Cns and peripheral Flashcards
The Central Nervous System (CNS)
Consists of the brain and the spinal cord
Composed of neurons and supportive glial cells
Interneurons are those neurons completely contained within the CNS
Sensory (afferent) and efferent neurons link interneurons to peripheral receptors and effectors.
The tissues of the CNS are divided into gray matter and white matter.
- Gray Matter – consists of numerous nerve cell bodies and mostly unmyelinated axons. The cell bodies are assembled in an organized fashion in both the brain and the spinal cord.
- White Matter – consists mostly of myelinated axons and contains very few cell bodies. Its pale color comes from the myelin sheaths that surround the axon.
- Bundles of axons that connect different regions of the CNS are known as tracts. Tracts in the CNS are equivalent to nerves in the peripheral nervous system.
Brain
The consistency of the brain and spinal cord is soft and jelly-like.
Must rely on external support for protection from trauma – brain encased in the cranium; spinal cord runs through the vertebral column.
3 layers of membrane, called the meninges, lie between the bones and the tissues of the CNS. The membranes are the dura mater, the arachnoid membrane, and the pia mater. They help stabilize and protect neural tissue.
Csf
The Cerebral Spinal Fluid (CSF) is the major protective component of the CNS.
The CSF along with interstitial fluid forms the extracellular environment for neurons.
The CSF is a salty solution that is continuously secreted by the choroid plexus.
The choroid plexus is similar to kidney tissue and consists of capillaries and transporting epithelium derived from ependymal cells.
The CSF serves two purposes:
Physical protection – provides buoyancy which gives the brain a lighter weight and provides a protective padding.
Chemical protection – creates a closely regulated extracellular environment for the neurons.
About 125-150 mL of CSF at any given time
The composition of CSF differs from plasma in the following ways: K+ conc. is lower, H+ conc. is higher (pH ~7.3), and the Na+ conc. is similar.
CSF contains very little protein and no RBCs
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The final layer of protection for the brain is the Blood-Brain Barrier.
Necessary to isolate the body’s main control center from potentially harmful substances in the blood and from blood-borne pathogens such as bacteria.
Highly selective permeability of brain capillaries shelters the brain.
Created by astrocytes
Brain
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Cerebellum – 2nd largest structure in the brain and functions in motor control (movement coordination), posture, and equilibrium)
Diencephalon – lies between the brain stem and the cerebrum and contains the centers for homeostasis
Cerebrum is skeletal movement and some sort of integration
Hypothalamus
Lies beneath the thalamus and is the center for homeostasis and contains centers for various behavioral drives, such as hunger and thirst.
Receives input from multiple sources, including the cerebrum and various sensory receptors.
Output from the hypothalamus also influences many functions of the autonomic nervous system, as well as a variety of endocrine functions.
Creates hormones to tell pituitary to secrete hormones
The Brain Stem
The oldest and most primitive region of the brain.
Cranial nerves originate along the brain stem and carry sensory and motor information for the head and neck.
Medulla oblongata – controls many involuntary functions such as blood pressure, breathing, swallowing, and vomiting.
Reticular formation – a diffuse collection of neurons that extends throughout the brain stem.
The limbic system
A region of the cerebrum which surrounds the brain stem.
It acts as a link between higher cognitive functions, such a reasoning, and more primitive emotional responses, such as fear.
The major areas of the limbic system are the:
- Amygdala
- Cingulate gyrus
- Hippocampus learning memory
The spinal cord
The spinal cord is the major pathway for information flowing back and forth between the brain and the skin, joints, and muscles of the body. In addition, the spinal cord contains neural networks responsible for locomotion.
Segments of the spinal cord give rise to a bilateral pair of spinal nerves.
Just before a spinal nerve joins the spinal cord, it divides into two branches called roots – the dorsal root and ventral root.
Dorsal afferent
Ventral efferent
Spinal cord
The spinal cord can function as a self-contained integration center for simple spinal reflexes.
Spinal interneurons may route sensory information to the brain through ascending tracts or bring commands from the brain to the cord through descending tracts.
Spinal Cord Organization:
Gray matter consists of sensory and motor nuclei (nuclei – a mass of nerve cells)
White matter consists of tracts of axons that carry information to and from the brain.
Pns
Provides links from and to world outside body All neural structures outside brain Sensory receptors Peripheral nerves and associated ganglia Efferent motor endings PIC
Sensory receptors
Sensory receptors (or sensors) are specialized cells, parts of cells, or complex multicellular receptors (such as the eye) that respond to changes in their environment (stimuli). Activation results in graded potentials that trigger nerve impulses Sensation (awareness of stimulus) and perception (interpretation of meaning of stimulus) occur in brain
Neural Reflex Pathways
Neural Reflex Pathways can be classified in 4 different ways:
By the efferent division of the nervous system that controls the response
Reflexes that involve somatic motor neurons and skeletal muscles are known as somatic reflexes.
Reflexes whose responses are controlled by autonomic neurons are called autonomic reflexes.
By the CNS location where the reflex is integrated
Reflexes integrated in the brain are called cranial reflexes.
Reflexes integrated in the spinal cord are called spinal reflexes and may not require input from the brain.
Neural Reflex Pathways
Neural Reflex Pathways can be classified in 4 different ways:
3. By whether the reflex is innate or learned
Many reflexes are innate; in other words, we are born with them and they are genetically determined. (Example: Knee jerk reflex)
Other reflexes are acquired through experience and called learned reflex or conditioned reflex. (Example: Pavlov’s dogs)
4. By the number of neurons in the reflex pathway.
The simplest reflexes are monosynaptic reflexes which have only two neurons: one afferent (sensory) and one efferent. Monosynaptic reflexes are used by somatic motor reflexes.
Most reflexes are polysynaptic reflexes which include one or more interneurons between the afferent and efferent neurons. Somatic motor reflexes can also be polysynaptic. All autonomic reflexes are only polysynaptic reflexes.
Somatic reflexes
Somatic Reflexes are also known as Skeletal Muscle Reflexes
Skeletal muscle reflexes are involved in almost everything we do.
Receptors that sense changes in joint movement, muscle tension, and muscle length feed this information to the CNS, which responds in one of two ways.
1. If muscle contraction is the appropriate response, the CNS activates somatic motor neurons to the muscle fibers.
2. If a muscle needs to be relaxed to achieve the response, sensory input activates inhibitory interneurons in the CNS, and these interneurons inhibit activity in somatic motor neurons controlling the muscle.
