Topic: 6.5 Option A

Nerve Signals Maintain Homeostasis Both the nervous system and the endocrine system control actions of the body and maintain homeostasis. Responses to changes in environment are made by electrochemical messages (relayed to and from the brain) or chemical messages (hormones - carried by the blood)

The nervous system is an elaborate communication system that contains about 85 billion nerve cells, (called neurons) that transmit nerve impulses (electrical signals) Memory, learning, and language are functions of the nervous system.

Vertebrate Nervous Systems 2 main divisions: CNS– central nervous system: the nerves of the brain and spinal cord - coordinating centre for incoming & outgoing info PNS – peripheral nervous system: all other nerves (called neurons) nerves that carry info in the form of electrical impulses between organs and the CNS

Relays info about environment to CNS Initiates appropriate response Controls skeletal muscle, bones, and skin Controls internal organs

Nerve Cells 2 types of cells: glial cells and neurons GLIAL CELLS (neuroglial cells): nonconduction; provide structural support and metabolism of the nerve cells. NEURONS: functional units of nervous system; conduct nerve impulses (action potentials) 3 types: sensory neurons, interneurons, motor neurons

Nerve A nerve is made of several individual neurons grouped together into a single structure (Kind of like a telephone cable: a protective sheath surrounding many individual wires)

Nerve Cell

Sensory Neurons Also known as afferent neurons Relay info from receptor cells and then pass it on to an interneuron Located in clusters called ganglia outside the CNS

Receptors Specialized cells that receive stimuli from the (internal or external) environment and pass on the information to sensory neurons. Ex: photoreceptors in your eyes respond to light Ex: chemoreceptors in your nose and on your tongue are sensitive to chemicals Ex: baroreceptors are pressure receptors in your skin that detect the fit of your clothes Ex: thermoreceptors in your skin respond to different temperatures

Interneurons/ Relay Neurons Link neurons within the body Found mainly in the brain and the spinal cord They receive nerve impulses from sensory neurons and pass them on to other parts of the CNS or motor neurons.

Motor Neurons Also known as efferent neurons Relay info from the CNS to the effectors (Effectors: muscles, organs, and glands – they produce responses to the stimuli)

Nerve Cell Parts DENDRITES: projections of cytoplasm Receive info (from sensory neurons or other nerve cells or receptors) and conducts nerve impulses toward the cell body. Cell Body: contains the nucleus, ER, Golgi, ribosomes, lysosomes, mitochondria

Nerve Cell Parts AXON: an extension of the cytoplasm Carries info and conducts nerve impulses away from the cell body (to other neurons or to effectors) Very thin – 100 axons could fit in a single hair strand

MYELIN SHEATH – a white coat of fatty protein and multiple phospholipid bilayers that covers many axons Prevents the loss of ions from the nerve cell If an axon has a myelin sheath, it is “myelinated” The myelin sheath is made of special glial cells called Schwann cells. Nodes of Ranvier: regularly occurring gaps between adjacent Schwann cells of the myelin sheath.

Nerve impulses move much faster along a myelinated nerve than a nonmyelinated one. The nerve impulse can jump from one node of Ranvier to the next – this is called saltatory conduction. (Conduction through a unmyelinated sheath is known as continuous conduction) Ex: Speed of conduction of 100m/s v..s 1m/s

MS – Multiple Sclerosis MS is caused by destruction of the myelin sheath. The myelin sheath hardens and produces scarlike tissue that prevents normal transmission of info. MS symptoms: double vision, speech difficulty, jerky movements, partial paralysis

Nerve fibers in the PNS contain a thin membrane called the neurilemma which surrounds the axon This promotes the regeneration of damaged axons. When you get a paper cut, neurons are severed. Feeling gradually returns to your fingers as the severed neurons are rejoined

Nerves within the brain that contain myelin and neurilemma are called white matter (b/c of whitish appearance) Nerves that lack both are called grey matter. These are found with the brain and the spinal cord Damage to grey matter is usually permanent

Nerve Circuits STIMULUS: a change in the environment (internal or external) that is detected by receptor cells and elicits a response. RESPONSE: a change in an organism often carried out by a muscle or a gland (called an effector)

Nerve Circuits EX: When you put your hand on a hot stove, the stimuli of the heat ultimately cause you to move your muscles in your arm to remove your hand from the stove. Receptor  sensory neuron  interneuron  brain  interneuron  motor neuron  effector Sometimes, these messages can be sent even quicker – you can move your hand from the hot stove before your brain even receives that information. This is a REFLEX. Reflexes are involuntary and often unconscious.

REFLEX ARC The simplest nerve pathway is the reflex arc. Most reflexes occur without brain coordination. Reflex arcs contain five essential components: 1. the receptor 2. the sensory neuron 3. the interneuron 4. the motor neuron 5. the effector

Withdrawal Reflex ex hand on a hot surface 1. Pain receptors in the finger detect the pain and activate sensory neurons. 2. The sensory neurons carry the impulse to the spinal cord via the dorsal root of a spinal nerve. The impulse is passed to a interneuron (relay neuron) in the grey matter of the spinal cord.

Withdrawal Reflex ex hand on a hot surface 3. The interneuron passes the impulse to a motor neuron. 4. The motor neuron carries the impulse out of the spinal cord via the ventral root to the muscles in the arm, which is the effector 5. The muscles (effector) contract and pull the arm away from the hot object.

Draw and label a diagram of a reflex arc for a pain withdrawal reflex. See page 536

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