Lesson 33 Lesson Outline: Nervous System Structure and Function Neuronal Tissue Supporting Cells Neurons Nerves Functional Classification of Neuronal Tissue Organization of the Nervous System Peripheral Nervous System Assemblages of Neurons Spinal Cord Objectives: References: Chapter 16: 387-428 Reading for Next Lesson: Chapter 16: 387-428
Nervous System Structure and Function Neuronal Tissue Made up of two cell types: Neurons Supporting Cells (nerve cells = neurons) (glial cells = neuroglia) Supporting Cells The supporting cells can make up more than 90% of the cells in the nervous system of some species. More complex organisms have more glial cells relative to neurons. These cells form a scaffolding or glue, which holds the tissue together. They assist the neurons by controlling the environment around them. Neurons Neurons are highly specialized. They are one of the two types of excitable cells in the body. The other one is muscle. Neurons conduct messages in the form of nerve impulses from one part of the body to another. All neurons have a cell body or soma and numerous thin extensions. There are two types of extensions (neuron processes): Dendrites Axons Dendrites Dendrites are the primary processes that receive information. They are input regions of the neuron. Note that cell bodies and axons can also receive information. Axons Are processes that can generate and conduct action potentials. They are processes that transmit information. They are the output regions of the neuron. They may be very short or very long depending on where they are conducting information. They form synapses at their terminals. Nerves Nerves generally consist of bundles of axons that travel together through the body.
Functional Classification of neuronal tissue Sensory neurons are afferent to CNS. Motor neurons are efferent to CNS. Interneurons are interconnections between other neurons. Organization of the Nervous System Nervous System Peripheral Nervous System Sensory (Afferent) Division Motor (Efferent) Division Visceral Somatic Somatic Nervous System Autonomic Nervous System Sympathetic Division Parasympathetic Division Brain and spinal cord. These are the primary integrating and control centre (although integration also occurs, to some extent, at every synapse). Peripheral Nervous System The peripheral nervous system consists of all the nerves: - cranial nerves arise directly from the brain. - spinal nerves arise from the spinal cord. The peripheral nerves are the communication lines between the CNS and the rest of the body. There are two functional sub-divisions of the peripheral nervous system: - sensory (afferent) nervous system - motor (efferent) nervous system
Sensory (Afferent) Nervous System The sensory nervous system conveys impulses to the CNS from sensory receptors. It has two sub-divisions: Somatic sensory neurons convey information from skin, muscles, joints (i.e. the body). Visceral sensory neurons convey information from the viscera. Motor (Efferent) Nervous System The motor nervous system conducts information from the CNS to the effectors (muscles, glands, etc.). It has two sub-divisions: The somatic nervous system: conducts information to skeletal muscles. The autonomic nervous system: conducts information to cardiac muscle, smooth muscle, and glands (i.e. viscera, glands, heart and blood vessels). The autonomic nervous system has two functional sub-divisions: - sympathetic nervous system - parasympathetic nervous system Autonomic Nervous System In the sympathetic nervous system the motor neuron releases norepinephrine (noradrenaline) at the effector site. The ganglia reside just outside the spinal cord. In the parasympathetic nervous system the neuron releases acetylcholine at the effector site. The ganglia reside close to the effector organ. Sympathetic Nervous System Parasympathetic Nervous System These two usually have opposite effects on the same visceral organs. If one stimulates, the other inhibits. Remember, however, that each stimulates some organs! Assemblages of Neurons Accumulations of nerve cell bodies = a ganglion Accumulations of ganglia = brain Reasons for accumulation = shorter distances for integration
Reasons for an anterior accumulation (brain) = sensory integration at the head end In vertebrates we see an anterior accumulation of nerve cell bodies into ganglia and an accumulation of ganglia to form a brain near the source of the most important sites of sensory input - near the sites that first encounter new environments. Definitions Grey Matter - accumulations of unmyelinated cell bodies and dendrites. White Matter - accumulations of myelinated axons. Nuclei and Tracts - synonymous with grey and white matter within the central nervous system. Central nervous system = brain and spinal cord. Peripheral nervous system = cranial and spinal nerves. The CNS is an integrating site for sensory input and motor output. Spinal Cord Embryonic Origins During embryonic development, the cells immediately adjacent to the neurocoel begin to differentiate through the process of neurulation. These are the actively mitotic stem cells. They become surrounded by cells that have been derived from this layer. Some of these cells become neurons and some become neuroglia. The neurons sprout dendrites and an axon and the axons grow out to form the outer layer of the developing spinal cord. Because the axons eventually become coated with myelin, the outer layer appears white and was named the "white matter". The inner layers, which are composed of non- myelinated cell bodies, are the "gray matter".
There are dorsal and ventral extensions of the grey matter - the dorsal and ventral horns. The dorsal horn contains the cell bodies of neurons receiving incoming sensory information, which they distribute elsewhere in the cord or to the brain. The ventral horns contain cell bodies of motor neurons that project out from the cord. The white matter contains the axons of nerve fibres projecting up and down the cord linking different levels of the spinal cord with each other and to the brain. Form and Function The spinal cord has two basic functions. It maintains simple reflexes at the segmental level - and it transmits information up and down the cord between segmental levels. Spinal Reflexes (intersegment communication) Sensory information arriving from the periphery synapses on cell bodies in the dorsal horns of the spinal cord. These are thus, second order neurons and are referred to as interneurons or association neurons. For reflex arcs, they carry the information to motor neuron cell bodies in the ventral horns on the same side, the opposite side, and at different levels in the cord. The motor neurons in the ventral horn project to effectors in the periphery. Many reflexes occur completely at the segmental level. One such classical example is the withdrawal reflex. This reflex excites flexors on the ipsilateral side and extensors on the contralateral side of the body leading to withdrawal of the stimulated limb and extension and transfer of weight to the unstimulated limb. The interneurons (association neurons) will also transmit information up the cord to the brain (ascending information) and can also receive descending information from the brain (descending information). Spinal Reflexes (intrasegment communication) Nerve fibres carrying similar information tend to travel together in nerve tracts and those carrying information up the cord are referred to as ascending tracts and those carrying information down the cord are referred to as descending tracts (most tracts are named in terms of where they originate and where they terminate - spinothalamic tract versus tectospinal tract).