anatomy chapter 11

Sensory input

Sensory input

Monitors changes that occur inside and outside the body

Integration

Processing & interpretation of input informationthe nervous system “decides” what response to make

Motor output (motor response)

Response is carried out

The Central Nervous System (CNS)

Composed of the brain and spinal cord

• Function: is responsible for interpreting sensory input and deciding

motor output

The Peripheral Nervous System (PNS)

Composed of nerves that extend from the CNS to the rest of the body

• Function: allows information to be sent between the CNS and the rest of the body

Neurons:

nerve cells that can respond to stimuli & transmit electrical

signals

Neuroglia (glial cells)

provide support and maintenance to neurons

Astrocytes (CNS)

most abundant, support & protect neurons

in CNS

• Star-shaped, with projections connecting to and wrapping around neurons, nerve endings, and surrounding blood capillaries

• Main functions:
• A) Provide nutrient supply for neuron cells

• B) Allows migration of young neurons • C) “Clean up” outside neuron cells

• Leaked K+ ions, neurotransmitter

Microglial cells (CNS)

Functions:

• A) Contact nearby neuron cells to monitor neuron health

• B) Migrate toward injured neurons & transform into a macrophage and phagocytize the neuron

Ependymal cells (CNS)

Most ependymal cells have cilia

• Function: lines central cavities of CNS to circulate cerebrospinal fluid (CSF) within cavities

Satellite cells (PNS)

Support & protect neuron cell in PNS

Oligodendrocytes (CNS) & Schwann Cells (PNS)

Wrap around thicker nerve fibers in CNS & PNS

• Function: myelin sheath creates an insulating covering for neurons

Cell body of neuron

portion of cell containing the nucleus • Function: plasma membrane can receive information from

surrounding neurons

• Most cell bodies are found in the CNS & are protected by bone

• Clusters of cell bodies in CNS are called nuclei, those in PNS are called ganglia

Dendrites

main receptive region of neuron

• A single neuron can have dozens of dendrites

• Function: provide increased surface area for incoming signals, convey incoming messages toward the cell body

Axon:

single, long “nerve fiber” extending from the cell body • The axon is the conducting region of the neuron

• Function: generates and transmits nerve impulses away from the cell body

• Bundles of axons in the CNS are called tracts, those in PNS are called nerves

• Axon branches at the end to form terminal branches & axon terminals

• Function: neurotransmitter released at axon terminal to pass the impulse to the next neuron

Myelin Sheaths

Functions: protects and electrically insulates long and/or large nerve fibers to increase speed at which impulses are transmitted

•Found only on axon portion of the neuron

•Not all axons are myelinated

Sensory (afferent) neuron

afferent neurons transmit signals from the body to the CNS

• Receptive endings of this neuron type can function as actual sensory structure, or are associated with larger sensory receptors

Motor (efferent) neuron

efferent neurons transmit motor response from CNS to the body

• Impulses travel to effector organs (muscle + glands)

Interneuron

lie between sensory and motor neurons • Function: pass signals through CNS pathways where

integration occurs
• Can connect to other interneurons → can communicate with

neighbors

Cause of change in resting membrane potential of neurons

Changing the permeability of the plasma membrane to one

(or more) ions

Leakage (non-gated) channels

always open, allow free flow of ions

Gated proteins:

part of protein forms a “gate” that must be opened before ions can move. Gated proteins can be:

• A) Chemically gated: only open when a certain chemical(neurotransmitter) binds to protein

• B) Voltage-gated: open & close in response to changing membrane potentials

Depolarization

decrease in membrane potential
• The inside of the membrane becomes less negative than

resting potential
• Excitation of a neuron

Hyperpolarization

increase in membrane potential
• The inside of the membrane becomes more negative than

resting potential • Inhibits a neuron

Graded Potentials

“Graded”magnitude varies directly with stimulus strength

• Strong stimulus = strong graded potential

•Graded potentials only occur over short distances • Current dies off quickly

•Can be depolarizing or hyperpolarizing

•Function: graded potentials are necessary to initiate an action potential

Action potentials

a very brief reversal of membrane potential (from -70 mV to +30 mV)

• Only produced by neurons and muscle cells
• Action potentials have a consistent strength and are long distance
• APs originate at the beginning of axon arising from cell body

(“trigger point”)

• Change in membrane potential from graded potential causes voltage-gated channels to open at this point

Na+ channel has two gates:

opens at depolarizatioLn oading... • B) Inactivation gate: blocks channel to

prevent Na+ movement

1) All voltage-gated channels are closed at the resting state (-70 mV)

Leakage channels are still open here!!!

