a&p: unit 4: nervous system

3 basic functions of nervous system

1. receiving sensory input (info from outside world through sensory organs and internal info monitored)
2. integration (making sense of input and deciding what to do about it
3. motor output (instructions on how to respond sent to muscles or glands)

2 basic divisions of nervous system

CNS: (central nervous system) brain and spinal cord; main "control center"
PNS: (peripheral nervous system) nerves that branch off from the CNS to communicate with the rest of the body

divisions of PNS

sensory and motor
within motor:
- somatic
- autonomic
within autonomic:
- sympathetic
- parasympathetic

afferent

(sensory) bring sensory information in to the CNS

efferent

(motor) brings motor instructions from the CNS to muscles or glands

somatic

voluntary

autonomic

involuntary

sympathetic

fight or flight
activates in response to stressor

parasympathetic

calm, cool, + collected
activates to promote normal body functions

neurons

main functional cells of nervous system
directly generate and transmit electrical signals (action potentials) for rapid communication within the body

glial cells

"entourages" of neurons
provide support, protection, nutrition, insulation, assist with signal transmission by neurons

glial cells in CNS (4)

astrocytes
microglials
ependymals
oligodendrocytes

astrocytes

"star-shaped" cells
most abundant
anchor neurons to their blood supply (capillaries)
regulate the movement of materials between blood and the neuron

microglial cells

"thorny"
immune defense
general maintenance and clean-up of the CNS (getting rid of damaged/unnecessary neurons)

ependymal cells

lining of fluid-filled spaces in CNS (including large fluid-filled spaces called ventricles in the brain)
produce the cerebrospinal fluid: in cerebrum (outer part of brain) and spine which fills in cavities and contributes to the cushioning of the brain/spinal cord

oligodendroycte cells

wrap around neurons and produce myelin sheath: fatty insulating barrier surrounding axons and help transmit signals faster

glial cells in PNS (2)

satellite cells
schwann cells

satellite cells

surround and support the cell bodies of neurons

schwann cells

wrap around the axons of neurons in the PNS to produce their myelin sheath

main parts of neurons (6)

dendrites
axons
axon terminals
myelin sheath
nodes
cell body

dendrites

short projections that receive messages

axons

projections that send messages (can be long or short)

axon terminals

small bulbs at the end of axons that release neurotransmittes

myelin sheath

fatty insulating material that covers axon

nodes

gaps in the myelin sheath

cell body

all typical business of keeping the cell alive

PNS vs. CNS
- bundle of axons (made of neurons)
- group of cell bodies

PNS:
- nerves
- ganglia
CNS:
- tracts
- nuclei

basics of a spinal reflex circuit

- stimulus is received by a receptor, which passes info along a sensory neuron
- interneurons in CNS (spinal cord) process info
- response is sent along motor neurons, which then reach an effector (muscle of gland) for a response

how do action potentials encode (produce) meaningful info when they are just an "on or off" signal

frequency + number of receptors generating the action potentials
ex: sensory receptors in skin that detect range of pressure
- gentle pressure: lower frequency + few receptors getting activated = fewer neurons sending action potentials at once
- stronger pressure: higher frequency + more receptors getting activated = more neurons sending action potentials at once

voltage

amount of potential energy (since opposite charges attract, separating charges on opposite sides of barrier = potential energy → when barrier is across cell membrane = membrane potential)

sodium-potassium pump basics

- uses ATP to push Na+/K+ against their concentration gradients
- pumps 2 K+ ions into the cell for every 3 Na+ ions out of the cell
- chemical gradient: more Na+ outside the cell and more K+ inside
- electrical gradient: more positive outside the cell and more negative inside

parts of the "wave" of action potential graph and basic info

1. initial stimulus (-70 mV → just before -50 mV); some Na+ channels open
threshold = (-50 mV) if graded potential goes from -70 to -50, action potential is triggered
2. depolarization: threshold met; voltage-gated Na+ channels open
3. repolarization: Na+ channels inactivated, voltage-gated K+ channels open
4. hyperpolarization: some K+ channels open → quickly returns to -70

pumps vs. membrane channels

pumps: active transport (use ATP) to push things against their concentration gradients
membrane channels: use passive transport to allow substances to travel down or with their concentration gradient

3 membrane channels

mechanically gated
ligand gated
voltage gated

mechanically gated

open in response to cell membrane being physically stretched
- pulls ion channels open → allows Na+ into cell, leading to a graded potential

ligand gated

ligand = chemical messenger that binds to detector molecules (receptors) → causes channel to open
- neurotransmitters = ligands released from one neuron which can then stimulate another neuron (connected via a synapse)

voltage gated

respond to changes in membrane potential
- when graded potentials cause the voltage across a membrane to increase from -70 to -50 mV, they open

what is a graded potential + what happens when threshold (-50 mV) is met

- initial depolarization of a neuron begins with ligand/mechanically gated channels allowing some Na+ in to create graded potential
- threshold met: voltage gated Na+ channels open, allowing Na+ ions to flow freely in = massive depolarization

axon hillock + what happens when threshold is reached

axon hillock (trigger zone) = first part of axon and where it begins
- contains lots of voltage gated ion channels
- where an action. potential is triggered from graded potential
- voltage gated Na+ channels open = depolarization

wave in more detail

- cell receives strong enough stimuli to reach threshold (-50 mV) + undergoes depolarization when action potential is triggered
- Na+ is so high and cell is so depolarized that it needs to "reset"
- voltage gated K+ channels open in response to depolarization and K+ moves out of cell (down its concentration gradient) to make cell's potential more negative and helps "reset" the resting potential = repolarization
- cell is briefly hyperpolarized when voltage becomes lower than -70 mV when K+ channels are slowly closing → quickly gets back to resting state (-70 mV)

resting state of a neuron

-70 mV

how an action potential is triggered

stimulus is strong enough to charge voltage from -70 mV (resting) to -50 mV (threshold for action potential) or higher

basics of how an action potential propages (travels like a wave) down the length of the axon

- axon has voltage gated Na+ and K+ channels along its length
- when one region of axon depolarizes, it triggers depolarization in adjacent region of axon
- wave of depolariztion travels down length of axon until it reaches the end
- at end: depolarization causes complex electrical and chemical events that cause neurotransmitters to be released from axon terminals
- whole process happens faster in myelinated axons as the action potential basically "jumps" from node to node