lecture 15 - animal nervous systems 2 Flashcards
Describe the different types of nerve cell
Nervous systems consist of three nerve cell types: sensory neurons, interneurons, motor neurons.
Sensory neurons receive and transmit information about environment or internal physiological state, interneurons process information received by sensory neurons, transmit it to motor neurons. Motor neurons produce suitable responses to the signals received.
Most sensory neurons are pseudounipolar (has one axon with two branches), some are bipolar (two extensions; one axon and one dendrite). Most interneurons and motor neurons are multipolar (several extensions; one axon and several dendrites)
Describe the organisation of animal nervous systems
Brain: centralized information-processing group of neurons
Ganglion: group of nerve cell bodies that process information from local region
Nerve and nerve cord: collection of bundled axons
Brain + main nerve cord = CNS
Long nerves + ganglions = peripheral nervous system
The organization and complexity of an animal’s nervous system reflects its lifestyle. Sessile animals (e.g. sea anemone) have a simple nervous system, active animals have a more complex nervous system. The only multicellular animals without a nervous system are sponges.
Describe ‘sensory-motor-interneurons’ in Hydra
Hydras have ‘all-in-one’ neurons that act as sensory neurons, interneurons, and motor neurons. They receive sensory information from the environment, directly stimulate muscle cells and nematocyte cells, and form connections between other neurons.
Describe the organisation of the human nervous system
Central Nervous System (CNS): Mainly interneurons
Peripheral Nervous System (PNS): Mainly sensory and motor neurons
The peripheral nervous system is organized into cranial and spinal nerves. Cranial nerves are located within the head, whereas spinal nerves run from the spinal cord to the periphery.
Afferent neurons send information towards the CNS, efferent neurons send information away from the CNS.
The central nervous system consists of brain and spinal cord.
Describe the peripheral nervous system
Sensory neuron cell bodies: ganglia
Motor neuron cell bodies: brain, spinal cord, ganglia
Most nerves contain axons from both sensory and motor neurons. Some cranial nerves (e.g. olfactory nerve, optic nerve) only contain sensory axons.
Sensory motor neurons are in contact with the cells in the spinal cord via the dorsal root, whereas the axons of the motor neurons leave the spinal cord via the ventral root. Dorsal root and ventral root join to form a nerve that contains both sensory and motor neurons
Made up of somatic nervous system (voluntary) and autonomic nervous system (involuntary).
What is the somatic nervous system?
sensory neurons that sense external stimuli and motor neurons that control voluntary muscle activation (skeletal muscle). Considered voluntary because it is under conscious control (but many reflexes are independent of conscious control)
What is the autonomic nervous system?
controls internal functions of the body (includes both sensory and motor neurons), usually act without conscious awareness
Describe the reflex circuit (somatic nervous system)
Reflex: automatic, rapid and predictable response to a stimulus
Simple reflex circuits: bypass the brain by directly connecting sensory neurons with motor neurons
quick reaction to stimulus
Describe the human knee-jerk reflex
Human knee-jerk reflex:
striking the patellar tendon with a hammer
stretching of extensor muscle
sensory neurons sense stretch
transmit stimulating signal to motor neurons that lead to extensor muscle contraction (has physiological role in running or landing from a jump)
Also example of coordination of muscles for coordinated movements. When extensor muscle contracts, flexor muscle should be relaxed. Achieved by reciprocal inhibition: neurons that sense extensor stretch do not only activate extensor muscle contraction, but inhibit contraction of the flexor muscle (via an inhibitory motor neuron)
Describe the autonomic nervous system
Autonomic nervous system: controls internal functions of the body (includes both sensory and motor neurons), usually act without conscious awareness.
Autonomic nervous system is further divided into sympathetic and parasympathetic division.
Sympathetic: arousal and increased activity (‘fight-or-flight’ response), e.g. increase in heart and breathing rate, glucose release by liver, inhibition of digestion
Parasympathetic: ‘rest and digest’. Stimulates digestion and metabolic processes.
Sympathetic nerves leave CNS from middle region of spinal cord, parasympathetic nerves leave from the brain and lower levels of the spinal cord
There are two groups of motor neurons in the autonomic nervous system: preganglionic and postganglionic neurons. The cell bodies of preganglionic neurons are located in the CNS, whereas the cell bodies of the postganglionic neurons are located in ganglia. Sympathetic ganglia form a chain close to the spinal cord, whereas parasympathetic ganglia are located in or near the effector organs.
Describe a reflex circuit (autonomic nervous system)
Reflex circuits of the autonomic nervous system are important for the control of internal body functions. Reflex circuits of the autonomic nervous system are similar to somatic nervous system reflex loops, but involve two efferent neurons. Some short reflexes (e.g. control of local gut motility) bypass the central nervous system, with interaction between sensory and motor neuron occurring in ganglion.
What are the functions of different regions of the brain?
Midbrain and hindbrain: basic body functions and behaviours
Forebrain: more advanced cognitive functions
Like the spinal cord, the brain consists of grey (neuron cell bodies) and white (neuron axons) matter. Fatty myelin of glial cells surrounding the axons make the white matter white
Describe the functions of glial cells in the CNS
- Structural and metabolic support for neurons
The CNS contains more glial cells than neurons. Glial cells have many functions, including physical support, providing nutrition, myelination, direction of migration of neurons during development, contribution to the blood brain barrier.
Astrocytes are the most abundant cells of the human brain.
