Chapter 13 - The nervous system

Question 1

Anatomy of a neuron

Answer 1

Question 2

Cell body (neuron)

Answer 2

AKA Soma, contains the nucleus, ER, golgi, ribosomes, etc. of the neuron

Question 3

Dendrites

Answer 3

The branched projections of a neuron that act to propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project.

Question 4

Axon Hillock

Answer 4

The portion of the neuron that connects the cell body (soma) to the axon. The impulses the neuron receives from all the dendrites are summed up at the axon hillock to determine whether an action potential will be initiated. All signaling from a neuron is initiated at the axon hillock.

Question 5

Axon

Answer 5

A long, slender projection of a nerve cell, or neuron, that typically conducts electrical impulses away from the neuron’s cell body. The function of the axon is to transmit information to different neurons, muscles and glands. Most mammalian axons are insulate by myelin.

Question 6

Myelin sheath

Answer 6

Insulating substance that surrounds axons. It is produced by oligodendrocytes in the CNS and Schwann cell in the periphery. Myelin prevents signal loss and increases the speed of conduction in axons. Action potentials cannot take place in areas of the axon that are myelinated.

Question 7

Nodes of Ranvier

Answer 7

Gaps between segments of myelin sheath where action potentials can take place, allowing for saltatory conduction.

Question 8

Synaptic (axon) terminal

Answer 8

AKA synaptic bouton. Ends of axons that form one side of the synaptic cleft; the location where neurotransmitters are stored. This structure is enlarged and flattened to maximize neurotransmission to the next neuron and ensure proper production and storage of neurotransmitters.

Question 9

Synapse

Answer 9

AKA synaptic cleft. The space betweeen the axon terminal of one neuron and the dendrite of another neuron where neurotransmitters are released.

Question 10

What is the pathology of demyelination?

Answer 10

Because myelin speeds the conduction of impulses along a neuron, the absence of myelin results in the slowing of information transfer. Multiple sclerosis (MS) is a common disorder where the myelin of the brain and spinal cord is selectively targeted for degradation by the immune system. Because so many different kinds of neurons are demyelinated, MS patients present with symptoms including weakness, lack of balance, vision problems, and incontinence.

Question 11

What common feature is shared by all neurons?

Answer 11

Despite the large variety of neurons in the body, including the fact that not all have the same structure or even a complete set of axons or dendrite, all neurons share the ability to signal chemically after electrical excitation, i.e. they are able to translate electrical signals into chemical signals.

Question 12

Resting membrane potential

Answer 12

The electrical potential difference (voltage) across the cell membrane of a neuron or muscle cell while at rest. It is typically on the order of –70 mV, and is generated by both negatively charged proteins within the cell and the relatively greater permeability of the membrane to K⁺ compared with Na⁺ (via the Na+/K+ ATPase pump).

Question 13

Na⁺/K⁺ ATPase

Answer 13

AKA Na⁺/K⁺ pump. A protien that hydrolyzes one ATP to transport three NA⁺ out of the cell for every two K⁺ it transports into the cell. The Na⁺/K⁺ ATPase is primarily responsible for maintaining the electrical and chemical gradient across the cell membrane.

Question 14

Diagram of Na+/K+ ATPase in the maintenance of resting potential

Answer 14

The Na⁺/K⁺ ATPase is also important for restoring the gradient after potentials have been fired.

NB: there are also channels for facilitated diffusion (Down the gradient) of Na⁺ and K⁺ across the membrane.

Question 15

Depolarization

Answer 15

Depolarization is caused by excitatory impulses inputs that make the cell less negative (more positive). If the axon hillock is depolarized beyond the threshold value (usually –55 to –40 mV), voltage-gated Na+ channels open, allowing Na+ to flow into the cell, further depolarizing the membrane and triggering an action potential.

Question 16

Action potential

Answer 16

A sharp change in the membrane potential of neurons or muscle cells caused by a change in the selective permeability to K⁺ and Na⁺ by voltage-gated ion channels. Action potentials occur when the axon hillock is depolarized beyond the threshold value, and are all-or-none events.

Question 17

What is the driving force during an action potential?

Answer 17

The Na⁺/K⁺ pump generates both an electrical and chemical gradient across the cell membrane. Depolarization allows Na⁺ to move into the cell, down both gradients.This sodium influx makes the cell potential positive, reaching apprioximately +35 mV at its peak.

Question 18

Know the different parts of the action potential graph

Answer 18

Question 19

How is membrane potential restored following an action potential?

Answer 19

Voltage-gated K⁺ channels will open at a sufficiently positive potential, driving potassium out of the cell. The movement of positive charges out of the cell will result in restoration of the negative membrane potential. This process is known as repolarization.

Question 20

Hyperpolarization

Answer 20

Hyperpolarization occurs after an action potential, when the efflux of K^{<span>+</span>} ions cause an overshoot of the resting membrane potential, making the cell more negative than during the resting state.

