Ch.10-Neural Tissue

Question 1

Describe the anatomical and functional divisions of the nervous system.

Answer 1

Central Nervous System(CNS) :

• Consists of the brain and spinal cord, located in the dorsal cavity.
• The CNS is the control centre of the nervous system, integrating all of its activities.
• Reflexes, past happenings, and current conditions determine how it will interpret sensory input and control motor output.

Peripheral Nervous System(PNS):

• Consists of the peripheral nerves connecting the CNS to other parts of the body.
• Primarily made up of nerves extending from the brain and spinal cord to the rest of the body.
• The cranial nerves transmit impulses to and from the brain, likewise the spinal nerves transmit impulses to and from the spinal cord.

Somatic Nervous System(SNS):

• Made up of somatic motor fibers transmitting impulses from the CNS to the skeletal muscles.
• Also known as the Voluntary Nervous System because our skeletal muscles are under conscious control.
• System does however also control involuntary contractions, such as those involved in reflexes.

Autonomic Nervous System(ANS):

• Contains visceral motor nerve fibers regulating glandular, cardiac muscle and smooth muscle activity.
• Generally not under conscious control hence known as Involuntary Nervous System.
• 2 subdivisions of the ANS are the sympathetic and parasympathetic divisions. Their actions usually oppose each other. When one division causes stimulation, the other inhibits its actions.

Question 2

List the basic functions of the nervous system.

Answer 2

Sensory receptors located at the ends of peripheral neurons provide the nervous system’s sensory functions.

They detect changes in the body’s internal and external environment and relay information, these changes may involve oxygen levels, temperature, light, sound, and many other types of information.

The information is converted into nerve impulses which are integrated so they can be processed to achieve the correct reaction. Motor functions then act on the integrated information.

Response structures called effectors are located outside the nervous system. When stimulated by nerve impulses, these effectors, which include muscles and glands, may contract, secrete, or perform other reactive functions. consciously controlled motor functions are handled by the somatic nervous system, which is in control of skeletal muscle.

Involuntary effectors, such as the heart, various glands, and the smooth muscle in blood vessels, are controlled by the ANS.

The nervous system helps maintain the body’s homeostasis by using its different divisions to respond to changes that occur.

Question 3

Describe the neuron and it’s important structural components.

Answer 3

• Neurons have a rounded cell body and extensions called dendrites and axons.
• Dendrites, which may be numerous, receive electrochemical messages.
• Axons send out electrochemical messages.
• Each neuron usually only has 1 Axon.
• Bundles of Axons constitute nerves.
• The cytoplasm of an axon is called ‘axoplasm’, this is surrounded by a specialised portion of the plasma membrane known as the ‘axolemma’.
• In the CNS, the axolemma may be exposed to interstial fluid or covered by neuroglial processes.
• Neuroglial cells provide insulation, physical support, and nutrients to the neurons.
• Neurons require continuous, abundant oxygen and glucose supplies due to their high metabolic rate. Without oxygen neurons don’t survive more than a few minutes.
• Neurons are similar in structure, but they vary greatly in size and shape.
• All neurons have a cell body or ‘soma’, dendrites and an axon.
• The cell body is made up a cell membrane, a granular cytoplasm or ‘perikaryon’, and organelles(lysosomes, a Golgi apparatus, mitochondria, and fine, thread-like ‘neurofibrils’)
• Neurofibrils are bundles of neurofilaments. The cytoskeleton of the perikaryon contains these, along with neurotubules.
• Well-developed Golgi apparatus forms either an arc or a circle around the nucleus.
• The cell body is where most biosynthesis occurs in the neuron. Therefore, it contains organelles that synthesise chemicals such as proteins.
• To maintain cell integrity and shape, microtubules and the neurofibrils form a structural network.
• throughout the cytoplasm are many sac-like Nissl bodies, also known as ‘chromatophilic substance’. These bodies are similar to the rough endoplasmic reticulum of other cells. Attached ribosomes synthesise protein.
• The centre of the cell body has a large, round nucleus with a nucleolus surrounded by cytoplasm.
• Certain neurons May contain pigments such as black melanin, a red pigment containing iron, or a gold-brown pigment called lipofuscin within the cell body.
• Most neuron cell bodies are located in the CNS and are protected by the bones of the vertebral column and skull.
• Nuclei are clusters of cell bodies in the CNS.
• Ganglia are clusters of cell bodies in the PNS, which lie along peripheral nerves.

