Nervous system: definition

master controlling and communicating system of the body -- cells communicate with each other and other systems by electrical (similar to action potential) and chemical (like AChl) signals
--rapid and specific
Made of CNS and PNS
**Endocrine system is the other

Nervous system: functions

1. Sensory input - receives stimuli; mostly brain but sometimes in the spinal column (Afferent pathway)
2. Integration - interprets stimuli and makes decisions (Control Center)
3. Motor output - responds to stimuli; activates effector organ (Effector pathway)

Central nervous system (CNS)

- brain and spinal cord
- integration and command center

Peripheral nervous system (PNS)

- nerves outside brain and spinal cord <--any structure outside brain and spinal cord but it is still related to the nervous system
- sensory receptors, cranial nerves, spinal nerves, peripheral nerves
- Carries messages to and from CNS

Sensory (afferent) Division of the PNS

- carries nerve impulses from sensory organs to CNS
- from skin, skeletal muscles (lactic acid accumulation makes muscles hurt), and visceral organs (stomach, bladder, etc...)

Motor (efferent) Division of PNS

- carries nerve impulses from CNS to effector organs
- 2 parts: Somatic (voluntary) and Autonomic (involuntary)

Somatic nervous system

- VOLUNTARY nervous system
- controls skeletal muscles

Autonomic nervous system

- INVOLUNTARY nervous system
- regulates smooth and cardiac muscles and glands
includes sympathetic and parasympathetic divisions - have opposite effects on organs

proprioceptors

where your body is in relation to itself
- the tiny little adjustments that prevent you from falling on your facy

What cell types make up nervous tissue?

Neurons and neuroglia

Neurons: definition

- transmit nerve impulses
- large, complex cells
- comprised of cell body and processes
- long lived and amiotic (don't divide)
- High metabolic rate - can't survive long without O2

Neuron Cell body

- has a nucleus and usual organelles
- active in synthesis or neurotransmitters
- well developed rough ER, ribosome, and Golgi apparatus
- Focus point for outgrowth of processes
* Most cell bodies are located in the CNS
* Most cell processes (AXONS) are located in the PNS

Neuron Processes

DENDRITES
- numerous, short, and branched processes
- receptive regions of the neuron
- Convey incoming messages toward cell body via GRADED POTENTIALS

AXONS
- singular long processes
- secrete neurotransmitters from axon terminals
- generate and transmit ACTION POTENTIALS (exactly the same as in muscle) along axon and away from cell body

Multipolar neuron

- structural classification
- one axon and many dendrites
- 99% of neurons - most of them are in CNS

Bipolar neuron

- structural classification
- one axon and one dendrite
- cell body is found in the middle
- rare, found in eyes and ears

Unipolar neuron

- structural classification
- One axon only - cell body is disassociated from the neuron (one, short process extends and attaches the cell body to a long process)
- rare, function as sensory receptors

Sensory neurons

- AFFERENT
- functional classification
- transmit ACTION POTENTIAL from skin, muscle, or internal organs toward CNS
- mostly unipolar and bipolar nuerons

Motor neurons

- EFFERENT
- functional classification
- carry ACTION POTENTIAL from CNS to muscles and glands <-- tells muscle to contract
- mostly multipolar neurons

Interneurons

- within CNS; between sensory and motor neurons
- mostly multipolar neurons
- most common function of neurons
** Do NOT convery message in/out of CNS, because they are already IN the CNS.

