The Nervous System

4 functions of the nervous system

1. Controls physiological parameters in the body

2. Works with the endocrine system to maintain homeostasis

3. Initiates voluntary movements

4. Is the origin of thought, memories, & emotions in the brain

The nervous system is organized into what 2 division? What do these divisions consist of?

1. Central Nervous System (CNS) - brain & spinal cord

   1. Peripheral Nervous System (PNS) - the rest of the nerves; responsible for afferent & efferent communication

4 components of the peripheral nervous system (PNS) & what they do

1. Receptors - bring info in

2. Afferent nerves - bring info to the CNS

3. Efferent nerves - bring info out of the CNS

4. Effectors - place that talks to brain & spinal cord and back

Afferent

Info coming INTO the CNS; arriving

Efferent

Info coming OUT of CNS; exiting

Visceral

Referring to organs, smooth muscle, cardiac muscle

Somatic

Referring to skin, skeletal muscle, or joints

Nerve

Structure of PNS that you can see with the naked eye that carries info in and out of the CNS

(The CNS does not contain any nerves!)

Autonomic nervous system

Involuntary nervous system pertaining to organs

2 divisions of the autonomic nervous system

1. Sympathetic division

2. Parasympathetic division

Sympathetic nervous system

Fight or flight response

Parasympathetic division

Conserves energy; rest & digest because it promotes rest and increases activity of digestive organs

Neuroglia (glial cells)

Supporting cells of the nervous system; they do NOT send signals, but instead, they take care of neurons

4 glial cells of the CNS

1. Astrocytes

2. Microglia

3. Ependymal cells

4. Oligodendrocytes

2 glial cells of the PNS

1. Satellite cells

2. Schwann cells

Astrocytes

Star-shaped cells in the CNS that are meant for guidance in early development; involved in maintaining ionic gradients & blood brain barriers (aka helps prevent toxins & germs from coming out of the blood & into the neuron environment)

Microglia

CNS cells that eat up any junk or germs they might find in between the cells

Ependymal cells

CNS cells that line the cerebral ventricles; have cilia to help circulate cerebrospinal fluid

Oligodendrocytes

CNS cells that have wrap around the axons of neurons; acts as the myelin sheath for neurons in the CNS

Satellite cells

PNS cells that surround the cell bodies to ensure a proper environment

Schwann cells

Cells that produce the myelin sheath for axons in the PNS

Neurons

Cells that live a very long time that don’t go through mitosis (amitotic); Have a high metabolic rate, meaning they are constantly making ATP, so they require lots of glucose, oxygen, and blood flow

5 structures of a neuron

1. Cell body (aka soma)

2. Dendrite

3. Axon

4. Axon hillock

5. Axon collateral

6. Telodendria

Cell body (soma)

Contains the nucleus and nucleolus; responsible for integration of signals

Dendrite

Branch-like extensions of the neuron that receiving electrical signals, carrying them to the cell body

Axon

Fiber-like extension that carries electrical impulses away from the cell body

Axon hillock

Cone-like attachment of the axon to the cell body

Axon collateral

Branches of the axon itself

Telodendria

Axon terminals that have synaptic knobs at their ends

3 components on a nerve fiber

1. Myelin sheath

2. Neurilemma

3. Nodes of Ranvier (myelin sheath gaps)

Myelin sheath

Made up of Schwann cells that insulates axons & speeds up the transmission of an action potential

Neurilemma

Plasma membrane of a Schwann cell

Nodes of Ranvier

Gaps in between each of the myelin sheaths

1. What is axonal transport?

2. What are the 2 kinds & what do they do?

1. A way to transport intracellular items along a neuron’s axon

2. Anterograde (amazon prime) - moving proteins from soma to axon terminal, similar to how regular impulses would move

3. Retrograde (amazon returns) - removing viruses from the axon terminal to the soma

3 types of neurons according to structural classification

1. Pseudounipolar

2. Bipolar

3. Multipolar

Pseudounipolar neurons (3)

• Cell body with a stunted process, then a really long process coming off of it

• Most sensory neurons are like this

• Cell bodies of these cells can be found in the dorsal root ganglion

Bipolar (4)

• 2 processes coming off of each side of the cell body

• 1 process serves a dendrite purpose

• 1 process serves as an axon

• Can be found in sensory systems (ex. retina of the eye)

Multipolar (2)

• What the standard neuron we think of looks like

• Neuron with soma, lots of dendrites, 1 axon

3 functional classes of neurons + what structural neurons encompass them & their functions

