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.
Mixed nerves
In the spinal cord, sensory and motor nerves are arranged in separate groups or tracts. Outside the spinal cord, when sensory and motor nerves are enclosed within the same sheath of connective tissue, they are called mixed nerves.
Neuroglia
The neurons of the central nervous system are supported by non-excitable glial cells that greatly outnumber the neurons. Unlike nerve cells, which cannot divide, glial cells continue to replicate throughout life. There are four types: astrocytes, oligodendrocytes, ependymal cells, and microglia.
Astrocytes
Astrocytes are a sub-type of glial cells in the central nervous system. They are also known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. In humans, a single astrocyte cell can interact with up to 2 million synapses at a time.
Oligodendrocytes
These cells are smaller than astrocytes and are found in clusters around nerve cell bodies in grey matter, where they are thought to have a supportive function. They are found adjacent to and along the length of, myelinated nerve fibers. Oligodendrocytes form and maintain myelin like Schwann cells in peripheral nerves.
Ependymal cells
These cells form the epithelial lining of the ventricles of the brain and the central canal of the spinal cord. Those cells that form the choroid plexuses of the ventricles secrete the cerebrospinal fluid.
Microglia
The smallest and least numerous glial cells, these cells may be derived from monocytes that migrate from the blood into the nervous system before birth. They are found mainly in the area of blood vessels. They enlarge and become phagocytic, removing microbes and damaged tissue, in areas of inflammation and cell destruction.
Response of nervous tissue to injury
Damage to neuroses can either lead to rapid necrosis with sudden acute functional failure or slow atrophy with gradually increasing dysfunction. These changes are associated with:
- hypoxia and anoxia
- nutritional deficiencies
- trauma
- infections
- aging
Peripheral nerve regeneration
Peripheral nervous system regeneration. Neuroregeneration in the peripheral nervous system (PNS) occurs to a significant degree. After an injury to the axon, peripheral neurons activate a variety of signaling pathways which turn on pro-growth genes, leading to the reformation of a functional growth cone and regeneration.
Neuroglial damage (Astrocytes)
When these cells are damaged, their processes multiply forming a mesh or ‘scar’, which is thought to inhibit the regrowth of damaged CNS neurons.
Central nervous system
The central nervous system consists of the brain and the spinal cord. These essential structures are both well protected from damage and injury; the brain is enclosed within the skull and the spinal cord by the vertebrae that form the spinal column. Membranous coverings known as the meninges provide further protection.
The meninges
The brain and spinal cord are surrounded by three layers of tissue, the meninges, lying between the skull and the brain, and between the vertebral foramina and the spinal cord. Named from outside inwards they are the:
- dura mater
- arachnoid mater
- pia mater
Dura mater
Dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. The other two meningeal layers are the arachnoid mater and the pia mater.
arachnoid mater
A fine, delicate membrane, the middle one of the three membranes or meninges that surround the brain and spinal cord, is situated between the dura mater and the pia mater.
Pia mater
The delicate innermost membrane enveloping the brain and spinal cord
Ventricles of the brain and cerebrospinal fluid
The brain contains four irregular-shaped cavities, or ventricles, containing cerebrospinal fluid (CSF). They are:
- right and left lateral ventricles
- third ventricle
- fourth ventricle.
The lateral ventricles
The lateral ventricles are the largest in the series of four interconnecting fluid-filled cavities within the brain
The third ventricle
The third ventricle is a cavity situated below the lateral ventricles between the two parts of the thalamus. It communicates with the fourth ventricle by a canal, the cerebral aqueduct.
The fourth ventricle
The fourth ventricle is a diamond-shaped cavity situated below and behind the third ventricle, between the cerebellum and pons. It is continuous below the central canal of the spinal cord and communicates with the subarachnoid space by foramina in its roof. Cerebrospinal fluid enters the subarachnoid space through these openings and through the open distal end of the central canal of the spinal cord.
Cerebrospinal fluid
Clear watery fluid which fills the space between the arachnoid membrane and the pia mater.
Functions of cerebrospinal fluid
CSF supports and protects the brain and spinal cord by maintaining a uniform pressure around these vital structures and acting as a cushion or shock absorber between the brain and the skull.
It keeps the brain and spinal cord moist and there may be an exchange of nutrients and waste products between CSF and the interstitial fluid of the brain. CSF is thought to be involved in the regulation of breathing as it bathes the surface of the medulla where the central respiratory chemoreceptors are located.
Brain
An organ of soft nervous tissue contained in the skull of vertebrates, functioning as the coordinating center of sensation and intellectual and nervous activity. Its parts are: (Diencephalon) -cerebrum -thalamus -hypothalamus (Brain stem) -midbrain -pons -medulla oblongata -cerebellum
Blood supply and venous drainage
The circulus arteriosus and its contributing arteries play a vital role in maintaining a constant supply of oxygen and glucose to the brain when the head is moved and if a contributing artery is narrowed.
Venous blood from the brain drains into the Dural venous sinuses and then downwards into the internal jugular veins
Cerebrum
The principal and most anterior part of the brain in vertebrates, located in the front area of the skull and consisting of two hemispheres, left and right, separated by a fissure. It is responsible for the integration of complex sensory and neural functions and the initiation and coordination of voluntary activity in the body.
Cerebral tracts
Tracts are neural pathways that are in the brain and spinal cord (central nervous system).
Tracts are formed by neurons synapsing onto one another, and these neurons can be classified as first-order, second-order, and third-order neurons depending on their location and order within the tract.
Basal ganglia
The basal ganglia are groups of cell bodies that lie deep within the brain and form part of the extrapyramidal tracts. They act as relay stations with connections to many parts of the brain including motor areas of the cerebral cortex and thalamus. Their functions include initiation and fine control of complex movement and learned coordinated activities, such as posture and walking. If control is inadequate or absent, movements are jerky, clumsy and uncoordinated.