Chapter 12: Nervous Tissue

Question 1

Homeostasis

Answer 1

a condition in which the body’s internal environment is maintained relatively constant.

Question 2

Nervous system has 3 basic functions:

Answer 2

1. Sensory function → sensory input from receptors detect changes inside & outside the body
2. Integrative function → sensory input is interpreted & coordinated with an appropriate motor response
3. Motor function → motor output to an effector organ such as a muscle or gland

Question 3

The nervous system has 2 main subdivisions:

Answer 3

Central nervous system (CNS)
-Brain & spinal cord

Peripheral nervous system (PNS)

• Cranial & spinal nerves, ganglia, & sensory receptors
• Can be further sub-divided into
  i) Sensory division
  ii) Motor division
  a) Somatic Nervous System
  b) Autonomic Nervous System

Question 4

Sensory Division

Answer 4

-Brings information from sensory receptors throughout the body to the CNS
-2 kinds of sensory info: somatic and special senses
> It carries somatic sensory info (touch, temp., pain, proprioception, etc.)
> Also carries special senses info (smell, taste, vision, hearing, equilibrium)
-Proprioception – sensory reseptors brings in info about location of body limbs and degree of tension in muscle/joints (tells what position head/limbs/torso is in)

Question 5

Motor Division

Answer 5

• Motor neurons conduct impulses from CNS to effectors (muscles & glands)
• This division is further sub-divided into:
  a) Somatic Nervous System
  b) Autonomic Nervous System

Question 6

Somatic nervous system

from Motor sys

Answer 6

• Under voluntary control
• Somatic = limb movements
• Motor neurons conduct impulses from CNS to skeletal muscles

Question 7

Autonomic nervous system

from motor sys

Answer 7

-Under involuntary control
-Autonomic = breahing, heart contractions (there are some skeletal muscle that are part of this like the diaphragm but theyre the exception, also postual muscle that hold up neck is another exception)
-Motor neurons conduct impulses from CNS to smooth & cardiac muscle, & to glands
-Autonomic nervous system is divided further:
> Sympathetic – “fight-or-flight”
> Parasympathetic – “rest-&-digest”
> Enteric - smooth muscle and glands of GI tract

Question 8

Two basic nervous cell types

Answer 8

1) neuron

2) Neuroglia

Question 9

Neurons

Answer 9

• Functional units of the nervous system
• Most are unable to undergo mitosis
• Require abundant supply of O2 & glucose
• Have properties of:
- irritability
- conductivity
• Generate & propagate action potentials (nerve impulses)

Question 10

Neuroglia

Answer 10

• Are supportive & protective cells that aid neurons
• Smaller & more numerous than neurons & are still capable of mitosis
• There are 6 types:
> found in the CNS:
- Astrocytes
- oligodendrocytes
- microglia
- ependymal cells
> found in the PNS
- Schwann cells
- satellite cells

Question 11

Nissl bodies

Answer 11

are clusters of rough ER and ribosomes: we think they’re present cause neurons are very metabolic and use lots of proteins so these are there to make extra

Question 12

neurofibrils

Answer 12

provide the neurons with shape and support (cytoskeleton)

Question 13

Myelinated v. Unmyelinated axon

Answer 13

covered by layers of neuroglial cell membrane (lipid/protein material)

• PNS→Schwann cell wraps around axon (only once)
• CNS→oligodendrocyte has many processes that wrap around many axons
• space b/t cells are Nodes of Ranvier

Unmyelinated
– are surrounded by a thin Schwann cell membrane that encloses several axons

Question 14

Ganglion

Answer 14

cluster of neuron cell bodies in the PNS

Question 15

Ion Channels

Answer 15

• Ions diffuse down an electrochemical gradient through a channel protein
• From areas of high concentration to areas of low concentration
• From areas that are charged to areas that are oppositely charged (i.e., +ve to –ve and from –ve to +ve)

Question 16

Are 4 kinds of ion channels

Answer 16

Leakage channels
Ligand-gated channels
Mechanically-gated channels
Voltage-gated channels

Question 17

Leakage channels

Answer 17

• Gates randomly open & close

* are more K+ leak channels than Na+ leak channels in cell membrane

Question 18

Ligand-gated channels

Answer 18

open & close in response to a certain chemical (eg. neurotransmitter, hormones, ions). Once the chemical (ligan – something that binds to something) binds to the receptor, the receptor will open up.

Question 19

Mechanically-gated channels

Answer 19

open or close in response to a mechanical stimulus (eg. vibration, touch, pressure, stretch). A physical action pops it open

Question 20

Voltage-gated channels

Answer 20

open in response to a change in membrane potential (voltage). (dif in electrical charge across the membrane)

Question 21

Resting Membrane Potential

Answer 21

• Difference in electrical charge exists across the cell membrane → the resting membrane potential (-70mV)
• Is maintained by an unequal ion distribution across the membrane
• Membrane potential. There’s a build up of negative charge between the cell memb and the cytoplasm. Outside of the membrane a positive charge is building up. This is only right beside the membrane.
• The NA is going to want to difuse into the cell via the leaking channels
• The K is gonna want to flow down the gradient to the otside of the cell. There more K then Na challels so theres more K going out then Na coming in
• End up with a negative cell interior due to net outward movement of positive charge (K+) & more negative proteins inside
• Na/K pump maintains these gradients (expels 3 Na+ for every 2 K+ that enters the cell, and thus removes more +ve charge than is brought in)

Outside of cell = extracellular fluid:

