Chapter 12: Nervous Tissue Flashcards
(30 cards)
Homeostasis
a condition in which the body’s internal environment is maintained relatively constant.
Nervous system has 3 basic functions:
- Sensory function → sensory input from receptors detect changes inside & outside the body
- Integrative function → sensory input is interpreted & coordinated with an appropriate motor response
- Motor function → motor output to an effector organ such as a muscle or gland
The nervous system has 2 main subdivisions:
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
Sensory Division
-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)
Motor Division
- 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
Somatic nervous system
from Motor sys
- Under voluntary control
- Somatic = limb movements
- Motor neurons conduct impulses from CNS to skeletal muscles
Autonomic nervous system
from motor sys
-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
Two basic nervous cell types
1) neuron
2) Neuroglia
Neurons
• 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)
Neuroglia
• 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
Nissl bodies
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
neurofibrils
provide the neurons with shape and support (cytoskeleton)
Myelinated v. Unmyelinated axon
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
Ganglion
cluster of neuron cell bodies in the PNS
Ion Channels
- 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)
Are 4 kinds of ion channels
Leakage channels
Ligand-gated channels
Mechanically-gated channels
Voltage-gated channels
Leakage channels
- Gates randomly open & close
* are more K+ leak channels than Na+ leak channels in cell membrane
Ligand-gated channels
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.
Mechanically-gated channels
open or close in response to a mechanical stimulus (eg. vibration, touch, pressure, stretch). A physical action pops it open
Voltage-gated channels
open in response to a change in membrane potential (voltage). (dif in electrical charge across the membrane)
Resting Membrane Potential
- 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
Generation of an Action Potential
- 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
Depolarizing Phase
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.
Re-polarizing Phase
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.
