Chapter 11 - Fundamentals of Nervous System & Nervous Tissue

Sensory Input (function of nervous system)

monitors changes that occur inside and outside the body

• collecting information about changes and sending them to another part of the body → brain or spinal cord

Integration (function of nervous system)

processing and interpretation of input information

• the nervous system “decides” what response to make

• “thinking part “ of nervous system

Motor Output/Response (function of nervous system)

response is carried out

• carried out to make some change from brain or spinal cord

• effector organ carries out motor response

The Central Nervous System (CNS)

composed of the brain and spinal cord

• 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

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

Neurons

nerve cells that can respond to stimuli and transmit electrical signals/impulses

• transmits the actual message from body

• without the ability to generate messages → neurons die

Neuroglia (glial cells)

provide support and maintenance to neurons

• keep neurons safe, functional, and protected

Nervous System Microanatomy

types of cells seen in the nervous system and their functions

• general structure of neurons

• classification of neurons based on function

Astrocytes of CNS (type of neuroglia)

projections from membrane surface connect to and wrap around neurons, nerve endings, and surrounding blood capillaries

• “star cells”

• provide nutrient supply for neuron cells

• allows migration of young neurons

  • allows brain to develop correctly

• “clean up'“ area surrounding neurons

Microglial Cells of CNS (type of neuroglia)

contact nearby neuron cells to monitor neuron health → is it functioning correctly?

• migrate toward injured neurons and transform into a macrophage and phagocytize the neuron

• destroying the stuff that “gets in the way”

Ependymal Cells of CNS (type of neuroglia)

most ependymal cells have cilia

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

  • important for protecting brain structures → liquid cushion

Satellite Cells of PNS (type of neuroglia)

support and protect neuron cell in PNS

• wrap around cell body of neuron in PNS → providing nutrients and protection

• similar to astrocytes in CNS

Oligodendrocytes of CNS (type of neuroglia)

wrap around nerve fibers in CNS

• creates an insulating covering called a myelin sheath for neurons

Schwann Cells of PNS (type of neuroglia)

wrap around nerve fibers in PNS

• creates an insulating covering called a myelin sheath for neurons

Cell Body (structure of neuron)

portion of cell containing the nucleus

• synthesizes proteins and neurotransmitters and plasma membrane can receive information

• most cell bodies are found in CNS and protected by bone

  • important because without the cell body, the neuron dies

Longevity Characteristic of Neurons

how long 1 individual neuron live

• basically lasts forever

  • because they’re amitotic

Amitotic Characteristic of Neurons

do not divide mitotically

• what you have is what you’re stuck with

  • problem = can’t reproduce 

    • brain damage → won’t be fixed; so loss of neurons is permanent 

Metabolism Characteristic of Neurons

active cell, always doing things = high metabolic rate

Nuclei

clusters of cell bodies in CNS

• heavily protected by bone

  • skull for brain

  • vertebrae for spinal cord

Ganglia

clusters of cell bodies in PNS

• not as well protected as nuclei in CNS

Dendrites (processes of neuron)

main receptive region of neuron

• provide increased surface area for incoming signals and convey incoming messages toward the cell body

  • makes neurons receptive → actually responding to sensory input

    • less dendrites = less likely to respond to sensory input

Axon (processes of neuron)

single, long “nerve fiber” extending from  the cell body

• conducting region of neuron

  • generates and transmits nerve impulses away from the cell body

Tracts

bundles of axons in CNS

Nerves

bundles of axons in PNS

Axon Terminals

axon branches at the end to form terminal branches

• the secretory region of the neuron

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

Myelin Sheaths

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

• myelination in the PNS (Schwann cells)

  • Schwann cells do NOT contact each other

Myelin Sheath Gaps

region of axon that is “exposed” due to absence of Schwann cell covering

Sensory (afferent) Neuron (neuron functional classification)

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 (neuron functional classification)

efferent neurons transmit motor response from CNS to the body

• impulses travel to effector organs (muscle or glands)

Interneuron (neuron functional classification)

found in between sensory and motor neurons

• pass signals through CNS pathways where integration occurs

• can connect to other interneurons → can communicate with neighbors

  • important for the brain to make the correct decision

Membrane Potential

a difference in electrical charge across the plasma membrane

• more positively charged outside the cell; negatively charged inside the cell

• resting membrane potential = -70 mV

  • neurons and muscle cells can change resting membrane potential through communication with other neurons

How do you change the permeability of the plasma membrane to one (or more) ions?

the use of ion channels in the plasma membrane

Ion Channels

selective transport proteins in plasma membrane that form “channels” to allow passage of ions into/out of cell

Leakage Channels

always open and allow free flow of ions across plasma membrane

• K+ leaks from inside cell to outside → creating negative charge inside cell

Gated Channels 

have a “gate” that must be opened before ions can move

Chemically Gated

only open when a certain chemical (neurotransmitter) binds to protein

• important when neurons are communicating with each other

Voltage Gated

open and close in response to changing membrane potentials

Mechanically Gated

open in response to physical deformation of receptor

Depolarization

when the inside of the cell become more positively charged compared to the resting membrane potential

• excitation of a neuron

• less of a difference of a charge → less potential

Hyperpolarization

when the inside of the cell becomes more negatively charged compared to the resting membrane potential

• inhibits a neuron

Graded Potentials

magnitude varies directly with stimulus strength

• strong stimulus = strong graded potentials

• only occur over short distances → current dies off quickly

• can be depolarizing or hyperpolarizing

• graded potentials are necessary to initiate a nerve impulse

  • occur on the dendrites and cell body

Action Potentials (Nerve Impulses)

can only be produced by neurons and muscle cells

• all have a consistent strength and occur over long distances

• ONLY depolarizing

  • membrane potential changes from -70 mV to +30 mV EVERY TIME

    • NEVER DIFFERENT

• necessary to send information from one neuron to another