VII. The Nervous System

A. Cells of the Nervous System

1.Neurons- cells specialized for transmitting chemical and electrical signals from one location in the body to another

a) Cell body- contains most of the cytoplasm, the nucleus and other organelles

b) Dendrites- convey signals to the cell body, Usually short, numerous and extensively branched

c) Axon- conducts impulses away from the cell body. They are tipped with synaptic terminals which release neurotransmitters

1)Neurotransmitters- chemicals that cross the synapse to relay a message to a new neuron

2) Synapse- gap between a synaptic terminal and the dendrites of another neuron

2. Types of Neurons: a) Sensory neurons- convey information about the external or internal environment to the central nervous system

b) Motor Neurons- convey impulses from the CNS to the effector cells

c) Interneurons- integrate sensory input and motor output (located in the CNS)

3.Supporting Cells- structurally support, protect insulate and assist neurons. They do not conduct impulses. They out number neurons 10-50X

a) Glial Cells- supporting cells of the central nervous

system

2) Oligodendrocytes- form myelin sheaths that insulate nerve processes

1) Astrocytes- encircle capillaries in the brain to control the ionic environment around neurons

b) Schwann cells- form myelin sheath in the peripheral nervous system to provide electrical insulation and speed the rate of nerve impulse transmission.

B. Transmission of Electrical Signals along a Neuron

1. The Origin of Electrical Membrane Potential a) Membrane potential- the difference in charge

between the cytoplasm and extracellular fluid due to a differential distribution of ions

b) Resting Potential- about -70 mV in a non-transmitting neuron

c) Ion Distribution1) Inside Cell: High K+, A-; Low Na+, Cl- = Negative Charge

2) Outside Cell: Low K+; High Na+, Cl- = Positive Charge

d)Sodium-Potassium Pumps actively move ions against their concentration gradient to maintain proper resting potential and counteract diffusion

2.Action Potential- rapid change in membrane potential caused by selective opening and closing of ion gates

1) Hyperpolarization- gates allow K+ to leave the cell causing the inside to become more negative

2) Depolarization- gates allow Na+ to enter the cell making the inside less negative

a)Gated ion Channels allow neurons to change their membrane potential in response to stimuli

b) Different ion channel affect the neuron:

c) An action potential has four phases

1)Resting State: no channels are open

Na+ gates are opened. K+ gates are closed. The influx of Na+ causes the interior of the cell to become positively charged

2) Large depolarizing phase-

3) Steep repolarizing phase-

Na gates close. K+ gates open. Loss of K+ causes cell interior to return to a negative state

4) Undershoot phase- is a time when membrane potential is temporarily more negative than resting state due to the lag in closing K+ gates

There is a Refractory Period- during undershoot phase. During this period the neuron is insensitive to stimulus and will not fire. This limits the maximum rate at which a neuron can fire

a)Strong depolarization in one area results in the depolarization of the neighboring area

b)Action potential does not travel down the axon but is regenerated at each position

c)The impulse travels in one direction due to the refractory period at the previous position

3. Propagation of Action Potential

4. Speed of Propagation of Action Potential

a) The larger the diameter of a neuron, the faster the impulse

b) Saltatory Conduction- the impulse jumps from one node of Ranvier to the next, skipping myelinated regions

c) Nodes of Ranvier- gaps in the myelinated sheath where ion gates are concentrated

C. The Synapse: Transmission between Cells

1. Synapse- tiny space between neurons that control communication between those neurons.

a) Presynaptic Cell- is the transmitting cell

b) Postsynaptic cell- is the receiving cell

2. Electrical Synapse- action potential spreads directly from presynaptic to postsynaptic cell via gap junction. Very uncommon

Chemical Synapse Animation

3.Chemical Synapse- a chemical called a neurotransmitter is released from the presynaptic cell and binds to receptors on a postsynaptic cells causing it to fire.a) An action potential arriving at the synaptic terminal at the end of an

axon causes Ca+2 to rush through voltage sensitive channels

b) The sudden in rush of Ca+2 causes synaptic vesicles which contain neurotransmitters to fuse with the presynaptic membrane releasing neurotransmitters into the synaptic cleft (the space between neurons)

c) Neurotransmitters diffuse to the postsynaptic membrane where they bind to specific receptors and trigger the opening of ion gates

d) This may cause hyperpolarization or depolarization depending on which ion gates are opened

e) The neurotransmitter is quickly degraded by enzymes and recycled to the presynaptic cells

4. Nervous Integrationa) One neuron receives signals from numerous

adjacent neuronsb) Excitatory postsynaptic Potential (EPSP)

are caused by neurotransmitters that open Na+ gates triggering depolarization

c) Inhibitory postsynaptic Potential (IPSP) are caused by neurotransmitters which open K+ or Cl- gates causing hyperpolarization

d) A single EPSP is rarely strong enough to trigger an action potential, although and additive effect, summation, from several terminals can trigger a neuron to fire

1) Temporal summation is when chemical transmission from one or more synaptic terminals occur so close in time that the additive effect causes the neuron to fire

2) Spacial Summation occurs when several neurons stimulate the postsynaptic cell at the same time and cause it to fire

3) EPSPs and IPSP can summate also, but each counters the other’s affect preventing the neuron from firing

Summation

D. The Vertebrate Nervous System

1. Peripheral Nervous system- consists of sensory and motor neurons

a) Somatic Nervous System (voluntary) carries messages from the central nervous system to skeletal muscles

b) Autonomic Nervous System controls involuntary functions and is divided into:1) Sympathetic Nervous System increases

function

2) Parasympathetic Nervous System decreases function

2. Central Nervous

a) Cerebellum - the part of the brain below the back of the cerebrum. It regulates balance, posture, movement, and muscle coordination.

b) Corpus Callosum - a large bundle of nerve fibers that connect the left and right cerebral hemispheres. In the lateral section, it looks a bit like a "C" on its side.

c) Frontal Lobe of the Cerebrum - the top, front regions of each of the cerebral hemispheres. They are used for reasoning, emotions, judgment, and voluntary movement.

d) Medulla Oblongata - the lowest section of the brainstem (at the top end of the spinal cord); it controls automatic functions including heartbeat, breathing, etc.

e) Occipital Lobe of the Cerebrum - the region at the back of each cerebral hemisphere that contains the centers of vision and reading ability (located at the back of the head).

f) Parietal Lobe of the Cerebrum - the middle lobe of each cerebral hemisphere between the frontal and occipital lobes; it contains important sensory centers (located at the upper rear of the head).

g) Pituitary Gland - a gland attached to the base of the brain (located between the Pons and the Corpus Callosum) that secretes hormones.

h) Pons - the part of the brainstem that joins the hemispheres of the cerebellum and connects the cerebrum with the cerebellum. It is located just above the Medulla Oblongata.

i) Spinal Cord - a thick bundle of nerve fibers that runs from the base of the brain to the hip area, running through the spine (vertebrae).

j) Temporal Lobe of the Cerebrum - the region at the lower side of each cerebral hemisphere; contains centers of hearing and memory (located at the sides of the head).