Nervous System. Human Anatomy & Physiology P. Wilson

Transcription

1 Nervous System Human Anatomy & Physiology P. Wilson 1

2 2 Types of cells in the nervous system: Neurons & Neuroglial cells Neuroglial (aka glial) cells perform functions that are vital to neurons by filling spaces, support neurons by providing nutrition and structural frameworks, producing myelin, and performing phagocytosis. In fact, neurons could not exists without the glial cells performing their various functions. 2

3 2 Types of cells in the nervous system: Neurons & Neuroglial cells Neurons are the structural & functional units of the nervous system. Neurons vary on size and shape but ALL neurons have 3 parts: a cell body dendrites an axon Mature neuron cells do not divide BUT some parts of the nervous system contain neural stem cells that can differentiate into neurons or neuroglia. 3

4 Neuron The cell body of a neuron consists of all the normal cellular structures. The receptive part of the neuron consists of short, highly branched structures called dendrites which receive nerve impulses from the axons of other neuron cells and transmit the impulse to their own cell body. A single axon carries the nerve impulse away from the cell body. 4

5 Myelinated vs Unmyelinated Myelinated axons have myelin sheaths are appear white. Masses of myelinated axons and their cell bodies form the white matter within the CNS Unmyelinated axons lack myelin sheaths are appear gray. Masses of unmyelinated axons and their cell bodies form the gray matter within the CNS 5

6 Gaps in the myelin sheath are called nodes of Ranvier; these gaps facilitate rapid conduction of nerve impulses called salutatory conduction 6

7 Organs of the Nervous System The organs of the nervous system can be divided into 2 groups: 1. The central nervous system (CNS) consists of the brain and the spinal cord. 2. The peripheral nervous system (PNS) consists of nerves (peripheral nerves) that connect the CNS to other body parts. 7

8 Functions of the Nervous System Together the CNS and PNS allow us to think, remember, move, to be aware of, and to respond, to conditions in the world in which we live. This is possible because the nervous system provides sensory, integrative, and motor functions to the body. 8

9 Sensory Functions Sensory receptors at the end of peripheral nerves gather information by detecting changes to the body s environment (both external & internal changes) then convert the information into nerve impulses that travel to the CNS. 9

10 Integrative functions To integrate means to combine parts into a whole, so In the brain, sensory nerve impulses are integrated into perceptions ( a sensation that is an interpretation of the combined & coordinated impulses) 10

11 Motor functions Once a decision (conscious or unconscious) is made, motor impulses travel to effector organs and a response to the sensory input is stimulated. Motor responses are carried out by effectors (a muscle or gland that effects change in the body). 11

12 PNS & Motor Functions The motor functions of the PNS can be divided into 2 categories: the somatic system is under conscious control; the somatic motor pathways lead to the skin and to skeletal muscles (aka voluntary muscles) the autonomic system controls effectors that are involuntary the heart, smooth muscles in blood vessels and in viscera, and glands 12

13 Neurons Classified by Structure 1) Bipolar neurons have only 2 processes: one arising from each end of their cell body; one is a dendrite and one is an axon. Both processes are structurally similar Neurons within specialized parts of the eyes, nose, & ears are bipolar 2) Multipolar neurons have many process arising from their cell body many dendrites and one axon; most of the neurons whose cell bodies lie in the brain and spinal column are multipolar. 13

14 Neurons Classified by Structure Unipolar neurons have a single process extending from their cell body; a short distance from the cell body this process divides into 2 branches. The cell bodies of these neurons aggregate in groups called ganglia; both branches really function as 1 axon the peripheral branch is associated with dendrites near a peripheral body part the other branch enters the brain or spinal cord. 14

15 Neurons Classified by Function Sensory (afferent) neurons carry nerve impulses from peripheral receptors to the CNS. located in peripheral body parts & dendrites associated with receptor cells in skin or sensory organs Interneurons carry nerve impulses from one part of the CNS to another. (These lie entirely with CNS) Motor (efferent) neurons carry nerve impulses from the CNS to effectors (stimulate muscles to contract & glands to secrete. 15

16 Cell Membrane Potential The membrane of a neuron is usually polarized (electrically charged), with an excess of negative charges on the inside of the membrane. This separation of charge (or potential difference) is called the resting potential. A threshold stimulus initiates a process that results in the depolarization of the neuron membrane. Very quickly, the neuron membrane repolarizes. This rapid change is called an action potential. 9-16

17 Action Potential A nerve impulse is created as depolarization creates a wave of action potentials (a localized bioelectric current) that move down the axon. 17