Question 4

Describe synapses.

Answer 4

The nervous system requires impulse transmission through neuron chains that are functionally connected by synapses.

A synapse is a junction between any two communicating neurons.

The actual gap between neurons is known as the synaptic cleft. Neurons conduct intracellular communication across these gaps.

Axodendritic synapses are between the axon endings of a neuron and the dendrites of other neurons. Axosomatic synapses are those between axon endings of one neuron and cell bodies(soma) of others.

Axoaxonal, dendrodentritic and somatodendritic synapses are less common, occurring (respectively) between axons, between dendrites, or between cell bodies and dendrites.

Neurons may have between 1,000-10,000 axon terminals, with synapses being stimulated by an equivalent number of other neurons.

A neuron carrying an impulse into a synapse is called a presynaptic neuron.

The neuron receiving this impulse is called a postsynaptic neuron.

The synaptic cleft is a fluid filled space that separates the pre and post-synaptic membranes. The process of the impulse crossing the synaptic cleft is called synaptic transmission.

Synaptic transmission occurs in one direction, carried by biochemicals(neurotransmitters).

Question 5

Describe synaptic transmission.

Answer 5

When a nerve impulse reaches the synaptic knob, the action potential opens the voltage gated calcium channels.
Calcium then enters the synaptic knob and reacts with proteins found in the presynaptic membrane and synaptic vesicle membranes, fusing them together, exposing the inside of the synaptic vesicle to the synaptic cleft, and allowing release of the neurotransmitters contained within.
Neurotransmitters then diffuse across the synaptic cleft to the neurotransmitter receptors.

Question 6

Discuss the events that occur at a chemical synapse.

Answer 6

Chemical synapses allow the release and reception of chemical neurotransmitters.

They are more common than electrical synapses and usually made of 2 parts: axon terminal and neurotransmitter receptor region.

Axon terminal is a knob-like structure of the presynaptic neuron, it contains synaptic vesicles-membrane bound sacs holding thousands of neurotransmitter molecules.

Neurotransmitter receptor region is located on the postsynaptic neuron’s membrane, this is usually located on the cell body or on a dendrite.

Synaptic cleft is fluid filled space separating pre and postsynaptic membranes, approx. 1-millionth of 1 inch in width.

electrical current from presynaptic membrane dissipates in synaptic cleft preventing nerve impulses being directly transmitted between neurons. Instead they are transmitted through chemical events based on the release, diffusion, and receptor binding of neurotransmitter molecules.

Neurons have unidirectional communication between them.

Question 7

List the major types of neurotransmitters

Answer 7

Acetylcholine

Amino acids(GABA and glutamic acid)

Monoamines(dopamine, histamine, norepinephrine and serotonin)

Neuropeptides(endorphins, enkephalins and substance P)

Gases(nitric oxide)

Question 8

Define action potential

Answer 8

Action potential is the basis for a nerve impulse.

It is based on the cell membrane reaching its threshold potential(sodium channels open and sodium diffuses freely inward), the membrane becomes depolarised, nearly simultaneously potassium channels open allowing potassium to diffuse freely outward causing the inside of the membrane to to become hyperpolarised(rapid sequence of depolarisation and repolarisation ).

Only cells with excitable membranes including muscle cells and neurons can generate action potentials.

Question 9

Explain the classifications of nerve fibers

Answer 9

Nerve fibers are classified by their diameter, degree of myelination, and speed of conduction.

There are 3 primary groups of nerve fibers: Group A,B and C fibers.

Group A fibers:
Mostly serve the joints, skeletal muscles and skin.
Primarily somatic sensory and motor fibers.
Largest diameter of all fibers with thick myelin sheaths.
Conduct impulses at speeds as high as 300mph.

Group B fibres:
Intermediate diameter.
Light myelination.
Conduct impulses at speeds averaging 30mph.

Group C fibres:
Nonmyelinated.
Smallest diameter.
Cannot create Salvatore conduction.
Conduct impulses at 2mph or less.