Biology 30 Alberta: Nervous System

What does the nervous system coordinate?

The internal environment and responds to external changes

Are the nervous system and endocrine system interrelated?

Yes

What are the two major divisions of the nervous system?

Peripheral nervous system (sensory and motor neurons) and the central nervous system (brain and spinal cord)

What is the motor component of the peripheral nervous system further divided into?

Somatic nerves (under conscious control) and autonomic nerves (under involuntary control)

Central nervous system (CNS)

The body's coordinating centre for mechanical and chemical actions (brain and spinal cord)

Peripheral nervous system (PNS)

All parts of the nervous system, excluding brain and spinal cord, that relay information between the CNS and other parts of the body

What are the two types of cells in the nervous system?

Neurons and glial

What are neurons?

Functional units; specialized to respond to chemical and physical stimuli, to conduct electrochemical signals and release chemicals that regulate body processes (conducts nerve impulses)

What are glial cells?

Non cunducting cells, give structural support, provide nutrients, and remove wastes from neurons (outnumber neurons 10X)

How many glial cells are there per neuron?

10

What four features are common to all neurons?

Dendrites, cell body (soma), axon, and terminal ends

What are dendrites?

Short, branching terminals that receive nerve impulses from other neurons or receptors and relay messages to the cell body

What is the cell body (soma)?

The site of metabolic reactions (contains nucleus)

What is the axon?

Conducts impulses away from the cell body (can range in length from 1mm to 1m). Some are enclosed in a fatty, insulated layer called myelin sheaths (myelinated axons appear white, unmyelinated axons appear grey). Myelin sheath is secreted by Schwann cells (type of glial cell). Small gaps between the sheath, Nodes of Ranvier, allow the impulse to jump along the axon

What are terminal ends?

Branched ends of axons, site of neurotransmitter release

What is a myelin sheath?

White coat of fatty protein, acts as insulation for the neurons

What is a Schwann cell?

A special type of glial cell that produces the myelin sheath

What are Nodes of Ranvier?

Regularly occurring gaps between sections of the myelin sheath along axon

What is the neurilemma?

Delicate membrane that surrounds the axon of some nerve cells, promotes the regeneration of damaged axons (not all nerve cells that have a myelin sheath have a neurilemma)

What are the three functional types of neurons?

Sensory (unipolar), motor (multipolar), and interneurons (bipolar)

Unipolar neuron

Sensory neuron (afferent)

Bipolar neuron

Interneuron

Multipolar neuron

Motor neuron (efferent)

What do sensory (afferent) neurons do?

Relay information to stimuli received by sensory receptors about the internal and external environment to the central nervous system for processing

What do interneurons do?

Links neurons to other neurons, found only in brain and spinal cord, integrate and interpret the sensory information and connect to outgoing motor neurons

What do motor (efferent) neurons do?

Relay information to the effectors (the cell or organ that responds to the stimulus). Muscles, organs and glands are classified as effectors because they produce responses

What are individual neurons organized into?

Nerves

What are reflexes?

Rapid, involuntary, and unconcious responses

What are the five components of a reflex arc?

A SENSORY RECEPTOR to detect the stimulus, a SENSORY NEURON to pass an impulse to an INTERNEURON, which relays the information to a MOTOR NEURON, which initiates an EFFECTOR

How quickly do reflexes travel?

Less than one second (some take less than 50 ms)

What do sensory receptors do in impulse transmission?

Receive stimuli from the internal and external environment and form nerve impulses

What do sensory neurons do in impulse transmission?

Transmit impulses from the sensory receptors to the interneurons (CNS)

What do interneurons do in impulse transmission?

Found in brain and spinal cord, act as integration and interpretation centre. Link neurons to other neurons (sensory to motor)

What do motor neurons do in impulse transmission?

Conduct impulses from the interneurons to the effectors

What do effectors do in impulse transmission?

Muscles, glands and organs that respond to the impulses from the motor neurons. Produce responses

What is membrane potential?

A voltage or electrical charge across the plasma membrane

What is the voltage difference across a nerve cell when it is not transmitting an impulse?

-70 mV

Which two mechanisms create the voltage difference across a nerve cell (when it is not transmitting an impulse)?

The membrane (impermeable to large, negative proteins and negative ions) and the sodium-potassium pump (pumps more Na+ out than it brings K+ ions in)

How do ions travel across the membrane of a nerve cell?

They rely on facilitated transport (using voltage gated ion channels)

Are there more K+ channels or Na+ channels in the membrane of a nerve cell?

