LIFESCI 7C Lecture Notes - Lecture 3: Sensory Neuron, Axon Hillock, Motor Neuron
Week 3
35.1 Nervous System Function and Evolution
● nervous system A network of many interconnected nerve cells.
○ Allows animals to sense and respond to environment, coordinate action of muscles and control internal
function of its body
○ neuron Nerve cell; the basic fundamental unit of nervous systems.
● Nervous systems first evolved in simple animals, allowing them to sense basic features of environment and
trigger movements useful for survival and reproduction
○ Growing complexity in organisms → growing complexity in nervous systems w/ greater abilities (i.e.
regulation of internal body functions in response to environment) and made possible more sophisticated
behaviors → increased fitness by improving ability to survive and reproduce
Animal nervous systems have three types of nerve cells
● sensory neuron A neuron that receives and transmits information about an animal’s environment or its internal
physiological state → response to temp, light, touch, odor, taste (physical and chemical features)
● interneuron A neuron that processes information received by sensory neurons and transmits it to motor
neurons in different body regions.
● motor neuron A neuron that, on receiving information from interneurons, effects a response in the body.
○ E.g. may stimulate muscle contraction to produce movement,
● Nervous system=fundamental to homeostasis (ability of animals, organs, and cells to actively regulate/maintain
a stable internal state)
● Most nerve cells have fiberlike extention (some receive, others transmit info) → forms a circuit → nervous
system, allowing info to be received, processed, and delivered
○ In most animals, interneurons form specialized circuits that may consist of a diverse array of nerve cells
having different shapes, sizes, and chemical properties
● ganglion (plural, ganglia) A group of nerve cell bodies that processes sensory information received from a local,
nearby region, resulting in a signal to motor neurons that control some physiological function of the animal.
○ Relay stations/processing points in circuits; ganglia evolved into brains
● brain The centralized concentration of neurons in an organ that processes complex sensory stimuli from the
environment or from anywhere in the body.
○ Complex neuron circuits, allowing brain to regulate broad array of behaviors as well as to learn and
retain memories of past experiences
Nervous systems range from simple to complex
● The organization and complexity of an animal’s nervous ssytem reflects its lifestyle
○ Sessile (immobile) animals have relatively simple sense organs/nervous systems
○ Active animals (e.g. anthropods have more sophisticated sense organs, brains, more complex n.
Systems
■ +-+such a system; instead, groups of cells specialized for functions respond to local
chemical/physical cues (
○ E.g. cells secialized in contracting or taking in water → closes when sponge is touched
● Simplest nervous systems found in cnidarians (radially symmetric animals like jellyfish or sea anenomes);
○ Their n system has few nerous arranged like a net, but no ganglia or central brain
○ Sensory nerousons sensitive to touch tell anenome’s motor neurons to react
● Flatworls are representatives of one of the earliest branching groups of bilaterians, well adapted for forward
locomotion; sensory neurons numerous in the front end, entire nervous system comparing inputs from different
sensory neurons and sending singals to appropriate muscles → more complex nerous system
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● In animals w/ organized nervous system → info is trasmitted to different regions along length of animal’s body by
distinct nerve cords and nerves (bundles of long fiberlike extensions from multiple nerve cells)
● Evolution of the brain is a key feature of vertebrate nervous systems, enabling organisms to to evolve complex
behaviorsr that rely on learning and memory (and even ability to reason);
○ animal brains become increasingly complex in the number of nerve cells, number of connections
between neurons, and variety of shapes/sizes of nerve cells and specialized connections between them
● Although animal nervous systems differ in organization and complexity, their nerve cells have fundamentally
similar molecular and cellular features and use the same mechanisms to communicate w/ neighboring cells
○ → mechanisms retained over course of evolution to code and reliably transmit info
○ Ability to receive sensory info from environment and transmit and process info within a nervous system,
is SHARED across a wide diversity of animal life, contributing to the success of multicellular animals
Case 7: What body features arose as adaptions for successful predation?
● cephalization The concentration of nervous system components at one end of the body, defined as the “front”.
○ Evolved independently multiple times in different animal groups → certain advantages
○ Thought to be an adaption for forward locomotion (since it allows animals to take in sensory info from
environment ahead as they move)
○ Nearness of sensory organs to central ganglia/brain → quick info processing → suitable response; brain
size and complexity increased as quality and amount of sensory info taken in increased
○ Considered adaptation for predation, allowing animals to better detect/capture prey
○ Particularly well-studied in vertebrates, where brain and many sense organs located in the head
35.2: Neuron Structure
● Neurons have extensions that receive information and relay it to the cell body. They also have other extensions
that transmit information away from the cell body.
● Although all neurons share this basic plan, they differ remarkably in size and number of extensions, according to
their specific function.
Neurons share a common organization
● All neurons have a cell body, from which emerge two kinds of fiberlike extensions; both can be highly branched,
enabling commmunication over large distances w/ many other cells
○ dendrite receives signals from other nerve cells or from specialized sensory endings; the input end of a
nerve cell.
