Nerve- Neuronal Endings

Question 1

What are the 4 main types of neuronal ending? *

Answer 1

1. Synapse- neuron to neuron
2. Motor end plate- neuromuscular junction
3. Muscle spindle- muscle fibre to muscle fibre
4. Sensory endings- signal back to CNS for processing

Question 2

What are synapses?

Answer 2

• Synapses are regions at the end of some axons that transmit the nerve impulse from one neuron to another
• They may connect with the dendrites, onto a cell body, or onto another axon or axon terminal, as well as into the bloodstream or diffusely into the adjacent nervous tissue
• Synapses can be chemical or electrical
• There are numerous synaptic connections between neurons e.g. Purkinje cells (cerebellum) have over 200,000

Question 3

What are the different types of synapses?

Answer 3

Axosecretory- Axon terminal secretes directly into bloodstream
Axoaxonic- A.T secretes directly into diff. axon
Axodendritic- AT ends on dendrite spine
Axoextracellular- A.T with no connection secretes into XC fluid
Axosomatic- AT ends on soma
Axosynaptic- AT ends on another axon terminal

Question 4

Important synaptic features

Answer 4

• Two neurons involved are pre and post synaptic neurons- Pre sends signal and post receives signal.
• Vesicles containing neurotransmitters, e.g Ach, will pass between these neurons
• Calcium regulated voltage channels present- involved in the passage of neurotransmitter from pre to post SN
• In chemical synapses, synaptic cleft present
• Sodium gated ion channels on the post synaptic neuron

Question 5

What are motor end plates?

Answer 5

The axon loses its myelination and forms a dilated termination –neuromuscular bulb (similar to a synapse) that sits within a trough on the muscle fibre surface. Several such bulbs form a motor end plate

A neuromuscular junction (myoneural junction, motor end plate) is a chemical synapse formed by the contact between a motor neuron and a muscle fiber.
•It is at the neuromuscular junction that a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction.

•Toxins that act on neuromuscular junctions: Botulinum toxin, Latrotoxin, snake venom, nerve gases, tetanus toxin

Question 6

What is a muscle spindle?

Answer 6

• Act as proprioceptors providing the CNS with data about the musculoskeletal system.
• Approximately 2 mm long and 0.1 mm wide.
• Changes in length (distension) of the muscle fibres caused by body movements are detected by the muscle spindles and the sensory nerves relay this information to the spinal cord.
• This helps maintain posture and regulates the activity of opposing muscle groups involved in motor activities such as walking.

Question 7

What is the stretch reflex?

Answer 7

• When muscles lengthen, the spindles are stretched. -The muscle spindle in turn sends an impulse to the spinal cord.
• This impulse results in the activation of more motor neurons at spinal level that send an impulse back to the muscle.

-This impulse tells the muscle to contract with greater force in order to decrease the speed at which the muscle is being stretched. This is known as the stretch reflex

Question 8

What are some unencapsulated receptors?

Answer 8

Merkel cells (skin, especially fingertips)- each associated w. expanded nerve endings which functino as tonic receptors for sustained light touch and for sensing an object’s texture.

Free nerve endings in papillary dermins and extending into lower epidermal layers, which primarly respond to high/low temperatures, pain and itching- also act as tactile receptors

Root hair plexes- web of sensory fibres surrounding bases of hair follicles in dermis that detects movements of hairs.

Question 9

What are some encapsulated receptors?

Answer 9

Meissner (tactile) corpiscles- elliptical structures
initiate impulses when light touch/ low frequency stimulia against skin temporarily deform their shape. Numerous in fingertips, palms and soles

Pacinian (lamellar) corpuscles- large oval structures, found deep in (hypo)dermis. Specialised for sensing coarse touch, pressure (sustained touch) and vibrations w distortion of capsule amplifying mechanical stimulus to axonal core where impulse is initiated.

Krause end bulbs- simple encapsulated, ovoid structures, with extremely thin, collagenous capsules penetrated by sensory fibre. Found primarily in skin of genitalia where they sense low frequency vibrations.

Ruffini corpuscles- stimulated by stretch (tension) or twisting (torque) in skin