More K+

What happens as K+ leaves the nerve cell?

It causes a build up of (+) charge on the outside because the negatively charged proteins or Cl- can't move across

The movement of ___ ____ is mainly responsible for creating electric potential

K+ ions

What would happen if movement of ions across a nerve cell membrane was left to diffusion alone?

The K+ concentration on both sides of the membrane would eventually even out

How many K+ and Na+ ions are moved by the sodium-potassium pump? In which direction? Using what?

K+: 2 back into cell

Na+: 3 out of cell

Transported by active transport using ATP energy

When does an action potential occur?

When the resting potential changes from -70 mV to -55 mV

What is the threshold potential?

-55 mV

What is the change from -70 mV to -55 mV called?

Depolarization

On myelinated axons, where does depolarization occur?

At the Nodes of Ranvier

When depolarization occurs, do voltage-gated Na+ channels open or close?

Open

When depolarization occurs and Na+ rushes into the axon, what is the new potential difference?

+35 mV

What does the change in voltage to 35 mV cause?

Na+ channels close and K+ voltage-gated channels open

What happens when K+ rush out of the axon?

They carry the positive charge out and repolarize the membrane to -90 mV

What is the 'overshoot' due to?

The K+ gates close slowly

What is the 'overshoot' period called?

The refractory period (or hyperpolarization)

Can action potentials be transmitted when the nerve cell is not at resting potential?

No (nerve cell must be at resting potential)

What does the speed of an action potential increase with?

The diameter of an axon

Do myelinated axons increse the speed of an impulse?

Yes (action potential jumps from node to node; saltatory conduction)

What is saltatory conduction?

The 'leaping' of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials

What are the spaces between neurons or a neuron and effector called?

Synapses

What is a presynaptic neuron?

The 'sending' neuron (neuron conducting impulses toward synapse)

What is a postsynaptic neuron?

The 'receiving' neuron (neuron transmiting impulses away from the synapse)

When an impulse reaches the terminal end, what must it do?

Cross the gap from the presynaptic neuron to the postsynaptic neuron

What does the action of an impulse crossing the synapse rely on?

Chemical messengers called neurotransmitters

What are neurotransmitters?

Chemical messengers (released at the end of a nerve fiber by the arrival of a nerve impulse and, by diffusing across the synapse or junction, causes the transfer of the impulse to another nerve fiber, a muscle fiber, or some other structure)

What are contained within the terminal ends of axons?

Tiny vesicles that contain neurotransmitters

What does the action potential reaching the synaptic knob cause?

Calcium channels to open and Ca2+ to move in

What triggers the release of neurotransmitters from synaptic vesicles?

The action potential reaching the synaptic knob, which causes calcium channels to open and Ca2+ to move in

Where do neurotransmitters (released from synaptic vesicles) release their contents into?

The synaptic cleft

What do neurotransmitters do after they have been released into the synaptic cleft?

They diffuse across the 20 nm gap and bind to receptors in the postsynaptic membrane

Where do neurotransmitters bind to?

The postsynaptic membrane (either on the dendrites of a neuron or the cell membrane of an effector)

What does the binding of neurotransmitters to receptors cause?

Specific voltage gates to open

What happens if neurotransmitters are excitatory?

Na+ gates open, causing a depolarization and continued transmission

What happens if neurotransmitters are inhibitory

K+ gates open, causing hyperpolarization and transmission stops

Why are inhibitory neurotransmitters important?

They allow information to be prioritized, and help to coordinate complex movements

What happens to a neurotransmitter once it has had an effect?

It is broken down by enzymes and the components are recycled

What is acetylcholine?

An important neurtransmitter in muscle control

What happens to acetylcholine after it has bound to its receptor?

It is broken down by the enzyme cholinesterase

What is cholinesterase?

An enzyme that breaks down acetylcholine

What does acetylcholine do when it is bound to a receptor (before it is broken down)

Creates a depolarization that initiates a muscle contraction

What would happen if acetylcholine is not broken down by cholinesterase?

Muscles will remain in a state of constant contraction, causing side effects like tremors, heart palpitations, or paralysis

What is summation?

The effect produced by the accumulation of neurotransmitters from two or more neurons (to meet the threshold level of the receptors in the dendrites)

What are some examples of neurotoxins?

Botulinium and tetanus

What are neurotoxins?

Bacterial proteins that are exocytosed into nerve endings where they inhibit the release of essential neurotransmitters