MCAT Bio/Biochem: Nervous and Sensory Systems

Soma

Cell body, responsible for normal cell activity

Dendrites

Wispy structures on the neuron, receive signals and transmit them toward the soma

Axon

Sends impulses away from the soma, toward the axon terminus

Axon hillock

Where the axon connects to the soma; most voltage-gated Na+ channels are where, where summation occurs

Axon terminus

Releases neurotransmitter upon the arrival of axon potential

Myelin

Insulator, increases the speed of the action potentials

* Impulse speeds can hit 120 m/s (virtually instantaneous)

Myelin composition in the CNS

Oligodendrocytes

Myelin composition in the PNS

Schwann cells

Nodes of Ranvier

“Naked”/unmyelinated points along the axon that allow the impulse to jump so it does not have to travel every mm of the axon (known as **saltatory conduction**)

Types of neurons

* Unipolar, bipolar: usually found in sensory organs, rarer
* Bipolar: found in the eyes
* Unipolar have the soma shoved to one side → dendrites become the axon; found in sensory cells, synapse with sensory cells

Resting membrane potential

\-70 mV

What is the function of Na+/K+ ATPases in maintaining the RMP?

Cell membrane essentially behaves as a capacitor; Na+/K+ ATPases move 3 Na+ out of the cell and 2 K+ in (net 1 positive ion out, makes the internal environment more negative)

* Collect sodium outside the cell and potassium inside, establish ion gradients
* Cells have potassium leak channels that allow potassium to leak OUT of the cell along its gradient

Depolarization

Moving away from the resting membrane potential in the POSITIVE direction

Hyperpolarization

Moving away from the RMP in the NEGATIVE direction

Repolarization

Return to rest potential

Equilibrium potential

Potential at which there is no driving force on an ion

* Na+ eq. potential: + 50 mV
* K+ eq. potential: - 90 mV

Action potential

Momentary change along one point on an axon, driven by voltage-gated channels

* All or none event
* Nerve impulse = traveling action potential

What happens in the first two phases (0 and 1) of the action potential?

**Phase 0**: cell is at resting membrane potential

**Phase 1:** input pushes the potential past threshold and voltage-gate Na+ and K+ channels open. VG Na+ channels (fast channels) open quickly, Na+ influx DEPOLARIZES the cell

What happens in the second two phases of action potential?

**Phase 2:** VG Na+ channels INACTIVATE and the VG K+ channels are fully open

**Phase 3:** K+ efflux repolarizes the cell

What happens in the last phase of the action potential?

**Phase 4:** VG K+ channels close slowly and extra K+ leaving hyperpolarizes the cell

(then, as potassium channels close, the sodium/potassium pump reestablishes the RMP)

Nerve impulse

* Traveling action potential
* Domino-like effect
* Influx of positive charge (Na+) opens VG channels downstream, pushing the next set past threshold
* *Note: action potential is unidirectional*

Synapse

Neuron-to-neuron or neuron-to-organ junction that neurotransmitter is released into

Absolute refractory period

Absolutely impossible to fire a second action potential

* Na+ channels are ***INACTIVATED*** at this time (different than being closed)
* Cell is too **positive (depolarized)**; near Na+ equilibrium potential

Relative refractory period

Possible but difficult to fire a second action potential

* Na+ channels are now **closed**
* Cell is too **negative** → further from threshold, near K+ equilibrium potential

Electrical synapses

* Physical connection between cell (ions flow directly from one cell to the next through gap junctions)


* Always excitatory, causes action potential in the postsynaptic cell
* Ion flow is bidirectional (either cell can be pre/post synaptic)
* Unregulated
* Ions flow from \[high\] to \[low\] ion concentration
* Relatively rare, very important in **cardiac muscle cells**

Chemical synapses

* Opposite to electrical
* Not physically connected, can be excitatory or inhibitory, unidirectional, regulated
* Involves presynaptic and postsynaptic neurons, and the cleavage of synapsin bonds between cytoskeleton filaments and vesicles containing neurotransmitters in the presynaptic cell
* Postsynaptic dendrite contains receptors for neurotransmitter

How is neurotransmitter released from a presynaptic cell?

1. The action potential arrives at the axon terminal and opens VG Ca2+ channels. Calcium influx breaks the *synapsin* bond between cytoskeleton filaments and vesicles of neurotransmitter
2. Vesicles of neurotransmitter migrate to the cell membrane, fuse with it, and are released into the synaptic cleft
3. NT diffuses across the cleft, binding to the receptors and opening the ion channel
4. Ions flow according to their gradient
5. Ion flow stops once the NT has been degraded or has been taken back up into the presynaptic cell

How many kinds of NT do neurons produce/respond to?

Neurons can only make one type of NT, but can respond to many

What happens to the NT once it is released into the cleft?

It can be recycled or broken down, can use medicines to change the amount of time NT spends in the cleft to adjust the response

What does the response of the postsynaptic cell depend on?

