Structures and Processes of the Nervous System

Chapter 8.2McGraw-Hill Ryerson

Biology 12 (2011)

Overview of Nervous System

Looks overwhelming, doesn’t it?

Here’s a chart that’s a little simpler

Central Nervous System: brain & spinal cord integrates and processes information sent by nerves from peripheral nervous system

Peripheral Nervous System: network of nerves that carry messages to CNS and send commands from CNS to the muscles and glands

Neurons are similar to other cells in the body– Surrounded by cell membrane– Have a nucleus that contains genes– Contain cytoplasm, mitochondria and

other organelles– Carry out basic cellular processes such as

protein synthesis and ATP production

Nerve Cells (Neurons)

Neurons are different by- Specialized extensions called dendrites and axons- Communicate with each other by electrochemical process- Contain some specialized structures (synapses) and chemicals (neurotransmitters)

Neurons• Most neurons consist of a

cell body and extensions called dendrites and axons.

• Dendrites carry impulses towards cell body

• Axons carry impulses away from the cell body

• Cell Body contains the nucleus

Neurons• Axons enclosed in fatty,

insulating layer called myelin sheath– Myelin sheath protects

neurons and speeds up rate of nerve impulse transmission

• Schwann cells form myelin by wrapping themselves around the axon

• Nodes of Ranvier are gaps in myelin sheath– Capable of electrical activity

Types of Neurons• Sensory neurons

– Carry nerve impulses from a receptor to the CNS– Have long dendrites and short axons

• Motor neurons– Carry nerve impulses from the CNS to an effector

• (ex. muscle or gland)– Have short dendrites and long axons

• Interneurons– Found completely within the CNS– Provide a link within the CNS between sensory neurons and motor

neurons– Have short dendrites and long or short axons

Reflex Arc

• Simplest nerve pathway• Occurs without brain coordination• Five components– Receptor– Sensory neuron– Interneuron in spinal cord– Motor neuron– Effector

Animation

Electrical Nature of Nerves

• Neurons use electrical signals to communicate with other neurons, muscles, and glands

• Signals = nerve impulsesCaused by changes in the amount of electric charge across a

cell’s plasma membrane

Resting Membrane Potential• Uneven concentrations

of Na+ (outside) and K+

(inside) on either side of neuron membrane– results in the inside of

the neuron being -70 mV

– Electrical charge inside of the cell is negative relative to outside of the cell

Resting Membrane Potential• 3 factors contribute to maintaining resting

membrane potential1) Large, negatively charged protein

molecules present inside the cell2) Ion-specific protein channels on cell

membrane allow passive movement of Na+ and K+

• K+ channels open at resting allowing K+ to leave cell, making inside cell negative relative to exterior

3) Sodium-Potassium pump actively transports Na+ and K+ in ratios that leave the inside of the cell negatively charged

Resting Membrane Potential• Sodium-potassium pump– Most important contributor to separation of charge– Every 3 Na+ transported out of cell, 2 K+ brought in

• Excess positive charge accumulates outside cell

Resting Membrane Potential

Polarization: The process of generating a resting membrane potential of -70mV

Action Potential

• A nerve cell is polarized because of the difference in charge across the membrane– More negative inside the cell than outside

• Depolarization occurs when the cell becomes less polarized (membrane potential is reduced to less than resting potential of -70mV)

• Action Potential causes depolarization to occur

Action Potential• Nerve signals are transmitted by

action potentials• Abrupt, pulse-like changes in membrane

potential (few ten thousandths of a second)• Can be divided into three phases

• Resting/polarized state• Depolarization• Repolarization

• The amplitude is nearly constant and is not related to the size of the stimulus. Therefore action potentials are all-or-nothing events.

Threshold potential: a certain level in membrane potential. Once it’s crossed action potential occurs (point of no return)

Action Potential timeline

Saltatory Conduction

Saltatory conductions• Nodes of Ranvier contain

many voltage-gated Na channels– Na moves into cell, charge

moves quickly through cytoplasm to next node

– Causes next node’s membrane to dpolarize to threshold• Previous node’s membrane in

refractory state (prevents action potential going backward)

– Depolarization initiated to conduct action potential

– This continues until pulse reaches end of neuron

Terminology• Synapse

– Region at which neurons come nearly together to communicate. (neuron or effector organ)

• Synaptic Cleft– Gap between neurons (at a synapse)– Impulses can not propagate across a

cleft• Synaptic Vesicle

– Packets of neurotransmitter in presynaptic neuron

• Presynaptic Neuron– Neuron sending a signal (before the

synapse)• Postsynaptic Neuron

– Neuron receiving a signal (after the synapse)

NeurotransmittersClassical transmitters are small molecules (often amino acid based)

Non-classical transmitters can be peptides or even gasses

• 5 general criteria:1) synthesized and released by

neurons2) released at the nerve terminal in a

'chemically identifiable' form3) the chemical should reproduce the

activity of the presynaptic neuron4) can be blocked by competitive

antagonist based on concentration5) active mechanisms to stop the

function of the neurotransmitter

Types of Neurotransmitters

Acetylcholine + muscles, learning, memory

Serotonin (a derivative of tryptophan)

+ sleep, relaxation, self esteem, too little = depression, perception

Norepinephrine (aka noradrenaline)

+ stress and fight/flight response, sympathetic NS:+BP & heart rate

Dopamine + prolactin (milk production), involved in pleasure, movement

Endorphins (-) pain, involved in pleasure

GABA (gamma aminobutyric acid)

(-) anxiety, too little in parts of brain can lead to epilepsy

Glutamate Most common NT, memory, toxic

Homework

• Pg 362 #1, 2, 3, 6, 7, 9, 10