18 All-or-nothing Response Nerve Impulse The result is that all impulses carried on a particular axon is of the same strength. Greater intensity of stimulation triggers more impulses per second, not stronger impulses. Refractory Period A brief period, following a nerve impulse, when a threshold stimulus will not trigger another impulse on an axon When an impulse (an action potential) reaches the (end of an axon) synaptic knobs of an axon, synaptic vesicles release neurotransmitter into the synaptic cleft. Stimulation of the nerve fiber stops when the neurotransmitter is degraded by an enzyme or reabsorbed by the presynaptic neuron. 18

19 Nerves A nerve is a bundle of nerve fibers held together by layers of connective tissue. nerves that carry impulses to CNS are referred to as sensory nerves nerves that carry impulses to muscles or glands are referred to as motor nerves most nerves include both sensory & motor fibers and are called mixed nerves 19

20 Nerve Pathways The routes that nerve impulses follow as they travel through the nervous system are referred to as nerve pathways. The simplest of nerve pathways are reflexes - automatic subconscious response to stimuli (changes within or without the body). Reflexes help maintain homeostasis by controlling involuntary processes such as heart rate & breathing Reflexes also carry out the automatic actions of swallowing, sneezing, coughing, & vomiting 20

21 Reflex Arc A reflex arc begins with a sensory receptor (dendrites of a sensory neuron or a specialized receptor cell in a sensory organ); next a sensory neuron carries the impulse to the CNS an interneuron in the CNS conducts the impulse to a motor neuron which transmits impulses from CNS out to an effector which responds by producing a reflex (behavioral action). 21

22 Knee Jerk! The knee-jerk reflex is an example or a simple reflex that utilizes only 2 neurons: a sensory neuron communicates directly with a motor neuron (no interneuron as a go-between!!) sensory neuron motor neuron 22

23 Bony Coverings The brain is covered by the cranial cavity of the skull. The spinal cord is covered by the spinal or vertebral column 23

24 Meninges membranes between bony coverings & soft tissue The dura mater is the outermost layer of the meninges; it is formed of tough, white fibrous connective tissue is attached to the inside of the skull contains blood vessels & nerves forms partitions that support & protect the brain & spinal cord. 24

25 Arachnoid mater The middle layer of the meninges it is a thin, web-like membrane that spreads over the surface of the brain & spinal cord subarachnoid space (between this mater and pia mater) contains the cerebrospinal fluid (CSF) Pia mater The innermost meninges is the pia mater. It is very thin; contains numerous nerves & blood vessels. Nourishes the cells of the brain Contains specialized masses that produce CSF. 25

26 Ventricles The brain has a series of interconnected cavities, called ventricles, that are continuous with the central canal of the spinal cord. The ventricles contain cerebrospinal fluid (CSF). Cerebral Spinal Fluid (CSF) The brain & spinal cord float in the CSF which supports and protects them by acting as shock absorbers. CSF also maintains electrolyte balance and serves as a pathway to the blood for excretion of cellular waste 26

27 Spinal Cord The superior boundary of the spinal cord is the inferior end of the cranium. The inferior boundary of the spinal cord is at the disk between the L1 and L2 vertebrae. There are 31 pairs of spinal nerves. The spinal cord has 2 major functions: 1. conducting nerve impulses 2. Serving as a center for spinal reflexes 27

28 Spinal Cord Ascending tracts The nerve tracts that carry sensory information to the brain are called ascending tracts. Injury to ascending tracts results in a loss of sensation in body parts distal to the injury Descending tracts The nerve tracts that conduct motor impulses from the brain to muscles and glands are called descending tracts. Injury to descending tracts results in a loss of motor function in body parts distal to the injury 28

29 2 Types of Descending Spinal Tracts Pyramidal tracts (aka corticospinal tracts) Carry impulses that control skeletal muscle movements. Extrapyramidal tracts control motor activities associated with maintaining balance and posture 29

30 The Brain The major parts of the brain are: the cerebrum (the largest portion of the brain) the cerebellum the diencephalon the brain stem 30

31 Structure of the Cerebrum Two large masses left & right cerebral hemispheres connected by a bridge of neural fibers called the corpus callosum. Surface has many ridges or convolutions (aka gyri) separated by grooves. A shallow groove is called a sulcus; a deep groove is called a fissure. 31

32 Lobes of the Cerebrum Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula The first 4 lobes listed correspond to the bones of the skull with the same names 32

33 Cerebrum The outermost layer of the cerebrum is the cerebral cortex, a thin layer of gray matter (myelinated or unmyelinated?). The cerebral cortex contains nearly 75% of all the neuron cell bodies in the nervous system. Beneath the cortex is a mass of white matter that makes up the bulk of the cerebrum. 33

34 Cerebrum Cerebral functions include: Sensory interpret sensory input & give rise to sensations or feelings Motor generate nerve impulses that control muscle activity Associative interpreting, reasoning, memory, & other higher brain functions 34

