous with the brainstem
Where does it extend in vertebrates?
It extends down the dorsal side
Nearly all of what passes through the spinal cord?
Nearly all nerves that innervate the viscera or muscles below the head pass through it
Nearly all sensory information from below the head passes through the spinal cord in the way to the brain
What does the spinal cord integrate?
What does a cross-section of the spinal cord reveal?
It reveals an outer white matter area containing motor and sensory axons and an inner gray matter area containing nerve cell bodies
How does sensory information enter the spinal cord?
It enters dorsally
Where are the cell bodies of these sensory neurons located?
They are located in the dorsal root ganglia
Which direction does all motor information exit the spinal cord?
All motor information exits the spinal cord ventrally
What are nerve branches entering and leaving the cord called?
They are called roots
How many regions is the spinal ford divided into?
In the direction of the brainstem to the tail, cervical, thoracic, lumbar, and sacral
What is the peripheral nervous system?
It consists of 12 pairs of cranial nerves, which primarily innervate the head and shoulders, and 31 pairs of spinal nerves, which innervate the rest of the body
From where do cranial nerves exit?
They exit from the brainstem
What about spinal nerves?
They exit from the spinal cord
What are the divisions of the PNS?
It has two primary divisions, the somatic and autonomic nervous systems
What do each of those contain?
They each have motor and sensory components
What is the Somatic Nervous System?
The SNS innervates skeletal muscles and is responsible for voluntary movement
What do motor neurons release?
They release the neurotransmitter acetylcholine (Ach) onto Ach receptors located on skeletal muscle
What does this lead to?
It leads to muscle contraction
What else is the somatic nervous system important for?
Besides voluntary movement, it also plays an important role in reflex action
What are the two kinds of reflexes?
There are monosynaptic and polysynaptic reflexes
What are monosynaptic reflex pathways?
They have a single synapse between the sensory neuron and the motor neuron
What is a classic example of this?
It is the withdrawal reflex
How does this work?
The first neuron (preganglionic neuron) has a cell body located within the CNS and its axon synapses in peripheral ganglia
What happens then?
Then the sensory neuron synapses with a motor neuron in the spinal cord
What does this do?
It changes the nature of impulses sent by the vestibular nerve to the brain
What occurs in polysynaptic reflexes?
In polysynaptic reflexes, sensory neurons synapse with more than one neuron
What is that?
It has specialized sensory cells called hair cells
What happens?
The injured leg withdrawals in pain, while the other leg extends to retain balance
What is the autonomic nervous system sometimes called?
It is sometimes called the involuntary nervous system
Because it regulates the body’s internal environment without the aid of conscious control
What does the ANS innervate?
It innervates cardiac and smooth muscle
Where is smooth muscle located?
In areas such as the digestive tract, the bladder, and bronchi
What does this imply the ANS is an important controller for?
It controls blood pressure, gastrointestinal motility, excretory processes, respiration, and reproductive processes
What are ANS pathways characterized by?
They are characterized by a two-neuron system
What does the second neuron do?
It has its cell body in the ganglia and then synapses on cardiac or smooth muscle
What is the ANS comprised of?
Two subdivisions called the sympathetic and parasympathetic nervous systems
What is the sympathetic nervous system?
It is responsible for the flight or fight response that ready the body for action
What does it basically do?
It basically does everything you would want it to do in an emergency situation
What do preganglionic neurons emerge from?
They emerge from the thoracic and lumbar regions of the spinal cord
What do they use as their neurotransmitter?
They use acetylcholine as their neurotransmitter
What do postganglionic neurons typically release?
They typically release norepinephrine
What dos the action of preganglionic sympathetic neurons cause?
It causes the adrenal medulla to release adrenaline into the bloodstream
What does the parasympathetic nervous system do?
It acts to conserve energy and restore the body to resting activity levels following exertion
What is this known as?
Rest and digest
What are examples?
It acts to lower hart rate and to increase gut motility
What is one very important parasympathetic nerve which innervates many of the thoracic and abdominal viscera?
It is the vagus nerve
Where do parasympathetic neurons originate?
They originate in the brainstem (cranial nerves) and the sacral part of the spinal cord
What do both the preganglionic and postganglionic neurons release?
They both release acetylcholine
What are the three types of sensory receptors to monitor its internal and external environment?
There are interoceptors, proprioceptors, and exteroceptors
What do interoceptors monitor?
They monitor aspects of the internal environment such as blood pressure, the partial pressure of CO2 in the blood, and blood pH
What do proprioceptors transmit?
They transmit information regarding the position of the body in space
Where are these receptors located?
They are located in muscles and tendons
Why are they there?
