Neural Control

Chapter 37

I. Nervous SystemsA. two kinds of generalized cells

1. neuronsa. basic cells of the nervous system; nerve cells

• one of the four basic tissue typesb. receive and transmit informationc. chemical signals electrical impulsesd. detection, integration, responsee. stimuli

The three fundamental phases of neural activity

2. glial cells a. neuroglia; several different typesb. structural support and framework

for neuronsc. produce myelin sheath

• oligodendrocytes and Schwann cellsd. some metabolic functions

• help nourish neurons, regulate Ke. act as selective barrier

• microglia and astrocytes

Fig. 31.6 Neurons and glial cells

B. neurons1. three categories

a. sensory neurons (afferent neurons)i. detection phaseii. receive information from external and internal environment

b. interneuronsi. integration phaseii. process and route information ("middle men")

c. motor neurons (efferent neurons)i. response phaseii. transmit information signals to organs, tissues, or muscles

• cause motor responses (reactions)2. myelin sheath

a. protective covering and electrical insulator of neuronsb. nodes (nodes of Ranvier) vs. internodes

3. structure of neuronsa. cell body (nucleus and organelles)

• produces neurotransmitterso chem. substances necessary for elec. impulses to be transmitted

b. dendritesi. receiving end of neuron (chemical signals)ii. receptor cells: larger/thicker dendrites of sensory neurons

c. axonsi. transmitting end of neuronii. chemical signals electrical impulsesiii.one axon branches (at ends) axon trees axon terminaliv. neuromuscular junctions

Fig. 37.4 Neuron anatomy

C. nerves• thousands of neurons surrounded by layers of tough connective tissue

Page 692 Nerve anatomy

II. Neural ImpulsesA. overview

1. electrical impulses are generated through action potentials • change in voltage across a membrane from -65 mV to +40 mV

2. polarized neuron depolarized repolarizationB. specifics

1. action potentials do not diminish with distancea. occur in axonsb. controlled ion movement, active transport, and diffusion

2. neurons are polarized in their resting statea. interior more negative than exterior (net negative charge)

• neg. charged proteins and Cl; pos. charged Na, K ionso inside neuron: neg. proteins, Cl-, K+

o outside neuron: Na+, some K+

b. Na/K ion exchange pumps maintain ion distribution• pump Na+ out and most K+ in

c. exterior positive (Na+), interior negative (proteins, Cl)• K+ acts as a “balancer”

d. resting potential = -65 mV

3. depolarizationa. begins as a change in permeability of plasma membrane

i. dendrites receive information as chemical signalsii. cell body releases a neurotransmitteriii. neurotransmitter affects ion gates and channels in axon membrane

• separate channels for Na+ and K+ ionsb. Na+ ion gates open Na+ ions rush inward

i. inrush changes balance of charges (-65 mV to +40 mV)ii. upper limit of change can vary between +30 and +40 mV

c. proceeds as rapidly moving wave across entire axond. threshold voltage (= about -40 mV)

i. amount of depolarization needed to start an action potentialii. action potentials are example of positive feedback

e. once started, the wave proceeds with constant magnitude and speedf. rate depends on strength of stimulus

i. typical action potential occurs in 2 millisecondsii. non-myelinated axons speed of 1 meter/second

• potential travels down axon, one small section at a timeiii. myelinated axons speeds up to 200 meters/second (450 miles/hour)

• saltatory conductiono gated ion channels are concentrated at nodes (nodes of Ranvier)o potential jumps over myelinated areas (internodes) faster, more efficient

Fig. 37.5 Resting potential and depolarization

Page 685 Saltatory conduction

4. repolarizationa. K+ ion gates open K+ ions move to outside of plasma membrane

i. causes an undershoot of resting potential (-80 mV)ii. what else is wrong now?

b. ion exchange pumps quickly redistribute K+ and Na+

• restores resting potential to -65 mVc. refractory period

i. time between depolarization and repolarizatonii. 2nd potential cannot occur unless exceedingly strong

Fig. 37.5 Repolarization and an entire action potential

An action potential

III. Communication Among Neurons and Between Neurons and MusclesA. synapses

1. axons linking to dendrites or cell body2. axons linking to organ, muscle, or tissue

B. synaptic cleft1. tiny gap between two neurons, where they meet at a synapse

• nerve impulses cannot cross this gap2. transmission across a synapse

a. accomplished by secretion of more neurotransmitters i. e.g., acetylcholine, serotonin, endorphin, dopamine, etc.

• can have other physiological effectsii. transmitting neuron secretes a

neurotransmitter across cleftiii. this starts a new action potential

in the receiving neuronb. very fast and versatile

Fig. 37.7a Synapses

Fig. 37.6 Synapse structure and function

C. neuromodulators1. molecules that block release of

neurotransmitters2. can also modify a neuron’s

response to a neurotransmitter3. e.g., endorphins block/modify

neuron responses act as natural painkillers

D. excitatory vs. inhibitory synapses1. excitatory

a. most synapses in bodyb. cause motor responses and stimulate neural activity

2. inhibitory a. inhibit motor responses and make them less likely to occurb. act as a control mechanism in bodyc. involve different neurotransmitters

Fig. 37.7b Excitatory and inhibitory synapses

E. reflex arcs1. simplest model of neural activity can involve as few as 2 or 3 neurons2. often bypass brain entirely

• operate through spinal cord only

Fig. 37.13 A reflex arc