Chapter 7: The nervous system Flashcards
What is the nervous system?
The nervous system detects and responds to changes inside and outside the body. Together with the endocrine system, it coordinates and controls vital aspects of body function and maintains homeostasis.
What does the nervous system consist of?
The nervous system consists of the brain, the spinal cord, and peripheral nerves. The structure and organization of the tissues that form these components enable rapid communication between all parts of the body.
How is the nervous system grouped?
The parts of the nervous system are grouped as follows:
- the central nervous system (CNS), consisting of the brain and the spinal cord
- the peripheral nervous system (PNS), consisting of all the nerves outside the brain and spinal cord.
Peripheral nervous system (PNS)
The peripheral nervous system (PNS) is the division of the nervous system containing all the nerves that lie outside of the central nervous system (CNS). The primary role of the PNS is to connect the CNS to the organs, limbs, and skin. These nerves extend from the central nervous system to the outermost areas of the body.
Somatic nervous system
Controls voluntary movement of skeletal muscles.
Autonomic nervous system
The autonomic nervous system, controlling involuntary processes such as heartbeat, peristalsis, and glandular activity. The autonomic nervous system has two divisions: sympathetic and parasympathetic.
Cells and tissues of the nervous system
There are two types of nervous tissue, neurons, and neuroglia. Neurons (nerve cells) are the working units of the nervous system that generate and transmit nerve impulses. Neurons are supported by connective tissue, collectively known as neuroglia, which is formed from different types of glial cells.
Neurons
The neuron is a nerve cell that is the basic building block of the nervous system.
Cell bodies
Cell bodies form the grey matter of the nervous system and are found at the periphery of the brain and in the center of the spinal cord. Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.
Axons and dendrites
Axons and dendrites are extensions of cell bodies and form the white matter of the nervous system. Axons are found deep in the brain and in groups, called tracts, at the periphery of the spinal cord. They are referred to as nerves or nerve fibers outside the brain and spinal cord.
Axons
Each nerve cell has only one axon, which begins at a tapered area of the cell body, the axon hillock. They carry impulses away from the cell body and are usually longer than the dendrites, sometimes if 100 cm.
Structure of an axon
The membrane of the axon is called the axolemma and it encloses the cytoplasmic extension of the cell body.
Myelinated neurons
The myelinated neurons are those neurons whose axons are covered with the layer of Schwann cell membranes called the myelin sheath. The myelin sheath plays the role in faster transmission of impulse or the action potential. The neurons with myelin sheath are found in the peripheral nervous system (Mostly in motor and sensory neurons).
Unmyelinated neurons
Unmyelinated neurons can be found in both the peripheral and central nervous systems in the group c nerve fibers, responsible for the transmission of secondary pain or itch. Unmyelinated nerve fibers can lose the nerve impulse during conduction.
Dendrites
These are the many short processes that receive and carry incoming impulses towards cell bodies. They have the same structure as axons but are usually shorter and branching. In motor neurons dendrites form part of synapses and in sensory neurons they form the sensory receptors that respond to specific stimuli.
The nerve impulse
The nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulses are mostly electrical signals along the dendrites to produce a nerve impulse or action potential. The action potential is the result of ions moving in and out of the cell.
The synapse and neurotransmitters
Synapse, also called neuronal junction, is the site of transmission of electric nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell (effector).
A chemical substance that is released at the end of a nerve fiber by the arrival of a nerve impulse and, by diffusing across the synapse or junction, causes the transfer of the impulse to another nerve fiber, a muscle fiber, or some other structure.
Nerves
A nerve consists of numerous neurons collected into bundles (bundles of nerve fibers in the central nervous system are known as tracts).
Each bundle has several coverings of protective connective tissue:
-endoneurium is a delicate tissue, surrounding each individual fiber, which is continuous with the septa that pass inwards from the perineurium
-perineurium is a smooth connective tissue, surrounding each bundle of fibers
-epineurium is the fibrous tissue that surrounds and encloses several bundles of nerve fibers. Most large nerves are covered by epineurium.
Sensory and afferent nerves
Specialized endings of sensory neurons respond to different stimuli (changes) inside and outside the body.
sensory receptors
These originate from the skin. They are pain, touch, heat, and cold. Sensory nerve endings in the skin are fine branching filaments without myelin sheaths. When stimulated, an impulse is generated and transmitted by the sensory nerves to the brain where the sensation is perceived.
somatic, cutaneous, or common senses
These originate in muscles and joints. Impulses sent to the brain enable perception of the position of the body and its parts in space-maintaining posture and balance
Proprioceptors senses
These originate in muscles and joints. Impulses sent to the brain enable perception of the position of the body and its parts in space-maintaining posture and balance
Autonomic afferent nerves
These originate in internal organs, glands, and tissues, e.g., baroreceptors involved in the control of blood pressure, chemoreceptors involved in the control of respiration, and are associated with reflex regulation of involuntary activity and visceral pain.
Motor or efferent nerves
Motor nerves originate in the brain, spinal cord, and autonomic ganglia. They transmit impulses to the effector organs: muscles and glands. There are two types:
- somatic nerves – involved in voluntary and reflex skeletal muscle contraction
- autonomic nerves (sympathetic and parasympathetic) – involved in cardiac and smooth muscle contraction and glandular secretion.