35 Cerebrum: Motor function Motor Functions major motor areas in frontal lobe just in front of central sulcus control skeletal muscles speech movements controlled by Broca s area frontal eye field controls voluntary movements of eyes and eyelids 35

36 Cerebrum Sensory Function Sensory parietal lobes, posterior occipital lobes, temporal lobes sensations from skin, vision, hearing Cerebrum Associative Function Associative frontal lobes, lateral portions of parietal, temporal, & occipital lobes memory, reasoning, judgment, emotion 36

37 Dominant or Non-dominant Dominant Hemisphere Used for languagerelated activities such as speech, reading, writing, and for complex intellectual skills that require verbal, analytical, and computational skills. In 90% of population left hemisphere is dominant. Non-dominant Hemisphere Used for motor tasks that require orientation of the body in space, understanding & interpreting musical patterns, and nonverbal visual experiences. Also controls intuitive thinking and emotional thinking. 37

38 Thalamus Diencephalon Functions in sorting & directing sensory information arriving from other parts of the nervous system. Perform services of both messenger & editor. (It is the executive secretary for the cerebrum!) Produces an awareness of pain, touch, & temperature 38

39 Diencephalon Hypothalamus Maintains homeostasis by regulating heart rate & rhythm, hunger & weight gain, and various other visceral functions Regulates secretions of various hormones, including those that stimulate sleep and wakefulness 39

40 Limbic System The limbic system includes the thalamus, hypothalamus, parts of the cerebral cortex, basal nuclei, & other deep nuclei. The limbic system controls emotional experience and expression recognizes threats to the organisms and produces emotions such as fear, anger, & pleasure, which leads to behaviors that increase chances for survival 40

41 Brain Stem Lies between the brain and spinal cord and connects the two structures. Three parts: midbrain pons medulla oblongata 41

42 Midbrain It is like a doorman to the cerebrum by conveying nerve impulses to & from the cerebrum. Several visual reflexes located here, including those that turn the eyes in concert with the head and auditory reflexes such as those that turn the head in the direction of a sound. 42

43 Pons Helps regulated rate & depth of breathing Relays impulses to the cerebellum and from the periphery to the higher brain centers 43

44 Medulla Oblongata Transmits all ascending and descending impulses to the spinal cord Control center for vital reflexes that an individual alive (heart, breathing) Also the center for reflexes like sneezing and coughing 44

45 Cerebellum Is the reflex control center it integrates sensory information concerning the position of the body in space, and for coordinating complex skeletal movements Damage to the cerebellum results in tremor, loss of precision in skeletal muscles, loss of muscle tone, abnormal gait, & loss of balance. 45

46 PNS 2 parts: Somatic & Autonomic The somatic system is under conscious control controls voluntary movements controls communication with the sense organs The autonomic system controls effectors that are involuntary concerned with the parts of our bodies that function involuntarily (without our awareness) 46

47 Divisions of the Autonomic Nervous System The sympathetic division Acts to prepare the body for energyexpending, stressful, or emergency situations (fight or flight) The parasympathetic division is most active under ordinary, restful conditions It acts to calm the body after the sympathetic division has been engaged Also, helps the body to conserve energy 47

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49 Green Boxes Did you know your body produces opiates? Pg 382. They are called What is a subdural hematoma? Page 391 CVA or TIA? Page 397 Is the patient in a coma or a persistent vegetative state? Page 407 Bell s Palsy: what is it? cause? Page

50 Myelin Myelin is a fatty, white substance (lipids & proteins). Myelin is found only on the axon fibers cell bodies, dendrites, & axon terminals are never myelinated so nerve synapses are ALWAYS in the gray matter Myelin is produced by neuroglial cells: in the CNS by oligodendrocytes in the PNS by Schwann cells 50

51 Myelin Myelin is electrically insulating it prevents the bioelectric current from leaving the nerve fiber (ie: no loss of signal ). Myelin s purpose is to increase the speed at which impulses move along the nerve fiber. Saltatory conduction (from the Latin saltare which means to jump or hop) Na + cannot cross the myelin sheath so the nerve impulse becomes less of a wave of action potential and more of a jump of action potential from node (of Ranvier) to node. 51

52 Myelin & Damaged Axons In the PNS myelinated axons can regenerate damaged fiber (although it may not be a perfect regeneration) In the CNS NONE of the axons (myelinated or not) can regenerated damaged fibers Only axons that are myelinated!!! NEVER cell bodies or dendrites!!! 52

53 Multiple Sclerosis (MS) MS is an autoimmune disease where a person s own immune cells attack the myelin sheath & strip it away Symptoms include fatigue, dizziness, tingling, gait (walking) problems, trembling seizures, pain, cognitive (information processing) issues 53