Receptors, not the neurotransmitter itself

* However, it takes more than one vesicle of NT to have a significant effect on the postsynaptic cell

Excitatory postsynaptic potential (EPSPs)

Input that pushes the potential towards threshold (Na+ or Ca+ influx)

Inhibitory postsynaptic potentials (IPSPs)

Input that pushes the potential away from threshold (Cl- influx, K+ efflux)

Summation

* Add up the input into the postsynaptic cell at the axon hillock. If the input pushes the potential past threshold, an AP is triggered
* Can be *spatial* or *temporal*
* Spatial: add up inputs from multiple sources
* Temporal: add up frequent impulses from a single source
* The closer the synapse is to the axon hillock, the greater its effect on summation

Afferent neurons

Carry sensory input from the peripheral nervous system (PNS) to the central nervous system (CNS); think __**approaching the CNS**__ → __**afferent**__

Interneurons

Involved in **integration (decision making);** entirely contained within the CNS

Efferent neurons

Responsible for motor output in the PNS; send commands out to the body → also known as motor neurons. Think __**exiting the CNS→ efferent**__

What are reflexes?

Rapid integration to avoid potential injury, do not require conscious input (ex: putting your hand on the stove, patellar tendon stretch reflex, bladder filling)

* Sensory (afferent) neurons carry information to the spinal cord (CNS)
* In the CNS, the sensory neuron synapses directly with a motor (efferent) neuron, which tells the muscle/body to contract (or some other command)
* In more complex reflexes, interneurons can be involved
* Afferent neurons in ***disynaptic reflexes*** can synapse on an interneuron, which synapses on an efferent neuron that sends out an inhibitory message (ex: telling a certain muscle to relax)

Spinal cord function

Primitive reflexes: walking, urination, sex organs

Function of the medulla oblongata

Basic vital functions; special respiratory/digest functions

* Not protected by blood-brain barrier, more sensitive to toxins in the blood
* Breathing, blood pressure, barfing (gag reflex)
* Connects spinal cord to the brain

Function of the pons

* Balance
* Facial movement

Cerebellum function

Smooths out and coordinates body movement (eye-hand coordination); inhibited by alcohol intoxication

What parts of the CNS compose the hindbrain?

The cerebellum, the pons, and the medulla oblongata

Midbrain function

* Visual and auditory startle reflexes
* Relay for auditory and visual stimuli, wakefulness
* When sleeping, auditory system remains active (safety)

Diencephalon function

* Consists of the epithalamus, the thalamus and the hypothalamus

Telencephalon/cerebrum function

Voluntary movement, sensorial function, olfactive function (smell), communication and language function, memory and learning function, and motor control

LImbic system function

Center for emotions

Pituitary gland function

Major endocrine organ; controls metabolism, growth, sexual maturation, reproduction, blood pressure

Epithalamus function

* Includes the pineal gland
* Secretes melatonin
* Sleep/wake cycles

Thalamus function

Sensory relay station

Hypothalamus function

* Maintains body homeostasis
* Controls pituitary gland

White matter

* Appears white
* Myelinated axons
* Cell-to-cell communication (send action potentials)
* Different name according to location in the body
* CNS-brain = tract
* CNS-cord = tract/column
* PNS = nerve

Grey matter

* Unmyelinated cell bodies and dendrites
* Somas and dendrites
* Integration (decision-making) occurs here
* Different names based on where it is found
* CNS-deep brain = nucleus
* CNS-brain surface = cortex (conscious mind)
* CNS-cord = horn
* PNS = ganglion

Lobes of the telencephalon/cortex

Frontal lobe, parietal lobe, occipital lobe, and temporal lobe

Frontal lobe function

Voluntary movement, problem solving

Parietal lobe function

General sensation and taste

Occipital lobe function

Vision

Temporal lobe function

Smell, hearing, and memories

Central nervous system (CNS)

One of two major division of the nervous system; contains the brain and the spinal cord

Peripheral nervous system (PNS)

* All nerves and sensory structures outside of the brain and spinal cord
* Can be divided into the somatic and autonomic nervous system

Somatic nervous system

Division of the PNS responsible for voluntary control of skeletal muscle

* Voluntary, skeletal muscle ONLY

Autonomic nervous system

Division of the PNS responsible for involuntary control of glands and smooth muscle; consists of the parasympathetic and sympathetic divisions

* Involuntary, all other organs

Sympathetic NS

Division of the ANS responsible for fight or flight

Parasympathetic NS

Division of the ANS responsible for the rest and digest functions

What neurotransmitter is used by the somatic NS? What is its effect on the CNS?

Acetylcholine ONLY; Ach is excitatory, will only cause muscle contraction

* *Note: in the somatic NS, one neuron travels from the CNS to the effector organ and is responsible for the release of Ach*

What neurotransmitters are used by the autonomic NS? What are their effects on the ANS?

