Nervous System BIO216 Lecture Notes PDF

BIO 216 THE NERVOUS SYSTEM LECTURE 1  THE NERVOUS SYSTEM The NS is formed of a big no. of cells, which are of 2 types: 1. Nerve cells = Neurons 2. Supporting cells = Glial cells 1. NEURONS  It is the basic structural unit of the NS.  It genera...

BIO 216 THE NERVOUS SYSTEM LECTURE 1  THE NERVOUS SYSTEM The NS is formed of a big no. of cells, which are of 2 types: 1. Nerve cells = Neurons 2. Supporting cells = Glial cells 1. NEURONS  It is the basic structural unit of the NS.  It generates electrical impulses → transmitted from one part of the body to another.  In most neurons: electrical impulses → release of chemical messengers (= neurotransmitters) to communicate with each other.  Neurons are integrators: their output = the sum of the inputs they receive from thousands of other neurons that end on them. 2  STRUCTURE OF NEURON Neurons occur in a wide variety of shapes and sizes, but they share common features. They all possess 4 parts: 1. Cell Body = Soma: It contains: - a nucleus - ribosomes → ptn. synthesis - mitochondria & other organelles → provide E & sustain metabolic activity of cells. 2. Dendrites: - Usually 5-7 processes (may be many more) - Usually highly branched (up to 400,000) → ↑ surface area. - Together with cell body, dendrites receive most input. - Transmit impulses toward cell body only. 3  STRUCTURE OF NEURON 3. Axon = Nerve Fiber: - Usually single & long (few µm to  1m). - Transmits impulses away from soma toward target cell. - Axon hillock or initial segment (= beginning of axon + part of soma where axon joins it) is the trigger zone where electric signals are generated in most neurons. Signals are then propagated along axon. - Axon may have branches = collaterals. - Near its end the axon undergoes branching. The greater the no. of branches, the greater the sphere of influence of a neuron. 4. Axon Terminal = Terminal Button: - Each branch of the axon ends in an axon terminal. - Responsible for the release of neurotransmitters (NT) from axon. NT diffuse out of the axon terminal to next neuron or to a target cell. 4  MYELIN SHEATH Axons of most (but not all) neurons are coated by a protective layer = myelin sheath  “myelinated neurons”. Myelin sheath is formed by the following cells: 1. In peripheral NS (PNS): by Schwann cells 2. In central NS (CNS): by oligodendrocytes. Schwann Cells - They are glia-like cells. - During embryonic development, these cells attach to growing axons & wrap around them → concentric layers of plasma membrane. - Myelin sheath of an axon is formed of many Schwann cells that align themselves along length of axon. - Nucleus is located in outermost layer. Each segment is separated from the next by a small unmyelinated segment called node of Ranvier. - Plasma membrane of Schwann cells is  80% lipid → myelin sheath is mostly lipid → appears glistening white to the naked eye. 5  MYELIN SHEATH (cont.) Function of myelin sheath: 1. Myelin sheath helps to insulate axons & prevents cross- stimulation of adjacent axons. 2. Myelin sheath allows nerve impulses to travel with great speed down the axons, “jumping” from one node of Ranvier to the next. Some nerve fibers are “unmyelinated”. Their axons are covered by a Schwann cell, but there are no multiple wrappings of membrane which produces myelin. These axons conduct impulses at a much lower rate. 6 MYELIN SHEATH 7  MYELIN SHEATH (cont.) Oligodendroglia = Olidodendrocytes - They are a type of glial cells. - They cover axons in central nervous system (CNS). - Unlike Schwann cells, they may branch to form myelin on up to 40 axons. 8  2. GLIAL CELLS = Supporting Cells - Act as supporting cells (glia = glue): They surround neurons & support them physically & metabolically. - They constitute 90% of cells in CNS. - There are 3 types: A. Astrocytes = Astroglia: Have small cell bodies & extensively branching processes. Functions: 1. Help regulate composition of extracellular fluid (ECF) in CNS. 2. Some of their processes form “end-feet”, which are close to cerebral blood capillaries → form a barrier around capillaries (blood- brain barrier) → prevents toxins & other substances from entering brain. 3. Sustain neurons metabolically (provide glucose, remove ammonia). 9  2. GLIAL CELLS B. Microglia: - Scavenger cells (=phagocytic cells) of CNS. - Activated by injury or inflammatory processes. - On activation, they migrate to area of injury to become macrophages & clean cellular debris. C. Oligodendrocyes = Oligodendroglia: Myelin-forming cells in the CNS 10 Glial Cells 11  B. ANATOMICAL ORGANISATION OF THE NERVOUS SYSTEM 12 ANATOMICAL ORGANIZATION OF NS (cont.) Central Nervous System (CNS): - Brain & spinal cord - Housed in bony structures: skull & vertebral column Peripheral Nervous System (PNS): - Transmits signals to & from CNS. - Consists of nerves that extend between brain & spinal cord skeletal, smooth & cardiac muscles and glands N.B. Nerves are bundles of nerve fibers. - The PNS consists of:  12 pairs of cranial nerves: originate in brain, pass thro’ openings in skull  31 pairs of spinal nerves: originate in spinal cord, pass thro’ openings in vert. column - Individual fibers in PNS may be processes of:  Sensory neurons or  Motor neurons 13  B. ANATOMICAL ORGANISATION OF NS (cont.)  Sensory (Afferent) Pathways: - Transmit nerve impulses from the periphery to the CNS. - Transmit information about:  the external environment (e.g., light, sound, touch, temperature or pressure) or  the internal state of the body (e.g., distension of viscera).  Motor (Efferent) Pathways: - Transmit impulses from CNS to effectors (muscles or glands) that perform the order.  Voluntary (Somatic) NS: - It consists of motor neurons that supply skeletal muscles. - It therefore controls voluntary functions of body.  Autonomic NS: - It innervates smooth muscles, cardiac muscle and glands. - It therefore controls involuntary functions of the body, e.g., heart rate, breathing, digestion (i.e., involuntary smooth muscles). - Autonomic nerves are further subdivided into sympathetic & parasympathetic divisions, which counterbalance each other. 14  C. FUNCTIONAL ORGANIZATION OF NS Functionally, neurons are divided into 3 main types: I. Afferent (sensory) neurons: - Transmit information from sensory receptors at the periphery to the CNS. N.B.: Receptors are specialized structures that respond to various physical & chemical changes in their environment, causing electrical signals to be generated in neurons. - Have an unusual shape:  No dendrites (do not receive input from other neurons)  Have a single process (considered to be an axon), which divides shortly after leaving the cell body: one branch, the peripheral process, ends at the receptors the other branch, the central process, enters the CNS to form junctions with other neurons. - Cell body & long peripheral process of axon are in PNS, only short central process of axon enters CNS. 15  C. FUNCTIONAL ORGANIZATION (cont.) II. Efferent (motor) neurons: - Transmit information from CNS to effectors (e.g., ms., glands, viscera). - Dendrites, cell body & a small part of axon lie within CNS, while most of the axon lies in the PNS. III. Interneurons: - Account for 99% of all neurons. - Lie completely within the CNS. - Transmit impulses between sensory & motor neurons, acting as neuronal bridges. - The no. of interneurons between certain afferent & efferent neurons varies according to complexity of the action. - Interneurons can act as signal changers, e.g., an excitatory input can be changed into an inhibitory output or no output at all. 16 C. FUNCTIONAL ORGANIZATION OF NS 17