NERVOUS SYSTEM Flashcards
CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) consists of the brain and spinal cord.
The brain is the part of the CNS that is located in the skull and contains about 85 billion neurons. The spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. The spinal cord contains about 100 million neurons. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.
Integrative control centres.
PERIPHERAL NERVOUS SYSTEM
The part of the nervous system that lies outside the central nervous system, consisting of nerves and ganglia.
The peripheral nervous system (PNS) (pe-RIF-e-ral) consists of all nervous tissue outside the CNS (figure 12.1a). Components of the PNS include nerves, ganglia, enteric plexuses, and sensory receptors. A nerve is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord. Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge from the spinal cord. Each nerve follows a defined path and serves a specific region of the body. Ganglia (GANG-glē-a = swelling or knot; singular is ganglion) are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. Ganglia are closely associated with cranial and spinal nerves. Enteric plexuses (PLEK-sus-ez) are extensive networks of neurons located in the walls of organs of the gastrointestinal tract. The neurons of these plexuses help regulate the digestive system (see section 24.3). The term sensory receptor refers to a structure of the nervous system that monitors changes in the external or internal environment. Examples of sensory receptors include touch receptors in the skin, photoreceptors in the eye, and olfactory receptors in the nose.
The PNS is divided into a somatic nervous system (SNS) (sō-MAT-ik; soma = body), an autonomic nervous system (ANS) (aw′-tō-NOM-ik; auto- = self; -nomic = law), and an enteric nervous system (ENS) (en-TER-ik; enteron = intestines). The SNS consists of (1) sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell, and (2) motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is voluntary.
The ANS consists of (1) sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, and (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. The motor part of the ANS consists of two branches, the sympathetic division and the parasympathetic division. With a few exceptions, effectors receive nerves from both divisions, and usually the two divisions have opposing actions. For example, sympathetic neurons increase heart rate, and parasympathetic neurons slow it down. In general, the sympathetic division helps support exercise or emergency actions, the ‘fight-or-flight’ responses, and the parasympathetic division takes care of ‘rest-and-digest’ activities.
The operation of the ENS, the ‘brain of the gut’, is involuntary. Once considered part of the ANS, the ENS consists of over 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal (GI) tract. Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons. Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. Enteric motor neurons govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs (such as acid from the stomach), and activities of GI tract endocrine cells, which secrete hormones.
NERVOUS SYSTEM - TISSUE
- Neurons (communicating cells)
* Neuroglia (supporting cells)
FUNCTIONS OF THE NERVOUS SYSTEM
Functions of the nervous system
The nervous system carries out a complex array of tasks. It allows us to sense various smells, produce speech, and remember past events; in addition, it provides signals that control body movements and regulates the operation of internal organs. These diverse activities can be grouped into three basic functions: sensory (input), integrative (process), and motor (output).
Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.
Integrative function. The nervous system processes sensory information by analysing it and making decisions for appropriate responses— an activity known as integration.
Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.
NEURONS
➢Highly specialised cells that communicate by sending nerve impulses (messages) from one body part to another
➢They have extreme longevity & an exceptionally high metabolic rate requiring a constant supply of O2 & glucose Comprised of: ➢Cell Body ➢Dendrites (Processes) ➢Axon (Processes)
Neurons are electrically excitable and can send electrical impulses (action potentials) to communicate
NUCLEI
name for clusters of cell bodies in the CNS
GANGLIA
name for clusters of cell bodies along the nerves (PNS)
GANG-glē-a) Groups of neuronal cell bodies lying outside the central nervous system (CNS). Singular is ganglion (GANG-glē-on).