Hyperpolarization may also be caused by inhibitory inputs, which make the neuron less likely to fire an action potential. Inhibitory inputs lead to hyperpolarization by making the cell more negative.

Question 21

Refractory periods

Answer 21

A period following repolariztion wherein normal stimulation will not cause an action potential. During an absolute refractory period no amount of stimulation will cause another action potential to occur. During a **relative **refractory period there must be grater than normal stimulation to cause an action potential because the membrane is starting from a more negative potential than the resting state.

Question 22

Impulse propagation

Answer 22

Movement of the action potential down the axon and to the synapse, initiating neurotransmitter release. This is a unidirectional process due to the refractory period following depolarization.

Question 23

Know the impulse propagation diagram

Answer 23

Question 24

What is the equation that determines the speed at which an action potential moves down the axon?

Answer 24

The speed of impulse propagation is determined by the insulator (resistivity), length, and cross-sectional of the axon: the longer the axon, the higher the resistance and the slower the conduction, the greater the diameter, the lower the resistance and the faster the conduction. In neurons, myelin serves as an insulator and provides an incredibly high resistivity.

Question 25

Saltatory conduction

Answer 25

Due to myelination, the axon membrane is only permeable to ions at the nodes of Ranvier. This speeds conduction by causing the action potential to jump from node to node along an axon.

Question 26

What is the order of signal transmission from one neuron to another

Answer 26

(presynaptic neuron) axon hillock → axon → synapse → neurotransmitter release → (synaptic cleft) → receptor (postsynaptic neuron) → Dendrite

Question 27

effector cell

Answer 27

A neuron that signals to a gland or a muscle

Question 28

What is the difference between intra- and inter- neuronal signal transduction?

Answer 28

• intraneuronal signal transduction - facilitated by electrical signals in an all-or-none fashion.
• interneuronal signal transduction - facilitated by chemical signaling in a modulated fashion, the strength of which depends on how much neurotransmitter is released at the presynaptic terminal.

Question 29

What determines whether a neuron is exitatory or inhibitory?

Answer 29

The specific neurotransmitter released by the neuron determines whether binding will result in depolarization or hyperpolarization.

Question 30

How is the duration of neurotransmission regulated?

Answer 30

Neurotransmitters are removed from the synaptic cleft, thereby halting signaling. This may occur either through enzymatic breakdown of the neurotransmitter (e.g. acetylcholinesterase) or by reuptake carriers, which recycle certain neurotransmitters from the cleft back into the presynaptic neuron (e.g. Dopamine Active Transporter). Other neurotransmitters may simply diffuse out of the area (e.g. nitric oxide).

Question 31

Neurotransmitters

Answer 31

Chemical messengers released from the synaptic clefts of a neuron that can bind and stimulate (or inhibit) a postsynaptic cell.

Question 32

Afferent neurons

Answer 32

Neurons that transmit information from the periphery to the brain or spinal cord. These are sensory neurons.

Mnemonic: My brain is the seat at the center of universe, why wouldn’t it be the reference point for information flow?

Question 33

Efferent neurons

Answer 33

Neurons that transmit information from the brain or spinal cord* to the periphery*. These are motor neurons.

Mnemonic: My brain is the seat at the center of universe, why wouldn’t it be the reference point for information flow?

Question 34

Interneurons

Answer 34

Neurons that are only involved in local circuits, transmitting information from one neuron to another. Interneurons are neither motor nor sensory.

Question 35

Major divisions of the nervous system (diagram)

Answer 35

Question 36

Nerve (and types)

Answer 36

A bundle of numerous axons. Nerves may be sensory, motor, or mixed.

• Sensory nerves - comprised of the axons of multiple efferent neurons, which relay information to the brain or spinal cord.
• Motor nerves - comprised of multiple comprised of the axons of multiple afferent neurons, which relay information from the brain or spinal cord to the periphery.
• Mixed nerves - a combination of both afferent and efferent axons.

Question 37

Ganglia

Answer 37

A cluster of neural cell bodies in the peripheral nervous system. Ganglia are contrasted from Nuclei, which are similar cell body clusters in the central nervous system.

Question 38

Central nervous system (CNS)

Answer 38

The central nervous system consits of the brain and the spinal cord. The CNS develops from the embryonic neural tube.

Question 39

Peripheral nervous system (PNS)

Answer 39

All neurons that are not part of the central nervous system (brain and spinal cord). The PNS is comprised of sensory and motor neurons that connect to the CNS, serving as a communication relay going back and forth between the brain and the extremities. Can be divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). Develops from the embryonic neural crest cells.

Question 40

White matter

Answer 40

A component of the central nervous system in the brain and superficial spinal cord, white matter consists mostly of glial cells (generate myelin) and myelinated axons. White matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.

Mnemonic: white matter appears pinkish white in brain sections due to the high lipid composition of myelin.

