Chapter 7 - The Nervous System Neurons and Synapses

nervous system

organ system consisting of trillions of cells in brain, spinal cord, periphery

functions of nervous system

• major control system

• rapid electrical signals for communication

• chemical communication too: neurotransmitters

• key role in maintaining homeostasis of most physiological variables

• sensation

• movement

divisions of the nervous system

central nervous system (CNS) and peripheral nervous system (PNS)

divisions for central nervous system

brain and spinal cord

divisions for peripheral nervous system

autonomic nervous system and somatic nervous system

divisions of autonomic nervous system

parasympathetic and sympathetic nervous system

somatic nervous system

controls skeletal muscle

autonomic nervous system

controls smooth muscle, cardiac muscle, and glands

• has 2 subdivisions

parasympathetic nervous system

“rest and digest”

sympathetic nervous system

“fight or flight”

afferent output

toward CNS, sensory

efferent output

to muscles/glands, motor

number of cranial nerves and spinal nerves

12 CN and 31 SN

neuron

nerve cell, basic structural and functional unit

function of neuron

• carries information in the form of electrical signals that move along the cell and to adjacent cells

• electrical signal (action potential) in a neuron causes it to release a neurotransmitter, a chemical messenger, to communicate with other cells

nerve

group of neurons in PNS

glial cells

supporting cells, help neurons function

dendrites

receive information from other neurons, typically from neurotransmitters (chemical), resulting in graded potentials

graded potentials

graded CHANGE in membrane potential, ie, the change in various sizes

axons

undergo action potentials (electrical) to deliver information, typically neurotransmitters (chemical), from axon terminals to other neurons or cells

glial cells

• surround soma, axon, and dendrites

• physically and metabolically support neurons

types of glial cells

• Schwann cells

• oliogodenderocytes

Schwann cells

form myelin on PNS axons

Oliogodendrocytes

form myelin on CNS axons

myelin

fatty sheath on axons

• myelinated neurons conduct action potentials more rapidly than non-myelinated neurons

afferent (sensory) neuron

carries info from tissues and organs to the CNS

efferent (motor) neuron

carries information from the CNS to effector cells

interneuron (association) neuron

connects neurons within the CNS

• integration and connection

effector neuron extra

• effects (cause) a change

• conveys electrical signals from the CNS brain/spinal cord to effector cells

  • ie: muscle, gland, (effector causes the response)

affector neuron extra

• affects (influences) what will happen

• conveys electrical signals from sensory receptor to brain/spinal cord

  • receptors in the skin, sensory structures (receptor receives a message)

  • ie: sensation in arm, light in eye

CNS

brain + spinal cord; interneurons

PNS

afferent neurons into the CNS + efferent neurons projecting out of the CNS

resting membrane potential of neuron

-70 mV

reasons for RMB

• ion concentration gradients

  • more Na+ outside cell

  • more K+ inside cell

  • more Cl- outside cell

  • fixed anions in the cell

• leak channels in membrane

  • always open

• sodium-potassium pump in membrane

ion gating

opening and closing of gated ion channels (in axon); changes permeability and polarization

depolarization

when the membrane potential becomes more positive or less negative

depolarization process

• due to positive ions entering the cell (when Na+ channels open)

• Excitatory, can result in an action potential

hyperpolarization

when the membrane potential becomes more negative

hyperpolarization process

• due to positive ions leaving the cell (when K+ channels open) or negative ions entering the cell

• inhibitory

repolarization

a return to the resting membrane potential (polarized state) when K+ channels are open and then start to close)

Na+ channels

are gated, closed at rest

K+ channels

either gated (closed at rest) or leak (always open), this means greater permeability at rest due to leak channels

initial depolarization

caused by neurotransmitters (from other neurons) binding to receptors on dendrites, causing a graded potential

action potential

A rapid, self-regenerative electrical signal in excitable cells, like neurons, that travels along the cell membrane to transmit information

• large depolarization to +30 mV electrical impulse

absolute refractory period

membrane cannot produce another AP because Na+ channels are inactivated

relative refractory period

occurs when K+ channels are open and neuron is hyperpolarized; requires stronger stimulus to reach threshold

conduction of APs

In an unmyelinated axon, an AP in the axon causes nearby Na+ channels to open, thereby depolarizing the adjacent axons where AP occurs

propagation of AP

is typically one-way because of the absolute refractory period follows the moving AP

saltatory conduction

In a myelinated axon, APs "jump from one node of Ranvier. Ion channels are abundant at the nodes, where Na+ influx occurs.

synapse

junction between a neuron and an effector cell, where a presynaptic neuron influences the electrical and chemical activity of a postsynaptic neuron or effector cell (in muscle gland)

presynaptic neuron

• A neuron conducts APs in its axon

• It sends the exitatory or inhibitory message (NT) to another neuron, at the synapse

postsynaptic neuron

• a neuron that receives the NT from another neuron, via a synapse.

• either excites (depolarized) and conducts APs in its axon, or is inhibited (hyperpolarized) by the NT.

• if excited, it will release NT when the AP reaches the end of the axon

neurotransmitters

chemical messengers released from presynaptic neuron to cause a response in postsynaptic neuron

NT excitatory effect

• depolarizes neuron due to Na+ or Ca2+ entry

• EPSP: excitatory postsynaptic potential

NT inhibitory effect

• hyperpolarizes neuron due to Cl- entry

• IPSP: inhibitory postsynaptic potential

Acetylcholine (ACh)

released by cholinergic neurons

• skeletal muscle excitation (movement), autonomic functions (both sympathetic and parasympathetic)

Catecholamines

released by adrenergic neurons

• based on tryosine (an AA)

• examples are dopamine, norepinephrine (NE), and epinephrine (E) (“adrenaline” + NE, E)

• mood, states of consciousness, movement, blood pressure regulation, and more.