• hi [Na+]
• hi [Cl-]
• Na+ diffuses in via Na+ leakage channels

Inside of cell = intracellular fluid:

• hi [K+]
• hi [proteins-]
• K+ diffuses out via more numerous K+ leakage channels

Question 22

Generation of an Action Potential

Answer 22

• An AP occurs in response to a threshold stimulus
• A neuron exhibits an “all-or-none” response
• Takes it from -70 to 0 then back down to -70 (depolarization)
• Threshold stimulus is anything that depolarizes the membrane and brings it to the treshhold. Once its brought from -70 to that level it’ll trigger an action potential. In most neurons the threshold to be reached is -55mV.
• Once its met, it triggers an AP. If this is triggers, the neuron exhibits an all-or-none response.
• The amplitude is the same every time

Depolarizing Phase
Re-polarizing Phase
After-hyperpolarizing Phase

Question 23

Depolarizing Phase

Answer 23

Na FLOWS INTO THE CELL
• A stimulus causes the membrane to depolarize to threshold
• Voltage-gated Na+ channels are quick to open upon depolarization
• Influx of Na+ causes membrane to depolarize even more (becomes less negative on the inside)
• More Na+ channels open & membrane potential becomes +30mV
• Na+ channels start to close

Some stimulis cause the membrane to depolarize the threshold (maybe a ligand has binded allowing something to come in, maybe mechanical channel stretched an allows a + charge in) and the -55mV is met.
Triggers the voltage gated Na channels to pop open and Na rushes in down [ ] gradient causes it to depolarize. + feedback, as some Na open more and more open. There time limited, the Na channels only open for a fraction of a second. At peak of the potential it’ll get to +30mV. Then they start to close.

Question 24

Re-polarizing Phase

Answer 24

K+ FLOWS OUT OF CELL
• K+ channels are slower to open upon depolarization (Na+ channels are now closing)
• K+ diffuses out of the neuron
• Membrane repolarizes (becomes more negative)

K channels are triggers by same initial stimulus that cause the Na channels to pop open. They’re a little bit slower and don’t start to open until the Na channels start to close. Cause K to disffuse out and causes the membrane to become more negatice (since K is flowing out). Were stopping + from coming in but allowing + to flow out. This goes on until it reaches -70 again.

Question 25

After-hyperpolarizing Phase

Answer 25

K+ KEEPS FLOWING OUT OF CELL
• Excess K+ leaves (K+ channels slow to close) & membrane hyperpolarizes (reaches -90mV)
• As K+ channels close, membrane potential returns to -70mV

Sometimes the K doesn’t close when its suppose to so too much K flows out and the cell actually reaches -90mV. Then K close and the cell slowly rebalances to the resting state which is -70mV. Everytime an action potential is generated it’ll look the same. It doesn’t matter what the stimuli was that triggers it, it’ll always be the same

Question 26

refractory period

Answer 26

• Neuron has a refractory period (RP) – time period when a second AP can’t be generated
• Refractory period is when a second AC can’t be generated. If a second stimulus came through, a new nerve impulse couldn’t happen.
• Ensures one way flow of transmission so that info get there as fast as possible and that each AP is a separate entity so that it doesn’t confuse cells.

• Absolute RP → not possible

• nothing can happen, physically impossible for another AP to be generated.
• Lasts from the time the channels open up to the end of the repolarization phase.

• Relative RP → requires a stronger stimulus

• From the end of the reporarization phase to the end of the hyperpolarizing phase.
• This is cause it’s not at -70, your down at -90mV so you need a stronger stimulus (bring in more positive charge) to get back to the threshold of -55mV.

Question 27

Conduction of an Action Potential

Answer 27

• AP is propagated towards the axon terminals
• Keeps its strength as it spreads along the membrane
• As Na+ flows inward, voltage-gated Na+ channels open in adjacent area of the membrane
• AP regenerates over & over at adjacent areas of the membrane
• can’t go backwards due to the refractory period

Question 28

Are 2 types of propagation:

Answer 28

1. Continuous conduction
   - Occurs along unmyelinated axons
   - A slow process
   - NRG expesive process since you need NA/K pumps all along the legth of the membrane
2. Saltatory conduction
   - Occurs along myelinated axons
   - Depolarization only at nodes
   - AP jumps from node to node
   - Much faster conduction

• Number of impulses per sec increases with intensity of stimulus
• The freq is going to increase with intensity of stimuli.
Ex. Light touch will have a certain about of impulses per second, whereas firm tough will have a higher rate of impulses per second.

Question 29

3 types of synapse

Answer 29

• axodendritic – the axon of one neuron is facing a dendrite
• axosomatic –axon with a soma
• Axoaxonic – axon interacting with other axon

Question 30

Transmission Across Synapses

Answer 30

• Synapse is unidirectional
• Electrical impulse can’t cross cleft
• converts to a chemical signal → involves release of a neurotransmitter
• Neurotransmitter binds to receptors on the membrane of the postsynaptic cell & generates an electrical signal, a postsynaptic potential, opens up Na ligangated channels
• Electrical –> chemical to cross the cleft (neurotransmitters) –> back to electrical

STEPS:

1. Nerve impulse comes down pre-synaptic neuron
2. hits the Ca gated channels, they open up, causing Ca to come in
3. cause the vesicles to fuse with the membrane (so converted to chemical signal)
4. releases the neurotransmitter into the gap, diffuses across the gap,
5. binds to the ligan gated channels on post synaptic neuron which opens up and allows + charge in
6. if enough positive comes in brings to threshold
7. triggers a new AP