What is the difference between nuclei and ganglia

cell body clusters in CNS versus PNS

What is the difference between fiber tracts and nerves

cell body clusters in CNS versus PNS

6 types of glial cells

1. Astrocytes
2. Microglia
3. Ependymal cells
4. Oligodendrocytes
5. Satellite Cells
6. Schwann Cells

Astrocytes

"STAR CELL"
- most abundant and versatile glial cells
- support, and anchor neurons to capillaries
- guide migration of young neurons
- control chemical environment by cleaning up released neurotransmitters <-- control them (if they're in the wrong place) by enzymes
**lots of processes make this cell look like a star
**some are attached to neurons and others are attached to capillaries
**brings blood vessels and neurons closer together - vascularizes neurons

Microglia

**specialized macrophage, only for the brain
- small cells with long spiny processes
- turn into macrophage when microorganisms are present - immune function <-- become more rounded with fewer processes
- responsible for monitoring and maintaining health of neurons

Ependymal cells

**look like epithelial cells
- squamous or columnar shape, some have cilia
- line cranial and spinal cavities to form a permeable barrier for cerebrospinal fluid
- beating of cilia helps circulate cerebrospinal fluid

Oligodendrocytes

**a cell with just a few processes
- wrap around thick CNS nerve fibers
- Produce insulating covering myelin sheaths around large axons in CNS

Satellite cells

a type of glial cell that surrounds cell bodies of the neuron in the PNS
- function unknown

Schwann Cells

a type of glial cell that forms myelin sheaths around larger axons
- function similar to oligodendrocytes

cerebrospinal fluid

the fluid within the cranial cavity that provides nutrients and cushioning for the brain.

How can you tell the difference between axon and dendrite?

axons usually have a myelin sheath

*dendrites and cell bodies are ALWAYS unmyelinated

Myelin sheaths: Definition and function

**produced in multiples (up to 60 segments) by oligodendrocytes in the CNS
Definition:
- whitish, fatty segments made of phospholipids that surround only the larger axons
Function:
- protects the axon
- insulates axons from one another <-- like live wires
- increase nerve impulse conduction speed <-- you can feel touch quickly

Form Myelin sheaths take in CNS

- formed by oligodendrocytes
- occur in multiple segments
- wrap concentric layers of plasma membrance and cytoplasm around axon (like roll cake)

Nodes of Ranvier

gaps between segments of myelin sheaths
- where axon branches emerge
- where action potentials land

Form Myelin sheaths take in PNS

- Formed by Schwann cells
- Each Schwann cell makes up one segment
- Attach themselves to the axon and then roll out (like dough) to wrap around it.
**Segments are fatter on longer atoms

Neurophysiology

**Neurons behave a bit like skeletal muscle - receive stimuli and respond (muscle responds by contracting)
- Neurons are highly excitable
- Ability to receive stimuli and respond
- When neurons are adequately stimulated* (GRADED POTENTIAL on dendrites and cell bodies), nerve impulses (ACTION POTENTIALS on axon) are generated
- regardless of stimulus, neuron type and location
*adequately because not all stimuli are created equal; some are too weak to stimulate the neurons

Membrane Potentials

**similar to ACTION POTENTIAL but much simpler
- resting neuron membrane is polarized at -70mV
- Changes due to electrolyte distribution
- Depolarization - inside becomes less negative
- Repolarization - inside returns to resting potential/neutral
- Hyperpolarization - inside becomes more negative

Graded Potential

short lived, local changes in membrane potential

- initial intensity varies
- intensity decreases with distance (from the beginning of dendrite to the cell body)
- occur only in dendrites and cell bodies (because axons deal with action potentials)
- not all stimuli are created equal; many are weak and die down before reaching axon and stimulating the neurons
- if the stimuli is strong enough it can initiate action potentials in axons.

Action Potential

- the principal means of neural communication
- occurs when GRADED POTENTIAL is strong enough
- occurs only in axons - aka nervous impulses
- do not decrease in strength over distance
- no change in intensity
*pretty much the same as in skeletal

Which is more important, action potential or graded potential?

They are equally important because you need graded to get action and you need action to talk to other cells.

How is Action Potential Conducted?

In unmyelinated axons - continous but slow. Electrical current moves forward and depolarizes next region.
In myelinated axons current is only conducted at nodes of Ranvier - fewer places for ions to enter through therefore they have to make bigger jumps.
**myelinated is 30 times faster than unmyelinated.