AXON (PART OF NEURON)
➢ Generates and conducts nerve impulses away from the cell body
➢ Begins at the Axon Hillock → Trigger Zone for the nerve impulse
➢ Any long axon is also known as a Nerve Fibre
➢ Axon terminals end in synaptic knobs containing neurotransmitters
Neurotransmitters are chemicals which transmit the message from one neuron to the next cell
DENDRITES (PART OF NURON)
Dendrites
➢ Short, tapering, highly branched extensions
➢ Receptive region of the neuron → receive signals from other neurons or sensory receptors
NEUROGLIA
Neuroglia: Connective Tissue of the NS
➢ There are six types of Neuroglia:
→Four in the CNS [Astrocytes, Microglia, Ependymal cells, Oligodendrocytes] → Two in the PNS [Satellite cells, Schwann cells]
➢ General functions of glial cells:
→To surround and support neurons
→To insulate one neuron from another →Tosupplynutrientsinordertopromoteneuronhealth &
growth
→To produce chemicals that guide young neurons to make connections
Examples of Neuroglia
Ependymal cells (CNS):
→ Line the central cavities of the brain and spinal cord
→ Are ciliated and help circulation of cerebrospinal fluid
Schwann cells (PNS):
→Forms myelin (white fatty sheath) around axons in PNS
→ Myelin insulates the axon and increases the speed of electrical conduction
with
SYNAPSE
Synapse
• The functional junction between two neurons, a neuron and muscle or gland
• Close! But do not actually touch and are separated by a 20-50 nm* gap called the synaptic cleft
*1 nanometer (nm) = 0.000000001 or 10-9 m
NEUROTRANSMITTER
Neurotransmitters are molecules in axon terminals released in response to a nerve impulse (action potential)
➢ Neurotransmitters send the message on by stimulating or inhibiting the next neuron (or muscle or gland cell)
• It is estimated that ~100 neurotransmitters exist
• Neurotransmitters can change the membrane potential by
opening ion channels– Graded potentials
Inhibition Excitation
• Neurotransmitters can also stimulate other activities in the next cell (e.g. expression of a gene, activation of enzymes)
WHITE MATTER
➢ White matter consists primarily of myelinated fibres bundled into large tracts
➢ Three types:
1. Commissural fibres → connect gray areas of the 2
hemispheres (also called the ‘corpus callosum’)
2. Association fibres →connect areas within the same hemisphere
3. Projection fibres → connect hemispheres with lower brain regions or spinal cord
White matter of spinal cord (outside) – myelinated fibres/tracts: Ascending tracts – take sensory information up to the brain Descending tracts - take motor information away from brain
CEREBROSPINAL FLUID
Cerebrospinal Fluid: Production and Circulation
➢ Cerebrospinal fluid (CSF) is found in and around the brain and spinal cord
➢ Formed from blood plasma → CSF contains less protein & different concentrations of various ions (Na+, Cl-, H+, Ca2+, K+)
➢ Formed within the Choroid Plexuses
➢ Clusters of thin-walled capillaries, enclosed by pia mater & ependymal cells
➢ Circulates in the ventricles & central canal of spinal cord then returns to the blood stream via the Arachnoid Villi
Cerebrospinal Fluid: Functions
1.
Mechanical protection
Provides buoyancy to CNS structures→makes it light and stops it collapsing in on itself
Provides cushioning / shock absorption
Nourishes the brain and spinal cord
Carries chemical signals such as hormones, sleep and appetite molecules/ optimum environment for neuronal impulses
AUTONOMIC NERVOUS SYSTEM
Parasympathetic:
• “Rest and Digest”
• Energy conservation
• Replenishes nutrient stores • Controls smooth muscle of
gastrointestinal and respiratory tracts
Sympathetic: • “Fight or flight ” • Increased alertness • Increased metabolism • Prepares body for an emergency situation • Stimulates glucose release from liver, increases cardiac output and reduces blood flow to gut.
The balance of parasympathetic and sympathetic activity is called autonomic tone
• This is controlled by the hypothalamus
• The activity can be altered depending on the situation – as the
activity in one increases, the other decreases.
• The autonomic nervous system contributes to homeostasis by regulating bodily processes in response to subconscious visceral sensory information
– E.g. The autonomic nervous system regulates heart rate in response to stimuli from the baroreceptors
– You don’t have to think about controlling your blood pressure!
ACTION POTENTIAL
action potential (AP) (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.
- The action potential is an “all or nothing” response
- There is a domino effect. Once depolarisation occurs this initiates other voltage gated channels to open - POSITIVE feedback!
- The action potential is regenerated again & again (propagated) along the axon until it reaches the axon terminal
Neurons are electrically excitable and can send electrical impulses (action potentials) to communicate
• An action potential is generated by the flow of ions as voltage gated channels open