Question 41

Grey matter

Answer 41

A component of the central nervous system in the brain and superficial spinal cord. Grey matter largely consists of unmyelinated axons, cell bodies, dendrites, and capillaries. It is primarily associated with processing and cognition,

Mnemonic: In living tissue, grey matter actually has a very light grey color with yellowish or pinkish hues, which come from capillary blood vessels and neuronal cell bodies.

Question 42

Hemispheres of the brain (diagram)

Answer 42

Question 43

Subdivisions of the brain hemispheres (hierarchy chart)

Answer 43

• Forebrain
  
  • Telencephalon
    
    • Frontal (upper front)
    • Parietal (upper rear)
    • Temporal (lower fron)
    • Occipital (lower rear)
  • Diencephalon
    
    • Thalamus
    • Hypothalamus
• Midbrain
• Hindbrain (brain stem)
  
  • Cerebellum
  • Pons
  • Medulla Oblongata

Question 44

Somatic Nervous System (SNS)

Answer 44

Division of the peripheral nervous system that is responsible for voluntary movement.

Question 45

Cerebral cortex

Answer 45

A large portion of the telencephalon, the cerebral cortex is comprised of highly convoluted grey matter and is responsible for the highest-level functioning in the nervous system, including creative thought and executive functioning. It also integrates sensory information and controls movement.

Question 46

Corpus collosum

Answer 46

The corpus collosum connects the brain’s left and right hemisphere and correlates their activity.

Question 47

Thalamus

Answer 47

Functions as the gateway to the brain for all efferent signaling; all ascending information is passed though the thalamus before being relayed to the cortex.

Question 48

Hypothalamus

Answer 48

An endocrine organ in the forebrain, directly above the pituitary gland and below the thalamus. The hypothalamus is capable of having organism-wide effects by regulating the pituitary through (paracrine) release of hormones. It can be thought of as the major gateway in the brain responsible for afferent hormonal signaling.

Question 49

Midbrain

Answer 49

Serves as a relay point between more peripheral structures and the forebrain, while receiving motor instructions from the forebrain and passing them to the hindbrain.

Question 50

Hindbrain: basic function and subdivisions

Answer 50

Responsible for many involuntary functions (e.g. respiration) and is comprised of the cerebellum, pons, and medulla oblongata. The structures of the hindbrain are conserved accross a wide variety of organisms.

Question 51

Cerebellum: function

Answer 51

Plays an important role in motor control by contributing to coordination, precision, and accurate timing. It receives input from sensory systems of the spinal cord and from other parts of the brain, and integrates these inputs to fine tune motor activity.

Question 52

Medulla oblongata

Answer 52

The part of the brain that controls such automatic functions as breathing, gastrointestinal tone, and heartbeat. The medulla is the most conserved part of the brain.

Question 53

Pons

Answer 53

The part of the brainstem that links the medulla oblongata and the thalamus.

Question 54

Peripheral Nervous System (PNS): divisions and functions

Answer 54

• Somatic Nervous System (SoNS) - associated with the voluntary control of body movements via skeletal muscles.
• Autonomic Nervous System (ANS) - acts as a control system that functions largely below the level of consciousness to control visceral functions. Often works in concert wiht the SoNS.
  
  • Sympathetic (SNS) - mobilizes the body’s nervous system’s fight-or-flight (stess) response.
  • Parasympathetic (PSNS) - responsible for stimulation of “rest-and-digest” activities of the digestive and excretory organs.

Question 55

Somatic Nervous System: function and mode of signaling

Answer 55

Responsible for voluntary muscular movement (via skeletal muscle). Acetylcholine release at the neuromuscular junction results in skeletal muscular contraction. The somatic nervous system has only one neuron between the CNS and the target organ. The somatic nervous system is the basis of not only voluntary movement, but also of reflexes, which are automatic.

Question 56

Reflex Arcs: Definition and Types

Answer 56

Muscular contractions regulated by the somatic nervous system that do not require input or integration from the brain to function.

• Monosynaptic Reflex Arc: Reflex pathway that has only one synapse between the sensory neuron and the motor neuron (e.g. knee-jerk reflex)
• Polysynaptic reflex arc: Reflex pathway with at least one interneuron between the sensory and motor neuron (e.g. withdrawal reflex)

Question 57

Autonomic Nervous System: function and mode of signaling

Answer 57

Responsible for both the “fight-or-flight” (sympathetic) and “rest-and-digest” (parasympathetic) responses. Does not require voluntary control. Cardiac muscle, smooth muscle, and glandular tissues are innervated by the autonomic nervous system.

The autonomic nervous system uses two (pre- and post-ganglionic) neurons in series to communicate between the CNS and the target organ. The specific neurotransmitter depends on the subdivision of the autonomic nervous system and the neuron:

• Sympathetic - Preganglionic: acetylcholine; Postaganglionic: norepinephrnie
• Parasympatheitc - Preganglionic and postganglionic: acetylcholine