BIOL 1040 - Chapter 28

Nervous System

 an animal organ system comprised of a network of specialized interacting cells (neurons) that functions in the detection, processing and response to internal and external stimuli. Functionally divided into two systems (CNS and PNS)

The Central Nervous System

comprised of the brain & spinal cord. 

Peripheral Nervous System

comprised of nerves outside the CNS that carry signals into & out of the CNS. Consists of

• Nerves – bundles of neurons wrapped in connective tissue.

• Ganglia – clusters of neuron cell bodies.

Three Main Functions of the Nervous system

1. Sensory Input – conduct signals from receptors to CNS. 

2. Integration – analysis & interpretation of sensory signals, & formation of appropriate responses. 

3. Motor Output – conduct signals from CNS to effectors that execute a response.

Parts of a Neuron (4)

Cell Body – contains the nucleus and other organelles. 

Dendrites – receive signals from sensory receptors or other neurons. 

Axon – portion of neuron that transmits impulses to other neurons or cells. 

Myelin Sheath – a sheath of cells that insulate the axon.

Function of a Neuron

perceives stimuli and transmits/conducts information (impulses) to other cells.

3 types of GLIAL Cells 

Astrocytes – transfer nourishment from capillaries to the neuron. 

Oligodendrocytes – form the myelin sheath around the axons of neurons & nerve fibers in CNS. 

Microglia – resident macrophages of the nervous system; they phagocytize infectious agents and neural debris.

Three Main Types of Neurons

Sensory neurons –  receive stimuli, convey signals from sensory receptors to the CNS. 

Interneurons – are located entirely in the CNS, integrate information & send it to motor neurons. 

Motor neurons – convey signals to effector cells.

Knee-Jerk Reflex Arc 

1. Sensory receptor detects stretch stimulus. 

2. Sensory neuron sends impulse to the CNS (spinal cord). 

3. Impulse is transferred from the CNS to motor neurons and interneurons.

Nerve Function Depends on Charge Differences Across Neuron Membranes. 

Outside = positive. 

Inside = negative. 

Resting potential = -70 mv. 

Created & maintained by the Na/K pump. 

[Na+] high outside neuron. 

[K+] high inside neuron. 

Collapsed by Na+ & K+ ion channels.

How does the Na/K pump work

1. Carrier has a shape that allows it to take 3 NA+

2. ATP is split and phosphat group attaches to carrier

3. Change in shape results and causes carrier to release 3 Na+ outside the cell

4. Carrier has a shape that allows it to take up 2K+

5. Phosphate group is released from carrier

6. Change in shape results and causes carrier to release 2K+ inside the cell

6 steps of “tracking an action potentional”

1. Resting state: Voltage gated Na+ and K+ channels are closed; resulting potential is maintained by ungated channels (+ and Na+ on the outside, - and K+ on the inside)

2. Depolariation: A stimulus opens some Na+ channels; if threshold is reached, an action potential is triggered

3. Additional Na+ channels open, K+ channels are closed; interior of cell becomes more positive

4. Repolarization: Na+ channels close and inactivate; K+ channels open and K+ rushes out. Interior of cell is more negative than outside

5. The K+ channels close relatively slowing causing a breif undershoot

6. return to resting state

The Action Potential Propagates Itself Along the Axon. 

1. Once an action potential is initiated, Na+ channels open, resulting in the influx of Na+ ions; this triggers a similar influx of Na+ in adjacent regions of the plasma membrane. 

2. Once the influx of Na+ is completed, Na+ channels close & K+ channels open, resulting in an efflux of K+ ions. 

3. Migrating action potential continues along the entire length of the axon; Na/K pump re-establishes correct membrane resting potential.

Impulse Transmission in Unmyelinated Neurons

occur mainly in the CNS as interneurons, where impulses travel much shorter distances between sensory & motor neurons.

Impulse Transmission in myelinated Neurons

occur mainly in the peripheral nervous system where impulses must normally travel further.

Synapse

a physical junction or relay point between the endings of the “sending neuron” and the “receiving cell”. Receiving cells can be another neuron, an effector cell or an endocrine cell. 

Synaptic integration

is the process by which a neuron combines multiple incoming signals (synaptic potentials) to determine whether or not to fire an action potential.

5 steps for synaptic integration

1. Action potential arrives at synaptic terminal. 

2. Action potential causes some synaptic vesicles filled with neurotransmitter to fuse with plasma membrane of synaptic terminal; Neurotransmitter is released into synaptic cleft. 

3. Neurotransmitter diffuses across synaptic cleft & binds to ion channel receptor proteins in receiving cell dendrite membrane. 