* Uses Ach or norepinphrine
* Can be excitatory or inhibitory
* 2 neurons from CNS to effector organ

Chain linkage/chain ganglions

* Act in the autonomic nervous system (sympathetic neurons)
* Chain of neurons allows for domino-like effect
* One fires, they all fire
* Compare to parasympathetic neurons, which have a long pre-synapatic neuron

Ganglion

Clump of grey matter outside the nervous system (cell bodies and dendrites)

ANS vs. CNS

See diagram below

Parasympathetic system effects and neurotransmitters

* “Rest and digest”
* Generally decreases body activity
* Reduces heart rate, blood pressure, respiratory rate
* Increases blood flow to digestive system
* Increases digestive activity
* Uses ACh at the organ level
* *Remember that ACh is NOT just inhibitory; stimulates the stomach and intestines to do what it needs to do in the context of the parasympathetic nervous system*

Sympathetic nervous system and neurotransmitters

* “Fight or flight”
* Generally increases body activity
* Increases heart rate, blood pressure, respiratory rate
* Increases blood flow to skeletal muscles
* Decreases blood flow to digestive system
* Decreases digestive activity
* Uses norepinephrine at the organ level
* *Not strictly excitatory; inhibitory effect on the digestive system, for example*

Effect of the sympathetic nervous system on the adrenal gland

* Directly stimulates the adrenal gland
* Releases ***epinephrine*** to increase and prolong sympathetic effects

Mechanoreceptors

Stimulated by physical shape changes (baroreceptors, Golgi tendon organs, touch receptors, etc.)

Chemoreceptors

Stimulated by chemicals (pH receptors, O2 receptors, taste buds, etc.)

Thermoreceptors

Stimulated by temperature (hot and cold receptors); respond to different temperature ranges

Nociceptors

Stimulated by pain (free nerve endings that respond to touch, chemicals, heat, etc.)

Photoreceptors/electromagnetic receptors

Stimulated by light (rods and cones); some animals have magnetic receptors for migration

Absolute threshold

The minimum level of stimulation required to activate a receptor

Difference threshold

Minimum difference in sensation that can be detected

Sensory adaptation

Ignore unchanging stimuli, can be re-triggered if stimulus changes or intensifies

Bottom-up processing

1. Sensory receptors register information
2. Sensory neurons send information to the brain
3. Brain identifies the information

Top-down processing

1. Brain applies prior knowledge and experience
2. Forms a holistic view of what’s going on

Iris

Colored part of the eye, regulates the diameter of the pupil

Lens

Biconvex structure that focuses light on the retina

Cornea

External transparent layer of the eye

Pupil

Black opening in the middle of the eye

Ciliary muscles

Muscles that regulate the curvature of the lens

Fovea

Responsible for extreme visual acuity

Retina

Layer at the back of the eye sensitive to light

Optic disk

Blind spot. Place on retina where optic nerve forms.

Optic nerve

Bundle of axons leaving the eye towards the brain

Organization of the retina

* Three layers of cells attached to the posterior wall of the retina
* Layer closest to the retina: rod and cone cells (photoreceptors)
* Middle layer: bipolar cells
* Furthest layer: ganglion cells, whose axons become the optic nerve

Direction of travel of light vs. nerve impulse

* Light travels from the optic nerve, to the ganglion cells, to the bipolar cells, to the rod cells, and to the retina
* The nerve impulse/signals travel in the opposite direction
* This is why there is a blind spot
* Where the axons of the ganglion cells pass back through the retina there can be no photoreceptors

Cone cells

* Color vision, requires bright light
* Clustered in the fovea centralis
* In humans: blue, green, red (400-700 nm)

Rod cells

Black and white vision, dim light

Mechanism of light perception (no light)

1. Na+ channels are open
2. Na+ enters and depolarizes the rod
3. Cell releases neurotransmitter onto the bipolar cells


1. Bipolar cell response depends on the kind of cell; **ON cells** respond only when there is light (will lead to an inhibitory response) whereas **OFF cells** will experience a stimulatory response

Mechanism of light perception (light)

1. Inhibition of Na+ channels
2. Na+/K+ pump polarizes the cell (hyperpolarization occurs)
3. Stop release of neurotransmitter


1. **ON cells** are inhibited by NT, so stopping the release of NT will activate these cells and the bipolar neurons will release NT onto the ganglion cell, which will fire an action potential
2. **OFF cells** are stimulated by NT release from the rods/cones, so stopping this release will inhibit the bipolar cells and ganglia will not fire an AP

Structures of the outer ear

Pinna, auditory canal, tympanic membrane

Structures of the middle ear

Malleus, incus, stapes (don’t want to __*MIS*__ a sound)

Structures of the inner ear

Semicircular canals, cochlea

Eustachian tube (auditory tube)

Connects the throat with the middle ear to allow drainage and pressure equalization