○ axon transmits signals away from the nerve’s cell body; the output end of a nerve cell.
■ axon hillock The junction of the nerve cell body and its axon where signals are summed
■ action potential if summing of signals is high enough, a brief electrical signal transmitted from
the nerve cell body along one or more axon branches.
■ End of each axon forms swelling called axon terminal, which communicates w/ a neighboring
cell through a synapse
● A space, the synaptic cleft, separates the end of the axon of the presynaptic cell and
neighboring postsynaptic cell; about 10-20 nm wide
○ neurotransmitter A molecule that conveys a signal from the end of the axon to
the postsynaptic target cell.
■ Arrival of nerve signal at axon triggers release of neurotransmitters
from vesicles in terminals (exocytosis), releasing them into the synaptic
cleft
■ neurotransmitters diffuse across synapse to bind to receptors on the
plasma membrane of the target cell,
■ Binding of neurotransmitters to receptors causes change in electrical
charge across postsynaptic membrane → signal continuation
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○ Some neurons do not synapse w/ other neurons but instead w/ other types of cells that produce
physiological response in animals (e.g. muscle cells, secretory glands)
Neurons differ in size and shape
● Neurons, depending on type, can vary in length, number of extensions from cell body, shape, degree of
branching in exensions,
● Signals from all dendrites determine the strength/frequency/timing of signals carried by neuron’s axon
○ Integration of information: process of bringing together info gathered from different sources
○ Degree of branching and number of synapses a neuron makes w/ neighboring nerve cells reflects how
info is processed/integrated by the cell as part of a larger circuit
Neurons are supported by other types of cell
● Neurons in many body regions (esp. In brain) are supported by other types of cells that do not transmit electrical
signals
○ glial cell A type of cell that surrounds neurons and provides them with nutrition and physical support.
■ Brain has more gilial cells than neurons!
■ astrocyte A type of star-shaped glial cell that contributes to the blood–brain barrier by
surrounding blood vessels in the brain and thus limiting the size of compounds that can diffuse
from the blood into the brain.
● Prevents pathogens/toxins in blood from entering brain
● ProtSupport endothelial cells that make up blood vessels in the brain!ection of brain
and spinal cord important as nerve cells have limited regenerative capacity after
damage
● Lipid-soluble compounds such as alcohol and certain anesthetics readily diffuse the
blood-brain barrier, affecting the functioning of the brain and an animal’s mental state,
○ But can also damage and destroy neurons
■ Can help orient neurons as they develop their connections → provide electrical insulation to
vertebrate neurons that allow nerve signals to be transmitted rapdily
35.3: Neuron Function
● Neurons send signals electrically from one end of the cell to another, not by diffusion of chemicals (too slow)
The resting membrane potential is negative and results in part from the movement of potassium ions
● Uneven charges inside and outside of neuron carriers: membrane potential, measured in volts
○ Without a signal, inside is more negatively charged than outside (has more negative ions)
○ Other cells have membrane potential, but only nerve and muscle cells respond to changes in
membrane potential → ELECTRICALLY EXCITABLE
○ movement of + and - ions across membrane → change in membrane potential → electrical signal
produced in nerve cell
● resting membrane potential The negative voltage across the membrane at rest, said to be polarized
○ Buildup of - ions inside and + ions outside; ranges from -40 to -85 milivolts (mV), most commonly about
-65 to -70 mV
○ Greater concentration of sodium (Na+) ions outside than inside and greater concentration of potassium
(K+) ions inside the cell than outside, distribution caused by sodium-potassium pump, which uses ATP
to move 3 Na+ out for every 2 K+ in, making the inside less positive (more negative) than the outside
● Exact value of RMP depends on movement of K+ back out of cell by passive diffusion via potassium ion
channels in membrane
○ At rest, K+ ion channels are open, and K+ moves out the cell whereas negatively charged ions (largely
proteins) remain inside the cell→ making inside even more negative!’
■ → at rest plasma membrane is most permeable to potassium!
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Document Summary
Nervous system a network of many interconnected nerve cells. Allows animals to sense and respond to environment, coordinate action of muscles and control internal function of its body. Neuron nerve cell; the basic fundamental unit of nervous systems. Nervous systems first evolved in simple animals, allowing them to sense basic features of environment and trigger movements useful for survival and reproduction. Animal nervous systems have three types of nerve cells. Motor neuron a neuron that, on receiving information from interneurons, effects a response in the body. E. g. may stimulate muscle contraction to produce movement, Nervous system=fundamental to homeostasis (ability of animals, organs, and cells to actively regulate/maintain a stable internal state) Most nerve cells have fiberlike extention (some receive, others transmit info) forms a circuit nervous system, allowing info to be received, processed, and delivered. In most animals, interneurons form specialized circuits that may consist of a diverse array of nerve cells having different shapes, sizes, and chemical properties.