Depolarization: voltage-gated Na+ channels open at the axon

Effect: Na+ freely enters the cell

• Inside of the cell becomes less negative

Membrane will reach a threshold voltage (-55 mV) as more Na+ enters the cell

Repolarization

This is when the action potential ends

Na+ gates close

• Na+ permeability drops rapidly

• Net influx of Na+ into cell stops completely

• **This causes the AP to stop rising!!

• Voltage-gated K+ channels open

• K+ leaves the cellrestores (-) internal charge of cell

) Hyperpolarization: excess K+ leaves cell

Result → inside of the cell becomes more negative than resting membrane potential

• While this happens→ Na+ activation gates have closed, inactivation gates reopen

Strong stimuli

impulses are sent more frequently

Weak stimuli

impulses sent less frequently

Refractory period

a period of time in which a second AP cannot be generated at an axon

Absolute Refractory Period:

Begins when Na+ gated channels open, continues until Na+ channels reset to their original state

• During this time, another AP cannot be generated in the area, no matter how strong the stimulus is

Importance:
• A) Ensures each AP is a separate, all-or-none event • B) Enforces one-way transmission of the AP

Relative refractory period:

Occurs after the absolute refractory

period

• Stimuli that are relatively weak cannot stimulate an AP, but an exceptionally strong stimulus can

Axon diameter

larger axon = faster conduction

Degree of myelination

more myelination = faster conduction

Continuous conduction

propagation in unmyelinated fibers
• Voltage-gated ion channels are adjacent for the entire length of the axon

Saltatory conduction

propagation in myelinated fibers

• Voltage-gated ion channels found ONLY in myelin sheath gaps

• AP generated in myelin sheath gap

faster

Synapse

Junction between two neurons that sends information from one neuron to the next

Presynaptic neurons

conduct impulses toward the synapse

• The neuron that is “sending” the message

Postsynaptic neurons

conduct signal away from the synapse

• The neuron that is “receiving” the message

Transmission of Action Potentials from One Neuron to Another

1) Action potential arrives at axon terminal of presynaptic neuron

2) Voltage-gated Ca2+ channels in terminal open in response to AP

• Ca2+ enters the axon terminal of presynaptic neuron

3. Synaptic vesicles in axon terminal fuse with membrane in response to Ca2+ influx

   • Neurotransmitter enters the synaptic cleft

4. Neurotransmitter crosses cleft, binds to proteins on postsynaptic neuron

5. Neurotransmitter binds receptors on the postsynaptic neuron membrane

   1. Binding causes ions channels to open •

   2. Ion flow generates a graded potential

6. Neurotransmitter in synaptic cleft is disposed of

Neurotransmitter can be disposed of by:

1) Reuptake of neurotransmitter by an astrocyte or by the pre-synaptic neuron

• 2) Degradation of neurotransmitter by an enzyme

• 3) Diffusion of neurotransmitter out of the synapse

Postsynaptic Potentials

the temporary change in membrane potential (i.e., a graded potential) of the postsynaptic neuron

Neurotransmitter binding cause graded potentials that vary in

strength according to

• Amount of neurotransmitter released

• How long neurotransmitter stays in synaptic cleft

Excitatory postsynaptic potential (EPSP)

Binding of neurotransmitter causes the

membrane to depolarize

A single EPSP cannot induce an AP alone

• Several EPSPs will summate (be “added together”) to generate an AP

• Two types of summation: • 1) Temporal summation

• 2) Spatial summation

Temporal summation

the postsynaptic neuron receives multiple EPSPs in rapid-fire order

Spatial summation

postsynaptic neuron receives multiple EPSPs at the same time

• EPSPs are “added together” simultaneously

Inhibitory postsynaptic potential (IPSP)

Binding of neurotransmitter causes the membrane to hyperpolarize

• K+ channels or Cl- channels open, making inside of cell more negative

Neurotransmitters

chemical signals produced in the cell body & is transported to the axon terminal

• Most neurons produce at least 2 types
• Neurons can release one or more neurotransmitters simultaneously

Effects

• Can be excitatory, inhibitory, or can be either depending on the receptor type they bind

Channel-linked receptors:

mediate fast synaptic transmission

• Receptors are ligand-gated ion channels

• When ligand binds → channel opens

• Na+ influx→ depolarization
• Cl- influx→ hyperpolarization

G-Protein Coupled Receptors

Response is indirect & prolonged • General process:

• A) Neurotransmitter binds to receptor
• B) G-protein is activated inside the neuron •

C) G-protein activates adenylate cyclase

D) Adenylate cyclase produces cyclic AMP (cAMP) • cAMP can have 1 of 3 actions:

• 1) Change membrane permeability by opening or closing ion channels in membrane

• 2) Activate specific genes in the cell nucleus • Cell can produce more or less proteins
• This is a metabolic change

• 3) Activate kinase enzyme
• Kinase begins to catalyze reactions • This is a metabolic change