1. Afferent (sensory; pseudounipolar & bipolar) - info toward CNS

2. Efferent (motor; multipolar) - info away from CNS

3. Interneuron (multipolar) - info within CNS

Cluster of cell bodies in the CNS

Nucleus

Cluster of cell bodies in the PNS

Ganglion

Bundle of nerve fibers (axons or dendrites) in the CNS

Tract

Bundle of nerve fibers (axons or dendrites) in the PNS

Fascicle

4 structures of nerves & what they are

1. Endoneurium - connective tissue covering each nerve fiber

2. Fascicle - a bundle of axons/dendrites

3. Perineurium - connective tissue surrounding each fascicle

4. Epineurium - connective tissue surrounding the whole nerve

3 functional classes of nerves & what they do

1. Afferent nerves - carry impulses towards the CNS

2. Efferent nerves - carry impulses away from the CNS

3. Mixed nerve - contains both afferent and efferent fibers

What 2 things determine electrical flow?

1. Ions inside & outside the cell

2. Permeability of the cel membrane

Excitable Cells (5)

1. Neurons & muscle cells

2. Overall, high concentration of K+, but low concentration of Na+

3. Outside - high concentration of Na+, low concentration of K+

4. Inside - high concentration of K+, low concentration of Na+

5. This unequal distribution is a resting membrane potential

Resting membrane potential of most neurons

-70 mV (polarized)

What is the net charge on the inside of the membrane vs. the outside?

• Inside - net negative charge

• Outside - net positive charge

Polarized

Inside of the cell is negative compared to the outside of the cell

Depolarized

The charge inside the cell moves closer to zero (more positive); excitatory event

Hyperpolarized

Inside of the cell becomes more negative than its normal resting membrane potential; inhibitory event

Axosomatic synapse

Synapse onto a cell body

Axodendritic synapse

Synapse onto a dendrite

Ligand-gated (chemically gated) ion channels

Stimulus (ligand) binds to receptor, causing a channel to open, allowing electrically-charged ions to move across the membrane
(ex. neuromuscular junction)

Depolarization in a ligand-gated (chemically-gated) ion channel (3)

• Inside of the cell is negative, outside is positive

• Na+ comes into the cell, bringing its positive charge

• Inside of the cell becomes more positive, meaning membrane potential moves closer to 0

Hyperpolarization in a ligand-gated (chemically-gated) ion channel (3)

• Inside of the cell is negative, outside is positive

• Cl- comes into the cell, bringing its negative charge

• Inside of the cell becomes more negative, meaning membrane potential becomes moves farther from 0

Graded Potentials

Membrane potentials we can change depending on how many channels we have

• Small number of open channels = small change in membrane potential

• Many open channels = big change in membrane potential

3 types of graded potentials

1. Postsynaptic potential

2. Receptor potential

3. Pacemaker potential

Postsynaptic potential

Graded potential produced by responding to binding of neurotransmitter & opening of a chemically-gated ion channel that could cause either excitatory (EPSP) or inhibitory postsynaptic potential (IPSP)

Receptor potential

Graded potential produced at the ends of afferent neurons when they are stimulated by light, heat, or mechanical energy (mechanically-gated ion channels)

Pacemaker potential

Graded potential involving leaky ion channels that are naturally open in certain specialized cells

Mechanism of chemical synapse (ligand-gated ion channels) (5)

1. Neurotransmitter binds to receptor (chemically gated ion channel)

2. Channel gate opens

3. Ions move in, causing depolarization/hyperpolarization (if Na+ is coming into the cell/if K+ is coming into the cell)

4. If a certain amount of positive charge reaches the axon hillock, and action potential will be initiated there

4 characteristics of graded potentials

1. They fade out (decremental)

2. They vary in amplitude (the more that open, the greater the change in membrane potential)

3. They can be bidirectional

4. They can be a depolarization or a hyperpolarization

Decremental property of graded potentials

They lose strength & die out as it moves away from initial sight of stimulus

Amplitude property of graded potentials

More channels = larger change in voltage

Bidirectional property of graded potentials

Current spreads in both directions from the site of stimulation

What type of polarization does EPSP inputs cause?

Depolarization

What type of polarization does IPSP inputs cause?

Hyperpolarization

What happens when the membrane potential reaches threshold?

Action Potential

Action Potential (AP)

All of northing depolarizations that occur when excitatory graded potentials reach the axon hillock; Depend on voltage-gated ion channels to continue the AP down the axon

4 characteristics of action potentials

1. Maintains strength for the entire length of the axon

2. Always have the same amplitude

3. Unidirectional; always flows along the axon, starting at the axon hillock & going to the axon terminal

4. Characterized by a sharp depolarization followed by a brief period of hyperpolarization

5 Steps of Action Potential

1. Resting state

2. Depolarizing phase

3. Repolarizing phase

4. Undershoot

5. Resting state