Chapter 34 The Nervous System: 3.5 Learning Objectives 3.5.3 Responses in the human 1. The nervous system: two-part division into the CNS and the PNS. 2. Neurons, name 3 types, give structure and function of parts. 3. Describe the electro-chemical movement of nerve impulses. 4. Define synapse and give its functions. 5. Discuss the activation and inactivation of neurotransmitters. 6. Describe the CNS. Giving the location and function of the main parts of the brain. 7. Draw and label a cross-section of spinal cord. 8. Name any one example of a nervous system disorder, one possible cause, prevention, and treatment. 9. Describe the Peripheral nervous system: location of nerve fibres and cell bodies. 10. Explain the role, structure and mechanisms of the reflex action. In animals there are two systems responsible for coordination of activities in the body: - The Endocrine System - The Nervous System The Nervous System allows for rapid responses to stimuli or changes in the environment The Nervous System consists of two parts. - The Central Nervous System (CNS): consists of the Brain and Spinal Cord. - The Peripheral Nervous System (PNS): consists of a network of nerves that carry messages between the CNS and the rest of the body. Nervous System Terms A stimulus is any change in your environment. A receptor is a nerve cell that detects the stimulus. A neuron is a specialised cell that carries electrical messages (impulses) around the body. An impulse is an electrical message that is carried along a neuron. An effector is part of an organism which carries out a response. There are four key stages involved in response to a stimulus: 1. Reception: detection of the stimulus by sense organs and nerves. 2. Transmission: The Message is carried from the neurons in the PNS to those in the CNS.
3. Integration: The Brain sorts and processes the incoming messages. 4. Response: Once the effectors receive a signal from the CNS they are stimulated to respond. Neurons (Nerve Cells): The basic units of the nervous system and carry electrical impulses around the body. 3 Types: Sensory (Afferent) neurons: take impulses from sense organs to CNS. Motor (Efferent) neurons: Take impulses from CNS to effectors. Interneurons: carry information between sensory and motor neurons. Function of Neuron Parts: Receptor: Organ or group of cells that detect stimulus. Nerve Endings: Connect sensory neurons to receptors. Dendrites: Fibres that carry impulses toward the cell body. Axons: Carry impulses away from cell bodies. Schwann Cells: Form the myelin sheath. Myelin Sheath: Insulate the electrical impulses. Nodes of Ranvier: Are gaps in insulation that speed up impulse speed. Cell Body: Forms the dendrites and axons that emerge from it. They produce neurotransmitter chemicals. - In sensory (afferent) neurons they are located outside the CNS. - In motor (efferent) neurons they are located inside the CNS. A Ganglion: A group of cell bodies outside the CNS (inside the PNS). A nerve or nerve fibre consists of a number of axons and cell bodies (ganglion) grouped together. (The nerves usually share Myelin sheaths).
Transmission of Nerve Impulses An axon branches into many axon terminals each with a neurotransmitter swelling on the end. These swellings release neurotransmitter chemicals which carry the impulse from one nerve cell to another. The movement of the impulse along the dendrite and axon involves the movement of charged particles called ions. At rest an axon has positive ions outside and negative ions inside. When a stimulus is received it has a domino effect and the charges are swapped, moving along the axon. The change in charge in one section causes a change in the next section. For a stimulus to be carried it must reach a certain threshold. The threshold is the minimum stimulus needed to cause an impulse to be carried. An impulse is either carried or not. The all or nothing law states that if the threshold is reached an impulse is carried, but if the threshold is not reached no impulse is carried. Sensitivity (mild Vs severe pain) depends not on the strength of the stimulus but on the number of neurons stimulated. Once a neuron is in the process of carrying one impulse it cannot carry another. This is similar to when dominoes fall you cannot push them again until you stand them up. The Refractory Period is a short timespan after a neuron has carried an impulse during which a stimulus fails to cause a response.
As mentioned earlier once the impulse reaches the end of an axon, the axon terminals release neurotransmitters to pass the impulse to the (dendrites) next neuron. Synapse: the region where two neurons come into close contact. The synaptic cleft: the tiny gap between two neurons at a synapse. Synapse Functions: 1. Pass impulse from one neuron to another or to an effector. 2. Control the direction of an impulse. 3. Act as valves. Neurotransmitters only on pre-synaptic end. 4. They prevent over stimulation of effectors (switch off impulses). 5. Some drugs block synapses and prevent transfer of impulses. Used in painkillers and psychiatric disorders. Neurotransmitters Neurotransmitters are chemicals released by swellings on axon terminals to pass an impulse from the axon on one neuron to the dendrite of another. - They are necessary as the electrical impulses (ions) cannot cross a synapse. - The enzymes required to make these neurotransmitters are made in the cell body. Over 60 neurotransmitters are known but the most common include: - Acetylcholine (Ach) - Noradrenalin - Dopamine Neurotransmitters are released by the axon terminals (pre-synaptic neuron), diffuse across the synaptic cleft and activate the dendrite receptors on the next neuron (post-synaptic neuron). - Once the process is complete enzymes digest the neurotransmitters, which are reabsorbed into the axon terminals and recycled. The Central Nervous System The CNS consists of the Brain and Spinal Cord. - Both are protected by bone and are covered by three membranes called meninges. - The space between the inner two layers is filled with cerebrospinal fluid which acts as a shock absorber and an exchange medium between the blood and brain.
The CNS - The Brain The CNS The Spinal Cord The spinal cord is located in the neural canal of the vertebrae. It is composed of nerve tissue and protected by the vertebrae and the meninges that protect it. It carries messages to and from the brain and sense organs. The sensory neurons enter the spinal cord through the dorsal route. The dorsal root ganglion consists of cell bodies and sensory neurons. The white matter contains axons. The grey matter contains cell bodies and dendrites. Interneurons connect sensory and motor neurons. The motor neurons leave the spinal cord through the ventral route. CNS DISORDER = PARKINSONS DISEASE Cause: Failure to produce a neurotransmitter called Dopamine. Symptoms: Inability to control muscle contractions. Trembling of hands, rigid legs, staring eyes. Prevention: None at the moment. Treatment: Physiotherapy to strengthen muscles. Treatment with drugs to mimic dopamine.
The Peripheral Nervous System The PNS consists of all the nerve fibres around the body (not including the brain and spinal cord). - The cell bodies of sensory nerves are located in the ganglia in the PNS. - The cell bodies of motor neurons are found in the CNS. A Reflex Action is an automatic, involuntary, unthinking response to a stimulus. - They are fast responses. - Generally involve flight or fight protection responses. - Also include responses such as blinking, breathing, control of blood pressure, pulling back if we get burnt, our eyes adjusting to a change in light. A reflex arc is a basic unit of response in the nervous system. - It involves receptors, nerves and effectors. - It involves such a fast reaction that although the brain is made aware of the impulse it does not control the response. The reflex response associated with burning your finger involves an impulse being sent to the spinal cord and being responded to, without the control of the brain.