4. An influx of (Na+) ions may trigger an action potential in the receiving cell. 

5. Neurotransmitter is taken back into sending neuron (or is degraded or diffuses away) & ion channels close.

Acetylcholine: 

Excites skeletal muscle contraction. 

Inhibits (slows) cardiac muscle contraction. 

Amino Acids: 

Glutamic acid – excitatory (CNS). 

Glycine & GABA (gamma aminobutyric acid) – inhibitory. 

Biogenic Amines (derived from amino acids): 

Norepinephrine (hormone) – excitatory (PNS). 

Serotonin & Dopamine – affect sleep, mood, attention & learning. 

Endorphins: 

Short chains of amino acids. 

Decrease perception of pain; promote euphoria.

Caffeine

blocks effects of inhibitory neurotransmitters (e.g. melatonin) that cause drowsiness. 

Nicotine

a stimulant that binds to & activates acetylcholine receptors. 

Mirtazapine

antidepressant that blocks the re-uptake of serotonin. 

Adderall

a combination drug of amphetamine salts used to treat attention deficit hyperactivity disorder (ADHD).  Blocks the re-uptake of dopamine & norepinephrine; referred to as a “study drug”. 

Cocaine

stimulant that promotes the release of dopamine & norepinephrine. 

Evolutionary Significance of the nervous system 

• Originated billions of years ago in prokaryotes. 

• The ability to detect changes in their environment & respond to such changes enhanced their ability to survive & reproduce.

Evolutionary Trends in the Nervous system

No symmetry → Radial symmetry → Bilateral symmetry.

No tissues → True Tissues → Nerve Net → bilateral nerve cords & ganglia → ventral nerve cord → dorsal nerve cord

Evolutionary Trends in the Organization of the Vertebrate Brain

Increased size, surface area & complexity. 

Preferential increase in size & complexity of forebrain/cerebrum.

Relative decrease in size of olfactory lobes. 

Increased relative size of forebrain in birds & mammals correlates with increased sophistication of behavior.

Central Nervous System (CNS)

exerts overriding control over the rest of the nervous system. 

• Brain – master control center. 

• Spinal cord – conveys information to the brain; integrates simple reflex responses. 

• Cavities filled with cerebrospinal fluid: Ventricles of the brain, central canal of the spinal cord & surrounding the brain.

Peripheral Nervous System (PNS) 

All nerves & ganglia outside the CNS. 

Cranial & spinal nerves. 

Connecting ganglia.

Cranial Nerves

twelve pairs of nerves that arise directly from the brain, not from the spinal cord, and pass through separate apertures in the skull.

Four Fluid-filled Spaces of the Brain

Ventricles, Central Canal & Cranial

filled with Cerebrospinal fluid. 

Aka the “lymphatic system of the brain”. 

Drains excess fluids & metabolic wastes from the CNS brought in by capillaries that service the CNS. 

Give neutral buoyancy to brain inside skull.

Meninges

Layers of connective tissue that protect the brain & spinal cord.

Blood-Brain Barrier

the selectively permeable barrier between the capillaries of the brain & the surrounding cellular matrix of the brain. 

Purpose of the blood brain barrier

Tight junctions between endothelial cells. 

Allows passage of nutrients & oxygen, but not metabolic wastes from other parts of the body. 

Helps to maintain a stable chemical environment for the brain. 

PNS

carries information to & from the CNS. 

Sensory Neuron

collect sensory information.

Motor Neurons  

Transmit responses to effector organs.

Motor System

carries information to & from skeletal muscles; controls voluntary contraction & relaxation of skeletal muscles.

Autonomic Nervous System

exerts involuntary control over cardiac & smooth muscles; controls digestive, cardiovascular, excretory & endocrine systems. 

Parasympathetic Division

Prepares the body for activities that gain & conserve energy (aka “Rest & Digest”). 

Sympathetic Division

prepares the body for intense energy-consuming activities (aka “Fight or Flight”). 

Enteric Division

includes neurons of the digestive tract, pancreas & gall bladder; controls secretion & peristalsis; regulated by sympathetic & parasympathetic divisions.

Food chain of the CNS

What is the frontal lobe in charge of

Motor

What is the pariential lobe for

Sensory

What is the Temporal lobe for

Hearing

What is the occipital lobe for

Vision

Cortex regions for processing (or controlling)

Somatic sensory information, including taste & smell (somatosensory cortex). 

Hearing & balance (auditory cortex). 

Vision (visual cortex).

Voluntary muscles (motor cortex).

Types of Communication: 

Spoken words = 7% 

Tone of voice = 38% 

Body language = 55%