They are there to tell the brain where the limbs are in space
Where else are they located?
They are also in the inner ear to tell the brain where the head is in space
What do exteroceptors sense?
They sense things in the external environment such as light, sound, taste, pain, touch, and temperature
What does the eye do?
It detects light energy as photons and transmits information about intensity, color, and shape to the brain
What is the eyeball covered by?
A thick, opaque layer known as the sclera, which is also known as the white of the eye
What is beneath the sclera?
The choroid layer, which helps to supply the retina with blood
What is the innermost layer of the eye?
The retina, which contains the photoreceptors that sense light
What does the cornea do?
It is transparent at the front of the eye, and it bends and focuses light rays
What do the rays then do?
Then they travel through an opening called the pupil
What is the diameter of the pupil controlled by?
It is controlled by the pigmented, muscular iris
What does the iris respond to?
It responds to the intensity of light in the surroundings
Where does light continue from there?
It continues through the lends, which is suspended behind the pupil
What is the lens?
It focuses the image onto the retina
What controls the shape of the lens?
The ciliary muscles control the shape of the lens
What is in the retina?
In the retina are photoreceptors that transducer light into action potentials
What are the two main types of photoreceptors?
Cones and rods
What do cones respond to?
They respond to high-intensity illumination and are sensitive to color
What do rods respond to?
They detect low-intensity illumination and are important in night vision
What do the cones and rods contain?
They contain three different pigments to absorb red, green, and blue wavelengths
What is the rod pigment called?
What do the photoreceptor cells synapse onto?
Bipolar cells
What then?
They synapse onto ganglion cells
What do the axons of the ganglion cells bundle to form?
They form the right and left optic nerves
What do the optic nerves do?
They conduct visual information to the brain
What is the point at which the optic nerve exits the eye?
It is the blind spot
What is the fovea?
It is a small area of the retina which is densely packed with cones, and is important for high acuity vision
How does the eye circulation system work?
It has its own circulation system. Near the base of the iris, the eye secretes aqueous humor, which travels to the anterior chamber of the eye from which it exits and eventually joins venous blood
What does the ear do?
It transduces sound energy pressure waves into impulses perceived by the brain as sound
What is the ear responsible for?
It is responsible or maintaining equilibrium balance in the body
What regions do sound waves pass through?
They pass through the outer ear, which consists of the auricle (pinna) and the auditory canal
What is at the end of the auditory canal?
At the end of the canal is the tympanic membrane (eardrum) of the middle ear
What does the middle ear do?
It vibrates at the same frequency as the incoming sound
What happens next?
The three bones or ossicles (malleus, incus, and stapes) amplify the stimulus, and transmit it through the oval window
What does the oval window lead to?
It leads to the fluid-filled inner ear
What does the inner ear consist of?
It consists of the cochlea and the semicircular canals
What does the cochlea contain?
It contains the organ of Corti
What does vibration of the ossicles do?
It exerts pressure on the fluid in the cochlea, stimulating the hair cells to transducer the pressure into action potentials
The three semicircular canals are each perpendicular to the other two and are filled with a fluid called what?
What is at the base of each canal?
A chamber with sensory hair cells
What does rotation of the head do?
It displaces endolymph in one of the canals, putting pressure on the hair cells in it
What does the brain do with this?
It interprets this information to determine the position of the head
What are the chemical senses?
Taste and smell
What do they do?
They transduce chemical changes in the environment, specifically in the mouth and nose, into gustatory and olfactory sensory impulses, which are interpreted by the nervous system
How does taste work?
Taste receptors, or taste buds, are located on the tongue, the soft palate, and the epiglottis
What are they composed of?
They are composed of approximately 40 epithelial cells
What does the outer surface of a taste bud contain?
It contains a taste pore, from which microvilli, or taste hairs protrude
Where are the receptor surfaces for taste?
They are on the taste hairs
What is interwoven around the taste buds?
A network of nerve fibers that are stimulated by the taste buds
What do these neurons do?
They transmit gustatory information to the brainstem via three cranial nerves
What are the four kinds of taste sensations?
Sour, salty, sweet, and bitter
How does smell work?
Olfactory receptors are found in the olfactory membrane
Where does it lie?
It lies in the upper part of the nostrils over a total area of about 5cm2
What are the receptors?
They are specialized neurons from which olfactory hairs, or cilia project
What do the cilia form?
They form a dense mat in the nasal mucosa
What happens when odorous substances enter the nasal cavity?
They bind to receptors in the cilia, depolarizing the olfactory receptors
What do axons from the olfactory receptors do?
They join to form the olfactory nerves
What do the olfactory nerves do?
They project directly to the olfactory bulbs in the base of the brain

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