Spinal Cord, PNS, and Synapses

Where does the spinal cord extend from?

From the foramen magnum to L1/L2

What are the two enlargements of the spinal cord?

Cervical and lumbar enlargements

What are the regions of gray matter in the spinal cord?

Dorsal horn, ventral horn, and lateral horn

What does the dorsal horn contain?

Sensory neurons/interneurons

What does the ventral horn contain?

Motor neurons

What does the lateral horn contain?

Autonomic neurons — Only present in thoracic and upper lumbar segments

What are the regions of white matter in the spinal cord?

Funiculi

What type of fibers are in the ventral roots?

Motor (efferent) fibers to skeletal muscles

What type of fibers are in the dorsal roots?

Sensory (afferent) fibers from receptors

What do the dorsal and ventral roots combine to form?

A spinal nerve

How many pairs of spinal nerves are there?

31 pairs

How many cervical spinal nerves are there?

8 pairs (C1-C8)

How many thoracic spinal nerves are there?

12 pairs (T1-T12)

How many lumbar spinal nerves are there?

5 pairs (L1-L5)

How many sacral spinal nerves are there?

5 pairs (S1-S5)

How many coccygeal spinal nerves are there?

1 pair (Co1)

What do mechanoreceptors detect?

Touch, pressure, vibration, stretch

What do thermoreceptors detect?

Temperature changes

What do photoreceptors detect?

Light (vision)

What do chemoreceptors detect?

Chemicals in solution

What do nociceptors detect?

Pain

What do exteroceptors respond to?

Stimuli from outside the body

What do interoceptors respond to?

Stimuli from internal organs and blood vessels

What do proprioceptors respond to?

Position/movement of muscles, tendons, joints

What does the brachial plexus provide?

Virtually all the nerves for the upper extremity

What spinal nerves contribute to the brachial plexus

Primarily C5-T1, with contributions from C4 and T2 in some people

Where does the brachial plexus emerge?

Between the anterior and middle scalene muscles

What are the five organizational levels of the brachial plexus (in order)

Roots —> Trunks —> Divisions —> Cords —> Branches

What are the five terminal branches of the brachial plexus?

1. Musculocutaneous nerve

2. Axillary nerve

3. Radial nerve

4. Median nerve

5. Ulnar nerve

What does the lumbar plexus provide nerves for?

The lower abdomen, anterior thigh, and medial thigh

What spinal nerves contribute to the lumbar plexus

L1-L4

What are the two major nerves of the lumbar plexus

Femoral nerve and obturator nerve

What does the sacral plexus provide nerves for?

The pelvis, posterior thigh, and nearly all of the leg and foot

What spinal nerves contribute to the sacral plexus?

L4-S4

Where does the sacral plexus emerge?

Through the pelvis

What is a synapse?

A junction that mediates information transfer from one neuron to another

What are the three locations of a synapse

1. Axosomatic

2. Axodendritic

3. Axoaxonal

What are the two types of synapses based on communication method?

1. Electrical synapse

2. Chemical synapse

What does the electrical synapse do

Uses gap junctions for direct ion flow; fast

What does the chemical synapse do?

Uses neurotransmitters to transmit signals across a synaptic cleft; most common

What do chemical synapses use to transmit signals?

Neurotransmitters

What are the three main components of a chemical synapse?

1. Axon terminal

2. Receptor region

3. Synaptic cleft

What is the axon terminal?

The presynaptic part that releases neurotransmitters

What is the receptor region

The postsynaptic part that receives the neurotransmitter

What is the synaptic cleft

The small gap between the axon terminal and receptor region; Seperates the presynaptic and postsynaptic neurons

What is a synaptic delay?

The brief time (0.3-5.0 ms) it takes for neurontransmitters to cross the synaptic cleft in neural transmission

Step 1 - What happens when the action potential reaches the axon terminal?

The action potential arrives at the axon terminal

Step 2 - What channels open in response to depolarization?

Voltage-gated Ca2+ channels open, allowing Ca2+ to enter the terminal (along with Na+ effects)

Step 3 - What does Ca2+ entry cause?

Causes synaptic vesicles to release neurotransmitters into the synaptic cleft via exocytosis

Step 4 - What happens after neurotransmitters are released?

Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane

Step 5 - What occurs when neurotransmitters bind to receptors?

Ion channels open, creating graded potentials which can be excitatory or inhibitory depending on the neurotransmitter

Step 6 - How are neurotransmitter effects terminated?

1. Reuptake by astrocytes

2. Degradation

3. Diffusion

What happens when a neurotransmitter binds during an excitatory postsynaptic potential?

The postsynaptic membrane depolarizes

Does an EPSP create an action potential?

No, it is a graded potential

What is the goal of an EPSP?

To bring the postsynaptic neuron closer to threshold, increasing the chance of generating an action potential

What happens when a neurotransmitter binds during an inhibitory postsynaptic potential

The postsynaptic membrane hyperpolarizes

Does an IPSP create an action potential?

No it is a graded potential

What is the goal of an IPSP?

To move the postsynaptic neuron farther from threshold, making it less likely to generate an action potential